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Recent studies suggest that, on average, quantitative genetic variation may not be lost within small populations as rapidly as neutral genetic diversity, but that levels of quantitative genetic variation can be highly variable among small populations Willi et al. Similarly, putatively neutral microsatellite loci are located in parts of the genome that are not subject to natural selection. As a result, allelic characteristics at these loci they may have little or no relationship to survival and fitness, and they tell us nothing about genetic changes at quantitative traits that might be occurring in the captive environment Reed and Frankham ; McKay and Latta Consequently, even if levels of neutral genetic diversity can be sufficiently maintained in captivity, caution must be exercised in interpreting such data for risk assessment and the ability of captive breeding programs to maintain fitness, a subject treated in detail in the next section.

A lengthy, two-sided debate surrounds the use of harvest augmentation, supplementation and captive breeding programs to either increase salmonid harvest levels, give a demographic boost to declining, at-risk populations, or to recover endangered salmonid populations, respectively. The debate is especially contentious with respect to whether or not hatchery- or captive-rearing, in general, can maintain attributes other than genetic diversity, namely fitness. A first predominant perspective argues that hatchery- or captive-rearing has negative impacts on the long-term persistence and fitness of wild salmonids.

Under this view, hatchery- or captive-rearing leads to unavoidable genetic changes within hatchery-raised salmonids, chiefly through domestication selection Box 1. Domestication selection results in a fitness reduction when hatchery- or captive-reared fish are then introduced into the wild and breed with wild fish. Such domestication selection can be reduced Table 2 , but it cannot be eliminated entirely Hindar et al. Theoretical work also suggests that domestication selection in the hatchery could have significant fitness consequences for a wild population in the case of supplementation programs, even if local, wild-born fish are used to generate hatchery fish each generation Lynch and O'Hely ; Ford ; Reisenbichler et al.

A corollary to this perspective is that hatchery programs, particularly hatchery augmentation and supplementation programs which have been the main focus of the debate, generally fail in their objective of maintaining fitness and of contributing to the natural productivity of wild salmonid populations Reisenbichler and Rubin ; Fleming and Petersson ; Reisenbichler et al.

A second and alternative perspective argues that hatchery- or captive-rearing of salmonids can maintain fitness within populations and play an important role in the supplementation of declining or recovery of endangered salmonid populations Brannon et al. A corollary to this perspective is that the genetic risks associated with hatchery- or captive-rearing have been overstated.

First, proponents of this view argue that, aside from theoretical studies on these genetic risks, the purported long-term effects of hatchery- or captive-rearing have little or no empirical basis Incerpi ; Rensel ; Brannon et al. Second, in many cases, apparent effects on wild populations have not been differentiated from the effect of management decisions involving the misuse of the hatchery fish Campton ; Rensel ; Brannon et al. Most notably, in many instances, hatchery fish from nonlocal rather than local source populations Box 1 were stocked into large geographic regions without consideration that they may not have been adapted to those areas Brannon et al.

To objectively evaluate the comparative strength of these divergent perspectives in the context of salmonid captive breeding programs, the evidence for each one must firstly be carefully sifted and presented see Appendix 1 for details of the literature search.

Particularly relevant are hatchery- or captive-rearing programs where i wild-born broodstock parents of hatchery fish are collected from a local river each generation, large numbers of their offspring are raised under captive conditions for a period of time, then released into the same local river, and where ii the lifetime fitness performance of the returning hatchery-born adults or their wild-born offspring versus wild adults can be directly evaluated in the wild. Under these conditions, one can most legitimately address the likelihood that current captive breeding procedures involving hatcheries will conserve fitness within populations.

Table 5 summarizes 30 laboratory studies that evaluated whether hatchery-rearing resulted in genetic changes in hatchery relative to wild salmonids. This list of studies by no means should be viewed as exhaustive as undoubtedly, some other studies have been inadvertently overlooked. The studies in Table 5 were not carried out in the wild, so they only address the potential for genetic changes incurred from captive breeding to have negative impacts on the persistence and adaptability of wild salmonids.

Additionally, many of these studies have been based on traditional supplementation practices see Table 1 ; footnotes of Table 5 and not necessarily on current captive breeding program procedures. Laboratory studies that have provided evidence for genetic changes or that found no evidence of genetic changes in phenotypic traits between hatchery-reared and wild populations of salmonid fishes.

Hatchery and wild populations were compared under common environmental conditions, unless otherwise noted. Of the 30 studies comparing hatchery and wild fish in Table 5 , only five compared hatchery fish derived from the same local population as the wild fish, and without confounding environmental and genetic differences or some degree of intentional artificial selection in the hatchery, which is not a typical element of captive breeding programs see Table 5 footnotes.

Of these five studies, three compared traits in hatchery and wild salmonids after one generation of captive breeding Dahl et al. Despite ample statistical power, only small, albeit significant, genetic differences were detected in two of three studies. Most significantly was a 2. In another study, trait differences that had been detected under hatchery conditions were not found when comparing hatchery and wild fish in the wild Dahl et al.

The other two studies compared traits in hatchery and wild salmonids after four to six generations of captive rearing Johnsson et al. Genetic differences were detected in three of four trait comparisons for juvenile growth rate and antipredator response. Finally, as expected, clear genetic differences between hatchery and wild fish were also detected when hatchery fish were nonlocal or had experienced intentional selection Table 5. Table 6 summarizes 20 studies that have directly evaluated the fitness performance of hatchery and wild salmonid fishes in the wild, with one additional study comparing fitness between fish with different degrees of captive-rearing Carofinno et al.

Again, this list of studies by no means should be viewed as exhaustive as undoubtedly, some other studies have been inadvertently overlooked. Likewise, many of these studies have been based on common supplementation practices rather than current captive breeding procedures see Table 1 ; footnotes of Table 6. Field studies within natural environments that have evaluated the fitness performance of hatchery and wild salmonid fishes. G , number of hatchery generations as in Table 5. Of these 20 studies comparing hatchery and wild fish in Table 6 , nine compared hatchery fish derived from the same local population as the wild fish.

Of these nine studies, three detected survival differences between hatchery and wild fish Reisenbichler and McIntyre ; Unwin ; Araki et al. However, the lifetime performance of second generation hatchery and wild fish in Reisenbichler and McIntyre differed in only two of four stream comparisons where hatchery fish survival was lower , and the Unwin study was confounded by rearing hatchery fish for 8—12 months in captivity before release into the wild from Brannon et al.

In addition, all studies finding no survival differences must be considered with caution because i hatchery fish were larger than wild fish when released into the wild Rhodes and Quinn ; Bohlin et al. On the other hand, unanimously, hatchery fish had inferior fitness when they were nonlocal or had been under intentional selection Table 6. To date, Araki et al. Based on steelhead trout Oncorhynchus mykiss , the program used wild-born broodstock parents of hatchery fish that were collected each generation and from which more numbers of offspring were raised in a hatchery for a period of time before being released into the same local river as 1-year old, juvenile smolts.

For a single generation, Araki et al. Consistent with what would be expected if captive breeding programs use local broodstock and minimize the time that individuals are kept in captivity, the authors found i no differences in reproductive success between local hatchery fish and wild fish, ii no differences in reproductive success between local hatchery-wild crosses and wild-wild crosses, but iii lower reproductive success in nonlocal hatchery fish relative to wild fish.

The results were therefore encouraging because they suggested that short-term captive-rearing programs of one generation might be capable of generating fish with quasi-equal fitness to that of wild fish. Still, Araki et al. The study also could not rule out the possibility that initial differences in rearing environments between the local hatchery and wild fish affected the former's fitness performance in the wild Araki et al.

Araki et al. The two chief results of the study were as follows. Second, relative to pure wild fish with no history of captive-rearing, and born and returning from sea in the same years a replication of Araki et al. The results of Araki et al. However, confidence intervals around point estimates of reproductive success were large in both Araki et al. This might account for the conflicting conclusions regarding whether one generation of captive-rearing leads to or does not lead to a loss of fitness in the wild.

If anadromous, hatchery-reared fish generate a greater proportion of nonanadromous offspring than anadromous wild fish, or vice-versa, then the relative fitness of hatchery-reared anadromous fish relative to wild fish in these studies would have been underestimated or overestimated, respectively.

Second, steelhead are often raised in hatcheries for a whole year to achieve a body size conducive to smoltifying which will increase survival chances in the wild Araki et al. This period of time in the hatchery is greater than in other salmonids e. Third, in many cases, hatchery- or captive-rearing programs for steelhead trout, and Chinook and coho salmon, accelerate growth rates and smoltification to achieve larger yearling smolts Mahnken et al. Fourth, results from supplementation or captive breeding programs that raise juveniles to the smolt stage in salmonids e.

A final informative study is that of Carofinno et al. The authors compared early life-history fry stage -to-smolt survival in the wild of fish derived from parents that had been raised in the hatchery to the fry stage H a versus fish derived from parents raised in the hatchery to age 1 H b.

These results were consistent with the hypothesis that the duration of time in the hatchery environment may increase the opportunities for domestication selection and hence reduce the fitness of fish released into the wild. However, it was unclear to what degree maternal effects might have affected the survival of fry from the two groups. Namely, the study assumed that the extra year of hatchery-rearing in mothers of H b fish had a negligible effect on their own offspring's survival relative to mothers of H a fish Carofinno et al. More recent simulations have shown that the severe loss of fitness in captive-reared steelhead trout Araki et al.

Other mechanisms associated with the captive-breeding process some already alluded to might also contribute to fitness declines, but these await empirical testing or exploration in salmonids or other taxa. First, manipulations during captive-rearing or breeding could elicit unusually high chromosomal abnormalities or epigenetic changes in salmonids, and thereby affect offspring fitness, O'Reilly and Doyle ; Araki et al.

Epigenetic changes such as alterations to DNA or mutations that affect gene regulation have been recently shown to have considerable effects in mammals Guerrero-Bosagna et al. For instance, typical rates of mutation, including in salmonids, are too low to generate large fitness effects over such short time-periods.

Still, even though a procedure such as equalizing family sizes has genetic and fitness benefits i. The procedure still has the potential to increase the likelihood that new mutations arising during the captive-breeding program will become fixed from domestication selection, in this case, because of a relaxation of natural selection in the captive environment Bryant and Reed ; Rodriguez-Ramilo et al. Nevertheless, the only empirical treatment of this topic involving fruitflies suggests that this may not be a great concern, even for large captive populations and long periods of captivity Rodriguez-Ramilo et al.

Third, maternal effects are common in early life history traits of salmonids Einum and Fleming ; Heath et al. These effects might influence the fitness of captive-reared fish if their mothers had experienced a period of time in the hatchery, as environmental variation in the captive environment relative to the natural environment may elicit plastic changes in reproductive investment. For instance, female salmonids raised in hatcheries tend to exhibit smaller egg sizes than wild females that are not necessarily genetically based Jonsson et al.

Finally, prevention of free mate choice for adults during captive-breeding might reduce the fitness of captive-reared offspring Berejikian et al. This may specifically inhibit sexual selection and the benefits gained from mating with differentiated partners in genes associated with improved immune responses [e. Indeed, in several salmonids, it appears that males and females seek out partners with maximal or at least intermediate MHC dissimilarity Landry et al. Although currently lacking critical empirical assessment in any salmonid or to my knowledge any other organism besides fruitflies , captive breeding programs adopting many recent procedures see details in Table 2 might reduce the severity of domestication selection or captive generations in a number of ways that could mitigate fitness reductions in captivity.

These procedures may be especially invaluable to programs dealing with the last remaining wild founders from a population that has become extirpated from the wild, given that some domestication selection in capacity is likely unavoidable in such cases. For instance, Atlantic salmon live-gene banking programs recently initiated in eastern Canada have individuals raised mainly or solely in the wild up to the end of juvenile stages, with the captive phase being the marine subadult-adult stage of the lifecycle because salmon are unable to survive in the wild at this stage for currently unknown reasons O'Reilly and Doyle In salmonids, wild exposure at the juvenile stages may be especially effective at reducing domestication selection, because this is a stage when mortality in the wild is especially high Waples ; Quinn These same programs also equalize family sizes in captivity and at the time of release into the wild O'Reilly and Doyle Theoretical and empirical studies King ; Allendorf , ; Frankham et al.

However, the only empirical study conducted to date on fruitflies did not find that the procedure minimized the loss of fitness upon the return of populations into the wild Frankham et al. Additionally, an inherent trade-off exists in subsequently equalizing family size following a period of exposure to the wild environment.

While this may increase levels of neutral genetic diversity in the successive captive broodstock, it may negate the fitness benefits accrued to the population from having natural selection disproportionately favour some families more than others during the period of wild exposure Box 2. Such a trade-off is perhaps one of the most perplexing issues facing captive breeding programs that attempt to conserve both genetic diversity and fitness, given that conserving each has its merits Box 2.

Owing to its potential advantages for reducing domestication selection in captivity, there is growing interest in having captive-bred individuals exposed to the wild for at least some portion of the lifecycle e. However, following a period of wild exposure, an unavoidable trade-off exists between retaining genetic diversity and fitness when generating the new captive broodstock. Casual arguments for conserving genetic diversity versus fitness might proceed as follows, and striking a balance between them may very well depend on the specific case:.

It is individuals from these better-surviving families that should be used disproportionately to generate the new captive broodstock. One cannot rule out that inter-family survival varies at different life history stages. Additionally, even with equalizing family sizes after wild exposure, the benefits of exposing genotypes within families to natural selection would still be gained. Furthermore, the disproportionate use of individuals from better-surviving families for generating the new broodstock would result in an irreversible loss of genetic diversity.

Some families would be under-represented and others potentially not represented at all. Such diversity may be important for the population to respond to future environmental change. Disproportionately using individuals from families with a greater fitness performance is most in line with what existing conditions in the wild can support. This practice should improve the likelihood that the reintroduced population will become self-sustaining. Captive-bred families favored by natural selection in the wild this year or the next might not be those favored several years or a decade down the road.

Cryopreserved sperm obtained from males in the founder or early generations of captivity could also be used to fertilize female eggs in subsequent generations Sonesson et al. This practice could mitigate the loss of fitness in captivity due to domestication selection or the relaxation of natural selection in captivity, by minimizing captive generations before reintroduction in the wild. The technique has been initiated in recently commenced live-gene banking programs of Atlantic salmon in Norway and Canada O'Reilly and Doyle , but like any tool, it has disadvantages that merit consideration as well discussed below.

Allowing captive-reared adults, or adults that have had some degree of captive-rearing, to also breed in the wild and thus have free mate choice, may generate offspring that have benefitted from sexual selection and whose parents have had exposure to natural breeding conditions and breeding grounds Berejikian et al. The procedure is currently being attempted as part of some Pacific salmon captive breeding programs Berejikian et al. Increasingly, hatchery-rearing procedures or environments are also being modified to more closely resemble the natural environment.

Modifications include reduced juvenile densities, overhead or submerged cover, naturally coloured substrate, antipredator behavior conditioning, subsurface rather than overhead feeding, and even net-pen rearing in natural environments Maynard et al. Reisenbichler pointed out that the effects of seminatural environments on potentially reducing domestication selection have not been empirically tested in salmonids, and he discussed two potential approaches for assessing this. Considerable uncertainty remains with respect to the short- and long-term fitness effects of captive breeding in salmonids, despite the numerous laboratory and field studies conducted to date on the performance of hatchery-reared and wild salmonids.

The most relevant studies to date also appear to have had limited statistical power to make general conclusions regarding whether or not one generation of captive-rearing can reduce fitness in the wild Araki et al. Caveats aside, the studies of Araki et al. Furthermore, as discussed by Hard and Waples , the power of even the most ambitious monitoring programs to statistically detect a captive-breeding effect on phenotypic and life history traits is likely very low because natural variability in the same traits is very high.

This means that the effects of captive-breeding might only be detected long after considerable harm to wild fish has occurred Waples On the other hand, for several reasons, the rate to which fitness was lost in Araki et al. Interestingly, fitness reductions in hatchery-reared salmonids detected in laboratory studies were not as strong as the Araki study 2. Studies involving nonlocal hatchery fish also suggest that fitness reductions will become elevated with increasing generations of manipulation or rearing in the captive environment see also Araki et al.

Indeed, many of the poorest performances of hatchery fish relative to wild fish involved nonlocal hatchery strains that had been in captivity for greater than five generations or that had undergone intentional artificial selection e. Finally, a major issue meriting further debate and study pertains to the trade-offs between maintaining genetic diversity and fitness of captive broodstocks Box 2 ; see also the section below on whether to use single versus multiple facilities to conserve genetic diversity and fitness.

For instance, there are clear fitness benefits to exposing individuals to existing conditions in the wild for some period of their lifecycle. There are also clear benefits to equalizing family sizes after a period of wild exposure to maintain neutral genetic diversity. Yet, this may also reduce the fitness benefits that were accrued during the period of wild exposure. Reintroduction attempts of a variety of captive-reared endangered species or populations into the wild have historically had mixed success Griffith et al.

Wolf et al. However, owing to the earlier dates in which a considerable portion of the studies within these reviews were conducted, many of these reintroduction attempts might have failed because the reintroduction programs did not account for all the prerequisites for success identified in later documentation, such as mitigating the factors originally leading to extirpation, behavioural deficiencies of the released animals, or improper release dates e. Additionally, it has only been widely recognized more recently that domestication selection may affect reintroduction success Frankham et al.

Thus, many historical captive breeding programs probably did not adopt procedures to reduce domestication selection or the loss of genetic diversity in captivity see Table 2. Seddon summarized a variety of definitions that have been considered regarding what constitutes a successful reintroduction. The definitions put forth have included i breeding by the first-wild born generation, ii a breeding population with recruitment exceeding adult death rates for 3 years, iii an unsupported wild population of a minimum of individuals, iv establishment of a self-sustaining population Griffith et al.

Evidently, the applicability of any one criterion might be limited depending on the life history characteristics of the species targeted for reintroduction Seddon I define a self-sustaining population as a population that persists for multiple generations in the absence of any human intervention, such as supplementation, artificial habitat enhancement or any degree of captive breeding or genetic modification.

In many ways, this definition is most in line with one of the ultimate goals of captive-breeding programs; that is, to re-establish a species in an area which was once part of its historical range IUCN The definition is also formulated with the hope that self sustainability will represent the long-term persistence of the reintroduced species, but does not assume that self sustainability is equated with long-term persistence. For instance, a salmonid population could be reintroduced as a self-sustaining population for several generations, but then a new threat might render it no longer viable e.

I also considered this issue from four additional contexts. First, was there any evidence that hatchery-reared fish in supplementation programs provided net long-term benefits to wild salmonid populations? These programs differ somewhat from reintroducing captive-reared salmonids into formerly occupied habitats, but they provide another context for assessing the potential for captive-reared lines to translate into self-sustaining populations. Third, how can one improve the chances of successful reintroduction if the wild environment has changed by the time the captive population can be reintroduced?

Fourth, was there any indication that particular salmonid species or life-history types may be more difficult to reintroduce successfully?

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This list of studies by no means should be viewed as exhaustive as undoubtedly, some other systems have been inadvertently overlooked. In 16 of 31 population systems, captive-breeding programs are too recent to assess whether they will ultimately be successful or not in translating into self-sustaining populations. In six of the remaining 15 systems, reintroductions have been unsuccessful at generating self-sustaining populations. Reintroduction failures have occurred even after 30 years of reintroduction attempts in some cases Table 7.

Reintroduction failure over this timeframe might not be too surprising given that many historical programs probably did not adopt procedures that are implemented in current captive breeding programs Table 2. However, the list of reintroduction failures also includes two captive breeding programs that incorporate many of these procedures e.

Importantly, not all of the obvious factors that were likely contributing to reintroduction failure had been removed in any of these six systems, regardless of whether current captive breeding procedures had or had not been adopted. While these factors were often multifaceted, it is noteworthy that environmental changes to habitat were implicated in all six systems with unsuccessful reintroductions Table 7.

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  7. Characteristics of salmonid reintroductions involving hatchery- or captive-reared fish. Conversely, there were no obvious habitat limitations in the nine population systems, where captive-breeding has led to apparently self-sustaining populations. Yet, in one case, artificial liming of rivers was required to reduce acidification induced by acid rain so that Atlantic salmon populations inhabiting them could be self-sustaining Hesthagen and Larsen In another case, successful reintroduction of sockeye salmon populations might have been driven by dispersal and gene flow from neighboring, healthy wild populations and not necessarily by captive-reared fish Withler et al.

    In four other cases, reintroduced populations might be becoming self-sustaining but they are all still dependent on supplementation Spidle et al. Environment Agency b , , Bosch et al. Consequently, there is little long-term evidence regarding whether captive-reared salmonids can or cannot be reintroduced as self-sustaining populations. This is either because i captive breeding programs that adopt a multitude of procedures to reduce domestication selection and the rate of loss of genetic diversity in captivity have been initiated too recently to assess the performance of captive releases in the wild, ii reintroduction failures were confounded by not having other threats removed that likely impeded reintroduction success, most notably, habitat loss or change, iii apparently successful reintroductions may have been confounded by other factors which could explain the success besides captive-breeding e.

    This estimate is based on the realistic amount of time required to initiate a captive-breeding program, carry out reintroduction attempts, and monitor postrelease success after multiple generations. Waples et al. Most programs 17 of 22 used hatchery fish from the local wild population for supplementation, but their data had not previously been summarized and published in the primary literature.

    For net long-term benefits to occur, Waples et al. Again, this situation is somewhat different from that of reintroducing captive-reared salmonids in an attempt to generate self-sustaining populations into formerly occupied habitats — it more typifies the situation where a captive-breeding program is initiated to supplement a rapidly declining population. Also, Waples et al. Bearing these caveats in mind, the major conclusions from the meta-analysis were as follows. Second, in the long-term, and in parallel to the observations and conclusions above, there is considerable uncertainty regarding the ability of supplementation programs to provide net long-term benefits to wild salmonid populations.

    As a result, these authors highlighted that the lack of empirical demonstration that supplementation provides net long-term benefits to wild salmonids should be a cautionary note to those considering initiating new programs or continuing existing ones Waples et al. These patterns are interesting for two reasons. Second, where salmonids have historically been capable of dispersing naturally, they have colonized all habitats currently suitable to them.

    The wild environment of captive salmonid populations might also change dramatically by the time fish can be reintroduced. The environment of the Bay may therefore be very different than that of say 15 to 20 years before its salmon populations collapsed, and these changes could have been the major reason for the collapse in the first place COSEWIC b. Krueger et al. Such an approach would presumably lead to a greater likelihood of that captive population evolving the capacity to respond to environmental change. To date, however, no empirical studies on any species have addressed this possibility Frankham , though research on this topic has recently been initiated within live-gene banking programs for Atlantic salmon populations in eastern Canada P.

    Still, one potential risk of this approach is that it could lead to an increase in straying to nontarget areas and thereby potentially affect other native populations. For instance, interbreeding of individuals between pink salmon populations resulted in increased straying rates to surrounding populations Bams In addition, and especially if the crosses will be carried out at a hierarchical level greater than subpopulations e.

    For instance, the advantages of generating greater genetic diversity in the released individuals might be outweighed by the possible disadvantages of outbreeding depression from mixing populations reviewed in Edmands Currently, there appears to be insufficient quantitative data on salmonid reintroductions to discern whether different species or life-history types vary in their chances of being successfully reintroduced into previously occupied habitats Table 7. However, if the ability of a salmonid species to be introduced successfully outside of its native range reflects its ability to be reintroduced into previously occupied parts of its native range, then two points merit consideration.

    First, anadromous populations, followed by lake migratory populations, may on average be more difficult to reintroduce than freshwater, resident populations. For instance, reviews of salmonid introductions suggest that anadromous salmonid populations do not transplant as well as freshwater species, perhaps because of their more complex requirements in having intricate life histories across multiple environments Withler ; Allendorf and Waples ; Utter Factors involved in freshwater salmonid declines might also be easier to rectify than those occurring across environments utilized by anadromous populations.

    Second, species such as rainbow trout and brown trout might be easier on average to reintroduce than species such as Atlantic salmon or several other Pacific salmon species, the former having been successfully introduced in many regions throughout the world where the latter have not Quinn ; references therein; Crawford and Muir One caveat of these predictions is that they assume the potential fitness consequences of captive-rearing are uniform across species and captive-breeding programs or even life-history variants within species.

    But as previously mentioned, this is likely not the case. A sensible but untested hypothesis is that captive-breeding programs elicit the greatest reductions in fitness in species or populations with the greatest life-history and habitat differences between captive and natural conditions Reisenbichler On the other hand, it appears that some supplementation programs, at least those involving juvenile releases, can achieve a measure of short-term success in terms of boosting overall numbers of fish Waples et al.

    It would also seem that many salmonid populations with long histories of intense supplementation have not become extinct or severely reduced in abundance. If fitness can be reduced so much and so rapidly by domestication selection, why have not many of these populations experienced rapid declines? Thus, an unresolved enigma in evaluating the likelihood that captive breeding programs can translate into self-sustaining salmonid populations, is whether, and how, increases to population abundance N provided by captive-rearing could offset reduced fitness in the wild of captive-reared fish and their progeny.

    Interestingly, there are numerous examples of the ability of salmonids to evolve rapidly in the wild over several generations Haugen and Vollestad ; Hendry et al. Certainly, then, the possibility exists that a reintroduced population based on captive-reared fish could re-adapt to the wild environment under a similar timeframe. Consider firstly a simple scenario where the original threats that led to the extirpation of a wild population have been removed and a one-time reintroduction of the captive-reared population is implemented.

    Owing to inevitable domestication selection in captivity, the captive-reared population has experienced a shift away from the wild optimum in quantitative trait variation related to fitness. Thus, it is now maladapted to the wild environment. Gomulkiewicz and Holt introduced a model examining conditions under which selection might prevent extinction of the captive-bred population upon reintroduction Fig. They considered whether such a population could evolve a sufficiently positive intrinsic growth rate r at abundance N below carrying capacity K before extinction from demographic stochasticity took place.

    Gomulkiewicz and Holt did not consider density-dependent effects but assumed that extinction risk was elevated below a threshold, critical population size Nc. In the context of attempting to reintroduce populations with captive-reared fish, the major implication of the model is that an initially maladapted reintroduced population with a negative growth rate could evolve a positive growth rate without going extinct, provided that: i genetic diversity was sufficiently high, ii fish were not too maladapted initially, and iii initial N was large relative to Nc to allow the reintroduced population to persist long enough for evolution to occur Fig.

    Note that these conclusions are also consistent with those in previous sections relating to the importance of maintaining as high a N e as possible in captivity Frankham et al. Note also, however, that there is an inherent tension between keeping N e and genetic diversity as high as possible and reducing domestication selection in captivity, a subject treated in detail in the next section. Potential relationships between reintroduced population abundance and extinction risk with or without evolution by natural selection, modified from Gomulkiewicz and Holt see also Kinnison and Hairston Population growth is density-independent and Nc represents a threshold abundance below which extinction risk is high.

    Without evolution, or when evolution cannot achieve replacement in the absence of gene flow, reintroduced populations decline to extinction A. Evolution is insufficient to prevent the reintroduced population from being at a high risk of extinction, but it allows the population to avoid extinction if the population persists B. Evolution is sufficient to prevent the population from being at a high risk of extinction C. Immigration and resultant gene flow allows the evolving population to avoid extinction more rapidly D than in its absence B. Immigration and resultant gene flow increases the susceptibility of extinction to the evolving population E than in its absence.

    All cases assume the same reduction in wild fitness within the captive-bred population before reintroduction. Gomulkiewicz and Holt's model thus also assumed that mechanisms exist that allow for positive population growth despite reintroduction of maladapted individuals, and similarly, that at some point following the initial drop in N from K , evolutionary contributions to population growth would not be countered by density-dependent factors Gomulkiewicz and Holt ; Tufto ; Kinnison and Hairston Unfortunately, empirical assessments of these assumptions are currently very limited in salmonids.

    For instance, analogous to reintroducing maladapted, captive-bred fish to a previously occupied habitat, Kinnison and Hairston and Kinnison et al.


    Under what conditions, then, could repeated reintroduction events increase the likelihood of successful overall reintroduction? On one hand, recurrent immigration from a maladapted, captive-reared source could demographically rescue a young, reintroduced population because the population literally never becomes extinct Holt Indeed, repeated influxes of immigrants have apparently been involved in some successful introductions or species invasions Lambrinos ; Roman and Darling On the other hand, immigrants would in general be maladapted to the local environment and resultant gene flow with the reintroduced population as it grows might constrain the effects of ongoing selection Fig.

    As a rough guide based on Gomulkiewicz and Holt , the reciprocal of the time a population first reaches low densities Nc following the initial reintroduction could be used as the frequency of gene flow episodes required for population persistence due to regular immigration or introductions. In short, assessments of the relative degree to which these opposing effects might affect reintroduction success are sorely needed. Whether single or multiple facilities are required to maintain both genetic diversity and fitness in captive breeding programs of endangered salmonids raises some important trade-offs to be factored in for biodiversity conservation.

    On one hand, to avoid significant losses of genetic diversity in captivity, captive populations must be kept at sufficiently large N e to slow the rate of loss of genetic diversity due to the genetic consequences of small N e Frankham et al. Yet, paradoxically, larger N e populations respond more readily to selection than smaller N e populations, all else being equal Robertson ; Weber and Diggins ; Allendorf and Luikart That is, a large N e facilitates adaptation by minimizing genetic drift, whereas a small N e increases genetic drift, which can hinder adaptation Crow and Kimura Consequently, while a larger N e is more advantageous than a smaller N e in the wild larger N e populations will on average be more capable of responding to environmental change than smaller N e populations , it might be disadvantageous in captivity larger N e populations may become more adapted than smaller N e populations to the captive environment.

    Nevertheless, Options 4—5 must be tempered with the fact that in small N e populations, one gets more genetic drift, in addition to some selection. Thus, a key issue for accommodating fitness and genetic diversity is not only the degree to which a captive population becomes adapted to the hatchery environment, but also the degree to which the selective regimes differ between the captive and wild environment. To throw more complexity into the different options, however, some theory Kimura and Crow ; Nei and Takahata ; see also Waples b predicts that Options 4—5 could also result in the maintenance of more overall genetic diversity and increase the overall N e compared to Options 1—3.

    This would only happen if no extinctions of the small populations occurred Kimura and Crow ; Nei and Takahata ; Lande ; Toro and Caballero Yet, such extinctions can arise in small captive breeding programs e. Thus, unless there is some means to avoid these captive population extinctions altogether, the potential genetic diversity benefits of Options 4—5 might not be realized.

    For salmonids, this would involve the maintenance of several small populations in captivity at one or multiple hatchery facilities, with translocations occurring only every several generations see Margan et al. To my knowledge, no empirical studies have tested whether the potential advantages of utilizing several small, isolated captive breeding populations with periodic mixture are upheld in salmonid captive breeding programs. These authors generated replicate populations and compared the genetic diversity and reproductive fitness of populations with the following N compositions: i 50 vs.

    Margan et al. The N compositions involving population subdivision e. Namely, cases involving subdivided populations that were then pooled, when compared to single large populations of equivalent total size, had lower inbreeding levels, significantly higher or similar reproductive fitness, and higher levels of genetic diversity i.

    There is only very limited empirical research to suggest that maintaining several small isolated populations with periodic mixing may be more effective at reducing losses of genetic diversity and fitness than maintaining a single large population. Periodic mixing might also reduce the risks associated with regular translocations e.

    Again, though, the tentative conclusion here is based on the assumption that no extinctions of the small populations occur in captivity. Although Frankham recently acknowledged that such a fragmentation regime had considerable merit, he did not recommend its application, perhaps because of the limited research on the subject. I now consider some potential pros and cons of these options as they may pertain to salmonids. This might have advantages in reducing i financial costs associated with translocations, ii the stresses that translocations impose on animals depending on the life-history stage of salmonid being translocated , and iii the potential asynchrony that might arise in breeding times and embryonic developmental times by using multiple facilities that realistically vary in their thermal regimes i.

    A first recommendation is that the small populations should not be so small that rapid inbreeding and loss of genetic diversity arises. A second recommendation, based on the results of Margan et al. Consequently, decisions to adopt such a strategy would have to weigh such benefits against their added financial costs, perhaps especially for i given the kind of space required to house adult salmonids.

    Finally, it is difficult to gauge how long the small populations should be maintained before pooling them. Inbreeding thresholds in salmonids are poorly characterized within species Wang et al. Yet, available data indicate that the fitness effects of inbreeding might be considerable in salmonid populations at a minimum of a half-sibling inbreeding coefficient without long histories of small population size Pante et al.

    As an overall cautionary approach, Margan et al. This may be unachievable in some cases unless pedigree information is available. Preceding summaries of certain sections in this review have suggested that salmonid captive-breeding programs may be unsuccessful in many cases because the root or purported causes of population decline or extirpation have not been mitigated. This implies that technical alternatives to hatchery facilities for conserving genetic diversity and fitness will also be unsuccessful unless at least some of the root causes of salmonid extirpation are corrected.

    Nevertheless, such technical alternatives may have practical utility in particular circumstances for conserving biodiversity. O'Reilly and Doyle recently reviewed the potential for cryopreservation techniques to reduce losses of genetic diversity and fitness in long-term live-gene banking programs.

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    Namely, cryopreserved sperm obtained from salmonid males in the founder or early generations of captivity could be used to fertilize female eggs in subsequent generations Sonesson et al. Because it can keep the genes within sperm largely intact for long periods of time hundreds to thousands of years; Stoss and Refstie , sperm cryopreservation has several advantages for biodiversity conservation. Second, the technique could minimize inbreeding and reduce domestication selection to captivity, as half of the gametes contributing to later generations would be obtained from individuals collected originally from the wild, or that had experienced only a single generation of captive rearing O'Reilly and Doyle Importantly, sperm cryopreservation techniques have been developed for a wide variety of endangered salmonids e.

    Sperm cryopreservation is not without its disadvantages. Because of its reduced viability relative to fresh sperm, more sperm than might be available through cryopreservation storage could be required to produce ample numbers of individuals that will in turn ensure modest numbers of mature adults for a live-gene banking program O'Reilly and Doyle Thus, cryopreserved sperm could not be depended upon to produce the last live-gene banking generation intended for release into the wild.

    How well can captive breeding programs conserve biodiversity? A review of salmonids

    Also, significant genetic divergence might occur between the founder and prerelease or release generations in live-gene banking programs O'Reilly and Doyle This could lead to outbreeding depression Box 1 in the release generations of a program if the cryopreserved sperm was not used within a few generations O'Reilly and Doyle Similarly, the wild environment might simply change during the generations of cryopreservation such that release generations may be maladapted to the wild by the time they are released.

    Finally, sperm cryopreservation cannot be viewed as a true alternative to hatcheries because it is necessarily dependent on breeding and rearing facilities. Techniques to preserve female eggs or fertilized embryos have not been developed for salmonids, so Thorgaard and Cloud and O'Reilly and Doyle reviewed two methods for reconstituting original wild populations from cryopreserved sperm. Either cryopreserved sperm from an extirpated population can be used to fertilize eggs from a nearby healthy population, or embryos can be produced with all-paternal inheritance androgenesis.

    The latter involves obtaining unfertilized eggs from females of a nearby extant donor population that are then irradiated to inactivate their genetic material, and then fertilizing them using cryopreserved sperm from the original native extirpated population Thorgaard and Cloud ; O'Reilly and Doyle The resulting androgenic diploids consist of DNA solely derived from the original native population by repressing the first cleavage division Thorgaard and Cloud Overall, these methods require considerable time and labour to reconstitute the original native gene pool, and suitable nearby extant populations may not be available to carry them out.

    Genetic changes associated with multiple generations of captive breeding and rearing will also arise when producing the final generation of juveniles intended for release into the wild. Finally, for androgenesis, the treatment used to block cleavage greatly reduces the survival of embryos, so additional crosses would likely be necessary with this method to retain heterozygosity and wild fitness. The most promising technical alternatives to captive breeding for conserving endangered salmonids are very recently developed surrogate broodstock technologies reviewed in Okutsu et al.

    These technologies involve the transplantation of primordial germ cells or spermatogonia from a target species into a related species, wherein the related species can then produce both viable sperm and eggs of the target species Okutsu et al. Okutsu et al. The authors were able to raise the injected masu salmon to maturity at which time the adults produced viable trout gametes.

    Nevertheless, intriguingly, the surrogated sperm and eggs when mixed created an F1 generation of normal trout, and this generation was subsequently able to produce a normal F2 generation of trout. For biodiversity conservation, the implication of Okutsu et al. I foresee five potential limitations of the technique. First, it is currently unclear how well the technique will work when adopted on different target and surrogate species of salmonids. The success rate of surrogate broodstock technologies might vary among species or even within species , or be considerably lower when using other species.

    Second, as in the case of sperm cryopreservation, the wild environment might simply change during the generations of cryopreservation such that captive-release generations may be unable to track selective changes in the wild by the time they are released. Third, the maternal environment of the surrogate might affect the performance of offspring. Fourth, there is potentially a political danger that efforts to protect endangered species habitat may be diminished if it is viewed that species can be brought back at any given future date.

    Fifth, chemicals and treatments involved in both surrogate broodstock technologies and sperm cryopreservation might generate epigenetic changes in captive-bred individuals. Epigenetic changes, such as alterations to DNA or mutations that affect gene regulation, have been recently shown to have considerable effects in mammals Guerrero-Bosagna et al. These changes might not be readily apparent in the hatchery environment but could have important fitness consequences when returning hatchery-fish into the wild P.

    Overall, such risks would have to be addressed if these techniques are to be considered sole alternatives to captive-breeding in endangered species restoration. Other alternatives to hatcheries for conserving species such as endangered anadromous salmonids might include A translocation to landlocked freshwater habitat, B transfer to other rivers that enter the sea, or C some mixture of artificial or semi-natural breeding from adult releases into natural river habitat, and then exclusive rearing of juveniles in freshwater and rearing of adults in sea pens, especially for those populations where marine survival is negligible.

    2018D2S3L3 Paulette Bloomer Genetic diversity in conservation management of freshwater fish species

    For instance, alternative A has been successful in generating new populations that act as safeguards against species extinction for endangered subspecies of cutthroat trout Oncorhynchus clarki spp. Cook, Dalhousie University, personal communication. However, though alternatives A and B do not necessarily require the extent of labour or resources as hatcheries, they may not be feasible in some cases.

    First, alternative A might not be applicable to some semelparous salmonids which show less evidence that they can support freshwater landlocked populations but see Laurentian Great Lakes chinook and pink salmon; Crawford Second, alternatives A or B also might not be justifiable if the endangered salmonid is nonnative and thus has the potential to impact native fauna, or if populations of the same species already exist there and interbreeding might occur.

    This is because the new environments, perhaps especially alternative A, might lead to potentially irreversible evolutionary change, or at least shifts in phenotypic trait distributions of populations. Finally, alternative C would likely still require some degree of hatchery support to assist in the artificial spawning of fish and to ensure a good representation of genetic diversity through the generations.

    This review on the extent to which captive breeding programs can conserve salmonid biodiversity reveals numerous trends and uncertainties. It also has several implications for ongoing salmonid captive breeding programs. Many of these implications are directly relevant to the assessments of captive breeding programs in other taxa, especially for species with indeterminate growth, high fecundities, or complex migratory lifecycles e.

    Encouragingly, for most captive breeding programs, neutral and perhaps quantitative genetic diversity within populations can be sufficiently maintained in captivity for several generations. Uncertainty over the longer-term also exists because programs adopting many procedures to reduce the loss of genetic diversity are still young, and these procedures have not been systemically evaluated for long-term effectiveness in salmonids and very rarely in other taxa.

    There is, nevertheless, great scope for current and future salmonid captive breeding programs to reduce the rate of loss of genetic diversity in captivity Table 2. In other words, maintenance of a large N e captive broodstock does not necessarily ensure the retention of genetic diversity pertaining to fitness. Though limited, the most relevant research suggests that quantitative genetic changes are likely manifested more rapidly than losses of overall neutral genetic diversity in captivity.

    There is also some indication that the magnitude of fitness loss increases as the duration in captivity increases. There is an unavoidable trade-off between reducing domestication selection during captive-rearing by having a period of wild exposure, and maintaining genetic diversity by equalizing family sizes of wild-exposed individuals when generating new broodstocks. What should be considered optimal in this regard merits serious discussion. Mechanisms reducing fitness in captivity and in the offspring of captive-wild matings are likely multifaceted, affecting behavior, swimming performance, imprinting, stress responses, growth, run-timing, developmental stability, developmental time to hatch, embryo size, maternal reproductive investment, body morphology and age-at-maturity, all of which may be linked to fitness.

    Identification of such mechanisms in specific cases could suggest ways to improve the chances of successful reintroduction in the long term. Owing to several confounding factors, there is currently little empirical evidence that captive-reared lines of salmonids can or cannot be reintroduced as self-sustaining populations. Research is sorely needed on whether the demographic advantages of increasing population abundance via captive breeding can outweigh the genetic disadvantages of losing fitness in captivity. There are biological pros and cons to maintaining captive broodstocks as either single or multiple populations within one or more hatchery facilities.

    This is especially the case when the objective is to retain both their genetic diversity and fitness. Astaxanthin is a powerful antioxidant that may be from a natural source or a synthetic trout feed. Rainbow trout raised to have pinker flesh from a diet high in astaxanthin are sometimes sold in the U. Trout can be cooked as soon as they are cleaned, without scaling, skinning or filleting. Medium to heavy bodied white wines, such as chardonnay , sauvignon blanc or pinot gris are typical wine pairings for trout.

    Rainbow trout is sometimes used as an indicator for water quality in water purification facilities. From Wikipedia, the free encyclopedia. This is the latest accepted revision , reviewed on 23 September Rainbow trout Adult female rainbow trout Conservation status. Walbaum , Eggs in gravel and rainbow trout alevin. See also: Salmon run. See also: Salmon in aquaculture. See also: Salmonid susceptibility to whirling disease.

    See also: Steelhead and salmon distinct population segments and Conservation status of British Columbia salmonids. See also: Artisan fishing , Recreational fishing , and Commercial fishing. See also: Salmon as food. Retrieved 1 August Retrieved Washington State Legislature. Trout and Salmon of North America. Tomelleri, Joseph R. New York: The Free Press. London: Richard Bentley.

    California Academy of Sciences. Archived from the original on Olympic National Park Report. Port Angeles, Washington. Government of Alberta-Fish and Wildlife Division. Archived from the original PDF on California Trout. Reviews in Fish Biology and Fisheries. Saturday Gazette-Mail. Charleston, West Virginia. The Free Press. Minnesota Department of Natural Resources. Oregon Department of Fish and Wildlife. R; Pottinger, T. Biology of Reproduction.

    Department of Agriculture. Wisconsin Department of Natural Resources. Alaska Department of Fish and Game. Marine Education Association of Australasia. July G3: Genes, Genomes, Genetics. Washington Department of Fish and Wildlife. British Columbia Ministry of Fisheries.

    Bethesda, Maryland: American Fisheries Society. Fish and Wildlife Service. Behnke from Trout Magazine. Globe Pequot. Our State magazine. Agricultural Resource Marketing Center. Agriculture and Agri-Food Canada. Monterey Bay Aquarium. BBC News. The New York Times. Sally's Place. Fish Disease Leaflet Geological Survey. Environmental Protection Agency. Deep Trout: Angling in Popular Culture. Oxford, United Kingdom: Berg.

    How well can captive breeding programs conserve biodiversity? A review of salmonids

    Trout Unlimited. Wild Salmon Center. November The Steelhead Society of British Columbia. Nature Precedings. Conservation Biology. Washington, D. Montana Outdoors. Journal of the Fisheries Research Board of Canada. Cal Trout. October Whirling Disease Foundation, Trout Unlimited.

    In Bergersen, E. January 13, The Trout Conservancy. Journal of Parasitology. Invasive Species Compendium. Centre for Agriculture and Bioscience International. Aquatic Invasions. Florida Integrated Science Center. North American Journal of Fisheries Management.

    Aquatic Nuisance Species Project. Biosecurity New Zealand. March Marine Fisheries Review. California Department of Water Resources. National Oceanic and Atmospheric Administration. Conservation Genetics. Santa Clara Valley Water District. North American Journal of Fisheries Management The Fisherman's World. New York: Random House. Public Land and Resources Law Review. In Yellowstone Park official introduced a fee permit policy to help pay the increased cost of protecting and enhancing this world-class fishery.

    Harry M. Abrams Inc. Few fish are caught here and tourists continue on to the West Yellowstone park exit, thinking that its too bad there are no fish in that pretty river that skirts the road.

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    Little do they know that they have been following one of the most fabled trout streams in the world! Trout Maverick. Montana Legislative Services. Wine Folly. May 20, Jihoceska Univ; Randak, T. Jihoceska Univerzita. Combs, Trey Steelhead Fly Fishing and Flies. Portland, Oregon: Frank Amato. Steelhead Fly Fishing.

    New York: Lyons and Burford Publishers. Gerlach, Rex Mechanicsburg, Pennsylvania: Stackpole Books. Halverson, Anders Review , Interviews Marshall, Mel New York: Winchester Press. McClane, A. New York: Times Books. McDermand, Charles Waters of the Golden Trout Country. New York: G. Putnam's Sons. Montaigne, Fen Reeling in Russia.

    New York: St. Martins Press. Scott and Crossman Freshwater Fishes of Canada. Bulletin Fisheries Research Board of Canada. Page Familiar Freshwater Fishes of America. Trout and char of the world. Trout - Salmonidae. Apache trout Cutthroat trout Gila trout Mexican golden trout Rainbow trout. Media related to Trout at Wikimedia Commons. Diseases and parasites in salmon Amoebic gill disease Ceratomyxa shasta Gyrodactylus salaris Henneguya zschokkei Infectious salmon anemia virus M74 syndrome Myxobolus cerebralis Nanophyetus salmincola Salmon louse Sea louse Salmon tapeworm Sphaerothecum destruens Tetracapsuloides bryosalmonae.

    Principal commercial fishery species groups. Carp Sturgeon Tilapia Trout. Eel Whitebait more Crab Krill Lobster Shrimp more Sea cucumbers Sea urchin more Commercial fishing World fish production Commercial species Fishing topics Fisheries glossary. Fish described in Taxa named by Johann Julius Walbaum. Namespaces Article Talk. Views Read Edit View history. In other projects Wikimedia Commons Wikispecies. By using this site, you agree to the Terms of Use and Privacy Policy. Oncorhynchus mykiss Walbaum , Kamchatkan rainbow trout.

    Western Pacific: the Kamchatka Peninsula, and has been recorded from the Commander Islands east of Kamchatka, and sporadically in the Sea of Okhotsk , as far south as the mouth of the Amur River. Anadromous forms are known as steelhead, freshwater forms as rainbow trout. Ocean and fresh water forms of coastal rainbow trout; a. Isolated in Lake Crescent , Washington. Redband forms.