Guide Biology of Sea Turtles, Volume 3

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These turtles occupy pelagic environments and are more likely to feed in areas where debris accumulates surface waters and convergence zones 9. Pelagic juveniles caught in longline fisheries operating in convergence zones in many parts of the world show relatively high rates of plastic ingestion 10 , 21 suggesting that high volumes of debris in seemingly healthy juveniles is a widespread phenomenon.

Our model would suggest these animals have relatively high mortality probabilities from plastic ingestion, between 0. This apparent contradiction may point to one bias in our data and analysis. Most of the animals we necropsied, and those included in StrandNet, were sourced from coastal areas where they have washed up either incapacitated or dead. These coastal animals have died from a mixture of causes, with the cause of death not strongly related to the likelihood of sampling the animal. The pelagic juveniles differ, as the cause of death is directly related to the chance of sampling an animal.

A quantitative analysis linking sea turtle mortality and plastic debris ingestion

Only animals caught in pelagic longlines were recovered and necropsied. This implies we have no samples of animals that have died due to plastic ingestion which would correspond to the longline caught animals. If plastic debris in offshore regions is smaller and more compactly shaped, it might be that there is a significant difference reduction in the likelihood of mortality due to ingestion of plastic debris in these regions.

This again suggests that our model should be used primarily in coastal regions, and if applied offshore should be considered an upper bound on the probability of mortality. Nearly species are now known to interact with anthropogenic debris 22 and as more species are investigated, the number continues to rise. As global plastic production increases, so too does our understanding of the ubiquity and impacts of anthropogenic debris on marine fauna such as seabirds 2 , 23 , fish 24 , 25 , marine mammals 2 , 26 , and a range of invertebrates 27 , 28 including corals The model has broad applicability and can be adapted for other taxa to understand dose responses to plastic ingestion for other marine taxa of interest.

Necropsies were performed on sea turtles obtained from across Queensland, Australia Fig. Of these, were green sea turtles Chelonia mydas , 52 were hawksbill turtles Eretmochelys imbricata , 30 were loggerhead turtles Caretta caretta , one was a flatback turtle Natator depressus and one was an olive ridley turtle Lepidochelys olivacea. There were also two unidentified deceased turtles. The turtles ranged from hatchlings through to adults CCL ranged from 4. Necropsies followed procedures described in Wyneken Cause of death COD was attributed where possible.

Contents of the gastrointestinal tract were sieved to extract any plastic present. When found, plastic was classified into material type, measured, counted and weighed. Animal ethics permits were not required to carry out necropsies, as animals were deceased. To investigate a potential dose-response relationship between debris and death, we first classified each death into four general causes: 1 unknown Ukn , where there was no clear evidence of or cause of death, including absence or very low levels of plastic in the digestive system; 2 known, non-plastic ingestion related KNP , where there was a clearly identifiable cause such as drowning in fishing gear; 3 indeterminate Ind , where there was substantial plastic debris present in the digestive system but also other possible causes, such as infection or propeller cuts; and 4 known, plastic ingestion related KP , where there was gut blockage, perforation, or other clear signs of plastic driven mortality.

This can be seen conceptually in Fig. All analyses were performed using R version 3. First we tested for differences in the amount of plastic debris measured as the count of items present in the four fate categories using a generalized linear regression model. We utilized a negative binomial error, due to over-dispersion in the data, which proved adequate based on a Chi square test. In addition to cause of death, we included age class and species as variables, as these can influence the frequency of debris ingestion 9.

We then assigned an interval value for the probability of death due to plastic ingestion, for each animal. Animals with known causes other than plastic ingestion KNP, e. Animals with either unknown causes, or indeterminate causes i. In order to accommodate the interval values for the unknown and indeterminate causes of death, we used a Monte Carlo technique. We randomly drew a value in the interval [0,1] for each observation in these two categories Unknown and Indeterminate , fit the model to the full dataset across all four causes Unknown, Known not plastic ingestion, Indeterminate, and Known plastic ingestion , and captured the estimated coefficient for the number of plastic items in the gut and its standard error.

We repeated this process 1, times, and report the distribution of the coefficient and its significance. We tested three alternative models for the relationship between plastic load and probability of death due to plastic. The simplest model accounted for the effect of gut volume on the relationship between chance of death due to plastic and the number of items by including the curved carapace length of the animals as a covariate, on the assumption that it is roughly proportional to gut volume.

There is a potential for animals of different age classes to be exposed to differing debris compositions. For instance, coastal adults might encounter mostly larger plastic debris that has recently washed into the ocean, while pelagic early juveniles might encounter smaller fragments that have degraded offshore. We tested two alternative models see SI for further detail in the Monte Carlo analysis. One model included the residuals from the relationship between the number of debris items found in a turtle and the dry weight of the items as a predictor, in addition to the total number of items.

This allowed us to capture the size distribution of the items, without the issues of collinearity that would arise if we included both the number and the total weight directly. The second alternative utilized the number of items divided by the curved carapace length, as a measure of the amount of debris in a turtle standardized for its size. We also included a factor for age class, which would allow the debris density i.

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For the regression analyses we aggregated the three adult subclasses into a single adult group, yielding 4 age classes: Hatchling, Post-Hatchling, Juvenile and Adult. Because the only way to determine if plastic was ingested is by necropsy, we restricted our analysis to include only animals that had been necropsied. The extent of data from StrandNet reported necropsies varied widely. Plastic densities were then classed as a Present, b Not present or c None noted; as many cases made no explicit mention of plastic debris presence or absence.


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When available, debris characteristics numbers and debris type were included. We also recorded the cause of death, as reported in the StrandNet record. Just as in the necropsy analysis we aggregated the causes of death in Strandnet to four categories, 1 cause of death unknown but with no or very low levels of plastic in the digestive system ; 2 known and not related to plastic ingestion; 3 indeterminate plastic was present in the gut but not clearly the sole cause of death ; and 4 plastic ingestion resulted in mortality.

We used logistic regression to estimate the probability that debris was found in an animal conditional on its assigned cause of death, the level of examination done, age class, and species. Jambeck, J. Plastic waste inputs from land into the ocean. Science , — Wilcox, C. Using expert elicitation to estimate the consequences of marine litter on seabirds, turtles and marine mammals. Policy 65 , — Schuyler, Q.

Risk analysis reveals global hotspots for marine debris ingestion by sea turtles. Global Change Biol. Ghostnet impacts on globally threatened turtles, a spatial risk analysis for northern Australia. Derraik, J. The pollution of the marine environment by plastic debris: a review. Laist, D. Impacts of marine debris: Entanglement of marine life in marine debris including a comprehensive list of species with entanglement and ingestion records.

Coe, D. Springer, New York Cornelius, S. Copeia 1 , — Fritts, T. Plastic bags in the intestinal tracts of leatherback marine turtles. Review 13 , 72—73 Gonzalez Carman, V. Young green turtles, Chelonia mydas , exposed to plastic in a frontal area of the SW Atlantic. Tomas, J. Marine debris ingestion in loggerhead sea turtles, Caretta caretta from the Western Mediterranean. Tourinho, P. To Eat or Not to Eat? Debris Selectivity by Marine Turtles. PloS One 7 10 , e Mistaken identity? Visual similarities of marine debris to natural prey items of sea turtles.

BMC Ecology 14 , 14 b. Lutz, P. Studies on the ingestion of plastic and latex by sea turtles, In: R. Shomura and H. Nelms, S. Plastic and marine turtles: a review and call for research. ICES J. Valente, A. Furthermore, turtles likely differ in their exposure to amounts and types of plastic, based on their feeding location in the water column and with respect to the coastal zone.

Plastic fragments on the water surface have been found to be larger near coastal zones and in the gyres, presumably due to their proximity to coastal sources or collection points in the gyres 1 , As such, one would expect turtles feeding in surface waters near coastal margins to be more likely to ingest larger fragments that would be more likely to cause mortality. This interaction between feeding location, plastic characteristics, and life history stage is reflected in our data.

Twenty-three percent of the juveniles and fifty-four percent of post-hatchling stage turtles in our necropsies ingested plastic, in comparison with fifteen percent of the sub-adults and sixteen percent of adults. The younger animals are feeding in the water column nearer the surface, and in some cases in coastal environments where debris is potentially bigger or in convergence zones where plastics accumulate. Thus, in using our results to translate rates of plastic ingestion into probability of mortality, it is important to consider both the life history stage of the animal and the location where it is feeding.

There have been discussions on sea turtles and debris selectivity. A study on the visual similarities between their natural prey items and the plastic debris ingested suggests sea turtles will actively seek out plastic debris that appear similar to their food sources, particularly flexible film-like items However, this does not preclude instances where ingestion occurs incidentally.

Regardless, post hatchlings and juveniles are shown to have higher incident rates of ingestion 9. These turtles occupy pelagic environments and are more likely to feed in areas where debris accumulates surface waters and convergence zones 9. Pelagic juveniles caught in longline fisheries operating in convergence zones in many parts of the world show relatively high rates of plastic ingestion 10 , 21 suggesting that high volumes of debris in seemingly healthy juveniles is a widespread phenomenon.

Our model would suggest these animals have relatively high mortality probabilities from plastic ingestion, between 0. This apparent contradiction may point to one bias in our data and analysis. Most of the animals we necropsied, and those included in StrandNet, were sourced from coastal areas where they have washed up either incapacitated or dead. These coastal animals have died from a mixture of causes, with the cause of death not strongly related to the likelihood of sampling the animal.

The pelagic juveniles differ, as the cause of death is directly related to the chance of sampling an animal. Only animals caught in pelagic longlines were recovered and necropsied. This implies we have no samples of animals that have died due to plastic ingestion which would correspond to the longline caught animals. If plastic debris in offshore regions is smaller and more compactly shaped, it might be that there is a significant difference reduction in the likelihood of mortality due to ingestion of plastic debris in these regions.

This again suggests that our model should be used primarily in coastal regions, and if applied offshore should be considered an upper bound on the probability of mortality. Nearly species are now known to interact with anthropogenic debris 22 and as more species are investigated, the number continues to rise. As global plastic production increases, so too does our understanding of the ubiquity and impacts of anthropogenic debris on marine fauna such as seabirds 2 , 23 , fish 24 , 25 , marine mammals 2 , 26 , and a range of invertebrates 27 , 28 including corals The model has broad applicability and can be adapted for other taxa to understand dose responses to plastic ingestion for other marine taxa of interest.

Necropsies were performed on sea turtles obtained from across Queensland, Australia Fig. Of these, were green sea turtles Chelonia mydas , 52 were hawksbill turtles Eretmochelys imbricata , 30 were loggerhead turtles Caretta caretta , one was a flatback turtle Natator depressus and one was an olive ridley turtle Lepidochelys olivacea. There were also two unidentified deceased turtles. The turtles ranged from hatchlings through to adults CCL ranged from 4. Necropsies followed procedures described in Wyneken Cause of death COD was attributed where possible.

Contents of the gastrointestinal tract were sieved to extract any plastic present. When found, plastic was classified into material type, measured, counted and weighed. Animal ethics permits were not required to carry out necropsies, as animals were deceased. To investigate a potential dose-response relationship between debris and death, we first classified each death into four general causes: 1 unknown Ukn , where there was no clear evidence of or cause of death, including absence or very low levels of plastic in the digestive system; 2 known, non-plastic ingestion related KNP , where there was a clearly identifiable cause such as drowning in fishing gear; 3 indeterminate Ind , where there was substantial plastic debris present in the digestive system but also other possible causes, such as infection or propeller cuts; and 4 known, plastic ingestion related KP , where there was gut blockage, perforation, or other clear signs of plastic driven mortality.

This can be seen conceptually in Fig. All analyses were performed using R version 3. First we tested for differences in the amount of plastic debris measured as the count of items present in the four fate categories using a generalized linear regression model. We utilized a negative binomial error, due to over-dispersion in the data, which proved adequate based on a Chi square test.

In addition to cause of death, we included age class and species as variables, as these can influence the frequency of debris ingestion 9. We then assigned an interval value for the probability of death due to plastic ingestion, for each animal. Animals with known causes other than plastic ingestion KNP, e. Animals with either unknown causes, or indeterminate causes i.

In order to accommodate the interval values for the unknown and indeterminate causes of death, we used a Monte Carlo technique. We randomly drew a value in the interval [0,1] for each observation in these two categories Unknown and Indeterminate , fit the model to the full dataset across all four causes Unknown, Known not plastic ingestion, Indeterminate, and Known plastic ingestion , and captured the estimated coefficient for the number of plastic items in the gut and its standard error. We repeated this process 1, times, and report the distribution of the coefficient and its significance.

We tested three alternative models for the relationship between plastic load and probability of death due to plastic. The simplest model accounted for the effect of gut volume on the relationship between chance of death due to plastic and the number of items by including the curved carapace length of the animals as a covariate, on the assumption that it is roughly proportional to gut volume.

There is a potential for animals of different age classes to be exposed to differing debris compositions. For instance, coastal adults might encounter mostly larger plastic debris that has recently washed into the ocean, while pelagic early juveniles might encounter smaller fragments that have degraded offshore.

We tested two alternative models see SI for further detail in the Monte Carlo analysis. One model included the residuals from the relationship between the number of debris items found in a turtle and the dry weight of the items as a predictor, in addition to the total number of items. This allowed us to capture the size distribution of the items, without the issues of collinearity that would arise if we included both the number and the total weight directly.

The second alternative utilized the number of items divided by the curved carapace length, as a measure of the amount of debris in a turtle standardized for its size. We also included a factor for age class, which would allow the debris density i. For the regression analyses we aggregated the three adult subclasses into a single adult group, yielding 4 age classes: Hatchling, Post-Hatchling, Juvenile and Adult. Because the only way to determine if plastic was ingested is by necropsy, we restricted our analysis to include only animals that had been necropsied.

The extent of data from StrandNet reported necropsies varied widely. Plastic densities were then classed as a Present, b Not present or c None noted; as many cases made no explicit mention of plastic debris presence or absence. When available, debris characteristics numbers and debris type were included.


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We also recorded the cause of death, as reported in the StrandNet record. Just as in the necropsy analysis we aggregated the causes of death in Strandnet to four categories, 1 cause of death unknown but with no or very low levels of plastic in the digestive system ; 2 known and not related to plastic ingestion; 3 indeterminate plastic was present in the gut but not clearly the sole cause of death ; and 4 plastic ingestion resulted in mortality. We used logistic regression to estimate the probability that debris was found in an animal conditional on its assigned cause of death, the level of examination done, age class, and species.

Jambeck, J. Plastic waste inputs from land into the ocean. Science , — Wilcox, C.

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Using expert elicitation to estimate the consequences of marine litter on seabirds, turtles and marine mammals. Policy 65 , — Schuyler, Q. Risk analysis reveals global hotspots for marine debris ingestion by sea turtles.

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Global Change Biol. Ghostnet impacts on globally threatened turtles, a spatial risk analysis for northern Australia. Derraik, J. The pollution of the marine environment by plastic debris: a review. Laist, D. Impacts of marine debris: Entanglement of marine life in marine debris including a comprehensive list of species with entanglement and ingestion records. Coe, D. Springer, New York Cornelius, S. Copeia 1 , — Fritts, T. Plastic bags in the intestinal tracts of leatherback marine turtles.

Review 13 , 72—73 Gonzalez Carman, V. Young green turtles, Chelonia mydas , exposed to plastic in a frontal area of the SW Atlantic. Tomas, J. Marine debris ingestion in loggerhead sea turtles, Caretta caretta from the Western Mediterranean. Tourinho, P. To Eat or Not to Eat? Debris Selectivity by Marine Turtles.