Seabird
As part of a grant provided to the World Seabird Union, digitization of the seabird life history database has now been complete! This is a 2013 update of Appendix 2 of the Biology of Marine Birds book by Schreiber and Burger. Updates to these data will be ongoing.
seabird
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The presence of "free" food in the form of offal and bait attracts many birds to fishing operations. In past years, more than 20,000 seabirds were hooked annually in groundfish fisheries off Alaska. Most birds taken during longline operations are attracted to the baited hooks when the gear is being set. These birds become hooked at the surface, and are then dragged underwater where they drown. Out of the total number of birds hooked, 75 percent are northern fulmars and gulls, although most regulatory and conservation attention is focused on bycatch of the endangered short-tailed albatross. With the use of seabird avoidance measures (e.g. paired and single streamer lines), seabird bycatch has been reduced four-fold.
Other than noted above, vessel operators using hook-and-line gear and fishing for groundfish in waters off the state of Alaska must refer to seabird avoidance measures in state regulations. See 5AAC 28.055.
Procellariiformes are the most threatened bird group globally, and the group with the highest frequency of marine debris ingestion. Marine debris ingestion is a globally recognized threat to marine biodiversity, yet the relationship between how much debris a bird ingests and mortality remains poorly understood. Using cause of death data from 1733 seabirds of 51 species, we demonstrate a significant relationship between ingested debris and a debris-ingestion cause of death (dose-response). There is a 20.4% chance of lifetime mortality from ingesting a single debris item, rising to 100% after consuming 93 items. Obstruction of the gastro-intestinal tract is the leading cause of death. Overall, balloons are the highest-risk debris item; 32 times more likely to result in death than ingesting hard plastic. These findings have significant implications for quantifying seabird mortality due to debris ingestion, and provide identifiable policy targets aimed to reduce mortality for threatened species worldwide.
While there are observations of seabird mortalities resulting from the ingesting debris13, and anecdotal evidence that ingestion has sublethal and lethal impacts on seabirds, a quantitative relationship is yet to be established. This is due to the difficulty of establishing a dose-response relationship between ingestion and mortality. In the absence of experimental feeding trials, the necropsy of wild seabirds collected deceased can provide the data to estimate a dose-response relationship. Wild seabirds die for many reasons including starvation, disease, injury, fisheries by-catch, and the ingestion of marine debris. We used seabirds that had an identifiable cause of death (e.g. fisheries, by-catch or advanced disease) as a control group (assuming their death was random with respect to the ingestion of marine debris). We compared these birds that died of indeterminate causes to those that were identified as having died due to marine debris ingestion (gut blockage, perforation or impaction). With this information, we investigated whether the ingested debris load is lowest in seabirds dying due to non-marine debris related causes, increasing for seabirds with indeterminate causes of death (which could potentially have died due to debris ingestion), and highest in seabirds that died from ingesting marine debris. Ultimately, we also used all seabirds in the dataset to estimate the relationship between the probability of death due to marine ingestion and the load of ingested marine debris. With this information we aim to determine whether there is a dose-response relationship between marine debris ingestion and seabird lifetime mortality, and if so, to predict the relationship between the load of ingested marine debris and the probability of death due to marine debris ingestion.
Quantity of marine debris ingested by seabirds by cause of death. The sum (left), cumulative weight (middle), and volume (right) of marine debris items ingested by Procellariiformes that have died from non-debris (KND) related causes, indeterminate (Ind) causes, and as a result of their marine debris ingestion (KD). An analysis of variance (ANOVA) and post-hoc pairwise t-test demonstrate that the sum of debris, volume of debris and weight of debris differ significantly between KND, Ind and KD birds.
Theoretical dose-response relationship between marine debris and seabird death. This model is adapted from Wilcox et al.14. We assigned each seabird death into three cause of death categories: (1) known, non-debris ingestion related (KND), where there was a clearly identifiable cause such as drowning as fisheries by-catch; (2) indeterminate (Ind), where there was marine debris present in the gut but also other possible causes, such as starvation and (3) known, marine debris ingestion related (KD), where there was a gut blockage, or other strong evidence of the ingested debris being the cause of mortality.
Slope of the relationship between probability of death due to marine debris ingestion and the debris load in the seabird. The top plot shows the distribution of slope estimates for the number of debris items in the gut, the lower plot shows the significance of these coefficients, from 1,000 Monte Carlo regression analysis samples.
Probability of mortality due to marine debris ingestion with increasing ingested marine debris load. Model results are based on the seabird species weight. Two models are shown, one based on Monte Carlo simulations. The first model assumes the cause of death has been assigned correctly, leading to animals with plastic ingestion as an assigned cause having a probability in the interval [1, 1] in the Monte Carlo process. The second model assumes plastic has been assigned incorrectly, leading to a probability in the interval [0, 1]. For each model we show the median (solid) and the extreme values (dotted) over 1,000 Monte Carlo simulations. The rug plot along the bottom of the figure shows the number of marine debris items ingested by each of the seabirds in our samples, with top showing seabirds that died of known non-debris ingestion causes, and beneath indicating those that died of either debris ingestion or were indeterminate.
All but one death resulted from obstruction in one of three locations; the gizzard and the small junctures leading in and out of the gizzard, causing either a large obstruction through to a complete blockage. The unique gizzard morphology of Procellariiform seabirds, (the gizzard is separated from the proventriculus by an isthmus juncture where hard items can become lodged and not easily regurgitated) puts Procellariiformes at higher risks of obstructions15. For this reason we can expect Procellariiformes to be at higher risk of mortality by marine debris than other species.
The composition of the marine debris influenced the probability of the likelihood of mortality, as observed in sea turtles. In turtles small hard plastic fragments may pass quickly through the gut with little incident14, but soft plastics are more likely to compact and contribute to a blockage or obstruction16. In seabirds not all ingested debris posed an equal risk of mortality. Hard plastics were responsible for only half (55%) of known and probable seabird deaths, but were the vast majority of items ingested. Soft plastic packaging, balloon fragments, rubbers and synthetic foams together accounted for only 5.4% of items but were responsible for 42% of probable and known mortalities. The ingestion of a soft item (10 confirmed or probable deaths from 140 items ingested) is 14 times more likely to result in death than the ingestion of a hard item (12 known/probable deaths from 2468 items ingested). This may be due to soft and pliable items resisting peristalsis and becoming obstructions17. Obstructions of soft pliable synthetics, including plastics and rubbers, have been recorded in a number of species including dogs18, cattle17 and birds19. In birds, such obstructions can cause death by enteritis, as well blocking the passage of food resulting in starvation19.
Balloons were the marine debris most likely to cause mortality. Where ingestion of balloons or balloon fragments were found, these fragments were the known or probable cause of death in 18.5% of balloon ingesting seabirds, with the ingestion of a balloon or balloon fragment is 32 times more likely to result in death than ingestion of a hard plastic fragment (5 known/probable deaths from 32 balloons ingested). Other studies have highlighted balloons as a high risk items for ingestion in other taxa20,21. Of particular concern is that seabirds may select for balloons when foraging because of their resemblance to prey, especially squid4. All balloons in this study were ingested by species that eat squid, suggesting these squid-feeding species are likely to have higher mortality rates. We suggest that reducing the presence of balloons and balloon fragments in the ocean would directly reduce seabird mortalities resulting from marine debris ingestion, and would have eliminated the 23% of confirmed KD deaths in this study for which balloons were cause. We propose that the most immediate solution to reduce seabird mortality from anthropogenic marine debris ingestion would be to reduce the amount of marine debris, particularly the number of balloons, entering the ocean.
In summary, we provide strong evidence that marine debris ingestion can cause death in seabirds, and that the probability of death increases with the number of items ingested. This finding has substantial transboundary implications for estimating mortality due to marine debris ingestion and consequently managing wildlife population declines. In addition, we highlight high-risk marine debris items, providing identifiable policy targets aimed at reducing mortality caused by high-risk debris items. Reducing the input of waste into the environment, in particular high-risk items, will undoubtedly reduce debris ingestion mortality in marine wildlife. 041b061a72