Species composition and size class of prey items found in the gut contents of piscivorous birds that have died of avian botulism

Background/Question/Methods

Avian mortality caused by type E avian botulism occurs more frequently and is more widely distributed in the Great Lakes since 1999. Piscivorous waterbirds have experienced the greatest mortality, presumably because they ingest fish that carry type E botulism neurotoxin (BoNT/E); however, important vectors for botulism neurotoxin into fish eaten by birds have not yet been identified. Dreissenid mussels and round gobies (Neogobius melanostomas) are thought to play a role in the transfer of BoNT/E to piscivorous birds in the Great Lakes because they are dominant in benthic habitats where the botulism bacteria likely germinates, and because they are important prey items of several of the birds species that regularly die in botulism outbreaks. Round gobies that are > 70mm feed mainly on dreissenid mussels in the Great Lakes. My research objective is to analyze stomach contents of birds that die during botulism outbreaks in order to (1) Identify common prey items in BoNT/E positive birds; (2) Determine whether there are differences in the diets of BoNT/E positive and BoNT/E negative birds; and (3) Use length estimates of gobies found in the stomach contents of BoNT/E birds to infer likely trophic pathways for the toxin.

Results/Conclusions

Preliminary analysis of birds that died in botulism related outbreaks at Sleeping Bear Dunes National Lakeshore from 2007-2011 showed that round gobies were present in 89% of Common Loons (n = 18) and 69% of Double-crested cormorants (n = 13). Furthermore, the mean estimated length of round goby found in the stomachs of loons and cormorants was 114.5mm ± 12.7mm and 110.95mm ± 13.17mm respectively. This suggests that gobies and mussels may be important in the transfer of BoNT/E to birds. A more extensive analysis of the diet composition, mean prey size, and diet overlap of botulism positive and botulism negative birds from 2011 and 2012 could infer likely pathways of BoNT/E in Lake Michigan food webs, which will hopefully allow managers to generate plans for reducing avian mortality caused by the toxin.