The importance of island surveys in documenting disease-related mortality and Botulism E in Great Lakes colonial waterbirds

Type E Botulism is an important source of mortality for waterbirds on the lower Great Lakes. The purpose of this study was to determine: 1) the spatial distribution of waterbird mortality and 2) if that mortality possibly impacted the breeding colonial waterbird populations in eastern Lake Ontario. Six islands in eastern Lake Ontario, Canada, were searched, July-November, 2004-2009, for dead/moribund waterbirds. Over 6600 dead/dying birds were located; five species accounted for > 98% of the birds found: double-crested cormorant (Phalacrocorax auritus), herring, ring-billed and great black-backed gulls (Larus argentatus, Larus delawarensis, Larus marinus, respectively) and Caspian tern (Hydroprogne caspia). Cormorants accounted for 65-78% of all birds annually. Mortality was greatest in 2005 (7.5%). Most carcasses (91%) were documented on four islands. Most necropsied carcasses (58%, N = 95) were confirmed/suspected to have died from Type E Botulism; it was the only mortality factor identified in all years and in all five main species. These results produced a different guild of affected birds from previous beached bird surveys; virtually no birds that roost on water (loons and waterfowl) were found. Deaths reported here had minimal impact to herring and ring-billed gull, double-crested cormorant and Caspian tern populations nesting in eastern Lake Ontario. However, they accounted for > 100% of the great black-backed gulls breeding there. The species was extirpated from Lake Ontario during the study period. When assessing mortality in aquatic birds, it is crucial to examine off-shore islands, where birds roost, to fully document this critical demographic parameter.