Links between type E botulism outbreaks, lake levels, and surface water temperatures in Lake Michigan, 1963-2008

Relationships between large-scale environmental factors and the incidence of type E avian botulism outbreaks in Lake Michigan were examined from 1963 to 2008. Avian botulism outbreaks most frequently occurred in years with low mean annual water levels, and lake levels were significantly lower in outbreak years than in non-outbreak years. Mean surface water temperatures in northern Lake Michigan during the period when type E outbreaks tend to occur (July through September) were significantly higher in outbreak years than in non-outbreak years. Trends in fish populations did not strongly correlate with botulism outbreaks, although botulism outbreaks in the 1960s coincided with high alewife abundance, and recent botulism outbreaks coincided with rapidly increasing round goby abundance. Botulism outbreaks occurred cyclically, and the frequency of outbreaks did not increase over the period of record. Climate change scenarios for the Great Lakes predict lower water levels and warmer water temperatures. As a consequence, the frequency and magnitude of type E botulism outbreaks in the Great Lakes may increase.     

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.     

Ecological tracers track changes in bird diets and possible routes of exposure to Type E Botulism

Dreissenid mussels have become important components of the Great Lakes biological community since their introduction in the 1980s, but much remains to be understood regarding their effect on energy and nutrient flows in pelagic systems. Here, we report a new method that tracks incorporation of resources of molluskan origin into food webs used by aquatic birds. Biochemical tracers (fatty acids and stable carbon isotopes) are used to characterize species associated with pelagic and benthic food webs in Lake Ontario. Our focus is on the polymethylene-interrupted fatty acids (PMI-FAs) because previous research identified mollusks as their primary source. We found that PMI-FA mass fractions were greater in organisms associated with benthic (e.g. round goby) versus pelagic (e.g. alewife) food webs. Double-crested cormorants that had recently consumed benthic prey fish, i.e. goby, had greater proportions of PMI-FAs in their blood plasma than birds which showed no signs of recent goby ingestion. We did not detect an increase in mass fractions of PMI-FAs in cryogenically archived cormorant eggs following expansion of dreissenid mussels in Lake Ontario. However, following the introduction and expansion of round goby in the lake, PMI-FAs were detected at greater levels in cormorant eggs. These results illustrate how only after dreissenid mussel-facilitated establishment of round goby was the full extent of exotic species disruption of food webs manifested in fish-eating birds. These food web changes may be contributing to negative impacts on aquatic birds exemplified by the emergence of Botulism Type E as a significant mortality factor in this ecosystem.