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Environmental Characteristics Associated with the Occurrence of Avian Botulism in Wetlands of a Northern California Refuge

Management Implications

The number of permanent wetlands (4) used in this study was small and all were located within a narrow geographic area (SNWR); therefore, the range of environmental conditions we measured was not very broad. We were able to analyze the data for simple associations between environmental parameters and botulism outbreaks, but not for complex associations that might be expected with variables like pH and redox potential, for which optimal ranges are likely to occur. For example, botulism outbreaks occurred in wetland enclosures with a sediment pH (pH S1) that ranged between 7.0 and 7.5 (Table 1), whereas a much broader range of pH was evident in enclosures where outbreaks did not occur (6.0–8.3). In spite of its limitations, the results of our study convey a preliminary indication of wetland characteristics that are associated with botulism outbreaks and provide a basis for future research and development of management strategies to reduce botulism losses. Most notably, we identified several environmental conditions associated with the occurrence of botulism outbreaks in high risk wetlands at SNWR, including higher temperature and higher invertebrate abundance or biomass. However, because these factors were not consistently higher in outbreak wetlands compared to nonoutbreak wetlands, they may have a more proximate effect in initiating an outbreak. Other wetland characteristics, such as redox potential, appear to play a greater role in determining whether a wetland is at high or low risk for botulism outbreaks, and these risk factors apparently vary annually.

Because wetlands tend to be seasonally dynamic, evaluation of environmental conditions that confer risk is complicated. Our results suggest several factors warrant further examination, including temperature, redox potential, pH, and conductivity. Future investigations should consider if characteristics of individual wetlands predetermine their risk for botulism outbreaks, and if other proximate conditions are required to initiate mortality. Also, more work is needed to determine how mortality rates from botulism are related to environmental conditions. Knowledge of these factors could be used to develop wetland management strategies that reduce the risk of botulism outbreaks and reduce mortality of waterbirds.

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