After adjusting Lake Ontario herring gull egg PCB concentrations for the influence of time, an analysis was conducted to explain the remaining variation in annual egg PCB concentrations. In years with cold winters and/or high alewife abundance, egg PCB concentrations were greater than predicted. PCB levels were also greater than predicted in years when alewife condition was low. Increasing the proportion of alewives in the gull's diet may lead to increased PCB levels in eggs. Stable isotope analysis ([15N]/[14N]) of herring gull eggs provided evidence supporting this hypothesis. Consumption of alewives by gulls (as influenced by gull metabolism, alewife abundance/condition, end alewife overwinter mortality) and alewife population characteristics (growth rates and age distribution) may be the keys to explaining fluctuations in Lake Ontario herring gull egg PCB levels.

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Journal Environmental Science and Technology
Hebert, C.E, Shutt, J.L. (J. L.), & Norstrom, R.J. (R. J.). (1997). Dietary changes cause temporal fluctuations in polychlerinated biphenyl levels in herring gull eggs from Lake Ontario. Environmental Science and Technology, 31(4), 1012–1017. doi:10.1021/es960408p