Management and Conservation Response of Double-Crested Cormorants to a Large-Scale Egg Oiling Experiment on Lake Huron

ABSTRACT We report on a management experiment examining the effects of large-scale egg oiling on double-crested cormorant nest abundance and measures of seasonal cormorant density (bird-days/km 2 ) from 2000 to 2005. We employed the staircase design to distinguish transient responses to management treatments from site and year effects that generally contribute to variation in populations. The response to egg oiling in Georgian Bay was as expected with a decline in nest abundance attributable to egg oiling. In the North Channel, nest abundance did not decline because of egg oiling but increased, reflecting either retention of nesting adults or recruitment to colonies. This surprising outcome may stem from fish escapement from pen rearing facilities in the vicinity of the oiling experiment in the North Channel. We observed no effect of egg oiling on the July-August seasonal density of cormorants. The strongest effect size was associated with site effects followed by year effects for nest abundance and seasonal density. The effect size of egg oiling on variation in nest abundance did not exceed 5% for any year in both the North Channel and Georgian Bay. Fish pen culture appears to affect coastal distribution of cormorants. ß 2011 The Wildlife Society

Data_Sheet_1_Seasonality can affect ecological interactions between fishes of different thermal guilds.docx

Seasonality could play a crucial role in structuring species interactions. For example, many ectotherms alter their activity, habitat, and diet in response to seasonal temperature variation. Species also vary widely in physiological traits, like thermal preference, which may mediate their response to seasonal variation. How behavioral responses to seasonality differ between competing species and alter their overlap along multiple niche axes in space and time, remains understudied. Here, we used bulk carbon and nitrogen stable isotopes combined with stomach content analysis to determine the seasonal diet overlap between a native cold-water species [lake trout (Salvelinus namaycush)] and a range-expanding warm-water species [smallmouth bass (Micropterus dolomieu)] in two north-temperate lakes over 2 years. We coupled these analyses with fine-scale acoustic telemetry from one of the lakes to determine seasonal overlap in habitat use and activity levels. We found that dietary niche overlap was higher in the spring, when both species were active and using more littoral resources, compared to the summer, when the cold-water lake trout increased their reliance on pelagic resources. Telemetry data revealed that activity rates diverged in the winter, when lake trout remained active, but the warm-water smallmouth bass reduced their activity. Combining stable isotopes and stomach contents with acoustic telemetry was a powerful approach for demonstrating that species interactions are temporally and spatially dynamic. In our case, the study species diverged in their diet, habitat, and activity more strongly during certain times of the year than others, in ways that were consistent with their thermal preferences. Despite large differences in thermal preference, however, there were times of year when both species were active and sharing a common habitat and prey source (i.e., resource overlap was greater in spring than summer). Based on our findings, important ecological processes are occurring during all seasons, which would be missed by summer sampling alone. Our study stresses that quantifying multiple niche axes in both space and time is important for understanding the possible outcomes of altered seasonal conditions, including shorter winters, already arising under a changing climate.</p

A New Thermal Categorization of Ice-Covered Lakes

Lakes are traditionally classified based on their thermal regime and trophic status. While this classification adequately captures many lakes, it is not sufficient to understand seasonally ice‐covered lakes, the most common lake type on Earth. We describe the inverse thermal stratification in 19 highly varying lakes and derive a model that predicts the temperature profile as a function of wind stress, area, and depth. The results suggest an additional subdivision of seasonally ice‐covered lakes to differentiate underice stratification. When ice forms in smaller and deeper lakes, inverse stratification will form with a thin buoyant layer of cold water (near 0°C) below the ice, which remains above a deeper 4°C layer. In contrast, the entire water column can cool to ∼0°C in larger and shallower lakes. We suggest these alternative conditions for dimictic lakes be termed “cryostratified” and “cryomictic.