PARENT SESSION
Contributed Oral Session 108: Aquatic Trophic Systems II
Wednesday, August 10, 1:30 PM - 5:00 PM, Meeting Room 524 C, Level 5, Palais des congrès de Montréal

Double-Crested Cormorant diet on boreal lakes: Implications for food web structure and fisheries management.

Earle, Suzanne^*,1, Paszkowski, Cynthia ¹, ¹ University of Alberta, Edmonton, Alberta, Canada

ABSTRACT- In lake ecosytems, knowledge of the direct and indirect effects of apex predators and piscivory is essential to managing fisheries and maintaining water quality. To determine if population increases of the Double-Crested Cormorant (Phalacrocorax auritus) are influencing ecosystem dynamics and trophic structure via top-down effects on boreal lakes, we used a combination of conventional diet, stable isotope analyses, and behavioural observations on three different lake communities in north-central Alberta, Canada. Regurgitation samples collected from bird colonies during 2003 and 2004 nesting seasons show a dominance of small bodied (55–80 mm) fishes including yellow perch, sticklebacks and spottail shiner. Yellow perch was the most frequently captured species (70%) and comprised the largest percent biomass (67%) in diets. Diet composition is likely a reflection of prey availability within the lake since netting data have also yielded a high abundance of juvenile yellow perch in Lac La Biche, the main foraging site. To provide diet information outside of the nesting season and a better understanding of longer term prey consumption and trophic linkages, we are comparing stable isotope ratios of ¹⁵N and ¹³C in liver and breast tissue of Double-Crested Cormorant with ratios in dorsal tissue samples of piscivorous (walleye, northern pike) and planktivorous (yellow perch, cisco) fish species. We expect cormorants to occupy a similar trophic level to predatory fish and to be foraging predominantly in the pelagic zone of large lakes. Our results suggest that new strategies for managing both fisheries and waterbirds may be essential to achieve a desired change in the current ecosystem state of many large boreal lakes.

Key words: Food web structure, Double-Crested Cormorants, Stable isotope analysis