This study is providing the first insight on the food sources supporting Hemimysis anomala in a lotic ecosystem where food webs differ from those of lentic ecosystems. Using carbon and nitrogen stable isotopes, the diet of Hemimysis was evaluated in multiple sites of the St Lawrence River where higher density swarms have been recently reported. We found that Hemimysis may feed on multiple food sources, including benthic and pelagic production, and from multiple trophic levels. Results from a mixing model applied to evaluate the importance of benthic versus pelagic food sources revealed site specific variations in diet. At the low flow sites located in the Montréal Harbour, Hemimysis fed primarily on pelagic production compared to the more open sites exposed to higher flow located outside of the harbor. This study indicates that Hemimysis may adapt its feeding behavior to environmental conditions and/or food source availability, potentially increasing its impacts on food web structure.

Savenkoff, C., H. Bourdages, D.P. Swain, S.-P. Despatie, J.M. Hanson, R. Methot, L. Morissette, and M.O. Hammill. 2004. Input data and parameter estimates for ecosystem models of the southern Gulf of St. Lawrence (mid-1980s and mid-1990s). Can. Tech. Rep. Fish. Aquat. Sci. 2529: vi+105 pp. Several groundfish stocks have collapsed in many areas of the northwest Atlantic over the last ten years. Mass-balance models are being used to reconstruct trophic flows through one of the areas, the southern Gulf of St. Lawrence ecosystem (NAFO division 4T) before (mid-1980s) and after (mid-1990s) the collapse of cod stock. The whole-system model of the southern Gulf of St. Lawrence is divided into 30 functional groups or compartments from phytoplankton and detritus to marine mammals and seabirds, including harvested species of pelagic, demersal, and benthic domains. We present here details of the input data (biomass, production, consumption, export, and diet composition) for each compartment used in the modelling. The successful development of ecosystem models proposed by the Comparative Dynamics of Exploited Ecosystems in the Northwest Atlantic (CDEENA) program will provide powerful new tools to evaluate the impact of human and environmental factors on a variety of Atlantic shelf ecosystems.

RÉSUMÉ Morissette, L., S.-P. Despatie, C. Savenkoff, M. O. Hammill, H. Bourdages, and D. Chabot. 2003. Data gathering and input parameters to construct ecosystem models for the northern Gulf of St. Lawrence (mid-1980s). Can. Tech. Rep. Fish. Aquat. Sci. 2497: vi+94 p. In the present study, we use Ecopath and inverse methods to reconstruct trophic flows through the whole northern Gulf of St. Lawrence ecosystem (NAFO zones 4RS) for the mid1980s period, prior to the groundfish stock collapses. This was a period of relatively constant biomass for the major species. The whole-system model of the northern Gulf of St. Lawrence is divided into 32 functional groups or compartments from phytoplankton and detritus to marine mammals and seabirds, including harvested species of pelagic, demersal, and benthic domains. We present here details of the input data (biomass, production, consumption, export, and diet composition) for each compartment used for modelling. The successful development of ecosystem models proposed by the Comparative Dynamics of Exploited Ecosystems in the Northwest Atlantic (CDEENA) program will provide powerful new tools to evaluate the impact of human and environmental factors on a variety of Atlantic shelf ecosystems.