Feeding Ecology of Juvenile Lake Sturgeon in the St. Lawrence River System

Abstract

We examined the diet of juvenile lake sturgeon Acipenser fulvescens at five sites in the St. Lawrence River, including two sites in major tributaries (Des Prairies and L'Assomption rivers), two main-stem sites located upstream (Verchères and Sorel-Berthier islands), and one site located downstream in the estuarine transition zone (ETZ). The benthic fauna was sampled at all sites except in the ETZ. The diets of juvenile lake sturgeon were highly diverse. Of the 74 invertebrate prey taxa identified, approximately 50 occurred in more than 5% of the stomach contents. Most of these prey taxa were of small size. In the three sites where the same sampling strategy and techniques were used, diet varied significantly with sampling site and month. In addition, diet varied significantly with fish length but independently of site and month. Lake sturgeon from the Verchères Islands exhibited the highest diet diversity, and fish situated in the ETZ exhibited the lowest. Except for one of the five sites (L'Assomption River), diet was dominated by amphipods. Chironomids predominated at L'Assomption River and were the second most numerous at the Verchères Islands, Sorel-Berthier Islands, and Montmagny sites. Other important prey were oligochaetes, ephemeropteran nymphs, trichopteran larvae, mollusks, and fish eggs. The benthos composition differed among sites. Positive values for Ivlev's electivity index were found for malacostracans and ephemeropteran nymphs at all four upstream sites and for dipteran larvae at one upstream site; mollusks were generally negatively selected at one upstream site. At the three sites where the same sampling strategy was used, food preference differed as a function of site and month. Within some site × month combinations, preference also varied with fish length. Our results (1) confirm that juvenile lake sturgeon are generalists and opportunistic benthic feeders, (2) reveal that diet composition is only partly determined by benthos availability, and (3) suggest that there is positive selection for drifting prey. Within the study area, lake sturgeon aggregate locally, but the presence of juveniles in these specific areas cannot be fully explained by their food habits.