Anadromous Sturgeons: Habitats, Threats, and Management

<em>Abstract.</em>—Atlantic sturgeon <em>Acipenser oxyrinchus</em> and lake sturgeon <em>A. fulvescens </em>live in sympatry in the St. Lawrence estuarine transition zone (ETZ). To describe their feeding ecology and compare their diets in this zone, sturgeons were sampled during the summer and fall of 2000 by trawling in the main channel and by gill netting in the shallower nearshore habitat. Stomach contents were sampled by gastric lavage of live specimens (trawling) and by digestive tract sampling (gill netting). Relative importance by taxonomic group was based on percent occurrence and percentage of the diet by number and weight for three sturgeon size-classes that corresponded to age-0, juvenile, and subadult stages. Spatial, seasonal, and life stage variations were observed in the diet composition of both sturgeon species. Age-0 fish of both species fed mainly on gammarids. Juveniles and subadults from both species fed mainly on oligochaetes and gammarids, but in opposite proportions: gammarids were the dominant prey for lake sturgeon and oligochaetes for Atlantic sturgeon. Subadult Atlantic sturgeon also fed on fish in the summer and on insects and mollusks in the nearshore habitat in fall. Vegetal matter was frequent and abundant in the stomach contents of Atlantic sturgeon subadults, especially in the nearshore habitat, and vegetal mass in the diet was correlated with gammarid biomass. In addition to gammarids, the lake sturgeon diet included insects, oligochaetes, and mollusks, whose proportions increased with sturgeon size-class. The proportion of amphipods decreased with size-class during both summer and fall. In the St. Lawrence ETZ, Atlantic sturgeon appear to be specialist feeders while lake sturgeon appear to be more often generalists. Diet diversity was higher in lake sturgeon, which fed on all of the 15 taxa identified in the macrobenthos of the ETZ; the diet of Atlantic sturgeon consisted of 10 taxa. In the main channel in fall, dietary overlap between Atlantic sturgeon and lake sturgeon was low for the juvenile and subadult life stages. The strong dependence of Atlantic sturgeon and lake sturgeon on oligochaetes and gammarids suggests that the areas where these benthic assemblages are found, near the freshwater–saltwater limit, are important feeding habitats for the age-0, juvenile, and subadult stages of both sturgeon species.