Modeling the Effect of Environmental Parameters on Feeding Ecology of the Shortnose Sturgeon in the Saint John River, New Brunswick

We examined the effects of spatial, temporal, and environmental parameters on the feeding habits of the shortnose sturgeon Acipenser brevirostrum . We sampled sturgeon gut contents using colonic flushing, a new, low-impact technique. We found that heavy feeding continued until at least November, when water temperature reached 8°C. In all seasons, foraging shortnose sturgeon used shallow water (2–8 m); however, in late fall this trend was even stronger, when 50% of the sturgeon were captured in less than 3 m depth. Over 60 families of prey items were identified from 166 colonic samples collected at four points between spring and late fall from seven locations throughout the lower Saint John River, New Brunswick. Samples from freshwater sites contained mainly Gastropoda, Sphaeriidae, Chironomidae, and Gammaridae. Samples from saline and brackish environments contained mostly Gammaridae, Corophiidae, softshell clam Mya arenaria , Cyathura polita , and Chironomidae. We used sensitivity analysis to determine the number of shortnose sturgeon required to obtain a representative sample of the diet. We found that 20–40 gut samples from sturgeons in both brackish and freshwater environments would suffice for a general description of the diet but that complete knowledge of the sturgeon diet and a comparison among several sites would require the sampling of hundreds of fish. The digestibility of prey varied from 77% of the ingested volume in a saline environment to only 6% in freshwater samples. Shortnose sturgeon from freshwater sites had higher ingested volumes and more prey per ingested volume than did those from brackish sites, which indicated further differences in feeding efficiency between the environments. Salinity was a significant parameter in the redundancy analysis of ingested prey items. In freshwater sites the significant factors were season, sturgeon length, ingested volume, number of ingested prey, and number of ingested taxa. In saline sites the significant factors were season, location, ingested volume, and the number of ingested prey. Our findings identified the spatial and temporal gradients in shortnose sturgeon foraging and provided insight into foraging efficiency in this species.