Food habits, dietary overlap and gastric evacuation rates of rockfishes (genus Sebastes)

Graduation date: 1983Presentation date: 1982-11-24Euphausiids constituted the major food of five co-occuring species of rockfishes (Sebastes spp.). Copepods, decapods, cephalopods, amphipods, fishes and other pelagic prey were also consumed but were\ud less important to the overall diet. Two species, S. flavidus and S. diploproa, were relatively euryphagous, utilizing a high number of prey taxa. The other species, S. pinniger, S. alutus and S. crameri, had a more restricted diet comprised mostly of euphausiids. The numerical composition of prey in the diet of all species was similar\ud due to the preponderance of the two dominant euphausiid species. Diet overlaps based on weight composition were high for S. pinniger, S. diploproa and S. alutus but were moderate for most comparisons involving S. flavidus and S. crameri. The diets of S. flavidus and S. pinniger were examined in more detail to explain some of the variability associated with their food habits. Both species exhibited peak feeding periods at the same times during the day and consumed about the same mean size of prey; although S. flavidus consumed a wider size range of prey. Neither species showed much variation in size or type of prey consumed throughout the size range of fish examined. There were significant seasonal, geographical and diel differences in food composition for both species which may be a function of varying food availability. The rates of gastric evacuation of two food types, squid (Loligo opalescens) and eulachon (Thaleichthys pacificus), were measured in the laboratory for adult black rockfish (Sebastes melanops), held nominally at 11 °C. Linear, logarithmic and square root regression models were applied to analyze the data expressed as both wet and dry weight proportions of the original meal. The linear model provided the best fit for the wet weight relationship of squid. The dry weight of squid and both the wet and dry weight relationships of eulachon were best described by a logarithmic function, or by a square root function when the regressions were forced through the original meal size. The instantaneous evacuation rates (about 6% hr-1) and stomach clearance times (about 75 hr) were similar for all relations except\ud those involving the wet weight of squid. The addition of the meal size as an independent variable improved the models in most cases\ud whereas the addition of the weight of the fish used in the experiments did not significantly improve the model in any instances