Influence of Abundance and Size of Age-0 Gizzard Shad on Predator Diets, Diet Overlap, and Growth

Abstract

Relationships among age-0 gizzard shad Dorosoma cepedianum and their predators were examined over 5 years (1987–1991) in Pomme de Terre and Stockton lakes, Missouri. to determine the influence of gizzard shad abundance and size on diets, diet overlap, and growth of predators. Within years, temporal changes in age-0 gizzard shad size affected predator diets and diet overlap of predators more than abundance. Age-0 gizzard shad were not important as prey until they grew larger than 25 mm total length, even though larval densities were much higher. They were the most common prey for most predators during June through October and diet overlap among predators was usually highest during that period. Lengths of age-0 gizzard shad ingested by predators resembled those available in the reservoirs throughout much of the season except in early summer, when predators ingested larger-than-average individuals, and in the fall, when smaller predators ingested smaller-than-average individuals. Among years, variations in summer production (kg/ha) and mean weight (g) of age-0 gizzard shad were significantly correlated with their importance in most predator diets and with diet overlap among predator species in Stockton Lake but not in Pomme de Terre Lake, possibly because age-0 gizzard shad were always abundant in Pomme de Terre Lake and annual variation in gizzard shad variables was less in Pomme de Terre than in Stockton Lake. One or both of these gizzard shad variables were usually significant in regression models explaining variation in growth increments of predators. When significant, production of age-0 gizzard shad was positively related to the speciesˈ importance in predator diets, to diet overlap among predators, and to predator growth, but mean weight of age-0 gizzard shad was negatively related to those variables. Because mean weight and mean density of age-0 gizzard shad were strongly negatively correlated, predators may have been responding more strongly to prey density than to prey size. Size of age-0 gizzard shad appeared to most strongly influence growth of smaller predators, whereas age-0 gizzard shad production most strongly influenced growth of larger predators. I conclude that changes in age-0 gizzard shad abundance and size can significantly influence predator diets, diet overlap among predators, and predator growth.