Patterns of diet and growth in co-occurring populations of largemouth bass and smallmouth bass

Largemouth bass Micropterus salmoides and smallmouth bass M. dolomieu both undergo ontogenetic diet shifts from insects and zooplankton (as juveniles) to fish and crayfish (as adults). This diet similarity leads to the expectation that the growth rates of co-occurring largemouth bass and smallmouth bass should covary positively. However, the relative abundance of the two species varies widely among lakes, suggesting that there are important ecological differences and making it difficult to predict how the individual growth rates of the two species will covary among systems. We quantified the diets and growth rates of largemouth bass and smallmouth bass in a set of lakes in New York State that varied in the relative abundance of the two species. Despite changing diets as a function of size, largemouth bass and smallmouth bass consumed similar prey across the size range. As expected based on diets, the growth rates of the two species were also positively correlated among lakes for most size-classes. However, the growth rates of small, invertebrate-feeding fish were negatively correlated with those of larger fish that fed on fish and crayfish. These results suggest that largemouth bass and smallmouth bass growth rates respond similarly across environmental gradients but that both species are composed of ecologically distinct stages.