A caloric-based evaluation of diet indices for largemouth bass

Selection of methods for quantitative description and assessment of food habits is a concern for trophic investigations. We used diet data for largemouth bass Micropterus salmoides, to compare a caloric-based approach with eight diet indices: percent frequency of occurrence, percent total number, percent total weight, mean relative number, mean relative volume, relative importance index, prey-importance index, and mean stomach fullness. Mean caloric contribution of stomach contents for each prey taxon was used as a standard to compare diet indices. Temporal differences in composition and caloric contents of largemouth bass stomach contents were apparent. Most diet indices provided similar assessments when diets were dominated by a single prey type (i.e., gizzard shad during June-October). However, diet indices evaluated provided dissimilar assessments of stomach contents when a variety of prey with differing caloric densities were consumed (e.g., April). Mean stomach fullness and percent by volume were significantly (p < 0.002) correlated (r = 0.94 - 1.00) with mean caloric contribution of largemouth bass stomach contents during all months. Unlike percent by weight, mean stomach fullness accounted for differences in fish size and stomach capacity. Thus, mean stomach fullness by prey type appears to be the most appropriate index when objectives include simplified caloric-based assessments of fish diets.