The cost of capturing prey:: measuring largemouth bass (Micropterus salmoides) foraging activity using glycolytic enzymes (lactate dehydrogenase)

We used muscle-derived lactate dehydrogenase (LDH) to measure effects of prey size and vegetation density on anaerobic foraging activity by largemouth bass (Micropterus salmoides). Largemouth bass (240-303 mm total length, TL) were fed bluegill (Lepomis macrochirus) prey (range 33-83 mm TL) in laboratory feeding trials. Prey selectivity experiments showed that small bluegills (<50 mm) were strongly preferred (>88%) over larger (>65 mm) individuals. Largemouth bass activity, as indexed by LDH, increased with increasing prey size and was 20% higher in fish feeding on large (mean size = 80 mm) versus small (mean size = 35 mm) bluegill. Bioenergetics modeling revealed. that food consumption was appreciably underestimated (29%-34%) for largemouth bass foraging on large bluegills (65 and 80 mm), implying that activity costs vary with prey size, consistent with LDH measurements. In contrast to prey size, vegetation density had little effect on anaerobic energy expenditure of largemouth bass. For two size groups of largemouth bass (mean = 244 and 316 mm) foraging on 50 mm bluegill, mean LDH activity was similar across simulated vegetation densities ranging from 70 to 350 stems m(-2). These findings highlight the importance of prey size on foraging costs by piscivores and the difficulties of accounting for activity level in bioenergetics models.