Evaluating How Local- and Regional-Scale Processes Interact to Regulate Growth of Age-0 Largemouth Bass

Abstract

Regional- and local-scale processes may interact to influence early growth and survival, thereby governing cohort strength. During summer through fall 1994–1996, we assessed how precipitation (a regional-scale process) and prey availability (a local-scale process) influenced piscivory and growth of age-0 largemouth bass Micropterus salmoides in five Ohio reservoirs (190–1,145 ha). We expected early growth to vary with the abundance and relative sizes of age-0 gizzard shad Dorosoma cepedianum. We collected age-0 largemouth bass and prey fishes every 3 weeks in each reservoir. In 1994, May precipitation was low (total = 4 cm), resulting in low mean daily reservoir discharge (x5 reservoirs = 3.6 m3/s). In four reservoirs, stable water levels may have led to successful largemouth bass reproduction and perhaps an early hatch. As such, age-0 largemouth bass in these systems were abundant, consumed gizzard shad, and reached large sizes by fall (15.3 g). In 1995 and 1996, high precipitation (total > 12 cm) and high reservoir discharge [x5 reservoirs = 13.8 m3/s (1995), 28.8 m3/s (1996)] in some reservoirs in May likely reduced largemouth bass abundances. Growth during these years was density dependent across reservoirs. When age-0 largemouth bass abundance was low, nonshad prey fish were consumed, and mean fall sizes were similar to those in 1994 (12.0 g). Conversely, fall weights (4.5–7.4 g) declined in reservoirs with increasing largemouth bass density. Surveying May precipitation in Ohio across 48 years revealed that conditions like those in 1994 occurred less than 15% of the time. Because gizzard shad should rarely be available and other prey fish species probably are limited, density-dependent processes should often regulate early piscivory, growth, and potentially, cohort strength in these systems.