Spatial and temporal heterogeneity of reservoir zooplankton abundance and effects on growth and survival of larval threadfin shad

Growth and survival of weekly cohorts of larval threadfin shad were examined in relation to water temperature and the spatial and temporal distribution of their zooplankton prey in a large, southeastern United States reservoir. Zooplankton heterogeneity was primarily attributable to differences in timing of spring production pulses along the long-axis of the reservoir. Peak zooplankton abundance occurred later at upstream sites than at downstream sites. This phenomenon could not be explained by either differential rates of warming in the two regions or by temporal variation in inflow rates. Environmental conditions encountered by threadfin shad larvae varied with location and timing of hatching. Prey availability to newly hatched cohorts ranged from 30 to 800 organisms/L, and water temperature ranged from about 15 to 28{dollar}spcirc{dollar}C. Daily growth rates of 31 weekly cohorts ranged from 0.39 to 0.78 mm/d and were related to prey density and water temperature for larvae up to 21 d old. Growth increased linearly with water temperature. There was a threshold level of prey density, below which growth increased with prey abundance and above which prey had relatively little effect. Cohort survival was significantly related to water temperature but not prey availability. However, survival was correlated with growth rate of 8-14 d old larvae, suggesting a mechanism whereby prey-mediated growth could regulate threadfin shad survival. The results of this study are generally consistent with the match/mismatch hypothesis. However, synchrony of fish hatching with peak prey density was a poor predictor of cohort growth or survival. Instead, a prey density of 160-290 organisms/L appeared optimal for growth. The relation between prey density and growth for larvae up to 3 weeks of age suggests that the influence of prey extended well beyond the period of first feeding. Threadfin shad spawned over a prolonged period that generally encompassed the relatively brief (and spatially variable) spring zooplankton pulse. About 50% of threadfin shad weekly cohorts hatched while prey density was at or above the estimated optimal level. A prolonged spawning season may overcome the problems of an unpredictable environment when viewed for an entire year class.