Manipulations of stocking magnitude: Addressing density-dependence in a juvenile cohort of common snook (Centropomus undecimalis)

We released hatchery-reared juvenile common snook (Centropomus undecimalis) to test the effects of augmenting age-1 abundance by 100% (high augmentation, n = 2) vs 10% (low augmentation, n = 2) in estuarine creeks of southwestern Florida. We monitored in-creek abundance of age-1 snook 1 month before releases in May 2002 to estimate wild snook density and stocking magnitude. All sampling used seining standardized for effort, gear efficiency, and depletion removal. After releases, sampling continued for 1 year. After 1 month, creeks with high augmentation showed a 126% and 74% increase in total age-1 abundance, and low augmentation creeks a 6% increase and an 18% decrease. Total age-1 abundance declined during fall in all creeks, but by winter, abundance increased again, comparable to earlier levels (132% and 67% above the pre-release estimates in high augmented creeks and 8% and 5% in creeks with low augmentation). While overall density was elevated in both high augmentation creeks, hatchery-reared snook in one creek experienced a 64-85% loss within 1 month after release; loss of hatchery-reared or wild snook was negligible in other experimental creeks. Pre-release density was not a good predictor of creek productive capacity, suggesting variation in habitat production and localized recruitment. Further work is needed to understand inter-cohort density-dependent interactions, food chain responses, and variation in habitat productivity.