Food, density, and microhabitat: factors affecting growth and recruitment potential of juvenile saltmarsh fishes

The relationships among microhabitat use, food habits, conspecific density and recent growth rate for estuarine-dependent juvenile spotted seatrout, Cynoscion nebulosus, and red drum, Sciaenops ocellatus, were studied to determine how nursery habitat influences early growth. Juvenile spotted seatrout and red drum were quantified along the marsh-edge ecotone from multiple drop samples, and their immediate environments characterized by a suite of physical and chemical variables along with substrate type and Spartina stem density. Recent daily growth of individual fish was modeled in a series of multiple regression analyses that considered the relative contributions of food, microhabitat, and conspecific density. The spotted seatrout model (p < 0.0001) included four independent variables, otolith radius, prey diversity, salinity, and a salinity-DO interaction term, and explained 67.9% of the variation in daily growth. All variables were significant (p < 0.05), and regression slopes were positive for all variables except salinity. The red drum model (p > 0.0001) included five independent variables, otolith radius, temperature, salinity, depth and substrate, and explained 63.3% of the variation in daily growth. All variables were significant (p > 0.05), and all regression slopes were positive. Fish size (as estimated by otolith radius) accounted for most of the variance in the spotted seatrout (60.2%) and red drum (44.8%) models, while the remaining environmental variables were significant and responsible for 7.7 and 18.5%, respectively. Density variables were not selected for either model, suggesting that density-dependence was not an important influence on recent daily growth. Generally, physico-chemical variables such as temperature, salinity, and dissolved oxygen contributed more to growth than diet or extrinsic factors such as grass stem density.