Density-dependent growth and mortality in an estuary-dependent fish: an experimental approach with juvenile spot Leiostomus xanthurus

The abundance of demersal marine fishes is a function of both pre- settlement processes that influence recruitment to benthic juvenile habitats, as well as post-settlement density-dependent processes that act during the juvenile stage. Few studies have investigated density-dependence for fishes that spawn offshore and recruit to inshore estuaries for the juvenile stage prior to returning to offshore waters as adults (i.e. estuary-dependent). We conducted 2 replicated experiments at different spatial scales to test for density-dependent growth and mortality in juvenile spot Leiostomus xanthurus, a common estuary-dependent species. In the small-scale experiment, we stocked spot in 1 m super(2) cages in a marsh creek at densities of 2, 5, and 10 fish m super(-2) and determined their growth and mortality after 51 d. In the large-scale experiment, we stocked spot in 79 m super(2) pond sections at 2, 5, and 10 fish m super(-2) and determined their growth and mortality after 52 d. We sampled benthic infauna at the end of the pond experiment to determine if prey availability mediated the effects of density on spot growth and mortality. Average spot growth rates decreased 83 to 97% and mortality increased 2- to 4-fold as density increased from 2 to 10 fish m super(-2). The density of benthic infauna at the end of the pond experiment was inversely related to spot density, consistent with competition for food as the underlying mechanism. Estimates of spot density compiled from the literature indicate that the density-dependent effects we observed occurred within the range of reported field densities. Our results provide strong experimental support for the hypothesis that density-dependent processes during the demersal juvenile stage in estuaries can modify patterns in the abundance of spot, and perhaps other estuary-dependent species, that are established prior to settlement.