Measures of juvenile fish habitat quality: Examples from a National Estuarine Research Reserve

Defining and quantifying essential fish habitat is difficult, perhaps parti cularly so in estuaries, which are typically dynamic. Yet we need habitat d ata to make informed decisions about the management of estuarine habitats a nd associated fish populations. Our ongoing efforts to resolve issues of fi sh habitat quality have been centered in the relatively unaltered Jacques C ousteau National Estuarine Research Reserve (the Reserve) in the Mullica Ri ver-Great Bay estuary in southern New Jersey, where extensive studies of fi shes and their habitats have been conducted during the last decade. Much of our effort to define essential fish habitat has focused on a variety of sh allow-water habitats (eelgrass, macroalgae, marsh creeks, unvegetated subst rates of different grain sizes) where it is easier to sample in a quantitat ive manner (e.g., using throw traps and beam trawls) and conduct experiment al manipulations (e.g., caging, deploying of artificial habitats). Although our studies in the Reserve have been extensive, they still have been focus ed on a relatively small component (less than 3%) of the fish fauna of the Reserve, including several species of economic importance. These species in clude winter flounder Pseudopleuronectes americanus, summer flounder Parali chthys dentatus, tautog Tautoga onitis, and black sea bass Centropristis st riata. This work has examined the period from larval ingress and settlement through the first year using a variety of complementary approaches. To dat e, these studies have included measures of habitat-specific distribution, a bundance, residence time, and growth. Attempts to identify both habitat-spe cific measures of mortality and sources of mortality have proven especially difficult for the migratory fishes typical of Middle Atlantic Eight estuar ies. In fact, this mobility, which occurs at seasonal, diel, tidal, and epi sodic (storms, upwelling, etc.) scales, makes it difficult to assess reside nce times and confounds attempts to measure habitat quality. The measures o f habitat quality that we have used suggest that there are species-specific and habitat-specific responses; however, data sets for multiple years are seldom available to confirm these responses. Efforts to quantify essential fish habitat will be limited in their effectiveness until interannual varia bility can be assessed.