The relative value of different estuarine nursery areas in North Carolina for transient juvenile marine fishes

Fishery Bulletin, April, 2003 by Steve W. Ross

Abstract--Offshore winter-spawned fishes dominate the nekton of southeastern United States estuaries. Their juveniles reside for several months in shallow, soft bottom estuarine creeks and bays called primary nursery areas. Despite similarity in many nursery characteristics, there is, between and within species, variability in the occupation of these habitats. Whether all occupied habitats are equally valuable to individuals of the same species or whether most recruiting juveniles end up in the best habitats is not known. If nursery quality varies, then factors controlling variation in pre-settlement fish distribution are important to year-class success. If nursery areas have similar values, interannual variation in distribution across nursery creeks should have less effect on population sizes or production. I used early nursery period age-specific growth and mortality rates of spot (Leiostomus xanthurus) and Atlantic croaker (Micropogonias undulatus)--two dominant estuarine fishes--to assess relative habitat quality across a wide variety of nursery conditions, assuming that fish growth and mortality rates were direct reflections of overall physical and biological conditions in the nurseries. I tested the hypothesis that habitat quality varies for these fishes by comparing growth and mortality rates and distribution patterns across a wide range of typical nursery habitats at extreme ends of two systems. Juvenile spot and Atlantic croaker were collected from 10 creeks in the Cape Fear River estuary and from 18 creeks in the Pamlico Sound system, North Carolina, during the 1987 recruitment season (mid-March-mid-June). Sampled creeks were similar in size, depth, and substrates but varied in salinities, tidal regimes, and distances from inlets. Spot was widely distributed among all the estuarine creeks, but was least abundant in the creeks in middle reaches of both systems. Atlantic croaker occurred in the greatest abundance in oligohaline creeks of both systems. Instantaneous growth rates derived from daily otolith ages were generally similar for all creeks and for both species, except that spot exhibited a short-term growth depression in the upriver Pamlico system creeks--perhaps the result of the long migration distance of this species to this area. Spot and Atlantic croaker from upriver oligohaline creeks exhibited lower mortality rates than fish from downstream polyhaline creeks. These results indicated that even though growth was similar at the ends of the estuaries, the upstream habitats provided conditions that may optimize fitness through improved survival.

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Offshore winter-spawned (OWS) fishes are a major component of the nekton of southeastern United States and Gulf of Mexico estuaries. Their larvae migrate across the shelf, enter estuaries, and the majority of juveniles reside for several months in shallow, soft bottom estuarine creeks and bays called primary nursery areas (PNAs). Very high concentrations of fishes in these PNAs suggest that they are valuable habitats, perhaps because they are good sources of food and shelter (Boesch and Turner, 1984; McIvor and Odum, 1988; Miltner et al., 1995). Despite similarity in some PNA physical characteristics, there is variability in habitats occupied (between and within species), especially with regard to salinity, tidal influence, accessibility (i.e. distance from inlets), and, perhaps, food and predator regimes (Weinstein, 1979; Ross and Epperly, 1985). Assessing the relative value of all PNA habitats to individuals of the same species is increasingly important (Weinstein, 1982; Sogard, 1992; Guindon and Miller, 1995; Beck et al., 2001). If PNA value varies, do most of the recruiting juveniles end up in the best habitats (Thresher, 1985)? Understanding variation in habitat quality during a major early life history phase should yield insight into causes of variability in year-class strength, particularly if juvenile fish distributions vary interannually. If PNA quality varies, then factors controlling variation in presettlement distribution are important to year class success because animals could be transported to habitats of unpredictable quality. If nursery areas have similar value, interannual variations in distribution across nursery creeks should have less effect on ultimate population sizes or production.

General estuarine distributions of two dominant OWS fishes, spot (Leiostomus xanthurus) and Atlantic croaker (Micropogonias undulatus), exhibit consistent patterns throughout their ranges. Juvenile Atlantic croaker routinely concentrate in oligohaline creeks or bays (Weinstein, 1979; Mercer, 1987a)--a pattern that suggests that the upstream regions are most valuable to this species. Spot, however, are more ubiquitously and variably distributed through the shallow PNAs (Ross and Epperly, 1985; Mercer, 1987b), perhaps indicating less dependence on a particular estuarine region. Despite these generalities, both species can be present in large numbers in almost any estuarine creek or bay over the full salinity range (e.g. Nelson et al., 1991). In general, juveniles of both species seem to avoid (or are unsuccessful in) more open water areas of estuaries during the early part of the nursery period.