384 



Abstract— Offshore winter-spawned 

 fishes dominate the nekton of south- 

 eastern 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 char- 

 acteristics, there is, between and within 

 species, variability in the occupation of 

 these habitats. Whether all occupied 

 habitats are equally valuable to indi- 

 viduals 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 distribu- 

 tion 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 {Leios- 

 tomus xanthurus) and Atlantic croaker 

 iMicropogonias undulatus) — two domi- 

 nant 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 biologi- 

 cal 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. 



Manuscript accepted 25 October 2002. 



Manusript received 31 December 2002 

 at NMFS Scientific Publications Office. 



Fish. Bull. 101:384-404 (2003). 



The relative value of different estuarine 

 nursery areas in North Carolina for 

 transient juvenile marine fishes 



Steve W. Ross 



NC National Estuarine Research Reserve 

 5600 Marvin Moss Ln. 

 Wilmington, North Carolina 28409 

 E-mail address: rosss(guncwil.edij 



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; 

 Mclvor and Odum, 1988; Miltner et al., 

 1995). Despite similarity in some PNA 

 physical characteristics, there is vari- 

 ability in habitats occupied (between 

 and within species), especially with 

 regard to salinity, tidal influence, acces- 

 sibility (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; Guin- 

 don 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)? Understand- 

 ing variation in habitat quality during 

 a major early life history phase should 

 yield insight into causes of variability in 

 year-class strength, particularly if juve- 

 nile fish distributions vary interannu- 

 ally. If PNA quality varies, then factors 

 controlling variation in presettlement 

 distribution are important to year class 

 success because animals could be trans- 

 ported to habitats of unpredictable 

 quality. If nursery areas have similar 

 value, interannual variations in dis- 

 tribution across nursery creeks should 

 have less effect on ultimate population 

 sizes or production. 



General estuarine distributions of 

 two dominant OWS fishes, spot (Leio- 

 stomus xanthurus) and A.i\a.ntic croaker 

 (Micropogonias undulatus), exhibit con- 

 sistent patterns throughout their rang- 

 es. 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 particu- 

 lar estuarine region. Despite these gen- 

 eralities, 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, ju- 

 veniles of both species seem to avoid (or 

 are unsuccessful in) more open water 

 areas of estuaries during the early part 

 of the nursery period. 



The main purpose of this paper is to 

 assess relative habitat value for two 

 dominant members of the OWS fish 

 group, spot and Atlantic croaker, across 

 a wide variety of North Carolina PNA 

 conditions. I assumed that fish growth 

 and mortality rates were direct reflec- 

 tions (integrators) of overall physical 

 and biological conditions in PNA habi- 

 tats. Therefore, 1 used early nursery 

 period age-specific growth and mortal- 

 ity rates of spot and Atlantic croaker, in 

 addition to distribution data, to assess 

 relative habitat quality, testing the hy- 

 pothesis that habitat quality varies for 

 these fishes across a broad range of typ- 

 ical PNAs in two very different estua- 

 rine systems. Growth and mortality can 

 be influenced by fish density; however, 

 Ross (1992) found similar growth and 

 mortality rates for spot and Atlantic 



