Ross: Relative value of different estuanne nursery areas for luvenile marine fishes in Nortfi Carolina 



387 



coasts estuaries (i.e. a drowned river valley). Diurnal lu- 

 nar tides (average range about 1.5 m) were a dominant 

 feature throughout the study area (Welch and Parker, 1979; 

 Pietrafesa and Janowitz, 1988). Ten stations were located 

 along a 50-km transect of the Cape Fear system (Fig. 1). 

 In the lower estuary, four polyhaline creeks were sampled 

 on the west side of the inlet (Oak Island). Two creeks were 

 sampled on opposite sides of the middle of the estuary and 

 four oligohaline creeks were sampled in the upper estuary 

 near Wilmington. All stations in this system were sur- 

 rounded by Spartina marshes. 



Field sampling 



All stations were sampled during daylight with two one- 

 minute tows (68.6 m each) of a small-mesh trawl (3.2-m 

 headrope length, 6.4-mm bar mesh wings and body, 3.2-mm 

 tail bag mesh). Catches from the two tows were combined 

 for the station sample. Surface and bottom salinities (near- 

 est %c) and water temperatures ( nearest °C ) were recorded 

 after each sample. Mean salinities and temperatures for 

 each area were analyzed for differences by using <-tests for 

 all possible combinations of area pairs. 



Sampling was designed to provide biological data during 

 the time of early residency in the nursery creeks, but before 

 significant emigration. Most recruitment of young juvenile 

 fishes into these creeks has ended by late-April, and some 

 fishes begin to emigrate by June— July (Weinstein, 1979; 

 Ross and Epperly, 1985; author's pers. obs.). To minimize 

 the influence of emigration on the calculation of growth and 

 mortality rates, sampling occurred during seven periods, 

 every other week from mid-March through mid-June 1987 

 (about 14 d between samples). Synoptic samples over this 

 large region were generated by assigning areas to four 

 crews for trawling during the same period of each sample 

 week. 



Newly recruiting OWS juvenile fishes of the 1987 year 

 class were sorted from the catches and preserved in the 

 field in 100% ethyl alcohol. About one month after col- 

 lection the fishes were identified, counted, and standard 

 lengths (SL) were measured to the nearest mm. Analyses 

 were limited to spot and Atlantic croaker, the two most 

 abundant species. Catch per unit of effort (CPUE) was 

 calculated by dividing the total number of individuals of 

 a species captured by the number of trawl tows in a time 

 period or area. Subsamples of these fishes representing 

 several collection dates and all areas were measured for 

 SL, blotted, and weighed to the nearest 0.01 g and were 

 used to develop a weight-length relationship using linear 

 regression. Differences in weight-length relationships be- 

 tween areas were assessed by using analysis of covariance 

 (covariate=logSL) in a general linear model procedure 

 (SAS Institute, 1988). 



Otolith aging 



Subsamples for aging were randomly selected from early 

 (early and mid- April) and late (mid and late May) dates 

 and from downstream (lower) and upstream (upper) areas 

 in each system. Sagittae were removed from these fishes, 



mounted on microscope slides with thermoplastic cement, 

 and polished (often on both sides) until thin sections were 

 obtained. Otoliths were viewed with oil immersion and 

 transmitted polarized light at magnifications between 500 

 and 625x, and images were projected through a video system 

 to a screen. Rings, presumed to be daily, were counted. The 

 formation of daily rings has either been validated (Peters 

 et al., 1978; Baldevarona, 1987; Siegfried and Weinstein, 

 1989) or assumed (Warien and Chester, 1985; Cowan, 1988) 

 for the two species in the size ranges used here. Even so, the 

 counted rings need not be deposited daily for growth rate 

 comparisons, nor is it necessary to know their periodicity. It 

 is required that groups offish being compared exhibit the 

 same ring formation periodicity over the time and space 

 of the comparison. Spot and Atlantic croaker do not form 

 growth rings until after yolksac absorption, about four to 

 five days after spawning (Peters etal., 1978; Warien, 1980). 

 Therefore, to estimate actual ages for mortality calcula- 

 tions, five days were added to the ring counts. 



Although it seems reasonable to assume that juvenile 

 spot and Atlantic croaker form daily sagittae rings, the 

 precision (repeatability) of ring counts and the ability to 

 identify daily rings needs addressing. Before aging the 

 samples used in this study, I examined several hundred 

 spot and Atlantic croaker otoliths. Counts by myself and 

 2-3 other otolith readers were compared. Our ring identi- 

 fications were compared to samples of known age spot and 

 Atlantic croaker provided by the National Marine Fisheries 

 Service (Warien^). These preliminary samples were used 

 as a training device to ensure that daily rings were ac- 

 curately identified and were not confused with shadows or 

 subdaily rings. Subdaily rings may not even be resolvable 

 at the magnifications (<625x) used in the present study 

 (Campana et al., 1987; Isely^). After aging the samples used 

 in this study, I re-aged a random selection of spot (without 

 knowledge of previous age assignments) and obtained a 

 mean difference in counts of 2.85 (SD=2.03,n=27). Because 

 Atlantic croaker otolith rings were usually easier to count, 

 I assumed that the above count difference was generally 

 similar for this species. I assumed that the ages reported 

 here had a count precision of ±3 days. I also assumed that 

 any aging errors were randomly distributed throughout 

 the samples and were not spatially or temporally biased. 



Growth and mortality 



Linear regression of the form Log,gSL = 6 -i- miage) was 

 used to model growth. The slope of this line, m, is the 

 instantaneous daily growth rate. Differences in growth 

 rates between areas were assessed by using analysis of 

 covariance (covariate=age) in a general linear model pro- 

 cedure (SAS Institute, 1988). Absolute and relative daily 

 growth rates were calculated by using values predicted 

 with the age-SL regression equation (Ricker, 1975). For 

 each sampling date, mean SLs were compared between all 



2 Warien, S. M. 1989. Personal commun. National Marine 

 Fisheries Service Beaufort Lab, Beaufort, NC 28516. 



' Isely, J. 1989. Personal commun. Zoology Dept., NC State 

 Univ., Raleigh, NC 27695. 



