WEINSTEIN ET AL.: RETENTION OF THREE TAXA OF POSTLARVAL FISHES 



reading <80% of the outside reading would indi- 

 cate that the inside meter had fouled. Of the 144 

 bottom samples collected, 71 had low inside meter 

 readings and 16 had high inside readings. An ad- 

 ditional 3 samples were discarded due to gear fail- 

 ure, leaving a total of 54 samples that could be 

 used to obtain an estimator for volume. 



A strong linear relationship between inside and 

 outside meter readings (r = 0.996, P<0.01) indi- 

 cated that an adequate linear predictor could be 

 obtained: 



Y = 0.913 Z 



where Y is the inside meter reading and X is the 

 outside meter reading, both in revolutions. The 

 standard error of the slope of this line is 0.009, and 

 the standard error of an individual estimate of the 

 inner reading is: 



SE = V'7.4729 X 10"^ X^ + 1.758337 x 10^ . 



The above estimator was used to obtain volumes 

 for all bottom samples in which the inside meter 

 reading was <80% of the outside reading. If a net 

 were actually clogged when we assumed that the 

 meter was fouled, this procedure would result in 

 an overestimate of the volume filtered and under- 

 estimate of the actual density of larvae present. 

 Thus, differences among strata would be even 

 larger than depicted in our data. 



Upstream and downstream nets at each depth 

 along the paired transects served as replicates in 

 the experiments. This survey constituted a facto- 

 rial design, with site, photoperiod, and tidal direc- 

 tion as main effects. Nonorthogonal factorial 

 analyses of variance ( ANOVA) (Searle 1971) were 

 performed for each taxon, date, and buoy (except 

 spot and Atlantic croaker at buoy 50 on 14-15 

 March). Examples of the analytical results are 

 reproduced in Appendix I to allow the reader to 

 follow our procedures. A posteriori multiple com- 

 parison procedures (Bonferroni ^-tests a = 0.05; 

 O'Neill and Wetherill 1971) were used to examine 

 station and depth differences and their interac- 

 tions with photoperiods and tides. Prior to analy- 

 sis, data were logarithmically transformed [logiQ 

 (10 + X)] in order to meet the homoscedasticity 

 requirement of ANOVA. 



A partial data analysis was performed for 5 

 April 1978 at buoy 50, deleting the last night set. 

 Either daytime or ebb data alone were used, de- 

 pending on the strata compared. However, data 



from all three valid sets were used to obtain an 

 estimate of sampling variability. 



All collections were preserved in 5% buffered 

 Formalin, and selected taxa were enumerated and 

 measured for standard length (SL). The latter 

 measurement was taken from the tip of the snout 

 to the end of the notochord or hypural plate. Sub- 

 sampling for lengths was employed when sorted 

 collections contained >100 individuals of a given 

 species. Data are presented herein on three taxa: 

 spot, Leiostomus xanthurus; Atlantic croaker, 

 Micropogonias undulatus; and flounders of the 

 genus Paralichthys . Flounders were counted but 

 not measured in this program. 



RESULTS 



Since the April sampling dates were near the 

 end of recruitment for winter-spawned species in 

 the Cape Fear estuary, the observed pattern of 

 distribution during this month should reflect the 

 selection of preferred nursery habitats. On 14-15 

 March, freshwater flow in the river exceeded 990 

 m^/s, and the salt boundary was below buoy 50, as 

 indicated by the absence of measurable salt in the 

 water column. On these dates, spot were entirely 

 absent at buoy 50 and only two Atlantic croaker 

 postlarvae were captured (Figures 3, 4). Floun- 

 ders, however, were abundant at buoy 50 during 

 this period. When subsequent sampling indicated 

 that the salt front was restored to its normal loca- 

 tion, about 6 km above buoy 50, catches of all taxa 

 (with one exception) were significantly greater 

 (P<0.05) upstream (Table 2). 



Diel Behavior 



Significant differences were only occasionally 

 detected among stations and were likely 

 influenced by local patterns of current and larval 

 transport. For this reason, these comparisons were 

 not considered further and were omitted from the 

 ANOVA summary (Table 3). Consistant trends, 

 however, were evident in several other compari- 

 sons involving depth, photoperiod, and their in- 

 teractions. For example, the 24 h mean abundance 

 across depths for spot and Atlantic croaker, and to 

 a lesser degree for flounders, was higher (see also 

 Figures 3, 4) at the bottom on the shoals and at 

 middepth and below in the channel, with essen- 

 tially no differences between buoys. Photoperiod, 

 on the other hand, influenced the catches of floun- 

 der in a consistent manner. Except for 14-15 



423 



