FISHERY BULLETIN: VOL, 86, NO. 2 



ments. On cruise IV, a Plessey Model 9040 CTD 

 was used for both temperature and salinity. On 

 cruise V, salinity samples were stored in bottles 

 and analyzed in the laboratory using a Guildline 

 Model 8400A Autosal, and temperature was 

 measured with reversing thermometers. 



Microzooplankton were collected using 5 L 

 (cruises I, II, and V) or 30 L (cruises III and IV) 

 Niskin bottles. During cruise I, 1 L samples of 

 untreated water were collected from 3 depths and 

 preserved for later analysis. During subsequent 

 cruises, samples were collected from up to 8 

 depths; four liters were poured gently through a 

 20 |xm sieve, backwashed with filtered seawater 

 into a sample jar, and preserved in a 5% formalin- 

 seawater solution. In the laboratory, all organ- 

 isms in each preserved sample were identified 

 and counted. In selected samples, the length and 

 width of the first 50 copepod nauplii were mea- 

 sured using an ocular micrometer. In addition, 

 during cruise I larger nauplii were collected with 

 0.1 m^ 64 |jLm mesh nets nested inside the 1 m^ 

 333 ^JLm mesh nets of a MOCNESS net (Wiebe et 

 al. 1976). Estimates of naupliar abundance from 

 these collections were used separately in the 

 analyses for patterns in distribution and abun- 

 dance. Lastly, from cruise I, the total number of 

 copepods in the shallow-water samples, total 

 number of copepods from the 333 jxm nets, and the 

 zooplankton displacement volume from the 333 

 (xm nets were analyzed for patterns in distribu- 

 tion and abundance. 



RESULTS 



Samples from the Mississippi River delta re- 

 gion, collected during 1981 (cruise I), 1982 (cruise 

 II), and 1983 (cruise III), were compared to deter- 

 mine if there was significant interannual vari- 

 ability within a region. Whole water samples and 

 samples retained on 20 ixm mesh sieves were 

 pooled for this analysis under the assumption 

 that nauplii in a whole water sample would be 

 retained on a 20 jjim mesh screen. The 64 [im net 

 samples were excluded from this analysis. This 

 test was made using average naupliar concentra- 

 tions in the upper 10 m only; naupliar concentra- 

 tions were typically lower in deeper waters, and 

 we did not have the same number of deep and 

 shallow stations for each of the 3 sampling years. 

 A 1-way analysis of variance indicated significant 

 variation between years within the Mississippi 

 River delta region {P <0.01). All 3 years were 

 statistically different. These results indicated to 



us that regional or other spatial comparisons can 

 only be made using samples collected during the 

 same year. Interannual comparisons for other re- 

 gions were not possible. 



Samples were collected from all 3 regions dur- 

 ing cruise I. Four tests for regional differences 

 during this cruise were made. First, regions were 

 compared based on naupliar concentrations in the 

 upper 10 m only (Table 2); based on whole water 

 collections F was significant at P < 0.05 and 

 based on samples collected with 64 \xm mesh nets 

 F was significant at P < 0.09. Second, regions 

 were compared based on naupliar concentrations 

 from all depths (Table 2); based on whole water 

 collections F was significant at P < 0.01 and 

 based on samples collected with 64 (xm mesh nets 

 F was significant at P < 0.22. Thus, whole water 

 samples collected on cruise I indicated significant 

 regional differences in naupliar concentration 

 but samples collected with 64 fxm mesh nets did 

 not. In addition, there were significant regional 

 differences in the concentrations of total copepods 

 (P < 0.01) and in zooplankton displacement vol- 

 ume (P < 0.05) during cruise I (data not shown). 

 Regional comparisons were not possible during 

 other cruises because all 3 regions were not sam- 

 pled in years other than 1981. 



Where possible, we also tested for onshore- 

 offshore gradients in naupliar concentrations in 

 the upper 10 m. Stations were categorized accord- 

 ing to bottom depth as shallow, <18 m, interme- 

 diate, 18-90 m, and deep, >90 m, and compari- 

 sons were made between these 3 depth categories. 

 A highly significant onshore-offshore difference 

 existed during cruise I for nauplii collected with 

 the 64 |xm MOCNESS nets (Table 3). During 

 cruise III samples were only collected in the Mis- 

 sissippi River delta region but onshore-offshore 

 differences within this region were significant 

 (Table 3). No significant onshore-offshore gradi- 

 ents were observed in naupliar concentrations de- 

 termined from whole water samples collected 

 from all 3 regions during cruise I, or in nauplii 

 from only the Mississippi River delta region dur- 

 ing cruise II (Table 3). 



Although regional differences in average con- 

 centrations of nauplii were not strong, the verti- 

 cal distributions were different. For example, sev- 

 eral stations at approximately the 50 m isobath 

 are compared in Figure 2. At station IV- 1 (Fig. 

 2a) ofT the coast of Texas, there was no marked 

 vertical heterogeneity in naupliar concentra- 

 tions, the maximum concentration was low (23 

 nauplii/L), and the average concentration was 



322 



