NOTE Kane and Anderson: Effect of towing speed on retention of zooplankton in bongo nets 



441 



42'N 



40'N- 



71 W 



69 W 



67 W 



Figure 1 



Locations surveyed with the RV Delaware II for effect of towing speed on 

 retention of zooplankton in bongo nets. Sampling was conducted from March 

 29 to April 1999 at two tow speeds: 1.5 and 3.5 knots. Locations are labeled 

 with the station number. 



cal and ordination techniques from the PRIMER 6.1.5 

 software package (PRIMER-E Ltd, Plymouth, UK). 

 Initially, data were log transformed and the biotic rela- 

 tionship between any two samples was represented by 

 the Bray-Curtis index (Bray and Curtis, 1957), which 

 measures the similarity (or dissimilarity) in species 

 composition. The triangular matrix of similarity be- 

 tween each sample was then classified into groups by 

 using two techniques: 1) hierarchical agglomerative 

 cluster analysis, and 2) nonmetric multidimensional 

 scaling (MDS). Clusters of samples that were found 

 to be statistically significant (P<0.05) by the similar- 

 ity profile test (SIMPROF) and also to be isolated by 

 low stress (<0.20) MDS ordinations, were judged to be 

 samples with similar zooplankton community structure. 

 The significance of the resultant groupings was tested 

 by using the nonparametric permutation procedure 

 ANOSIM (analysis of similarity). 



Results 



Nine taxa dominated the zooplankton community cap- 

 tured during the survey (Table 1). All dominant taxa 

 were common to both tow speeds and their mean abun- 

 dance ranked in nearly the same order. The only differ- 

 ence was found for the copepods Temora longicornis and 

 Oithona spp., which were the seventh and eighth most 

 abundant taxa in the 1.5-knot tows, and eighth and 

 seventh most abundant in the 3.5-knot tows (Table 1). 



The mean and median abundance levels of the dominant 

 species were not significantly different (P>0.05) between 

 tow speeds. 



The copepodite stage distributions of the dominant co- 

 pepods were equally represented at the two tow speeds. 

 Mean abundance levels of all Calanus finmarchicus life 

 stages and Pseudocalanus spp. stage-V copepodites were 

 nearly identical from both hauls (Fig. 2) and not signifi- 

 cantly different (P>0.05). Slightly higher mean numbers 

 of late-stage copepodites of Metridia lucens, Centropages 

 typicus, and Centropages hamatus, and adults of Pseu- 

 docalanus spp. were captured at the higher tow speed 

 (Fig. 2), but the values also were not significantly differ- 

 ent (P>0.05). These similar mean numbers for the vari- 

 ous life-stages indicate that there was no substantial 

 difference in the size of particles retained or extruded 

 by the nets at the two tow speeds. 



Ordinations of zooplankton community structures 

 from the two speeds were very similar. Cluster analysis 

 of data from both tow speeds produced five significant 

 (P<0.05) groupings that were also evident in the low- 

 stress MDS plots (Fig. 3). These station clusters for the 

 two tow speeds were nearly identical, both reflecting the 

 different depth strata and areas sampled during the 

 survey. The two large clusters were essentially inshore 

 and offshore station groupings, and the remaining three 

 were different deepwater (>200 m) sites (Figs. 1 and 3). 

 The community composition of the samples collected at 

 the two speeds was not significantly different (ANOSIM 

 procedure, P=0.40). 



