442 



Fishery Bulletin 105(3) 



3.5 knot tows 



1 .5 knot tows 



25000 



12500 



Pseudocalanus spp. 



Calanus finmarchicus 



8000 



4000 



o 

 o 

 o 



S 9000 



o> 4500 



Adults Stage Stage Stage 

 V IV III 



Metridia lucens 



0.74 



0.32 0.38 



Adults Stage Stage 



V IV 



5000 



2500 



Centropages typicus 



25000 



12500 



Adults Stage Stage 



V IV 



Pseudocalanus spp. 



Figure 2 



Comparison of mean abundance at 3.5 and 1.5 knot 

 tow speeds for different life stages of the five dominant 

 copepods sampled during the cruise. Only the mean 

 abundance of adults and copepodite stages (denoted as 

 stage III, IV, or V) that were captured quantitatively 

 with the 0.333-mm mesh nets are shown. Mean values 

 for the two tow speeds were not significantly different 

 (P values from <-test results are given above each tested 

 pair). Error bars represent one standard error. Please 

 note that y-axis scales are different. 



1 .5 knots 



Figure 3 



Nonmetric multidimensional scaling (MDS) plots pro- 

 duced after calculation of Bray-Curtis similarity index 

 on log-transformed zooplankton station abundance data 

 obtained with the bongo sampler towed at (A) 1.5 knots 

 and (Bi 3.5 knots. Encircled groups are stations strongly 

 grouped by cluster analysis. The stress value is a mea- 

 sure of the strength of the MDS plot and is considered 

 meaningful if <2.0. 



In contrast to species abundance, mean and median 

 displacement volume values from the 1.5-knot tows 

 were significantly different (P<0.05) from the faster 

 tows, averaging 40% higher (Table 1). Because mean 

 and median total zooplankton counts at the two speeds 

 were not significantly different (P>0.05), the increased 

 biomass could be caused by items not counted in slower 

 tows. These could include phytoplankton, Cnidaria frag- 

 ments, and general detritus that would have been ex- 

 truded through the nets at higher speed tows. 



Discussion 



The degree of similarity between the overall catches 

 at the two speeds was remarkable given the variation 

 normally associated with replicate plankton tows (Wiebe 

 and Holland, 1968). None of the survey mean and median 

 abundance values of dominant taxa were significantly 



