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



443 



different and the rank abundances were nearly identical. 

 Furthermore, nearly identical life-stage distributions 

 were derived for the dominant copepod species at the 

 two tow speeds. There was no significant effect of tow 

 speed on zooplankton community structure. 



Catches in most tows at a single location showed 

 good agreement between the catches of taxa at the 

 two speeds, although occasionally there were large 

 differences between species abundance. One reason 

 for this discrepancy may be sampling error attributed 

 to water movement. Although the vessel returned to 

 the exact location where the 1.5-knot tows began, the 

 same specific water parcel was not precisely sampled 

 by the faster tow. Furthermore, the patchiness of zoo- 

 plankton also makes it difficult to sample exactly the 

 same community with duplicate tows. Nonetheless, 

 these differences were averaged out when several sta- 

 tions were pooled and mean or median abundances 

 were calculated. 



Direct comparison between displacement volume 

 measurements made at the two speeds is not recom- 

 mended. It appears that at the slower tow speed unac- 

 counted material is collected that is extruded through 



the net meshes at higher speed tows. Filtration pres- 

 sure across the mesh varies with the square of the 

 approach velocity; thus, 3.5-knot tows will exert a 

 pressure approximately 5.4 times higher than the 1.5- 

 knot tows (Tranter and Smith, 1968). Because this 

 study was conducted at or near the height of the spring 

 phytoplankton bloom in the region, sample biomass in 

 the slower tow was likely elevated by the retention of 

 larger diatom species that dominate the spring bloom 

 on Georges Bank (O'Reilly and Zetlin, 1998). The re- 

 sults indicate that sample displacement volumes for 

 the 3.5-knot tows conducted during early spring should 

 be increased by 40% before comparison to levels for 

 the 1.5-knot tows. Owing to the widespread seasonal 

 and spatial variability of plankton populations, caution 

 should be exercised in applying this conversion factor 

 to other data sets. Colton et al. (1980) also reported 

 that displacement volumes for replicate 3.5-knot tows 

 were lower than those for 1.5-knot tows, but they found 

 the difference was only 20.8%. Their study was con- 

 ducted in the same region as ours, but samples were 

 collected at one location in early November over a 

 five-day period. Further comparative studies in other 



