TABLE 2. The estimated average numbers of zooplankters per 6.5-minute sampling interval entrapped 



in phytoplankton on the filter screens in 1961 compared to the average numbers retained in the 

 collecting bucket per interval in 1958 



^ Period 1 had 45 sampling intervals, period 2, 18, and period 3, 36 for a total of 99. At end of 

 each period all material adhering to main filter screens was removed and preserved for later enumer- 

 ation. 



^ 129 sampling intervals. 



on several occasions, showed that a small 

 portion of the zooplankters entering the filter- 

 ing funnels are trapped in this material. The 

 numbers of zooplankters in three such phy- 

 toplankton samples, as estimated by volu- 

 metric subsampling with replacement, are 

 given in table 2. Since the phytoplankton was 

 accumulated over different numbers of 6.5- 

 minute sampling intervals in each case, the 

 estimates are summed and divided by the total 

 number of intervals for the three samples 

 and expressed as average numbers per sam- 

 pling interval. 



Copepods, larvaceans, and doliolids were 

 the only zooplankters found in the phyto- 

 plankton. The larvaceans are somewhat more 

 abundant than the copepods, while the doliolids 

 are far less abundant. Comparison to the 

 average numbers of such organisms present 

 in the 129 samples collected in 1958 suggests 

 that the percentage loss of copepods is negli- 

 gible, but that^the loss of larvaceans may be 

 higher than 4» percent. If entrapment of 

 larvaceans in the phytoplankton proves to 

 be this high in terms of the matching 1961 

 samples, the towed pump and shipboard filter- 

 ing unit would have to be considered unsuitable 

 for making quantitative collections of this 

 group. No worthwhile assessment can be 

 made of entrapment of doliolids because they 

 are virtually nonexistent in the 1958 samples. 



Except for larvaceans, then, it appears that 

 the loss of zooplankters through escapement 

 and entrapment is probably not more than a 

 few percent, and that loss of accuracy through 



mixing between consecutive samples is prob- 

 ably negligible. For most purposes such 

 minor losses can be disregarded, but if 

 necessary the losses from escapement and 

 entrapment can be measured directly so that 

 corrections can be applied to sample esti- 

 mates. 



DISCUSSION 



The towed pump and filtering unit was 

 designed for the purpose of making estimates 

 of known precision of small zooplankters for 

 areas of specified sizes. Operations during 

 the fall of 1961 indicate that the system 

 performs this task satisfactorily. Zooplankters 

 between the lengths of 0.2 mm. and 12.0 mm. 

 (approximately 1/2 inch) are collected quanti- 

 tatively and the collection of samples that 

 are discrete for consecutive 6.5-minute inter- 

 vals of vessel travel at cruising speed is a 

 simple procedure. A number of such small 

 samples can be taken as subsamples from a 

 given area. 



The present operational status of the towed 

 pump and shipboard filtering unit, particularly 

 of the collector and the hose-cable assembly, 

 was achieved only after considerable experi- 

 mentation and modification. The towed pump 

 and hose-cable suspension are patterned after 

 a model originally designed and built for the 

 Bureau of Commercial Fisheries Biological 

 Laboratory, La Jolla, by the Fisheries Instru- 

 mentation Laboratory of the Bureau of Com- 

 mercial Fisheries Biological Laboratory, 



17 



