Whether the size of the zone of suction 

 fluctuates during actual towing due to agitation 

 of the water by the movement of the vessel 

 itself is not known, but it is at least reason- 

 able to assume that the orifice of the collector 

 is always preceded by a field of suction 

 rather than a field of back pressure. Back 

 pressure, which is often produced by clogging 

 in plankton nets, is undesirable because it 

 may delay the passage of some organisms into 

 the orifice or even deflect them from it. 

 Suction, on the other hand, will insure that 

 organisms are transported instantly from 

 the orifice through the hose to the filtering 

 unit. 



Wiborg (1948) suggests that the faster mov- 

 ing organisms will succeed in avoiding the 

 currents at the mouth of the suction hose of 

 pump collectors. He concluded, after com- 

 parative tests with the Clarke-Bumpus sam- 

 pler and the Nansen closing net, that a pumping 

 rate of at least 200 liters per minute is 

 necessary for adequate sampling. Though this 

 figure may have some merit for pumps that 

 are virtually stationary, as was the one used 

 by Wiborg, it cannot be arbitrarily applied 

 to a pump intake that is moving through the 

 water at a good rate of speed. The size of 

 the suction zone, which depends on the ratio 

 of the rate of intake to the rate of travel, 

 would be less for a moving collector than for 

 a stationary one. Undoubtedly the movement 

 of the collector, rather than the small zone 

 of suction, would be the major cause of avoid- 

 ance with an instrument towed at 9 knots. 



A second reason given by Wiborg (1948) 

 for the 200 liter-per-minute minimum pump- 

 ing rate is that at lesser rates it takes too 

 long to collect sufficiently large numbers of 

 some kinds of organisms. During the 1961 

 fall surveys the towed pump and shipboard 

 filtering unit, with a delivery rate of 92 

 liters per minute, collected early stages of 

 copepods and even the adult stages of 

 Calanus heligolandicus in numbers sufficient for 

 density' estimates of good precision. 

 Euphausiids occur in much lower, but still 

 sufficient numbers. The same is true of 

 chaetognaths. Fish eggs and larvae, on the 

 other hand, are among the organisms that 

 occur too rarely in the samples to yield 



abundance estimates with a satisfactory de- 

 gree of precision. It is possible that these 

 would occur in greater numbers in other 

 areas and during other seasons of the year. 



The upper size limit of organisms col- 

 lected by the towed pump and shipboard 

 filtering unit is indicated by the condition 

 of the euphausiids and the fish larvae in the 

 samples. Both are mutilated if they are 

 more than a half inch in length, and virtually 

 no organisms longer than three-quarters of 

 an inch occur in the samples. This may be 

 directly related to the physical characteristics 

 of the pump, i.e., diameter of orifices and 

 spacing of impeller blades. 



The lower size limit of organisms collected 

 is suggested by the size composition of or- 

 ganisms escaping from the filtering unit. The 

 greatest detectable escapement was through 

 the filtering screen of the collecting bucket. 

 On five occasions the water remaining in the 

 funnel at the end of a 6.5-minute sampling 

 interval was collected in a jar when the 

 friction cap was removed from the bucket 

 screen. The number of organisms in each 

 escapement sample was estimated by vol- 

 umetric subsampling with replacement. Each 

 sample was stirred in 1,000 cc. of water and 

 12 aliquots of 25 cc. were removed, examined 

 and returned successively. The mean num- 

 bers per aliquot were multiplied by 40 to 

 obtain estimates of the numbers in the sam- 

 ples. 



The estimated average numbers of copepods 

 escaping for five samples are tabulated in 

 three size categories in table 1 and illustrated 

 for eight size categories in figure 12. They 

 are not adjusted for differences in the volume 

 of water filtered in each of the five samples, 

 but volume varied very little between samples. 



Percentage escapement and along with it 

 the minimum size for quantitative collection 

 cannot be definitely established until the re- 

 lation between the size composition of es- 

 caping copepods can be compared to the size 

 composition of the copepods retained by the 

 filters during the five pertinent sampling 

 intervals. These samples have not yet been 

 counted. However, the numbers of copepods 

 present in 129 samples collected in the same 



14 



