small copepods supplied, on the average, 74 

 percent of the total organic matter in the 

 stomach contents of sardines and that all 

 crustaceans supplied nearly 89 percent. The 

 size range of small copepods is not explicitly 

 stated, but it may be surmised from the data 

 in the above report that the small category 

 includes organisms up to about 1 mm. in 

 length. To collect organisms of the size 

 indicated, filtering screens with mesh open- 

 ings of 100 microns (/j) or perhaps even less 

 must be used. 



The study also requires that estimates of 

 known precision be obtainable for areas as 

 small as 20 square miles or as large as 

 several hundred square miles. This can be 

 accomplished efficiently by subsampling, i.e., 

 taking several small, discrete samples within 

 the specified area, while the vessel is 

 traveling at cruising speed. 



The study further requires that areal esti- 

 mates be made for more than one depth or 

 that they represent some vertical range. To 

 accomplish this the sampling instrument must 

 be capable of collecting samples over some 

 range of depth rather than at a single depth. 



A towed pump and shipboard filtering unit 

 appeared to offer the best possibility of 

 satisfying these requirements. Filtering rate 

 and filtering efficiency can be independent of 

 filter mesh size, and they can be directly 

 measured. With the proper mechanical ar- 

 rangement, discrete samples representing 

 very short segments of the vessel track can 

 be taken at vessel cruising speed. If the 

 pump is suspended at the end of a hose it 

 can be lowered to different depths. 



Although self-contained high speed samplers 

 such as the Isaacs high speed sampler (Ahl- 

 strom, Isaacs, Thrailkill, and Kidd, 1958), the 

 Gulf III sampler (Gehringer, 1952), and the 

 Hardy plankton recorder (Hardy, 1939) can be 

 towed at different levels, they do not adequately 

 retain zooplankters less than 1 mm. in length. 

 Filtering screens with meshes fine enough to 

 collect these smaller organisms would ad- 

 versely effect filtering rate and filtering effi- 

 ciency by intensifying clogging. The Isaacs sam- 

 pler and the Gulf III sampler, furthermore, are 

 unsuitable for the collection of short interval 



samples because they must be retrieved and 

 serviced to terminate each sample. The Hardy 

 recorder does resolve a continuous strip col- 

 lection into an intergrade series of small sam- 

 ples, but there is a practical limit to how small 

 these samples can be and it is necessary to 

 make assumptions about filtering rate and fil- 

 tering efficiency. 



Other workers have resorted to pumps for 

 plankton sampling from time to time, because 

 this seemed to be the best way to collect 

 numerous small samples in time and/or space 

 to study the variability of plankton density. 

 Aron (1958) lists 17 investigators in addition 

 to himself who have used pumps to collect 

 plankton. Cassie (1959) has also made ex- 

 tensive use of a pump to collect plankton. 

 However, none of the systems used by these 

 investigators would have fulfilled the require- 

 ments outlined above. Although several of 

 them have been used to sample from various 

 and considerable depths, this was possible 

 only from vessels that were drifting or moving 

 very slowly. Collier (1957) is the only in- 

 vestigator listed by Aron who sampled with 

 a pump while traveling at vessel cruising 

 speed. He achieved this by using an inboard 

 pump with the intake protruding through the 

 hull of the vessel, an arrangement that offers 

 no potential for sampling at different depths. 



The towed pump and shipboard filtering unit 

 described in this report was designed to retain 

 zooplankters, particularly crustaceans, as 

 small as 100 or 200 /J in length and to take 

 samples that would be discrete for time 

 intervals as short as a few minutes. It was 

 designed also with the potential for collecting 

 over some vertical range, though no attempt 

 has yet been made to realize this capability. 



DESCRIPTION 



The entire system is shown schematically in 

 figure 1 as it is installed on the Black Douglas. 

 It is composed of (1) a collector containing a 

 pump and motor towed by a suspension unit 

 consisting of a hose through which runs a 

 steel cable for support and an electric line 

 to power the pump, (2) a winch and auxiliary 

 guide wheel, and (3) a shipboard filtering unit. 

 The towed hose is a 150- foot length of 2-inch 

 internal diameter (I.D.) single- jacket firehose. 



