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several depths simultaneously, thus recording both the vertical and horizontal 

 distribution of the plankton. Although it would obviously be very difficult to 

 cause the recorders to tow at great depths from vessels traveling at cruising 

 speeds, it should be possible to sample the deeper levels from research vessels 

 traveling at slower speeds. At speeds of 2 to 4 knots five instrunnents could 

 probably be towed at five different depths. 



Clarke-Bumpus Plankton Sampler - Over 150 instruments of this design have 

 been built and supplied to more than 60 laboratories in various parts of the 

 world. The plankton sampler has been reported to be very useful at least for 

 certain types of work with certain types of plankton. In addition to whatever 

 intrinsic merit the instrument may have, it has the advantage that it represents 

 a standard quantitative procedure available for every laboratory and makes pos- 

 sible an exact comparison between the results of plankton investigations in dif- 

 ferent parts of the world. The revised account of the sampler published by 

 Clarke and Bumpus (1950) gives further infornnation for the adjustments and cal- 

 ibration of the instrument. Additional refinements have been developed since 

 1950, and anyone having any difficulty with the operation of the instrument 

 should consult the authors or the maker. 



Two minor difficulties have been reported in the use of this instrument. 

 In regions where jelly-fish, salps, or sea-weed is abundant these organisms 

 may cling to the wire and interfere with the operation of the messengers. The 

 other difficulty is the failure of the instrument to catch a sufficient quantity of 

 plankton in regions where plankton is not abundant. It is believed that a scaled 

 up model of the instrument could be built for such areas. By increasing the 

 opening of the net to a diameter of about 11 inches, the area of the opening and 

 hence the volume filtered per unit of time could be increased fivefold. In the 

 case of larger instruments it would be desirable to streamline them to reduce 

 horizontal resistance if this is feasible. 



In our plankton studies on Georges Bank off Cape Cod plankton samplers 

 equipped with No. 2 silk net were hauled at speeds of about 2 knots for periods 

 of 25 to 40 minutes. Ordinarily between 10 and 20 cubic meters of water were 

 filtered during each tow by each sampler. But filtered volumes as high as 30 

 cubic meters were recorded and, in the case of severe clogging, the filtered 

 volumes were reduced to as little as 5 cubic meters. On Georges Bank the 

 tows with the plankton sampler produced catches of about 1 cubic centimeters 

 of plankton measured by displacement per cubic meter of water filtered, equal 

 to about 1 gram wet weight or 0. 1 gram dry weight. Ordinarily 10 to 20 cubic 

 centimeters of plankton were obtained per haul and this amount is obviously suf- 

 ficient for a quantitative estimation of the more abundant constituents of the 

 plankton. In regions where the plankton is much less abundant, or for a study 

 of the very scarce constituents of the plankton, larger volumes are needed. In 

 such situations the sampler must be towed for longer periods or a larger model 

 nnust be built. 



Comparison with simultaneous hauls made at the same time with larger 

 nets indicated that organisms of the size and activity of sagittae and copepods 

 did not escape the relatively small opening of the instrument in any selective 

 fashion. The plankton sampler can be towed for as long as is desired, thus 

 avoiding the objections of spot samipling. Even with relatively short hauls the 

 samipler obtains a far more representative picture than that obtained by many 

 other methods. In fresh water the sampling of zooplankton is sometimes done 

 by means of water samples taken directly or by means of the Juday plankton 

 trap. Vertical net hauls are of course spot sampling as far as the horizontal 

 direction is concerned. The use of the plankton pump is favored by some lab- 



