210 FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



45° above the first net and 28° below it. The depth ranges of fishing would then be 

 0-18 meters and 22-44 meters, respectively, for the upper and lower nets. Since the 

 course of plankton nets through the water usually is undulating (Russell, 1925, pp. 

 603-604), the theoretically unfished gap between the nets and the theoretically 

 stepwise character of hauls would both be practically obliterated and the sampling 

 virtually uniform, except for the greater depth range covered in unit time by the 

 lower net. The latter was taken into account in the subsequent treatment of data. 



During the eighth and ninth cruises when the hauls were made with a 2-meter 

 net, only one such net was used, and at the deeper stations it was sent down to a depth 

 roughly equivalent to that reached by the deeper of the two nets employed on earlier 

 cruises, so that the single, oblique haul of the 2-meter net sampled through approxi- 

 mately the same strata as the two nets of the preceding cruises. 



Measurement oj quantity of water strained by the nets. — It is obvious that two 

 variables, speed of towing and degree of clogging, seriously modify the flow of water 

 through plankton nets, causing variations in the catching capacity. To eliminate these 

 sources of variability, a current meter was installed in the mouth of the net to measure 

 the flow. The utility of current meters in measuring the volume of water passing 

 through a plankton net depends on whether or not the flow past the meter is equal 

 to or proportional to the average flow of water into the net. By towing, at usual 

 speeds, a standard net with a current meter in the center of the mouth and another 

 meter at the periphery, it was found (William C. Herrington, unpublished notes) 

 that the flow past these two positions differed less than 10 percent. Since these 

 positions were such as to register the maximum difference in rate of flow, if any ex- 

 isted, this evidence was taken as indicating uniform flow into all parts of the mouth 

 of the net. Hence we regarded the registration of flow past the meter as directly 

 measuring the flow tlirough the entire opening. 



The instrument used for measuring the flow consisted of the propeller mechanism 

 and revolution counter from a dismembered Ekmann current meter, turning five to 

 six revolutions per meter of flow at usual towing speeds. For precise determination 

 the meter was calibrated over the range of towing speeds. The total revolutions 

 turned during a tow were converted to speed by dividing by the duration of the haul, 

 in seconds; and the equivalent rates of flow were found from the calibration graph. 

 These are the rates used in the specimen computation of table 15. 



While the current meter was used as a standard procedure, there were times when 

 mechanical difficulties prevented proper registration. To provide basic data for 

 comparable treatment of hauls made on such occasions, records were taken periodi- 

 cally, during each haul, of the towing wire's angle of stray and of the ship's speed as 

 measured by timing the progress of the ship past a chip cast alongside. An estimate 

 of the extent to which the net was clogged was made at the end of each haul. Rela- 

 tions between these observations and flow past the current meter gave average factors 

 by which angles of stray or ship's speed could be translated to terms of equivalent 

 current meter measurements. This afforded means of estimating the flow on those 

 hauls which were not accompanied by reliable current-meter records. All the hauls 

 of cruise I, and 5 percent of the hauls on subsequent cruises were of this class. For 

 these hauls there was some error of estimate which may have been considerable for 

 individual instances, but were, we believe, of random nature tending to balance each 

 other, and so could have introduced very little inaccuracy into the general results, 

 based on averages of a number of stations. 



