BIOLOGY OF THE ATLANTIC MACKEREL 



215 



nets, excepting that no current meter was employed to measure the flow of water 

 through the 2-meter net. In lieu of this measure, the speed of towing was measured 

 by timing the travel of the ship past a chip cast alongside. It was later found from 

 a statistical analysis of the relation between chip speed and flow through meter nets 

 as measured by the current meter, that tbe force of the wind modified the chip speed 

 materially. From the relationship established, a schedide of adjustments was applied 

 to the apparent cbip speed, to convert it to an approximation of true towing speed. 

 This apparent flow was used instead of a current meter reading. Because of the sub- 

 stitution of a deduced value based in part on average performance instead of on actu- 

 ally measured value, the two members of individual pairs of hauls are not strictly 

 comparable, but the average, or sum, of the 19 hauls with each type of net is not subject 

 to this fault. 



From the distribution of sizes of larvae caught by the respective nets (table 16), 

 it is obvious that the smallest sizes of mackerel larvae were almost entirely lost through 

 the coarse meshes of the 2- meter net; that the 6- to 9-mm. sizes were incompletely 

 retained; and that sizes from 10 mm. upward were fully retained by the larger net. 



Two conclusions may be drawn from the comparison: (1) the catches of the two 

 nets, per unit volume of water strained, are virtually identical for larvae 10 mm. and 

 upward, and nearly so for the 7- to 9-mm. sizes, hence no material distortion can have 

 resulted from the pooling of data from the two types of nets, according to the methods 

 employed in this report. (2) Both types of net must have taken essentially all the 

 larvae of sizes 10 to 22 cm. in length that chanced to be in their path, for if any larvae 

 tended to dodge the nets they would surely have been relatively more successful in 

 eluding the 1-meter net, and thus lowered its catch of the elusive sizes in relation to 

 that of the 2-meter net. The closeness of the paired values for the size range specified 

 is eloquent evidence this did not take place. It is to be regretted that no such paired 

 hauls are available for the later cruises, when catches of still larger larvae might have 

 indicated the upper size limit for effective catching of larvae by plankton nets. 



Table 16. — Comparison of numbers of larvae caught by 1-meter nets and by 2-meter nets at identical 



stations of cruises VI atid VII 

 [Catches of both nets were converted to the basis ot straining 17.07 cubic meters per meter of depth fished] 



