Fishery bulletin of the Fish and Wildlife Service - NOAA

BIOLOGY OF THE ATLANTIC MACKBBEL 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 the force of the wind modified the chip speed
materially. From the relationship established, a schedule of adjustments was applied
to the apparent chip 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 substitution
of a deduced value based in part on average performance instead of on actually
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 hi 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 8-meter nets at identical
stations of cruises VI and VII
[Catches of both nets were converted to the basis of straining 17.07 cubic meters per meter of depth fished]
Length of larvae (millimeters)
3 ... .
4
s
и 7 ..
8 . .
9
10
11 .
2-meter
net
Number
0.39
.61
1.76
7.40
17.33
28.10
20.28
13. 76
13.17
1-meter
net
Number
6 214
230
143
56
33
25
37
24
14
12
13
14
16
16
17
18
20
22
Length of larvae (millimeters)
2-meter
net
Number
12.84
9 50
5 86
3.14
.48
.48
.09
.09
1-meter
net
Number 13
10j4
1
1