Fishery bulletin of the Fish and Wildlife Service - NOAA

214 FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE
TABLE 15.—Specimen computation of factors for adjustment of haul to standard basis of straining ,17.07
cubic meters of water per meter of depth fished and for ascertaining contamination of catch of the
lower net in passing through the upper stratum
Item • . : .
[The data relate to station 21491]
1. Length of line payed out . . _ .
2, Ayftrnge stray * - - - , ' , -
3. Stratum fished - -
5. Time fished (exclusive of time spent by the lower net in passing through
the upper stratum).
6. Rate of flow through not (from current meter) . .
7. Clogging (on ftrb'tiwy scale, see text) - - -
10. Totnl flow (item я tlmm )te"i 9)
11. Standardflow Atem 4 times 17.07— V. :
13. Time spent by lower net in passing through the upper stratum
14. Flow through net while passing through the upper stratum (item 9 times
item 13).
16. Factor to be applied to catch of upper net to find the number of organisms
caught by lower net while passing through the upper stratum.
Unit
Meters
Degrees from verticaL...,.
Meters
Meters
Seconds ,
Meters per second
Meters per second.
Meters РвГ
Meters . . —
Seconds -
Meters
Upper net
0-2«
£1.3
0-16
16
865
О.Б74 1
0.574
496
348
0.70
Lower net
26-65
' , 35.0
20-46
- * • .-- 16
'980
1
-0.007
0.567
656
Correction for contamination.—The nets were lowered and raised without closing.
Consequently when two nets were used, the portion of the catch of the lower net taken
during its passage through the stratum fished by the upper net may be considered as a
contamination. The amount of this contamination was computed from the known
average concentration of mackerel eggs and larvae in the upper stratum, the known
time spent by the lower net in passing through this stratum and the assumed flow
through the net (the same as that registered by the current meter installed in the
upper net after correction for clogging). The computations were made for each stage
of egg and length of larva, and the resulting numbers subtracted from the catch of the
lower net (table 14). In all instances, the corrections were substantial, and at many
stations approximated the entire catch of the lower net. Important numbers usually
remained after the correction at those stations where the upper net did not fish down
to the thermocline and the lower net fished in the stratum above the thermocline for
a time in addition to the time spent while it was being payed out and hauled back
through this stratum. As might be expected from consideration of the laws of
random sampling, the amounts to be subtracted were sometimes in excess of the
amounts caught in the lower net. When this occurred, differences were negatively
added to the catch of the upper net, these instances of over-correction offsetting otnef
instances of under-correction, leaving the average undisturbed.
Relative catch of 1-meter and 2-meter nets.—A comparison of the catching efficiency
of 1- and 2-meter nets is afforded by 19 instances during cruises VI and VII where
both nets were hauled at the same station. The hauls were made, and the results
were converted to the standard basis by the methods already described for both
348
0.63
127
72
0.21