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 



[The data relate to station 21491] 



Item 



Unit 



Upper net 



Lower net 



Length of line payed out 



Average stray 



Stratum fished 



Thickness of stratum fished 



Time fished (exclusive of time spent by the lower net in passing through 

 the upper stratum). 



Rate of flow through net (from current meter) 



Clogging (on arbitrary scale, see text) 



Adjustment for clogging 



Adjusted flow (item 6 plus item 8) 



Total flow (item 5 times item 9) __. 



Standard flow (item 4 times 17.07— ) _ 



Meters 



Degrees from vertical. 



Meters 



Meters 



Seconds 



Meters per second. 



Meters per second. 

 Meters per second. 

 Meters 



Meters 



Factor for adjusting to standard haul (  . fl ) . 



Time spent by lower net in passing through the upper stratum 



Flow through net while passing through the upper stratum (item 9 times 



item 13). 

 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. 



Seconds. 

 Meters.. 



0-25 



51.3 



0-16 



16 



865 



0.574 

 1 



0.574 

 496 



0.70 



25-55 



35.0 



20-45 



16 



1 



-0. 007 



0.567 



556 



348 



0.63 



127 

 72 



0.21 



Correction Jor 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 other 

 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 



