282 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



' Although the amount of water strained by our 

 midwater trawls was not metered, it can be esti- 

 mated fairly accurately if we can accept certain 

 assumptions; namely, that (1) the hauls were 

 made at uniform speed; (2) the nets were 100 

 percent efficient, i.e., all of the water entering 

 the mouth of the net passed on through; and 

 (3) the amount of water strained was, there- 

 fore, proportional to the area of the mouth open- 

 ing. It must be admitted that these assumptions 

 may include some error, but we believe that for our 

 purposes here they can reasonably be applied to 

 the Isaacs-Kidd trawls. The nets had a high ratio 

 of length to mouth opening and were of relatively 

 coarse mesh, except for the cod end. There was 

 never any e^-idence of clogging. Vessel speed was 

 adjusted during the tows to yield a wire angle of 

 70°-72°, and the actual hauling speed was gen- 

 erally maintained within a range of 4 to 6 knots. 

 If we accept 5 knots as the average hauling speed, 

 in 1 hour the 6-foot trawl would strain 25,663 cubic 

 meters of water and the 10-foot trawl would strain 

 76,043 cubic meters. 



We have calculated average values in terms 

 of number and volume of organisms in the total 

 catch, for each of the four trawls for each major 

 geographic region investigated (table 7) . The re- 

 sults show considerable variation, part of which 

 is influenced by the large numbers of euphausiids 

 and barnacle larvae obtained in the North Pacific 

 with the two Isaacs-Kidd trawls and by a few 

 large catches, also of euphausiids, made in the 

 equatorial Pacific with the 6-foot beam trawl. The 

 volume of catch varies generally with the size of 

 the trawl. Wlien evaluated in terms of volume of 

 catch per imit of mouth area, we find that the two 

 Isaacs-Kidd trawls were on the average about 

 equal in catching efficiency; they showed no 

 marked superiority over the 6-foot beam trawl or 

 the 1-meter ring trawl when used in Hawaiian 

 waters, and were even slightly inferior to the 

 6-foot beam trawl when operating in the equa- 

 torial Pacific. 



The average individual size (ml.) of organisms 

 in the catch varied inversely with the relative 

 amount of fine-meshed netting used to line the 

 cod ends of the trawl nets. The net with the 

 largest mouth diameter caught, on the average, 

 the largest organisms. The net with the smallest 

 mouth diameter did not catch the smallest organ- 



Table 7. — Number of organisms and volume of catches 

 made tcith the four tratcls, in relation to size of mouth 

 opening of the trawl net and to geographic region 



[Number of quantitative hauls in parentheses] 



' Average volume of catch divided by average number of organisms. 



isms, however, because its cod-end liner was of 

 slightly larger mesh than that used in the other 

 three trawls. 



On Smith cruise 27 to the central North Pacific, 

 the 6- and 10-foot Isaacs-Kidd trawls were used 

 on consecutive hauls on three successive nights 

 for the purpose of comparing the catches obtained 

 with the two sizes of gear. The results of the test, 

 summarized in table 8, indicate that the catch 

 of the larger trawl was about three times as great,- 

 with respect both to volume and to number of or- 

 ganisms, as that of the smaller trawl. When 

 judged in terms of catch per unit of mouth area, 

 the two trawls took essentially the same volume 

 and number of organisms. The kinds and sizes 

 of organisms in the collections were also similar 

 for the two trawls. 



A checklist of organisms identified in the catch 

 of the four trawls is given in appendix tables 5 to 

 8. In general, the greater the number of hauls 

 and the more regions sampled by a trawl, the 

 longer the list. A study of appendix tables 5 to 



