TRAWLING FOR FORAGE ORGANISMS IN CENTRAL PACIFIC 



281 



identifications being made to the most precise 

 degree that seemed practicah 



Tlie number of individuals and displacement 

 volume were determined for each kind or group 

 of organisms identified. The minimum and maxi- 

 mum lengths, in millimeters, were recorded for 

 each category of organisms. In some instances 

 the total number of the more nimierous organisms 

 in the less-than-2-cm. category was estimated from 

 the number in a subsample. 



In examining the resulting data it was evident 

 that those organisms less than 2 cm. in longest 

 dimension usually constituted a very small fraction 

 of the total volume of the catch. Of greater im- 

 portance was the infrequent occurrence of organ- 

 isms of large bulk. It was decided, therefore, to 

 give in the summarj- tables (appendix tables 1 to 

 4) the numljer of organisms and the volume of the 

 total catch, expressed in terms of 1 hour of hauling, 

 witli the two size categories combined but with 

 added reference to the occasional occur'rence of 

 large numbere of small organisms, such as euphau- 

 siids, and the infrequent bulky forms, such as 

 jellyfish or large fish, that greatly influenced the 

 volume of the catch or the total number of indi- 

 viduals in it. 



The nature of the trawling gear and the 

 methods used did not permit or justify an exact 

 quantitative evaluation of the catch. The amount 

 of water strained on each haul was not metered. 

 Vessel speed during hauling, however, was fairly 

 uniform for each trawl, since an attempt was made 

 to hold the wire angle within a narrow range of 

 values. Although most hauls were for a period of 

 1 hour, the time varied somewhat, as indicated in 

 appendix tables 1 to 4, and this variation was 

 taken into account. Other possible sources of 

 variation in the data were those relating to the 

 hour of hauling and to depth of haul. The major- 

 ity of tlie hauls, however, were made at night 

 within 1 to 3 hours after sunset and sampled the 



upper ocean layer between the surface and 400 

 meters. 



Detailed lists showing composition of the 

 catches for each of the four trawls are given in 

 appendix tables 5 to 8. Because of the great 

 variety of invertebrates and fishes in the collec- 

 tions, it was not possible to give equal attention 

 or bring the same skill to the identification of all 

 groups. Major effort was spent on the eupliau- 

 siids, shrimps, and stomiatoid and myctophid 

 fishes because of their prominence in the collec- 

 tions. References used in the identification of the 

 different groups are listed in the appendix (p. 

 304). Berg's (1947) system of classification and 

 nomenclature was used in most respects for the 

 family names of the fishes. 



Where appropriate, the data were subjected to 

 statistical analysis and the results expressed in 

 terms of the probabilit}' that the event occurred 

 by chance alone. We regarded events with 

 P^0.05 as not occurring by chance and therefore 

 of significance. 



CATCHING ABILITIES OF THE FOUR 

 TRAWLS 



The catching ability, or efficiency, of a midwater 

 trawl is related to many factors, including size of 

 the mouth opening, mesh size of the net, and haul- 

 ing speed, and also to the amount of disturbance 

 or turbulence directly ahead of the net caused by 

 the towing cable and bridle lines. These general 

 features, except turbulence, are summarized in 

 table 6 for the four trawls. The 1-meter ring 

 trawl, which had the largest mesh and was towed 

 at a slightly higher speed than the other three 

 trawls, produced the poorest catches, both in num- 

 ber and in volume of organisms. As shown in 

 figure 6, this trawl also had the most obstruction 

 inunediately ahead of the net ; the other three 

 trawls were niucli better designed in this respect. 



Table 6. — Factors affecting catching ability of the four trawls used 



