MIDWATER TRAWLING FOR FORAGE ORGANISMS IN THE CENTRAL 



PACIFIC, 1951-1956 



By Joseph E. King and Robert T. B. Iversen, Fishery Research Biologists 

 Bureau of Commercial Fisheries 



Since the inauguration of field work early in 

 1950, the staif of the Bureau of Commercial Fish- 

 eries Biological Laboratory, Honolulu,' lias con- 

 ducted studies on the oceanography, productivity, 

 and fisheiy resources of the central Pacific Ocean. 

 Although not a major activity of the Laboratory's 

 research program, midwater trawling has received 

 considerable attention. Four kinds of trawls were 

 tested and employed to a varying extent on 22 

 cruises in the central Pacific from 1951 to 1956. 

 Trawling was not conducted, however, with the 

 expectation of discovering new fishery resources 

 of commercial importance, but rather to sample 

 quantitatively the forage organisms which are the 

 basis and support of fishery stocks in general, and 

 particularly those organisms utilized by the tunas. 



Midwater trawling has been conducted along the 

 west coasts of Canada and the L^nited States by 

 a number of institutions employing different types 

 of gear. The Fisheries Research Board of Can- 

 ada, Biological Station, Nanaiino, British Colum- 

 bia, pioneered in the development of a commercial- 

 type trawl for use in the herring fishery (Barra- 

 clough and Johnson, 1956). This trawl, or one of 

 its modifications, has been used by the Bureau of 

 Commercial Fisheries in fishei-y explorations in 

 the eastern Pacific (IT.S. Fish and Wildlife Serv- 

 ice, 1956 ; Schaefers and Powell, 1958) . The Cali- 

 fornia Department of Fish and Game has also 

 tested a commercial-type midwater trawl of 

 somewhat difi'erent design (Pacific Fisherman, 

 1953). The Isaacs-Kidd midwater trawl was de- 

 veloped at Scripps Institution of Oceanography 

 to collect bathypelagic fishes and invertebrates 

 (Devereaux and AVinsett, 1953). This type 



1 Formerly Pacific Oceanic Plsliery iDTesUgatlons. 



The senior author is presently Assistant Chief. Bninch of 

 Marine Fisheries, Washington 25, DC. 



Approvi'il fiir publication, Novenilier 30, l'J61. Fishery Bulle- 

 tin 210. 



of trawl has been employed by Scripps staff 

 membei-s on numerous cruises in the eastern 

 Pacific and has also been used recently in tlie 

 northeastern Pacific by personnel of the Uni- 

 versity of Wasliington Department of Ocean- 

 ograpiiy to relate variations in the abundance of 

 plankton and nekton to other oceanographic fea- 

 tures (Aron, 1959). Some of the studies men- 

 tioned here have resulted in discoveries and infor- 

 mation that have been of great value and interest 

 to the commercial fishing industry, and all have 

 made worthwhile contributions to our scant knowl- 

 edge of the abundance and distribution of animal 

 life in the mid-depths of the ocean — a life zone 

 that presently contributes little in the way of 

 liuinan food ("Wli it el eat her, 1957; Powell, 1958). 



Specific objectives of the trawling program 

 were as follows: 



(1) To obtain a measure of the abundance and 

 distribution of potential tuna food; (2) to obtain 

 an estimate of the standing crop of forage organ- 

 isms; (3) to sample juvenile tunas; (4) to learn 

 something of tuna feeding behavior, e.g., depth 

 of feeding, diurnal variation, and selectivitj'; and 

 (5) to evaluate the general catching abilities of 

 diiferent midwater trawls in the highly trans- 

 parent waters of the tropical and subtropical 

 Pacific. This report describes and evaluates the 

 results of our trawling studies with respect to these 

 objectives. 



H. J. Mann, of the Honolulu laboratory, pre- 

 pared the engineer's drawings of the trawls and 

 helped the authors with the detailed descriptions 

 of the trawls. From the engineer's drawings, 

 Tamotsu N'ai<ata prepared the drawings and otiier 

 figures that appear in this paper. Isaac Ikehara 

 and Allen Shimomura assisted in the sorting, 

 counting, volume measurement, and identification 

 of the organisms in the collections. 



271 



