TRAWLING FOR FORAGE ORGANISMS IN CENTRAL PACIFIC 



295 



are in press or in manuscript form, describing the 

 food of tunas in the central Pacific (Welsli, 1949; 

 Keint jes and King, 1953 : King and Ikehara, 1956 ; 

 Tester and Nakamura, 1957; Yuen, 1959; Naka- 

 mura, MS.: ^ Iversen, in press: and Waldron and 

 King/) These reports all emphasize the great va- 

 riety of food that tunas consume, but point out 

 certain differences related to the species and size 

 of tuna and the depth, distance from land, season, 

 and area of capture. Although adult tunas eat 

 a number of organisms that might be classed as 

 plankton (e.g., euphausiids, amphipods, and 

 stomatopod and crab larvae), the bulk of their 

 food consists of nekton, such as fishes, squids, and 

 shrimps. 



By means of the midwater trawl, we hoped to 

 sample at a trophic level ''closer" to the tunas than 

 was possible with the 1-meter zooplankton nets. 

 For this purpose, however, our procedures had a 

 basic weakness: the trawls were operated almost 

 entirely at night, since the catches were very poor 

 during daylight hours, but our tuna fishing was 

 carried out during the day because the few night- 

 time fishing stations had yielded very poor results. 

 Therefore, the only comparison which our data 

 afford is that between forage organisms captured 

 at night and the stomach contents of tunas cap- 

 tured during the day. It would seem theoretically 

 possible, however, as a result of the diurnal migra- 

 tion of the forage organisms, that the deep-swim- 

 ming tunas caught in the daytime at depths of 

 100 to 500 feet may have been feeding on the 

 same organisms that were taken at night in the 

 trawl when it was fishing near the surface. De- 

 spite these basic differences in the data, we thought 

 it worthwhile to make some general sununaries 

 and comparisons of trawl catch and tuna food, and 

 also to examine station-to-station variations in 

 these factors, as observed on certain cruises where 

 both variables were evaluated. 



In table 9 we summarized the number of taxo- 

 nomic categories of organisms found in th^ trawl 



catclies and in tuna stomachs. Although such a 



I 



« Nakamura. E. L.. MS. Food and feeding habits of Mhrquesnn 

 skipjack (Katsuwonus pelamis). Bureau of Commercial Fish- 

 eries Biological Lalioratory, Honolulu. 



* Waldron, Kenneth U., and .Joseph E. King. Food of skip- 

 jack in the central Pacific. U.S. Fish and Wildlife S«»rvlc*'. 

 Bureau of Commercial Fisheries Biological Laboratory. Honolulu, 

 (Experience Paper No. 3G, Section No, 5. FAO World Scientific 

 Meeting on the Biology of Tunas and Related Species, La Jolla. 

 Calif,), July ia62. 



sunmiary does not show the specific kinds of ani- 

 mals involved it does indicate that about twice as 

 many phyla or subpliyla were found in the trawl 

 catches as were represented in the tuna stomachs. 

 It is also evident, however, that a great variety of 

 forms was found both in tlie trawl catches and in 

 tuna stomachs. 



An attempt is made in tal)le 17 to illustrate the 

 major differences and similarities in the composi- 

 tion of the trawl catches and of tuna stomach 

 contents. Every item was listed that occurred in 

 10 percent or more of either the trawl collections 

 or the stomachs of four species of tuna. Again, 

 the data demonstrate the greater variety of orga- 

 nisms characteristic of the trawl catches, which 

 seems to indicate that the tunas were not feeding 

 at random but were exercising some degree of 

 selection. The gi-eatest similarity between the 

 trawl catches and the tuna stomach contents was 

 the occurrence in both of Amphipoda, Phronima 

 sp., Stomatopoda, Euphausiidae, Decapoda (Crus- 

 tacea and Mollusca), Enoploteuthidae, Salpidae, 

 Tunicata, Stomiatidae, Paralepididae, Mycto- 

 phidae, Gempylidae, and CoUi/bm drachme. 

 Some of the major differences were the high oc- 

 currence in the tuna stomachs of crab larvae, 

 squids of the families Ommastrephidae and 

 Loliginidae, and such fishes as Gempylus serpens 

 and representatives of the families Bramidae and 

 Acanthuridae, as contrasted with the low oc- 

 currence of these organisms in the trawl catches. 

 On the other hand, it is evident from the table 

 that a large number of animals of frequent oc- 

 currence in the trawl catches were not found in the 

 tuna stomachs. 



If the percentage of occurrence of the different 

 taxonomic groups listed in table 17 for the Isaacs- 

 Kidd trawls is plotted as a function of their oc- 

 currence in tuna stomachs, we obtain a series 

 of distributions (fig. 22) which show, in 3 out of 

 10 instances, some indication of an invei-se correla- 

 tion. Organisms occurring with the highest fre- 

 quency in the trawl collections were usually rare 

 in the tinia stomachs, whereas those forms occur- 

 ring most frequently in the tuna stomachs were not 

 generally common in the trawl catches. (In fig. 

 22 we plotted points only for those organisms 

 which occurred in both the tuna stomachs and 

 the trawl catches.) 



On a few cruises, tuna longline fishing and mid- 



