TRAWLING FOR FORAGE ORGANISMS IN CENTRAL PACIFIC 



301 



Table 21. — Collection data for six juvenile tuna captured hy midwater trawl in the central Pacific, 195Z-56 



trawl catches and the Salpidae were present in 

 lesser volume; in the tuna food the Pyrosomatidae 

 were lacking, but the Salpidae were present in 

 about the same volume as in the trawl samples. 

 Sauries {Colohihis sp.), the largest single item in 

 the albacore stomachs examined, were not taken in 

 the 6-foot Isaacs-Kidd trawl. On cruise 2-2, a lan- 

 temfish {TarJetanheania sp.) ranked large in 

 volume in the albacore food, but was not taken in 

 the trawl. Other kinds of lanternfish were cap- 

 tured in the trawl on cruise 23, however, that did 

 not occur in the tuna stomachs. 



On the basis of these data we conclude that the 

 6-foot Isaacs-Kidd trawl did not sample the im- 

 mediate food of yellowfin and albacore tuna in 

 the equatorial Pacific and central North Pacific. 

 With respect to the food of other tuna spex?ies, such 

 as bigeye {Parathunnus sibi) and skipjack 

 {Katsuwonus pelamis), we do not have sufficient 

 information collected concurrently with midwater 

 trawl catches to provide within-cruise or station- 

 to-station comparisons. For these species our best 

 data are those given in table 17 and figure 22. 



MIDWATER TRAWLS AS SAMPLING 

 DEVICES FOR JUVENILE TUNAS 



Larval tunas of several species have been cap- 

 tured in the central Pacific in 1-meter zooplanktoii 

 nets with mesh apertures about 0.65 mm. in width 

 (Matsumoto, 1958; Strasburg, 1960). The young 

 tunas captured in this manner have ranged from 

 3 to about 12 mm. in length, but specimens larger 

 than 6 mm. were uncommon in the catch. The 

 number captured per 30-minute haul has varied 

 from none to several hundred. Both of these au- 

 thors concluded that diurnal differences in the 

 catch of tuna larvae resulted primarily from ver- 

 tical migrations and only secondarily from the 



increased ability of the larvae to dodge the net 

 during the daytime. 



The almost complete absence from the plankton 

 catches of young tuna more than 12 mm. in length 

 (Strasburg, 19G0) could possibly be attributed 

 to their living in a different habitat or level of the 

 sea than that sampled by the plankton nets or to 

 tlieir ability to dodge the net. Since the plankton 

 net sampled the mixed layer and usually beyond, 

 to depths as great as 200 meters and sometimes 

 greater, and the hauls were spaced over a wide 

 range of latitude and longitude at all seasons of 

 the year, it would seem reasonable to believe that 

 the environment occupied by the young tunas was 

 being sampled, but at sizes above 12 mm. they 

 easily escaped the 1-meter net. 



It was our hope that by means of midwater 

 trawls we would capture juvenile tunas of lengths 

 above 12 mm. which were able to elude the plank- 

 ton nets. In 274 hauls made with the four mid- 

 water trawls described in this report we captured 

 only six juvenile tunas, which ranged from 18 to 

 60 mm. in length. Three of the six were caught in 

 the 10-foot Isaacs-Kidd trawl ; the remainder were 

 taken in the other three trawls. The collection 

 data for these six specimens are given in table 21. 



On cruise 15 of the Smith, 7 yellowfin, 36 skip- 

 jack, and 13 unidentified tuna larvae were cap- 

 tured in hauls of 1 -meter zooplankton nets between 

 latitude 6° N. and the Equator along longitude 

 140° W. (Matsumoto, 1958). Four hauls with the 

 6-foot Iieam trawl made in this area on cruise 15 

 yielded only 1 juvenile tuna, a skipjack (Katsu- 

 wonus pelamis) 18 mm. in length (table 21). 

 There was evidence, therefore, that considerable 

 numbers of larval tunas, and most likely juvenile 

 tunas, were in the area but were not capt'ired by 

 the beam trawl. We expected the larvae to escape 

 through the relatively coai-;e meshes of the trawl 



