Juvenile Tunas Collected by Midwater Trawling in Hawaiian Waters, July‐September 1967

A small‐mesh midwater trawl was towed 83 times in Hawaiian waters during July‐September 1967. Objectives were to develop techniques for collecting juveniles of skipjack tuna and other scombrids and to provide information on their distribution. A systematic trawling routine was followed in July and August off the island of Oahu. Deep. (100 m) tows were alternated with shallow (20 m) tows during the day (1200–1800 hours), night (2000–0200 hours), and morning (0400–1000 hours) for the first 57 stations. The last 26 stations included tows, primarily at shallow depth, off the islands of Molokai, Lanai, Hawaii, and Oahu. A total of 995 juvenile scombrids, representing six species, was collected. The catch included 578 skipjack tuna (Katsuwonus pelamis), 180 yellowfin tuna (Thunnus albacares), 133 frigate mackerel (Auxis sp.), 46 bigeye tuna (T. obesus), 25 kawakawa (Euthynnus affinis), and 7 albacore (T. alalunga). The largest catches of juvenile tunas were made in the offshore Oahu area about 56 km from shore in July and August and off Hawaii in September. Tows in these areas averaged about 18 tunas per tow. In contrast, tows in the inshore Oahu area about 7 km from shore averaged only three tunas per tow. Shallow tows at night tended to catch the most juvenile skipjack tuna and deep tows at night, the fewest. Catches during the morning were not significantly different from those at night, but shallow‐morning tows tended to be less productive and deep‐morning tows to be more productive than corresponding tows at night. Daytime catches were about the same in shallow and deep tows. The differences in the catch rates for skipjack tuna between shallow and deep tows at different times were assumed to be the result of diel vertical migration. Juvenile yellowfin and bigeye tunas either did not undertake did vertical migrations or their movements were not extensive. Almost all the bigeye and yellowfin tunas were taken in shallow tows. Skipjack tuna measured 7–47 mm standard length; most were 9–15 mm long. Skipjack tuna were significantly larger in deep tows (average 14.4 mm) than in shallow tows (average 11.1 mm) during all three towing periods. Although the apparent tendencies were not significant, night and morning tows at the deeper depth also tended to catch larger skipjack tuna than day tows. It was tentatively concluded that the smaller skipjack tuna live primarily in the upper isothermal layer, whereas the larger individuals tend to occur in deeper water.