PRELIMINARILY RESULTS OF THE RELATIONSHIP BETWEEN CATCH RATIO OF BIGEYE TUNA (Thunnus obesus) TO TOTAL CATCH AND DEPTH OF UNDERWATER STRUCTURES OF FADS

The main target species of Japanese purse seine operated in the tropical waters of the Pacific Ocean is skipjack tuna (Katsuwonus pelamis) and yellowfin tuna (Thunnus albacares) consisted of nearly 98 % to their total catch weight and the remaining is bigeye tuna (Thunnus obesus) according to logbook. The recent study of the vertical distribution of the three species around drifting FADs (fish aggregating devices) indicates that the skipjack tuna distribute relatively shallower depth layer. A typical FAD consists of floating foundation and underwater structures, which is used-up fishing net. Therefore shortening the length of the underwater structure of FADs (depth of FADs) may be effective to reduce the bigeye tuna catch. Relationship of the species composition of purse seine catch and the depth of FADs was investigated by port samplings and by log book, of 17 sets and 65 sets, respectively from May to June 2007. The analysis was a preliminary because of small sample sizes in both data sets. The clear relationship between the presence/absence of bigeye tuna catch and the depth of FADs was not obtained by generalized linear model (port sampling; Chi-Square 0.11, P = 0.7365 and log book; Chi-Square 2.20, P = 0.1376). It was found that the depth of FADs had no sigfinicant effect on the ratio of bigeye tuna catch to total catch per set (port sampling; F1, 4 = 2.57, P = 0.1839 and log book; F1, 23 = 0.19, P = 0.6711). The effects of the depth of FADs for the amount of skipjack and yellowfin tuna catch were also investigated and both the catches were not significant with the depth of FADs.