Inferences on Tuna Behavior from Data in Fishermen's Logbooks

Abstract

Abstract Tuna seiner's logbooks representing 34,263 sets on tuna were examined to find indications of tuna behavior. The data were tabulated in four ways to show (1) number of purse seine sets made by time of day, (2) changes with time of day in percentage of sets that were successful in catching tuna, (3) changes with time of day in species composition of the catch, and (4) changes in school size with time of day. The frequency distribution of number of sets by time of day is nearly a normal curve which does not fit an assumption of equal availability of tuna at all hours. Greatest number of sets was at midday. The percentage of sets which was successful in catching tuna was higher at night than in the day. Also, success for yellowfin and skipjack was about 10% lower in early morning than the rest of the daylight period. Relatively few skipjack tuna were caught at night. Mixed species schools were even less frequent at night than expected on the basis of reduced frequency of skipjack, suggesting a tenden...