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Inferences on Tuna Behavior from Data in Fishermenˈs Logbooks
Author(s) -
Whitney Richard R.
Publication year - 1969
Publication title -
transactions of the american fisheries society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.696
H-Index - 86
eISSN - 1548-8659
pISSN - 0002-8487
DOI - 10.1577/1548-8659(1969)98[77:iotbfd]2.0.co;2
Subject(s) - tuna , yellowfin tuna , skipjack tuna , fishery , morning , daylight , geography , thunnus , time of day , biology , zoology , fish <actinopterygii> , botany , physics , optics
Tuna seinerˈs 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 tendency for separation of mixed species schools at night. Average size of tuna schools was the same at night as in the day except for schools of yellowfin tuna in the area south of 15°N, which were significantly smaller at night. Bluefin tuna schools increased in size from an average of 6 tons at 0700 hr to about 30 tons at 1900 hr, but the average daylight school was the same size as the night. Interpretations and inferences from the data were attempted in terms of tuna behavior and behavior of fishermen with the objective of providing hypotheses that will be useful in designing studies of tuna behavior and of vessel operations. Among the hypotheses developed are: (1) Tuna make diurnal vertical movements which tend to bring them up toward the surface at midday and down into the deep waters at night. Skipjack may tend to go deeper than yellowfin tuna. (2) Decreased visibility of the purse seine at night accounts for the increased percentage of sets that are successful. Decreased activity of the fish might also explain the higher success. (3) Tuna schools maintain themselves at night by visual means, thanks to light from a number of possible sources. Some breakdown of schools may occur at twilight as a result of low light levels.