Annual fluctuations in the catch by purse seine fishery and the age composition of skipjack tuna katsuwonus pelamis linnaeus in the tropical seas of the western pacific ocean
Bulletin of Tohoku National Fisheries Research Institute 51: 89-106
The purpose of this paper is to study annual fluctuations in the fishing effort, the catch by purse seine fishery and the biological characteristics of skipjack tuna in the vicinity of northern Papua New Guinea. Recently, the total purse seine fishery catch has increased rapidly and amounted to 130 thousand metric tons annually after 1983. The skipjack tuna catch accounts for 70% of the total catch. Tha annual range in fork-length of skipjack tuna caught by purse seiner was mostly from 25 to 70 cm with three modes appearing at 30, 40 and 50 cm. The fork-length of most fish was from 40 to 50 cm, but the percent of small fish in the 30-40 cm range has increased since 1981. The growth curve of skipjack tuna in the tropical seas was estimated by connecting of the fork-length modes. The age composition of skipjack tuna caught by purse seiner and pole and liner were estimated based on the growth curve. Since 1976, the pole and line catch has consisted mainly of 3-year-old fish whereas the purse seine catch were contained mostly 2-year-old fish until 1980. Because the small fish (30-40 cm) increased after 1981, the major component shifted to 1-year-old fish in 1987. It is necessary, therefore, to continue watching the influence caused by the large-scale catch of young skipjack tuna.