若狭湾西部海域におけるタチウオの漁業生物学的研究 ＶＩ 若狭湾西部海域におけるタチウオの年齢と成長

Abstract

Favourite preys for ribbon fish Trichiurus lepturus (1.9-397.0mm in anal length) were described, based upon the progressive changes in the gut contents with growth. 512 ribbon fish were caught by larva net, beach seine, beam trawl, set net, purse seine, gill net and Danish seine during the years from 1980 to 1983 in the western Wakasa Bay. The occurrence method and the number method were used in the analysis of the food habit of the fish.The ribbon fish of less than 20mm in anal length was zooplankton feeder. The food habit of the fish 20-100mm length was carnivorous and the main foods of the fish were Natantia, Mysidacea and Pisces (mainly anchovy larvae). The fish of more than 100mm length had the piscivorous food habit and the main foods were anchovy and sardine.A particular size of the ribbon fish, which started feeding on other fishes in the western Wakasa Bay, was smaller than that of the East and Yellow China Sea, the adjacent waters of Tsushima and Osaka Bay. In common with each area, however, the fish of more than 250mm in anal length had the piscivorous food habit.The ribbon fish even at the early post larval stage had high feeding ability and positively fed on copepods such as Paracalanus parvus and Paracalanus sp., while the foods of 8 species at the same stage appeared in the same survey area were phytoplankton, nauplii and copepodites. This ability of the post larval ribbon fish seemed to be closely related to larger size of the egg and the larvae hatched than those of 8 species. Because the ribbon fish spawned smaller number of eggs than those of 8 species, the high ability of feeding at the early post larval stage is very important for their effective survival.The main gut contents of the fish of less than 10mm in anal length were pelagic copepods such as Paracalanus parvus, Paracalanus sp. and Clausocalanus sp. and 10-20mm length were females of Acartia erythraea which made swarms and generally occurred on or slightly above the bottom near the shore. The change of the main food from the pelagic copepods to the epibenthic copepod suggested that the ribbon fish of 10-20mm in anal length shifted their habitats from offshore to the bottom layer near the shore.