日本海西部におけるシイラ漁況の研究-VIII

Abstract

Large and medium sized dolphin, 50-150cm in fork length, are usually found with juwenile fishes in their stomachs. The major preys are Engraulis japonica, Upenous bensasi, Petroscrites trossulus, Oplegnathus fasciatus, and monoacanthids. In order to compare the fauna of prey species with that in the sea, the author examined the juveniles taken from the surface layer of the sea by two methods, horizontal towing plankton net under control to avoid drifting sea weeds; and dipping up juvenile fishes together with drifting sea weeds. The results are summarized as follows. 1. Juveniles dipped up with sea weeds were fewer in number of species, but more numerous in number of individuals, than juveniles taken by the horizontal towing (Table 2). 2. Juveniles of Engraulis japonica and Upenous bensasi were not collected with drifting sea weeds. 3. Juveniles of Petroscrites trossulus, Oplegnathus fasciatus, and monoacanthids dipped up with sea weeds were larger in size, and more numerous, than those of the same species taken by horizontal towing (Tables 3 and 4). Juveniles of these species preyed by the dolphin resembled, in body length, those of the same species dipped up together with sea weeds.