若狭湾産マアジの系群に関する研究

Abstract

Our analysis and comparison of various morphological and ecological characters of the jack mackerel caught in different localities of Wakasa Bay have led us to the conclusion that this fish living in Wakasa Bay consists of two races. One called “Kuroaji” (black jack mackerel) by fishermen seems to be an offshore migrant, apparently characterized by having well-elongated body with its back darkened, the body depth being about 4.6 in fork length (Fig. 1; Table 1). The other called as “Kiaji” (yellow jack mackerel) by fishermen has its body yellowish dorsally and rather deep, the body depth being less than 4.2 in fork length (Figs. 1, 2; Table 1). Besides these differences, the two races are clearly distinguished from each other in the following points. 1) Inner opaque area of the otolith is somewhat wider in “Kuroaji” than in “Kiaji” (Figs. 4C, 4D). 2) None of the ova of the fish examined has been found to be ripe throughout the year. “Kuroaji” has its ova usually smaller than 0.20mm. in diameter, while “Kiaji” has them less than 0.1mm. in diameter. An analysis of the length frequency indicated that the two races of the fish have their respective peak of spawning between January and March (Kuroaji) and between June and July (Kiaji). 3) Generally “Kuroaji” grows up faster than “Kiaji” (Table 2). The monthly growth rates are estimated as 16.5mm. in fork length, and 3.8g. in body weight for “Kuroaji”, and 9.6mm. in length and 2.2g. in weight for “Kiaji” during the first one year; and 4.9mm. and 5.1g. for the former, and 3.5mm. and 4.2g. for the latter during the second year. 4) Small pelagic animals, such as crustacea, cuttle-fish and some others, seem to be ordinary food for “Kuroaji” (Table 3), while “Kiaji” is likely to feed on small coastal fishes, mainly on anchovy ranged from 23mm. to 78mm. in body length. The other minor stomachcontents of “Kiaji” were found to be sand eels, young jack mackerels, prennant lantern fishes and unicorn cods. 5) In most of “Kiaji” over 180mm. in fork length, their stomachs, pyloric caeca, livers and intestine were infested on the surface with encysted tetrarhynch cestode larvae (Fig. 7). This parasite was not found in “Kuroaji”.