Spawning seasons of adult and growth of 0-year-old deepsea smelt Glossanodon semifasciatus in Tosa Bay, Pacific coast of Shikoku

Abstract

The deepsea smelt Glossanodon semifasciatus is one of the most important fishes for the demersal fishery in the Pacific coastal waters of central and southern Japan. In the present study, the spawning season in Tosa Bay was estimated to be from January to April (spring spawning group), and also at a lower frequency in October to November (autumn spawning group) based on gonad somatic index changes and stages of the ovaries of adult fish landed by commercial fisheries in Tosa Bay. Furthermore, the abundances of 0-year-old fish of the two seasonal spawning groups were investigated by a trawl survey. The spring spawning group was much more abundant than the autumn spawning group. The growth of 0-year-old fish was traced by the modes of the histogram of the specimens by measuring the Scale Length (SCL) of fish caught by a monthly trawl survey using a research vessel from April 2002 to March 2005. The smallest specimen sampled was about 19 mm SCL in mid April 2002 at the depth of 150 m. Although the histograms consisted of polymodal classes and at least three or more growth lines were traced, the main mode of SCL of the spring spawning group from 2002 to 2003 was 25 mm in April, 50 mm in May, 75 mm in June, 90 mm in August, 105 mm in October, 110 mm in November and about 140 mm in March in the next year. The growth rate of the 0-year-old fish in Tosa Bay is higher than that estimated for the sub-population in the Sea of Japan, possibly due to the higher temperature of the nursery ground of the Tosa Bay. The recruitment index of the spring spawned group fluctuated between years however reasons for the fluctuations require further investigation.