GROWTH, MATURITY AND POPULATION-STRUCTURE OF THE BAGRID CATFISH, PSEUDOBAGRUS-ICHIKAWAI, IN THE TAGIRI RIVER, MIE PREFECTURE, JAPAN

A small population of the bagrid catfish, pseudobagrus ichikawai, was investigated during the period from April 1988 to December 1989 in the Tagiri River, Mie Prefecture, Japan. The catfish were nocturnally act ive from spring to autumn, especially from June to September. During t he daytime and in winter, they hid in crevices along the vegetated riv er shores or under boulders on the river bed. Analyses of seasonal cha nges in size distribution and growth patterns of 202 individuals revea led that the fish grew mainly in summer. New recruits, which appeared first in autumn, attained about 40 mm in standard length (SL) by July of the following year. At age 2 years, some females exceeded 60 mm SL, being mature, whereas males probably mature at 3 years, reaching abou t 100 mm SL. The maximum sizes recorded were 93.5 mm SL for females an d 108.0 mm SL for males. Sexual size dimorphism was apparent in immatu re, 1 year-old, becoming pronounced in older fish. The sex ratio of ad ults was largely female-biased. Although it appeared that spawning too k place in July of both years, no recruits representing the 1988 year- class were captured. Differences occurred between 1988 and 1989 in gro wth rates, maturation of 2-year-old females, and population structure. The possibility of the species having a polygynous mating system, wit h the population size and age structure being unstable owing to floods , is suggested.