Sex ratios in relation to age and body size in “Honmoroko”, Gnathopogon caerulescens

Abstract

The age composition, and age- and size-dependent sex ratios of mass samples of “Honmoroko”, Gnathopogon caerulescens, were examined in Lake Biwa. The fish ranged from 0 to 2 years in age, but were mostly age 0 (97.9 %). The proportion of females was 48.6 % for age 0, 73.2 % for age 1, and 90.0 % for age 2 fish. Age 0 fish were significantly male biased, while ages 1 and 2 were significantly female biased, though the grand total showed a balanced sex ratio. The proportion of females was 48.0 % or less among age 0 fish in size classes of less than 9 cm in standard length (SL) and significantly different from a female:male ratio of 1:1 in such size classes, except for the 4–5.99 class. In size classes of 9 cm SL or greater, the proportion of females among age 0 fish was greater, more than 71.3 %, with significant differences from a balanced sex ratio for all such classes. These results suggest that G. caerulescens can be regarded as essentially an annual fish and that its spawning habits ought to result in a higher mortality for males than females, thus leading to female-biased sex ratios at ages 1 and 2.