Reproductive strategy and interspecific competition in the Lake-living gobiid fish isaza,Chaenogobius isaza

Abstract

Isaza, Chaenogobius isaza, is a small gobiid fish endemic to Lake Biwa. It lives offshore throughout almost the entire year, showing a remarkable diel vertical migration. In early spring, males and females migrate to lake shore to spawn under stones. Spawning season is limited to a very short span of time in early spring, late April to early May. During this short spawning season, the male is supposed to have only 2 brood cycles at maximum. At each brood cycle, the male is strictly monogynous, never accepting additional females. Males therefore show a marked mate choice, choosing a larger female regardless of the size of the male himself. Females also choose larger males. However, males are supposed to not waste time in male-male fighting in the natural spawning area. That this very short spawning season and its occurrence in early spring is primarily to avoid interspecffic competition with another littorally reproducing goby Yoshinobori, Rhinogobius brunneus, is experimentally demonstrated.Isaza, Chaenogobius isaza (Fig. 1), is a small

Why egg-caring males of Isaza (Gymnogobius isaza, Gobiidae) refuse additional females: preliminary field observations

“…This is not the case on Isaza (Hidaka and Takahashi 1987). Nest size seems not to be a factor limiting brood size (Hidaka and Takahashi 1987;D. Takahashi, personal observations).…”

“…Takahashi, personal observations). Negative effects such as destruction of rock nests or egg cannibalism by females that rarely forage in the breeding sites will be implausible (Hidaka and Takahashi 1987). Thus, monogamy of Isaza males seems to be inconsistent with mating system theory, and the reason why males never accept additional females is not understood.…”

“…Isaza inhabits the offshore area and forages on zooplankton in the water column in Lake Biwa in seasons other than its breeding period, within 1 month from early or late April (Kobayashi et al 1950;Hidaka and Takahashi 1987). Breeding starts when the water temperature increases to over 10°C, and the annual variations in the start of reproduction probably depend on the variable water temperature.…”

“…In Isaza Gymnogobius isaza (previously named Chaenogobius isaza), a small freshwater goby endemic to Lake Biwa, Japan, males care for eggs in a mass under a nest stone until hatching. Females choose large mates, probably while visiting a number of nests (Hidaka and Takahashi 1987). Thus, the theory of mating systems, that males mate with many females to increase their reproductive success, will predict polygyny in this fish.…”

“…Thus, the theory of mating systems, that males mate with many females to increase their reproductive success, will predict polygyny in this fish. After spawning with a female, however, males never accept additional females to spawn until hatching of the first clutch (Hidaka and Takahashi 1987), and males monogynously mate with single females in one brood cycle. Such male rejection of additional females has rarely been reported in gobioid fishes where females visit nests at which males exclusively guard eggs.…”