Abstract
The daytime distribution of benthic juveniles of the genus Gymnogobius was described based on discrete depth sampling using a small trawl-net from 2 m to 40 m depth in Lake Biwa during July and August 1996. A total of 9,366 individuals were collected in this survey, most were G. isaza (93.0%) with a few G. urotaenia (3.9%) and morphologically unidentified specimens (3.1%). Juveniles of G. urotaenia were distributed in littoral areas shallower than 10 m depth, where the bottom is just at or above the thermocline. In contrast, juveniles of G. isaza were distributed in more off-shore areas, deeper than 10 m depth, and the density was highest depth of 10 m to 20 m i.e. just at or below the thermocline. The unidentified specimens were collected at depths 10 m to 20 m. In principal component analysis based on five morphometric characteristics (pre-anus length, caudal peduncle length, caudal peduncle depth, eye diameter and interorbital width), the unidentified specimens showed intermediate morphometric characteristics between G. isaza and G. urotaenia. This result suggests the possibility that they are hybrids between these two species.
