Population size and distribution of Pseudobagrus ichikawai, an endangered bagrid catfish, in the Kawaura River

Abstract

The population size of the endangered bagrid, Pseudobagrus ichikawai, in the Kawaura River and its tributary, the Tsuzuya River (Nagara River system), was estimated by a combination of night-visual-census and mark-releaserecapture methods in 1996. The fish were distributed in patches in reaches of 6 km (Kawaura R.) and 2 km (Tsuzuya R.) out of 11km surveyed. Visual counts in all 235 pools in the surveyed area and mark-release-recapture experiments in five selected pools indicated that a correction factor of ca . 4.3 times was necessary for the former, a total of 1460±100 SE fish being present in pools . The total population including that in riffles was estimated as being less than twice that in the pools, with 40-50% of the total population constituting the reproductive group.The effective population size was likely to be less than this proportion, owing to a sex ratio bias towards females. Larger pools tended to be occupied by greater numbers of fish, both the number and density of fish in a pool being positively correlated with the number of fish in neighboring pools . These results suggested that movement between the pools is important for the persistence of the P. ichikawaipopulation, which is presently somewhat fragmented by many weirs on the river.