The Wet-Season Movements and General Biology of Some Important Fish Species at the Great Fault Line on the Lower Mekong River of the Lao P.D.R.

摘要

Nocturnal wet-season fish movements during the months of May and June have been studied in 1994, 1996, 1997 and 1998 in one rocky channel, Hoo Som Yai (HSY), traversing the Great Fault Line (GFL) on the Mekong River in Southern Lao P.D.R. The primaryaim of the study was to obtain an overview offish movements in relation to changes in local hydrological conditions and gather information on general biology of the species at the time of the migrations based on reproductive and dietary investigations. The studies were carried out in cooperation with local artisanal fishers using specialized bamboo wing traps set into rapids. Movements at the GFL are bi-directional, but mainly in an upstream direction. Upstream movements are dominated by catfishes (Pangasiidae and Siluridae) and downstream movements are anecdotally reported to be dominated by a group of mainly small Cyprinid species that also form an important fishery in some years. Two Pangasid catfishes, Pangasius conchophilus and Pangasius larnaudiei form the mainstay of the fishery in HSY. Hydrological conditions in HSY were recorded during all studies and related to standardized catch (CPUE). Length-weight relationships are provided for the most important migratory species. Biological examinationof migratory species from the study site and local markets has provided limited information regarding the nature of the migratory movements. Adult Pangasid catfishes are considered to be on their spawning migration, but destination habitats are unknown.Large numbers of juvenile P. conchophilus appear to be making a different type of migration, perhaps for dispersal and / or feeding. Based on anecdotal evidence, several small Cyprinid species make a downstream movement just after, or just before spawning. Movement at the GFL appears to be dependent on local hydrological conditions. Threshold flow-volumes may trigger the first large runs in May or early June, but further movements appear to be dependent on freshets and the numbers of fish waiting to move below the GFL. Standardized catch (CPUE) was found to be significantly related to flow volume passing down HSY in 1996 (P=0.0002) and 1997 (P = 0.0000). No significant relationship was found in 1994 (P = 0.82) and 1998 (P = 0.16). Using correlation, Standardized catch (CPUE) was found to be significantly related to flow volume in 1996 (P = 0.0000), in 1997 (P=0.0000) and in 1998 (P = 0.0000). No significant relationship was found in 1994 (P = 0.20).