THE MIGRATORY BEHAVIOR OF JUVENILE AND ADULT SALMONIDS IN RELATION TOAN ESTUARINE BARRAGE

There is wide concern regarding the effects that estuarine barrages ma y have on estuarine environments and the species which utilise them. O f particular concern are the possible effects on diadromous fish which move between fresh and salt water. This paper presents results from a 3-year investigation (1993-95) of the migratory behaviour of both juv enile and adult salmonids undertaken in the vicinity of a barrage on t he River Tawe in South Wales, which was completed in 1992. The localis ed upstream movements of acoustically tagged returning adult salmonids (n = 28) were investigated in the immediate vicinity of the barrage a nd associated fish pass using a High Resolution (HiRes) tracking syste m. The behaviour of salmonid smelts (n = 52), tagged with miniature ac oustic transmitters, were also monitored during their spring seaward e migration within the impounded estuary. The upstream movement of adult salmonids appeared to be delayed by the presence of the barrage with many fish holding station close to the barrage structure. Of these, ab out half were clearly attracted by the plume of water from the fish pa ss, but few fish ascended the barrage by this route. Approximately one third of those fish detected below the barrage were subsequently dete cted upstream in the impoundment, most moving past when the barrage wa s over-topped. The movement of fish over the barrage was significantly unimodal with respect to the tidal cycle with a mean passage time of 4 minutes after high water. Once in the impoundment fish often remaine d close to the barrage (< 100 m) for several hours or days during whic h time movements appeared largely random. The movement of salmonid sme lts through the impounded estuary and into the lower estuary, downstre am of the barrage, was not continuous. Smelts held position within the impoundment immediately upstream of the barrage. The movement of smel ts upstream of the barrage was predominantly random, and fish that suc cessfully negotiated the structure did so by migrating through the shi p lock or over the weir during an over-topping flood tide. The migrato ry behaviour patterns of juvenile and adult salmonids are compared wit h previous studies in unmodified river estuaries, and discussed in rel ation to the possible impact of barrages on salmonid migration.