The post-stocking behaviour of hatchery-reared brown troup (Salmo trutta L.)

Abstract

Stocking, transfer and introductions of hatchery-reared salmonids are commonly used to enhance recreational or commercial fisheries and to preserve or re-establish threatened populations (Cowx, 1994). Whilst a lot of effort has been directed toward understanding the production and stocking methods of hatchery-reared salmonids, relatively little is known about the post-stocking survival and behaviour of these fish (Hickley, 1994). In particular there is little available information concerning the post-stocking dietary habits, dispersion and behaviour of hatchery-reared brown trout; particularly following release into standing waters. Consequently, a study was instigated to examine the relative post-stocking dispersion and temporal changes in the diets of hatchery-reared brown trout released to support a commercial recreational fishery, Carron Valley Reservoir. Further laboratory studies examined if resident brown trout display a prior-resident competitive advantage over stocked conspecifics, if hatchery-reared brown trout display non-cost effective aggressive behaviour and the short-term changes in the feeding efficiency of naive hatchery-reared brown trout when experiencing novel prey. 1000 commercially produced hatchery-reared brown trout were marked with a subcutaneous alcian blue tattoo and released into Carron Valley Reservoir at the start of the 1999 fishing season. Stomach samples were taken from angler recaptured hatchery-reared fish along with consecutively captured resident brown trout. The diets of the two groups were compared to assess the relative post-stocking temporal changes in the diets of the hatchery-reared brown trout. This study found hatchery-reared brown trout to consume lower weights and numbers of prey, and they appeared to exhibit a preconditioned `look up' dietary response to surface prey immediately following release than resident conspecifics. It was further observed that although hatchery-reared brown trout did not immediately adapt to natural diets, their relative foraging efficiency increased over the sample period. In laboratory experiments naive hatchery-reared brown trout further demonstrated the improved feeding efficiency with experience. 1000 commercially produced hatchery-reared brown trout were tagged with a combination of Visible Implant (VI) and `Floy' style tags prior to release during the 1998,1999 and 2000 fishing seasons. An angler survey programme was instigated to record the reported recapture positions of these tagged fish in order to assess the post-stocking gross dispersion patterns of hatchery-reared brown trout in Carron Valley Reservoir. A further 3 trout were radiotagged to elucidate the fine scale post-stocking dispersion of these fish. This study found hatchery-reared brown trout to disperse quickly from their respective release sites, although over a relatively restricted area. Individual hatchery-reared brown trout were observed to exhibit high levels of activity immediately following release, during which period they covered relatively large total distances within a relatively restricted area. In an artificial stream environment, established wild brown trout displayed a prior-resident competitive advantage over later introductions of both hatchery-reared and wild conspecifics. Established wild fish initiated more aggressive acts and maintained home stations closer to a point source of feed than introduced trout. Introduced hatchery-reared brown trout were more aggressive and exhibited a lower mean specific growth rate than simultaneously stocked wild conspecifics, suggesting that excessive expenditure of energy for unnecessary aggression may contribute to the poor post-stocking survival in hatchery-reared brown trout.