The effects of intra- and inter-specific competition on the distribution of stocked juvenile Atlantic salmon, Salmo salar L., in relation to depth and gradient in an upland trout, Salmo trutta L., stream

The relative effects of inter- and intra-specific competition on the distribution of stocked salmon in relation to depth and gradient were investigated in an upland stream during two summer sampling periods. The stream was divided into two areas by an impassable fish barrier, and trout were removed from the upstream section prior to 2 years of salmon stocking. A small amount of trout re-immigration to the cleared area occurred. Under sympatric conditions in the downstream section the fry of both species were significantly more abundant in shallow water, whereas the yearling and older fish tended to inhabit the deeper, slow-flowing areas. Under near allopatric conditions in the cleared section salmon fry changed their distribution to include deeper areas, with shallow, fast-flowing water becoming the least preferred habitat. This trend for a wide distribution of salmon fry over all the available habitat in the upstream section was maintained in the second year when parr were present but trout remained at very low densities. It was therefore concluded that high inter-specific competition from trout was responsible for restricting the distribution of salmon fry to shallow habitat in the control area. Intra-specific competition from older salmon apparently only affected the growth and survival of salmon fry. The regulating mechanisms involved in these inter- and intra-specific effects are discussed in terms of competition for stream resources and predation. Recommendations based on the findings are made for stocking and habitat management of salmonid waters.