No abstract

The distribution of various age classes of salmon and trout was assessed in upland streams by electrofishing. Water depths and site gradients were measured and correlated to fish densities. The fry of both species were significantly more abundant in shallow water; up to 75.3% of salmon fry and 72.2% of trout fry were captured in sites of mean depth < 20 cm.Older trout were found mainly in the deeper areas, with a maximum of 7.4% captured in sites < 20 cm mean depth. Yearling fish were found in all the depth-ranges sampled, but with a tendency for higher numbers in mid-range depths. There were similar correlations in the abundance of each age class with the actual areas of shallow, mid-range and deep water habitat available within sites. Correlations of fish density with gradient indicated that trout were limited in their distribution to areas of lower flow, whereas salmon were not. Since depth and gradient were significantly negatively correlated, there was an apparent preference oftrout for slightly deeper habitats than the equivalent year classes ofsalmon. The observed habitat segregation is discussed in terms ofcompetition and selection.

Abstract. Counts were made of cormorants, Phalacrocorax carbo (L.), feeding on the River Bush. County Antrim. Northern Ireland during the post‐dawn period on three occasions. Two of the counts during May 1986 indicated that up to 264 birds may have been feeding at least once per day throughout the catchment during the salmon, Salmo salar L., smolt run. The number of feeding birds had dropped to an estimated 61 by the time of the third count on 1 July 1986. Stomach samples from shot birds showed that upstream feeding was concentrated on wild smolts and brown trout. Salmo trutta L. However, cormorant predalion downstream from the salmon hatchery at Bushmills was restricted solely to hatchery smolts. Estimates of the total daily predation rates were calculated at 653–1214 wild smolts. 107–231 hatchery smolts and 422–785 brown trout. The possible impact of this level of predation on the salmonid stocks of the river was assessed.

The relative effects of inter-and intra-specific competition on the survival and growth of stocked salmon were investigated in an upland trout stream during summer and winter 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. Salmon fry stocked into the cleared area survived more than twice as well and grew significantly larger than those stocked into the area containing trout and older salmon. Intra-specific competition from older salmon in the second year of stocking in the cleared area significantly reduced the survival and growth of the O + salmon. However, these were still significantly larger and survived better than those in the control area where inter-specific competition from trout was maintained. Some immigration of trout to the cleared area occurred; these showed greatly enhanced growth rates compared to those in the control area, reflecting low intra-specific trout competition in the former. Inter-specific competition effects of older salmon on both trout fry growth and survival were also detected, although the latter did not become apparent until the winter. This is discussed in terms of the relative importance of biotic and abiotic regulating mechanisms. Evidence of allopatric niche segregation is also discussed, since salmon in the cleared area did not have a biomass equivalent to that in the area which also contained trout.

The relativeeffects ofinter-and intra-specificcompetition on thedistribution ofstocked 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 shallew 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.