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STOCKING VERSUS NATURAL RECRUITMENT IN ATLANTIC SALMON (SALMO SALAR L.) AND BROWN TROUT (SALMO TRUTTA L.): THE SIGNIFICAP7CE OF REDUCED STOCKING IN THE RIVER, L~ERDALSELVA, WESTERN NORWAY. S.J. Saltveit, Zoological Museum, Freshwater Ecology and Inland Fisheries Laboratory, University of Oslo, Sarsgt. 1, 0567. Oslo 5, Norway. Since the regulation of the river Lwrdalselva, western Norway, in 1974, a large number of juvenile Atlantic salmon and brown trout (sea trout) have been stocked above the natural reach for anadromous fish to compensate for possible loss in salmon production. In 1988 the numbers of Atlantic salmon stocked were drastically reduced, while stocking of sea trout was terminated. Between 1982 and 1987 the annunal mean number of Atlantic salmon yearlings stocked was 545 000, while in 1988, 1989 and 1990 the mean number stocked was 116 000, an average reduction of about 80~. In comparison with density estimated for yearlings (O+) in 1984 and 1985, the densities of O+ in_21989 and 1990 were reduced by 80-90~, i.e. frog 42.7 and 33.2 fish. 100 m in 1984 and 1985 to 4.3 and 6.0 fish. 100 m in 1989 and 1990. A strongly significant correlation (P~0.001) was found between the numbers of Atlantic salmon yearlings (0+) stocked and the corresponding density of 0+ in the river. The reduction in salmon density occured even though fish ladders allowed natural reproduction. The lack of natural reproduction was due to few salmon moving up through the fish ladders and the fact that very few of these were females (20~). Only a small decline was observed in the density of brown trout yearlings, and the density of older fish did not decline in the same way as for older salmon. There are both anadromous and non migratory brown trout, and far more sea trout of, which 70~ are females, negotiate the f i ch 1 aAr7arc JUVENILE COMPETITIVE BOTLENECK IN THE PRODUCTION OF BROWN TROUT IN HYDROELECTRIC RESERVOIRS DUE TO INTRASPESIFIC HABITAT SEGREGATION. Ola Hegge, Trygve Hesthagen & Jostein Skurdal. Oppland County Environmental Administration, Statsetatenes hus, N-2600 Lillehammer, Norway We studied resource utilization and growth of brown trout in 7 norwegian hydroelectric reservoirs with supplementary stockings, by benthic and pelagic gillnett sampling. In lazge, deep (>_ 35 m) reservoirs, without planktivore competitors, brown trout segregated according to size, as small individuals (< 200 -250 mm) mainly occurred in benthic habitats while larger individuals utilized upper strata of pelagic waters. The habitat use of brown trout was mainly benthic in shallow reservoirs with higher benthic food production and in reservoirs with plankton feeding whitefish. In large, deep reservoirs, the shift from benthic to pelagic habitat was accompanied by an increase in growth. This indicates that the conditions for young age classes may be a bottleneck in the capacity to produce brown trout in hydroelectric reservoirs with limited benthic feeding conditions, despite an ample access to food in pelagic habitats. Thus, when evaluating the possibility of increasing brown trout production by supplementary stockings, it is necessazy to consider food and growth conditions for all age/size groups of brown trout. 10