Survival of stocked hatchery-reared brown trout, Salmo trutta L., fry in relation to the carrying capacity of a trout nursery stream

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Abstract. During the period June 1982-84 hatchery-reared brown trout, Salmo trutta L., fry were Stocked into stretches of the Owendoher. a trout nursery stream on the east coast of Ireland. These experiments were designed to examine the survival of stocked fry and to estimate the carrying capacity of the system. During the first year fry were stocked into sectors already supptorting wild fish at densities normal for the system. In the following year fry numbers were artificially reduced prior to stocking with the hatchery-reared fish. Mortality of the stocked fry was high after release with less than 33% of the fish surviving beyond the first 3 weeks. No stocked fish survived after October 1982. In the second year, however, 2-9% of the fish survived. The best survival rates were achieved where wild fry numbers were lowest. Regardless of the initial stocking density the various experiments yielded autumn fry densities (0.07-0.7 fish/m2) similar to those at unstocked sites (0.1-0.62 fish/m2). Stocking did not increase recruitment to the 1+ group and again 1+ densities (0.15-0.35 fish/m2) similar to unstocked sites (0.07-0.39 fish/m2) were obtained at the end of each year. These results suggest that spawning and recruitment in the Owendoher yield population densities approaching the maximum carrying capacity of the stream. The system appears to support a maximum summer fry density in the region of 1 fish/m2 and a maximum autumn density of 0.7 fish/m2.