Response of resident brown trout, Salmo trutta L., and rainbow trout, Oncorhynchus mykiss (Walbaum), to the stocking of hatchery-reared brown trout

Two strains of hatchery-reared adult brown trout, Salmo trutta L., [208-334 mm total length (TL); n = 591] were individually marked and released into a limestone stream. The estimated survival after one month (86%; n = 508) w as comparable to that for resident brown trout and rainbow trout, Oncorhync hus mykiss (Walbaum), (89%; n = 771), but declined to 14% (n = 83) after 8 months compared with 52% (n = 451) for resident trout. The movement of resi dent trout out of stocked stretches was higher (14%) than from control site s (5%), but the population size in both individual sites and the overall st udy area were unaffected. The growth of resident brown trout was unaffected by stocking, but rainbow trout showed lower growth rates in stocked versus unstocked stretches both one and 8 months after stocking (P < 0.002).