STOCKING EXPERIMENTS WITH 0+ AND 1+ TROUT PARR, SALMO-TRUTTA L, OF WILD AND HATCHERY ORIGIN .1. POST-STOCKING MORTALITY AND SMOLT YIELD

Wild and hatchery‐reared 8–12‐month‐old (5–8 cm) trout, Salmo frurta L., were stocked in tributaries of the River Gudenb. Mortality was examined by means of electrofishing. Repeated electrofishing and handling caused a small increase in mortality. The daily instantaneous mortality rate Z was high during the first 2 months after stocking, ranging from 0.0070 for wild trout to 0.0326 for domestic trout at a stocking density of one trout per m2 and from 0.0206 (wild trout) to 0.0888 (domestic trout) at a stocking density of two trout per m2. Two months after stocking, Z decreased drastically ranging from 0.0007 (wild trout) to 0.0067 (domestic trout). When stocked, first‐generation hatchery trout showed Z equal to domestic trout. Wild trout resident in the experimental stream were negatively affected by the introduction of domestic trout and wild trout from another stream. at a stocking density above the carrying capacity. It is concluded that the higher mortality of domestic trout was caused by changes in food, feeding and exercise, possibly combined with the lack of selection in the hatchery. Smolt yield at age 2+ was 3.2% (0+ trout stocked in the fall)‐7.0% (1 + trout stocked in the spring) of the domestic trout stocked (approx. one‐sixth to one‐third of natural populations) and 65.2–68.7% of the domestic trout present before the smolt run. For first generation hatchery trout of wild origin the corresponding figures were 7.3% (age 0 +) and 93.4%, and for wild trout introduced to the experimental stream they were 11.1% (age0 +)and39.8%. Copyright © 1991, Wiley Blackwell. All rights reserved