Growth, survival, and habitat use of naturally reared and hatchery steelhead fry in streams: effects of an enriched hatchery rearing environment

Abstract

After hatchery-reared salmonids are released into the wild, their survival and performance are frequently lower than those of wild conspecifics. Additionally, negative effects of hatchery fish on wild fish are cited as factors affecting the recovery of salmonid populations. Alternative hatchery rearing environments and release practices have been proposed to mitigate both problems. We investigated the postrelease growth, survival, habitat use, and spatial distribution of hatchery steelhead Oncorhynchus mykiss fry reared in conventional and enriched environments and compared their performance with that of naturally reared steelhead fry from the same parent population in two streams. Average instantaneous growth rates differed between streams but not among the three rearing groups. The survival of naturally reared fry was significantly greater than that of both types of hatchery fry (relative survival ¼ 0.33) but did not differ between the conventional and enriched environments. Naturally reared fry grew and survived equally well regardless of the type of hatchery fry with which they were stocked. Supplementation increased fry population size in all stream sections but produced hatchery-biased populations. Steelhead fry preferred pool habitat within stream sections, but pool use was affected by an interaction between rearing environment and stream. Hatchery fry had more clumped spatial distributions than naturally reared fry, which were affected by a significant interaction between rearing type and stream. Hatchery rearing type and stream had no effect on the spatial distribution of naturally reared fry. We conclude that (1) hatchery steelhead fry released in streams grow as well as naturally reared fry but do not survive as well, (2) enriched hatchery environments do not improve postrelease growth or survival, and (3) upon release, fry raised in enriched hatchery environments affect the growth and survival of naturally reared fry in much the same way as fry reared in conventional hatchery environments.