A Review of Recent Studies Investigating Seminatural Rearing Strategies as a Tool for Increasing Pacific Salmon Postrelease Survival
Desmond J. Maynard, Thomas A. Flagg, Robert N. Iwamoto, and Conrad V. W. Mahnken
Abstract.—Traditional hatchery salmonids lack many behavioral and morphological attributes needed to survive after release (Maynard et al. 1995). The Seminatural rearing concept hypothesizes that exposing hatchery salmonids to natural habitats, foods, predators, and currents will induce them to develop the wild behavior, physiology, and morphology needed for postrelease survival. The paper reviews recent studies investigating the efficacy of this concept. Rearing salmonids in seminatural rearing habitat, with natural fluvial substrates, structure, and overhead cover, usually improves survival. Supplementing hatchery fish diets with live foods often enhances their ability to hunt live prey. However, utilizing automated underwater feeders to feed Chinook salmon Oncorhynchus tshawytscha in a more natural manner did not alter their depth preference, response to novel visual stimuli at surface, or predator vulnerability as predicted. In most, but not all cases, conditioning salmonids to avoid predators improves their postrelease survival. Exercise usually improves growth and health, but does not always increase postrelease survival. Fisheries managers can use the increased survival successful seminatural rearing strategies offer to increase recruitment to the fishery and spawning population, reduce competitive impacts on listed stocks, or simply reduce operational costs.