Salmonid stocks have been depleted quantitatively and qualitatively, and much of their freshwater habitat has been destroyed. Extensive enhancement programs through hatchery rearing have not restored these stocks, largely because their evolutionary heritage and the discrete stock structure
that it has engendered are poorly understood. Salmonids show a wide range of life-styles and high phenotypic plasticity. The simplest are those of pink salmon, Oncorhynchus gorbuscha, of which some populations spawn in brackish tidal waters and never use fresh water, and the most complex
those of steelhead trout O. mykiss, which exhibits more than 30 life-history types, from anadromous to landlocked. Between these two extremes, life-styles range from those with some elements emancipated from the sea through some that complete their life cycles wholly within fresh water.
Successful enhancement of salmonid stocks requires an understanding of the nature of this flexibility and of the controls over maturation that determine the life-cycle pattern that individuals will adopt. Hence, understanding fish life histories depends on understanding the control of fish
reproduction. Maintaining this developmental diversity in rehabilitation of depleted populations depends on breeding programs designed to maintain genetic continuity, creation of the appropriate range of environmental opportunities during early rearing, and conservation of a sufficiently diverse
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