SALMONID LIFE-HISTORY EVOLUTION AS A CONSTRAINT ON MARINE STOCK ENHANCEMENT

Salmonid stocks have been depleted quantitatively and qualitatively an d much of their freshwater habitat has been destroyed. Extensive enhan cement programs through hatchery rearing have not restored these stock s, 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 simple st are those of pink salmon, Oncorhynchus gorbuscha, of which some pop ulations spawn in brackish tidal waters and never use fresh water, and the most complex those of steelhead trout O. mykiss, which exhibits m ore than 30 life-history types, from anadromous to landlocked. Between these two extremes, life-styles range from those with some elements e mancipated from the sea through some that complete their life cycles w holly within fresh water Successful enhancement of salmonid stocks req uires an understanding of the nature of this flexibility and of the co ntrols over maturation that determine the life-cycle pattern that indi viduals will adopt. Hence, understanding fish life histories depends o n understanding the control of fish reproduction. Maintaining this dev elopmental diversity in rehabilitation of depleted populations depends on breeding programs designed to maintain genetic continuity, creatio n of the appropriate range of environmental opportunities during early rearing, and conservation of a sufficiently diverse freshwater habita t.