Coastal Cutthroat Trout as Sentinels of Lower Mainland Watershed ...

29of wild and hatchery juvenile cutthroat, each from wild brood sources. Although wildand hatchery juveniles grew at similar rates in the hatchery, hatchery fish grew moreslowly in the wild. Further, hatchery-reared fish matured earlier and exhibited a highfrequency (50%) of residual non-migrants. Thus, maturing “residuals” of a similar sizeof these two trout species in the same habitats are prone to spawn together and formhybrids. Further, a sharp decline in steelhead smolt-to-adult survival in the ocean overthe past decade has resulted in much fewer steelhead spawners, which may be leading togreater potential for mixing with cutthroat spawners on spawning sites.Costello and Rubridge (2004) suggest that this high incidence of hybrids is indicative ofan apparent lack of selection against them, which should otherwise occur. They furthersuggest it may be a combined result of habitat degradations and hatchery fishintroductions. They also submit that from an evolutionary genetics perspective, hybridswarms represent possible extinctions because although their genes are maintained, theyare not likely to be in the unique combinations of the original native populations. Thus,habitat protection and restoration along with minimizing steelhead stockings in cutthroathabitats, and visa versa, are management actions that would reduce genetic risks ofhybridization.In summary, coastal cutthroat trout demonstrate highly plastic life histories, which varyin dominance of a life history in any geographic region and perhaps even groups ofstreams (Northcote 1997). The general assumption is that migratory behaviors aregenetically fixed, but they may be rather plastic, and only fixed within wide bounds.Thus, if environmental conditions change rapidly, adaptive behavior can be rapidlymobilized. Alternatively, it may only take a few generations to shift such behavior.Timing of spawning, for example, is highly heritable, yet it can be rapidly shifted in afew generations as it was at Chambers Creek, Washington. After a few generations peaktiming of spawning occurred in January instead of March-April, and such a timing wouldbe maladaptive in the wild. Accordingly, for management purposes related to fishculture, a conservative approach is to avoid mixing of discreet stocks and specieshydridization by: (1) assuming that cutthroat stocks are genetically different even ifcurrent molecular tools cannot distinguish among them; (2) minimizing residualismimpacts through selection of low risk stocking locations; and (3) minimizingopportunities for cultured cutthroat and steelhead to hybridize by adequately separatingstocking locations (although if a hatchery located within the watershed, the latter is achallenge).3. POPULATION DYNAMICS OF COASTAL CUTHROAT TROUT3.1 Cutthroat Trout Smolt YieldsResearch studies in the Pacific Northwest which document the population dynamics ofcoastal cutthroat trout are sparse because most are directed at salmon and steelhead.Most information is from other studies where cutthroat trout were not a priority and thussmolt yields per unit of stream, age-specific survival rates, and stock-recruitmentrelationships are unavailable. Cutthroat smolt yields per unit stream length are not