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AFSC/ABL: 1996 Brood year Steelhead growth and early life-history transitions
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Heritabilities of growth, precocious maturation and smolting were measured in 75 families
of juvenile steelhead or rainbow trout Oncorhynchus mykiss, progeny of within and
between line matings (crosses) of wild, anadromous steelhead and wild, resident (lake)
rainbow trout originally derived from the same anadromous stock 70 years earlier.
The tagged yearling progeny were combined by line in common freshwater rearing containers
and graded into three categories: mature, smolt or rearing (undifferentiated) at age
2 years. Heritabilities of precocious male maturity, smolting and growth were moderate
to high, and the genetic correlation between growth and smolting was low. Smolting
and precocious male maturity were highly variable among families within lines and
significantly different between lines. Each of the four lines produced significant
numbers of smolts at age two. Smolting and maturation were negatively genetically
correlated, which may explain the persistence of smolting in the lake population despite
strong selection against lake smolts; balancing selection on male maturation age may
help to maintain variation for smolting. The high heritability of smolting, coupled
with the inability of smolts that leave the lake to return to it indicates that the
genetic potential for smolting can lie dormant or be maintained through a dynamic
interaction between smolting and early maturation for decades despite complete selection
against the phenotype. The results have significant implications for the preservation
of threatened anadromous stocks in fresh water and the inclusion of resident fish
of formerly anadromous populations, currently trapped behind long-standing barriers
to migration, as one component of the same population.
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