Data from: Sex-specific life history affected by stocking in juvenile brown trout

Salmonids are a socioeconomically and ecologically important group of fish
that are often managed by stocking. Little is known about potential
sex-specific effects of stocking, but recent studies found that the sexes
differ in their stress tolerances already at late embryonic stage, i.e.,
before hatchery-born larvae are released into the wild and long before
morphological gonad formation. It has also been speculated that
sex-specific life histories can affect juvenile growth and mortality, and
that a resulting sex-biassed demography can reduce population growth. Here
we test whether juvenile brown trout (Salmo trutta) show sex-specific life
histories and whether such sex effects differ in hatchery- and wild-born
fish. We modified a genetic sexing protocol to reduce false assignment
rates and used it to study the timing of sex differentiation in a
laboratory setting, and in a large-scale field experiment to study growth
and mortality of hatchery and wild-born fish in different environments. We
found no sex-specific mortality in any of the environments we studied.
However, females started sex differentiation earlier than males, and while
growth rates were similar in the laboratory, they differed significantly
in the field depending on location and origin of fish. Overall,
hatchery-born males grew larger than hatchery-born females while wild-born
fish showed the reverse pattern. Whether males or females grew larger was
location-specific. We conclude that juvenile brown trout show sex-specific
growth that is affected by stocking and by other environmental factors
that remain to be identified.