4. Discussion
Using molecular markers, we could identify captive-bred juveniles of 56
experimentally produced families and compare them to wild-born juveniles
of the same cohort. The captive-bred 0+ dominated in number (by a factor
5.3), had a significantly more balanced sex ratio, and were on average
larger than wild-born 0+. The reason for these differences remains
unclear but could be linked, for example, to different stress levels
during embryogenesis, the timing of stocking relative to the timing of
emergence of wild-born, or size differences at the time when exogenous
feeding starts. Given these many possible reasons, it may even be a
general rule that captive-bred and wild-born fish of the same cohort
differ in growth and survival because of differences in early life
history (Palejowski et al. 2022).
We used a panel of > 1 million SNPs to calculate the
kinship coefficients of 47 parental combinations that resulted from our
experimental breeding. These 47 kinship coefficients (minus one outlier)
may well reflect the average inbreeding coefficient per experimental
full-sib family in a population that does not seem to suffer from
elevated levels of inbreeding, as concluded from measurements of hybrid
vigour that included our study population (Clark et al. 2013;
Stelkens et al. 2014) in crosses of populations that are
genetically distinct (Stelkens et al. 2012). Nevertheless, the
variation in kinship coefficients that we observed could be used to
predict female growth in the wild. Females grew slower with increasing
inbreeding coefficients. No such effects could be observed among males.
We conclude that the susceptibility to inbreeding depression is
sex-specific at this juvenile stage.
We sampled the juveniles about 6 months after release into the wild.
Sexual maturity and first breeding are expected at the end of their
second or third year of life. Therefore, the sex-specific effects of
inbreeding that we observed cannot be explained by the sex-specific
stress that is expected during the mating season. Moreover, because sex
chromosomes of brown trout are largely homomorphic (Guiguen et
al. 2019), as is typical for lower vertebrates (Beukeboom & Perrin
2014), they are not expected to contribute significantly to sex-specific
inbreeding depression (Vega-Trejo et al. 2022). The effects we
found here are therefore best explained by sex differences in life
histories. In the case of brown trout, these sex differences are rather
cryptic. Little is known about sex differences in morphometry or
behaviour at such early life-history stages even though the brown trout
is a common and well-studied species. However, recent studies on brown
trout and grayling revealed that the sexes differ in the timing of gonad
development. Females generally develop their gonads earlier than males
while males in turn grow faster than females during that time (Maitreet al. 2017; Palejowski et al. 2022). The size difference
that was found before in other populations (Palejowski et al.2022) could be confirmed in the present study but was overall small
(around 3%). This difference was linked to the variance in kinship
coefficients and may even be caused by the variance in kinship
coefficients because the sex difference in size was not apparent in
families with small inbreeding coefficients. We therefore predict that
increased inbreeding within a population will accentuate the sex
difference in growth.
The overall sex ratio in our re-captured sample was about equal, and
there was no significant effect of inbreeding on recapture rates for the
different families. Based on the rate of experimentally produced fish
among the sampled ones (80.3%), the total number of fish that could be
sampled, and the number of fertilized eggs that were used for this
study, the overall mortality of the experimentally produced fish over
their first 9 months of their life was 78.4% or less if some fish had
escaped sampling. Because embryo mortality was low in a parallel study
on the same families (Wilkins et al. 2017), the mortality in the
present study reflects the acute stress during stocking and/or the
selection during the fish’s first spring and summer in the wild.
Somewhat comparable levels of mortality during these first months have
been observed in nearby populations of brown trout (Palejowski et
al. 2022), i.e., the level of selection in our study system seems not
extra-ordinary.
As is typical for studies on wild populations, quantifying likely
effects of emigration remained difficult. In our study system, upstream
emigration was not possible. Downstream emigration into the larger
stream (Rotache) was possible. However, if migration happens in this
species, it typically starts at later developmental stages and is then
often sex-biased, with females being more likely to migrate than males
(Forseth et al. 1999; Nevoux et al. 2019). The most
parsimonious explanation for the observed overall equal sex ratio in the
captive-born fish, and the non-significant correlations between
recapture rates and inbreeding, is therefore that there was no
sex-specific mortality and no sex-specific emigration, and that
inbreeding depression only affected growth but did not lead to increased
mortality in the hatchery-produce fish. The pattern was different in the
wild-born 0+ who grew smaller than the hatchery-produced 0+ and had a
male-bias sex ratio, suggesting that wild-born females suffered from a
higher mortality than wild-born males during their first spring and
summer. It is possible that a combination of sex-specific inbreeding and
larger hatchery-born competitors led to sex-specific mortality among the
wild-born.
Inbreeding coefficients can show significant heritability in small and
structured populations (Neff & Pitcher 2008; Nietlisbach et al.2016). This prediction is supported by the significant correlation that
we found between parental inbreeding coefficients and the average
kinship coefficients, i.e., the expected inbreeding coefficients of
their offspring. If parental inbreeding coefficients predict offspring
inbreeding coefficients, and if inbreeding depression during the
spawning season affects intra- and inter-sexual selection (i.e., giving
inbred individuals a selective advantage), natural spawning would be
expected to reduce the average inbreeding coefficient of the next
generation. However, the one extreme kinship coefficient that we
recorded would not be avoid through effects of inbreeding on health and
vigour because the parents of this sib group had average and very
similar inbreeding coefficients, suggesting that they could have been
brother and sister that would not be able to avoid each other without
some form in kin recognition.
Our experimental breeding also allowed to test for general maternal and
paternal effects on juvenile growth. We found significant maternal but
not paternal effects, suggesting that juvenile growth is affected by
maternal environmental effects such as egg size and egg content. The
absence of significant paternal effects is either due to limited
statistical power or suggests that heritability of growth is small when
measured in juveniles recaptured from the wild. Paternal effects on
offspring growth are, however, frequently observed in brown trout larvae
when studied under controlled laboratory conditions, revealing
significant heritability of growth in this species (Marques da Cunhaet al. 2019; Nusbaumer et al. 2019). The limited number of
recaptured juveniles per each of the 60 full-sib families did not allow
to test for possible effects of dam x sire interactions on growth.
In conclusion, inbreeding did not significantly affect mortality of
juvenile brown trout that had been stocked into the wild as larvae.
However, female growth during their first spring and summer in the wild
was reduced with increasing inbreeding coefficients. No such effect
could be observed in males who even could grew on average about 3%
larger than females during that time. Inbreeding depression is hence
sex-specific around the time of gonad formation and long before intra-
or inter-sexual selection are expected to cause sex-specific
(male-biased) inbreeding depression.