Higher to be critical in determining the

Higher light exposure was associated with a shorter incubation
period, smaller YSA, and similar embryo size compared with individuals exposed
to only light during a seasonal diel period and absence of light. These
findings are consistent with previous studies on the impact of temperature on coregonid
egg development and metabolic rates (Edsall
and Colby 1970, Colby and Brooke 1973, Karjalainen et al. 2015, Karjalainen et
al. 2016).

In addition, increasing light
exposure during incubation led to faster growth in larvae. Higher growth rates
could be reflected by a higher yolk consumption rate and conversion efficiency
of yolk into tissues. In this experiment, environmental constraints from light seemingly
induced a partitioning of yolk consumption towards maintaining elevated levels
of cellular activity from photosensitive organs instead of development and growth,
which ultimately accelerated hatching.

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Our study demonstrates for the
first time that light exposure during incubation potentially intensifies the known
impact of temperature on coregonid embryonic life-history traits, as reflected
by the significant effects of light on incubation period and yolk-sac consumption.
This suggests a potential synergistic relationship between temperature and
light and the existence of a critical threshold on light tolerance during
incubation. In coregonids, winter severity (i.e., water temperatures and ice cover) and subsequent inter-annual changes
in light attenuation is likely to be critical in determining the response of climate
change in early-life stages.

Environmental conditions
experienced during embryogenesis can have compounding effects that influence
later life-history traits, survival, and reproductive success (Jonsson
and Jonsson 2014). In fish, smaller young-of-year
have a lower overall fitness than their larger conspecifics (Kamler
2008). Larval emergence with a smaller
YSA can be associated with reduced condition (i.e., lipid content) and survival, high vulnerability to predators,
and reduced performance in intraspecific competition (Louhi
et al. 2011). Hence, further consequences during
the first season of growth are expected and should be further explored.

Overall, our results demonstrate
a relationship between light exposure and life-history traits, such that
exposure to light would be considered to have dramatic impacts on the embryonic
development and survival of larval cisco. As we used an extreme, unnatural range of light exposure, additional
studies of light intensity naturally experienced by the species in Lake Superior
would have important implications for the conservation of this population and
beyond. In light of climate change, these results advocate the necessity for
future studies to re-examine the effects of common stressors in freshwater
species under different thermal scenarios.