Despite temperature effects on gonad development, timing of spawning is remarkably flexible in Atlantic cod

At high latitudes, early life stage survival of fish is often associated
with how spawning time relates to the timing of the spring bloom. With
ocean warming, basic physiological rates of ectotherms, like fish, will
speed up - including gonadal development rates which dictate spawning
time. Since warmer water is thought to influence the spring bloom timing
differently than that of fish spawning time, the two may fall out of
synchrony in the future. The precise mechanisms between temperature and
gonadal development and spawning time have, however, been difficult to
disentangle. Here, we take advantage of a series of independent laboratory
experiments measuring individual oocyte development up to or near spawning
for 153 Atlantic cod (Gadus morhua) kept between 3 and 12 °C. From these
data, we derive a predictive, mechanistic equation for daily oocyte growth
rate as a function of temperature and oocyte developmental status
(diameter). The vitellogenic oocyte growth follows an accelerating
pattern, and the model predicts that spawning can advance by up to 7 days
per 1 °C increase. Within-treatment variation is, however, of comparable
magnitude to between-treatment temperature effects. The model was also
tested in the field by back-calculating oocyte development of 82 fish
(2018-2021) sampled at two locations along the Norwegian coast, using
daily ambient temperatures from telemetry tags during vitellogenesis as
model input. We find that Atlantic cod are able to initiate vitellogenesis
over a period of several months in late summer and autumn, as well as
regulate the oocyte development rate across a wide range of temperatures –
both leading to significant phenotypic plasticity in spawning phenology.