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Seasonal variation of brain size in a freshwater top predator

1. Teleost fishes occupy a range of ecosystem and habitat types subject
to large seasonal fluctuations. Temperate fishes in particular, survive
large shifts in temperature, light availability, and access to certain
habitats across seasons. Yet, there is limited understanding of how
behavioral responses to a seasonally shifting environment might shape,
or be shaped by, the nervous system. 2. Here we quantified variation in
relative brain size and the size of five externally visible brain
regions in a freshwater top predator, lake trout (Salvelinus namaycush),
across six consecutive seasons in two different lakes. Acoustic
telemetry data from one of our study lakes was collected during the
study period from a different subset of individuals and used to infer
relationships between brain size and seasonal behaviors (habitat use and
movement rate). 3. Our results indicated that lake trout relative brain
size was larger in the fall and winter compared to the spring and summer
in both lakes. Larger brains coincided with increased use of nearshore
lake habitats and increased horizontal movement rates by lake trout in
the fall and winter based on acoustic telemetry. The telencephalon
followed the same pattern as whole brain size, while the other brain
regions (cerebellum, optic tectum, olfactory bulbs, hypothalamus) were
only smaller in the spring. 4. Seasonal shifts in total brain size might
reflect greater underlying changes in the size of the telencephalon.
These findings provide evidence that flexibility in brain size could
underpin shifts in behavior which could subserve functions associated
with differential habitat use during cold and warm seasons and allow
fish to succeed in seasonally variable temperate environments.