DOI: 10.5061/dryad.0cfxpnw35
title: Seasonal variation of behavior and brain size in a freshwater fish

description: Teleost fishes occupy a range of ecosystem and habitat types subject to
 large seasonal fluctuations. Temperate fishes in particular, survive large
 seasonal shifts in temperature, light availability, and access to certain
 habitats. Mobile species like lake trout (Salvelinus namaycush) can
 behaviorally respond to seasonal variation by shifting their habitat
 deeper and further offshore in response to warmer surface water
 temperatures during the summer. During cooler seasons, use of more
 structurally complex nearshore zones by lake trout could increase
 cognitive demands and potentially result in a larger relative brain size
 during those periods. Yet, there is limited understanding of how such
 behavioral responses to a seasonally shifting environment might shape, or
 be shaped by, the nervous system. Here we quantified variation in relative
 brain size and the size of five externally visible brain regions in lake
 trout, 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).  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 habitats and increased horizontal movement rates 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.  These findings provide evidence that flexibility in
 brain size could underpin shifts in behavior, which could potentially
 subserve functions associated with differential habitat use during cold
 and warm seasons and allow fish to succeed in seasonally variable
 environments.
description: <em>Study Sites and sample collection</em>
 Lake trout were collected
 seasonally from Lake of Two Rivers (hereafter referred to as “Two
 Rivers”), Ontario, Canada (45<sup>o</sup>34’42.6” N,
 78<sup>o</sup>29’0.4” W; 274 ha surface area, 38 m maximum
 depth) and Lake Opeongo (hereafter referred to as “Opeongo”), Ontario,
 Canada (45<sup>o</sup>41’46.8” N,
 78<sup>o</sup>22’27.8” W; 5800 ha surface area, 49.4 m maximum
 depth), each located within Algonquin Provincial Park. Unlike Two Rivers,
 Opeongo supports pelagic prey fish (e.g., lake cisco: <i>Coregonus
 artedi</i>). Mature lake trout were sampled seasonally from fall
 2017 to winter 2019 (Table 1) using trap nets, gill nets, and
 angling equipment. The fish were euthanized immediately upon capture via
 severing of the spinal cord, and their lengths and weights collected
 (under approved University of Toronto Animal Use Protocols). Fish heads
 were then removed and placed in labelled containers with 10% neutral
 buffered formalin (Fisher Scientific Inc., New Jersey, USA).
 Fish that possessed undeveloped gonads were considered
 immature and removed from analysis. Following previous conventions (Guzzo et
 al. 2017), the period after ice-off but before mean surface temperature
 (&lt;6 m depth) exceeded 15˚C was denoted as spring, summer was
 defined as the period during which surface temperatures reached or
 surpassed 15 ˚C, fall began when lakes cooled to ≤ 15˚C and lasted until
 winter, defined as ice-on to ice-off. Water temperatures were
 measured throughout the upper 6 m of the water column in each lake using a
 string of data loggers (HOBO Temp Pro H20-001, Onset, Cape Cod, MA)
 deployed over the deepest point of each lake (Table 1). <em>Brain Mass and Region
 Volumes</em> Following procedures from Edmunds et al.
 (2016b), the brains were removed and trimmed of cranial nerves, and the
 spinal cord was cut at the obex. Each brain was blotted thoroughly to
 remove excess formalin and was then weighed on an analytical balance
 (Fisher Scientific Accu-124D), to a resolution of 0.0001 g to obtain brain
 mass, which was used to estimate brain size. Pictures were taken of the
 dorsal, left, and ventral sides of the brain using an Olympus SZ61
 dissection microscope and a Canon Powershot G9 digital camera and PSREMOTE
 v1.7 software (Breeze and Breeze 2009). A calibration grid was included in
 each picture. The height, length, and width of brain regions visible on
 the resulting images (olfactory bulbs, telencephalon, optic tectum,
 cerebellum and hypothalamus) were measured using the measuring tool in
 ImageJ software (Rueden et al. 1997). Regional volumes (mm3) were
 estimated using the ellipsoid formula: V = (L x W x H) π/6, providing us
 with estimates of size for each brain region. Volume measurements were
 conducted by the same person (EJV) and measurement accuracy was assessed
 by repeating measurements of five randomly selected brains ten times
 (Appendix S1: Table S1). <em>Acoustic
 telemetry</em> Acoustic telemetry data were available
 from Lake of Two Rivers throughout the study period, albeit for a
 different subset of lake trout individuals (n=9). Fixed location reference
 tags within the array provided a measure of array performance throughout
 the study period. Estimated positions based on the Vemco (InnovaSea, WA,
 USA) positioning algorithm were compared with known positions of the tags
 to quantify positioning errors across seasons and years using methods
 proposed in Smith (2013). Reference tags at 5 m depth had a lower average
 error than tags placed at 18 m depth. Over 95% of detections at 5 m had a
 positional error of less than 6 m vs. 86% at the 18 m depth. Overall mean
 error was 2.41 m vs 5.24 m at 5 and 18 m depth, respectively. Additional
 details about the telemetry array setup, performance, and fish tagging can
 be found in (Appendix S2 of manuscript).
description: Metadata for the datasets are located in the README file, as well
 as written in the header of the associated R files. R
 files were written in R studio, and require packages for usage.