Using an acoustic telemetry network established in North America’s largest lake, we identified two distinct Walleye movement strategies corresponding to different life history modalities. These movement strategies were highly conserved between years and split with almost 50% of the tagged population adopting either migratory or resident strategies. Consistent with the POLS hypothesis, more exploratory migrant fishes achieved larger body sizes than residents. Larger body size in Walleye typically confers greater fecundity (Baccante & Reid 1988), with the largest or oldest females in the population having disproportionately high reproductive success (Venturelli et al. 2010). Thus, fitness advantages in this migratory Walleye population may be achieved by reaching larger body sizes more quickly, conferring greater fecundity.
However, our study did not support all life history patterns predicted by PLOS. Whereas POLS predicts lower survival in migrants, multistate mark-resight modelling indicated no differences in survivorship between migrant and resident Black Bay Walleye. Because Black Bay is a fish sanctuary north of Bent Island (where all fish overwinter) and is closed throughout the bay to commercial Walleye fishing (Furlong et al. 2006), this removes a major source of adult mortality. Similar survivorship between groups should lead to an increased proportion of migratory Walleye in the Black Bay population due to the fitness advantage of increased size. This, perhaps, contributed to the Walleye population growth in the region observed in the FWIN netting program (Berglund 2014).
Our findings indicate that thermal habitat conditions act as an environmental control on the timing of long-range Walleye migrations. Habitat use of migratory and resident Black Bay Walleye differed noticeably from August to October, when the water outside of Black Bay reached thermal and optical optima and Walleye occupancy outside the bay increased. Outside of late summer/early fall, water temperatures outside of Black Bay were below optimal levels for Walleye, and both migratory and resident fish shared the inner basin habitat. This apparent strong dependence of Walleye migration on thermal conditions suggests that major changes in weather or climate may greatly impact the degree of migration observed in Lake Superior, such that in warmer years with longer summers (predicted to occur more frequently with climate change), migration opportunities could increase.
So, what is the driver of annual migratory patterns in Black Bay Walleye? Given that migratory fish tend to grow larger and faster than residents, we speculate that migration is driven by access to larger-bodied, energetically-dense coregonids such as Cisco (Coregonus artedi), as opposed to the need to evade poor thermal-optical habitat conditions in Black Bay. Indeed, large regions inside Black Bay always fell within the optimal thermal-optical regime for Walleye between May and October, and we found no significant relationship between TOHA and Walleye occupancy within Black Bay. Access to larger, more energy dense prey has been shown elsewhere to provide increased growth efficiency and maximum size in Walleye (Kaufman et al. 2009). Optimized forage intake accessed through improved food quality and larger prey size can lead to a greater energetic surplus despite increased metabolic costs to migration (Roff 1988; Rennie et al. 2012). Interestingly, the period (Aug-Oct) where Walleye leave Black Bay coincides with the period when female Walleye increase ovary development (Henderson et al. 1996).
Male and female Black Bay Walleye showed systematic sex-based space use differences in detailed Walleye netting surveys. We found that adult female Walleye were more likely to be captured outside of Black Bay than males, whereas adult male Walleye were more likely to be captured inside Black Bay than females. The timing of the netting survey coincides with the period when migratory Walleye leave Black Bay, supporting the hypothesis that females are more common among migratory fish than males. Indeed, the difference in growth exhibited between migratory and resident Walleye closely resembles the sexual size dimorphism displayed in this species (Henderson et al., 2003; Rennie et al. 2008). Notably, immature Walleye (which lack sexual growth differences) did not show the same differential spatial use pattern as sexually mature Walleye, suggesting a greater exploratory behaviour in mature (female) fish only. In previous studies examining Walleye movement, females travelled greater distances than males to access cooler, deeper water (Raby et al. 2018; Matley et al. 2020). While hypotheses surrounding sex-biased movement behaviors or sexual segregation related to life history require further testing, one plausible explanation is that the sex benefiting most from resource acquisition (i.e., female Walleye) is the one that migrates to find those resources.
The tendency of mature female Walleye to be captured outside Black Bay (with no similar pattern observed in juveniles), combined with patterns in growth differences strongly suggest that migrating Walleye in our study are predominantly female. Because immature male and female Walleye grow at similar rates before maturation (Rennie et al. 2008), survival should only differ for mature fish. However, despite apparent sex-based differences adult Black Bay Walleye movement patterns, survival rates appear to be similar. This may be due to the lack of significant commercial harvest of Black Bay Walleye – which was historically a major source of mortality for this population until 2003 (Furlong et al. 2006; Berglund 2014) – providing an equalizing effect on death rates between male/female or resident/migrant individuals.
Distinct movement patterns of migration and residency corresponded to life history differences that were largely predicted by POLS. Spatial state transitions depended on the state to which fish were previously classified, but there was no time dependence of movement patterns based on multistate mark-resight modeling. This was despite the predictable observed patterns of out-migration observed between August and October, and the congregation in the north end of the bay each winter and spring. Space use by migratory Black Bay Walleye did not differ from that of residents for much of the year, with all Walleye congregating in the north end of the bay during the winter months and spring spawning. Within both migratory and resident Black Bay Walleye groups we found variation in movement patterns; some fish made direct movements between positions, while others made frequent back and forth trips. Given our telemetry array design and study objectives, we evaluated the maximum outbound extent of movement, but not the total movement (which in highly mobile resident individuals could potentially be greater than migrants). In winter, receivers were removed from shallow regions of Black Bay to avoid ice damage, contributing to a decline in resight probability during this time. Detection simulations along receiver lines (which were in place over winter) indicated that a fish crossing them would be detected 99% of the time based on 2016 and 2017 deployments, suggesting Walleye movement throughout the Bay was minimal during this period.