Timing and synchrony of migration in a freshwater fish: Consequences for survival

Abstract

Animal migration is one of the most spectacular and visible behavioural phenomena in nature with profound implications for a range of ecological and evolutionary processes. Successful migration hinges on the ability to exploit temporary resources (e.g. food) and evade threats (e.g. predators) as they arise, and thus the timing of migration is often regarded as a dominant predictor of individual migratory success. However, with the exception of intensively studied taxa (mainly birds), relatively few studies have investigated inter-individual annual and seasonal variation in migratory timing and performance, or tested predictions on how migration across high and low predation-risk habitats may exert selection on migratory timing. In particular, studies that assess the survival consequences of variation in migratory timing remain rare, which is most likely due to the logistical challenges associated with monitoring survival success and population-level characteristics simultaneously. Here, we address the above-mentioned questions using roach Rutilus rutilus, a fish that migrates from lakes characterised by high predation risk into low-risk streams during winter. Specifically, we used individual-based tracking of roach in two European lake systems over multiple migration periods (9 and 7 years respectively), to obtain highly detailed (year-round scheduling, repeat journeys and the fate of individuals) data on the variability/synchrony of migratory timing in spring and autumn respectively. We report seasonal differences in the variability of migratory timing, with lower variance and higher migration synchrony in spring lake arrival timing as compared to autumn lake departure timing. Furthermore, the timing of autumn migration is more variable across years than the timing of spring migration. Second, we find that later arrival to the lake habitat is positively associated with apparent survival from 1 year to the next, whereas we found no effect of lake departure timing on survival probability. These findings represent rare evidence showing how intraspecific variation in timing in migratory fish differs across years and seasons, and how variation in timing can translate into survival consequences for prey in systems characterised by high predation risk.

Keywords: aquatic ecology; individual differences; migration; movement ecology; predation risk; timing.

FIGURE 1

Temporal migration patterns in roach in the two study lakes, lake Krankesjön, Sweden (a), and lake Søgård, Denmark (b). The large circle represents the year and small solid circles on the perimeter represent the average timing of migration of the population for a specific year. Blue circles represent autumn migration timing and red circles spring lake arrival timing. Bold error bars outside the large circle denote 95% CI of autumn and spring migratory timing across years respectively.

FIGURE 2

Relative timing of roach autumn (a; blue symbols) and spring migration (b; red symbols) in lake Krankesjön (left panel) and lake Søgård (right panel). The solid vertical line denotes the average timing of migration for all roach individuals within a specific year and the symbols represent the relative deviation from the average for survivors (filled triangles) and non‐survivors (open circles) within a given migration season (y‐axis). In autumn, survivors and non‐survivors do not differ in migratory timing, whereas in spring survivors migrate back to the lake later than non‐survivors.