Abstract
BackgroundMigrations allow animals to find food resources, rearing habitats, or mates, but often impose considerable predation risk. Several behavioural strategies may reduce this risk, including faster travel speed and taking routes with shorter total distance. Descriptions of the natural range of variation in migration strategies among individuals and populations is necessary before the ecological consequences of such variation can be established.Methodology/Principal FindingsMovements of tagged juvenile coho, steelhead, sockeye, and Chinook salmon were quantified using a large-scale acoustic tracking array in southern British Columbia, Canada. Smolts from 13 watersheds (49 watershed/species/year combinations) were tagged between 2004–2008 and combined into a mixed-effects model analysis of travel speed. During the downstream migration, steelhead were slower on average than other species, possibly related to freshwater residualization. During the migration through the Strait of Georgia, coho were slower than steelhead and sockeye, likely related to some degree of inshore summer residency. Hatchery-reared smolts were slower than wild smolts during the downstream migration, but after ocean entry, average speeds were similar. In small rivers, downstream travel speed increased with body length, but in the larger Fraser River and during the coastal migration, average speed was independent of body length. Smolts leaving rivers located towards the northern end of the Strait of Georgia ecosystem migrated strictly northwards after ocean entry, but those from rivers towards the southern end displayed split-route migration patterns within populations, with some moving southward.Conclusions/SignificanceOur results reveal a tremendous diversity of behavioural migration strategies used by juvenile salmon, across species, rearing histories, and habitats, as well as within individual populations. During the downstream migration, factors that had strong effects on travel speeds included species, wild or hatchery-rearing history, watershed size and, in smaller rivers, body length. During the coastal migration, travel speeds were only strongly affected by species differences.
Highlights
Migration is an important life history strategy in many animals, where individuals move among habitats at specific times of year or at specific ontogenetic stages to gain access to food resources, reduce predation risk, or find mates
There generally appear to be species-specific migration routes through the inner coastal Strait of Georgia, British Columbia (B.C., Figure 1), with some species typically exiting the Strait northwards through Queen Charlotte Strait [1,2], and other species typically southwards through Juan de Fuca Strait [3], with both routes leading to outer coastal waters
We considered an interaction between fork length (FL) and Fraser River or non-Fraser watershed (FnF) as a fixed effect, which permitted different slopes of the relationship between travel speed and FL for these watershed categories
Summary
Migration is an important life history strategy in many animals, where individuals move among habitats at specific times of year or at specific ontogenetic stages to gain access to food resources, reduce predation risk, or find mates. Variation within populations may exist, with some individuals moving in one direction and others moving in another Such divergence in migration routes could be caused by various navigational cues (reviewed in [5]) or genetic factors. Salmon have been hypothesized to have two ‘‘zip codes’’ [6], with population-specific ocean feeding grounds in addition to their freshwater spawning grounds, and it is possible that similar variation in migration phenotypes could occur within populations. Descriptions of the natural range of variation in migration strategies among individuals and populations is necessary before the ecological consequences of such variation can be established
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