Straying Can Make a Salmon a Big Splash in a Small Pool
Migration to natal sites for the purposes of reproduction (philopatry) has been observed across -many aquatic taxa and is particularly evident among fishes and marine turtles. Philopatry is not limited to one life history strategy either, driving scientists to ponder what initiated and maintains this behaviour (for theories related to salmon see Stearns and Hendry 2004). For example, the benefits or consequences of philopatry in species that migrate to their natal sites, spawn and die afterwards (semelparity) are likely different compared to species that undertake multiple spawning migrations throughout their lives (annually or with periodicity; iteroparity). Further complicating matters, philopatry can be imperfect, where the success rate of natal homing among the population of interest is less than 100%. Scientists refer to the proportion of individuals in a population that migrate to non-natal sites and reproduce with conspecifics as “strays.” For example, Lake Sturgeon Acipenser fulvescens in the Laurentian Great Lakes are iteroparous and migrate to natal sites to spawn, but straying to nearby rivers has been observed in wild populations (3.5% natural stray rate; Homola et al. 2010). Similarly the iteroparous leatherback sea turtle migrates to natal beaches, but Eckert et al. (1989) observed that the proportion of strays among populations on nesting beaches ranged from 0-27.3% each year. In the case of leatherbacks, the dynamic nature of nesting beaches is thought to influence the degree of philopatry. Compared to semelparity, it is easier to theorize what fitness benefits straying may confer in iteroparous species: an increase in the overall fitness of the receiving population due to an increase in genetic diversity, or an increase in reproductive success of the stray avoiding unfavorable conditions at the natal site (e.g., leatherback sea turtle), etc.
In the April edition of Fisheries, Bett et al. (2017) address the issue of straying in a semelparous species and provide an indepth review of the suspected, and theorized, causes and consequences of straying (spawning with conspecifics in non-natal locations) by Pacific salmon.
The authors begin by providing a well-reasoned context that answers the question: why does the rate of straying matter, especially to small populations of Pacific salmon? Bett et al. (2017) explain that many species of Pacific salmon exhibit significant local adaptation that is often detectable at fine spatial scales. Any stray that spawns and introduces foreign genetic material could result in introgression and the loss of the local adaptive advantages, reducing overall population fitness, a consequence that is greater for small populations.
Using a case study of Sockeye Salmon Oncorhynchus nerka in British Columbia, Canada, the authors suggest that the size of the population that is producing strays (donor population), the rate of straying from the donor population, and the size of the population receiving the stray (receiving population) can affect the magnitude of potential consequences of strays spawning with non-natal conspecifics. Based on the case study and an evaluation of the literature the authors suggest that the proportion of strays (rate of straying) is likely influenced by genetics, anthropogenic activities, and several environmental factors including conspecific interactions, location of spawning habitat and distance between donor and receiving populations, rearing conditions, water flow, and temperature.
With respect to temperature, the authors suggest that “increased straying may be a form of behavioural thermoregulation whereby migrating individuals seeks thermal refugia in non-natal streams.” Given the warming trend of many rivers and streams within the Pacific salmon range, further increases may result in an increase in stray rate among populations.
Among the plethora of anthropogenic activities that occur within the range of Pacific salmon, the authors identify four that may influence the environmental conditions and or the behaviour response of individuals and thus promote straying: climate-change, hatcheries, transplant stocking, and hydroelectric dams. For example, hatchery reared salmon may be prone to straying due to reduced homing ability as a result of release or rearing practices.
Armed with an understanding of the factors that may promote straying, Bett et al. (2017) then outline the potential consequences for the semelparous Pacific salmon. First, they suggest that straying may be vitally important to small receiving populations if it results in demographic rescue (e.g., addition of new individuals to a population that was not self-sustaining) or through evolutionary rescue (e.g., introduction of a trait such as increased thermal tolerance). The benefits do not come without risk however, including genetic homogenization of the receiving population resulting in the dilution or extirpation of locally adaptive traits, reduced population level fitness, increased competition for spawning sites, or increase to exposure to foreign parasites.
While the authors are unable to provide a unifying theory for straying (e.g., when, how many, or exactly what outcomes to expect), Bett et al (2017) further the discussion by providing a succinct review of the phenomena in Pacific Salmon in the context of a resource management in a changing world.
By Rebecca Dolson-Edge
Eckert, K., L. et al. 1989. Inter-nesting migrations by leatherback sea turtles (Dermochelys coriacea) in the West Indie. Herpetologica 45 (2): 190-194.
Hendry A.P., and S. cC. Stearns, editors. 2004. Evolution illuminated: salmon and their relatives. Oxford University Press, Oxford.
Homola, J.J., et al. 2010. Genetic assessment of straying rates of wild and hatchery reared lake sturgeon (Acipenser fulvescens) in Lake Superior tributaries. Journal of Great Lakes Research 36:798-802.