EVOLUTIONARY ECOLOGY

Decades of research by Alaska Salmon Program scientists has offered numerous insights into the mechanisms that drive variation in sockeye salmon age and phenotype (body size and shape). Environment and habitat play prominent roles, as does the evolutionary effect of bear predation. The following is a brief summary of what ASP scientists have learned from in-depth research since 1946.

The average size of mature salmon is among their more obvious phenotypic traits, and is widely known to vary among populations, along with the age of the salmon. In Wood River salmon populations, salmon spawning in larger rivers are generally larger and older than those spawning in smaller streams. Subsequent sampling found systematic variation in adult size at age as well as age among populations in the Kvichak River system.

There are many benefits to being a large salmon.  Potential reproductive success in females increases with body size – in general, larger females produce more and larger eggs than do smaller females. Thus selection on females would be expected to favor larger and older females. Observations of courtship behavior indicate that larger males, especially those that are deep-bodied for their length, are more likely to be chosen by a female and are also more likely to outcompete other males for a mate or preferred habitat.

However, natural selection also opposes large size and old salmon, in part because each year spent at sea (e.g., 3 vs 2 for most sockeye salmon) is another year when mortality can occur, precluding any reproductive success. In addition, some forms of selection on the spawning grounds favor smaller rather than larger salmon, including the risk of stranding in shallow water in some streams.

The risk of predation by brown bears occurs in many streams, and especially the smaller (narrow and shallower) ones. Bears tend to kill larger than average salmon. In addition, in small streams where salmon are easier to catch, bears tend to kill newly arrived salmon, and this is important because those salmon may not have spawned, and so the predation reduced or eliminated their reproductive success. In contrast, a salmon killed shortly before it would have died of senescence would experience little or no reduction in reproductive success. This selection against salmon that delay reproduction is likely the explanation for the variation in longevity (absent any predation) among populations that differ in predation risk.  Thus age, size at age, and shape of mature sockeye salmon reflect the contrasting patterns of natural and sexual selection.

– Blair et al. 1993. Variation in life history characteristics and morphology of sockeye salmon in the Kvichak River system, Bristol Bay, Alaska. Transactions of the American Fisheries Society 122:550-559. – Carlson et al. 2007. Predation by bears drives senescence in natural populations of salmon. Public Library of Science One 2:e1286. – Carlson and Quinn. 2007. Ten years of varying lake level and selection on size-at-maturity in sockeye salmon. Ecology 88:2620-2629. – Carlson et al. 2009. Does variation in selection imposed by bears drive divergence among populations in the size and shape of sockeye salmon? Evolution 63:1244-1261. – Cunningham et al. 2013a. Selecting for the phenotypic optimum: size-related tradeoffs between mortality risk and reproductive output in female sockeye salmon. Functional Ecology 27:1233-1243. – Cunningham et al. 2013b. Size-selectivity of predation by brown bears depends on the density of their sockeye salmon prey. American Naturalist 181:663-673. – Gende et al. 2004. Brown bears selectively kill salmon with higher energy content but only in habitats that facilitate choice. Oikos 104:518-528. – Mathisen and Gunnerød. 1969. Variance components in the estimation of potential egg deposition of sockeye salmon escapements. Journal of the Fisheries Research Board of Canada 26:655-670. – Quinn and Buck. 2001. Size and sex selective mortality on adult Pacific salmon: bears, gulls and fish out of water. Transactions of the American Fisheries Society 130:995-1005. – Quinn et al. 2017. Diverse foraging opportunities drive the functional response of local and landscape-scale bear predation on Pacific salmon. Oecologia 183:415–429. – Quinn and Foote. 1994. The effects of body size and sexual dimorphism on the reproductive behaviour of sockeye salmon (Oncorhynchus nerka). Animal Behaviour 48:751-761. – Quinn et al. 2001. Balancing natural and sexual selection in sockeye salmon: interactions between body size, reproductive opportunity and vulnerability to predation by bears. Evolutionary Ecology Research 3:917-937. – Quinn et al. 1995. The influence of life history trade-offs and the size of incubation gravels on egg size variation in sockeye salmon (Oncorhynchus nerka). Oikos 74:425-438. – Quinn and Kinnison. 1999. Size-selective and sex-selective predation by brown bears on sockeye salmon. Oecologia 121:273-282. – Quinn et al. 2001. Influence of breeding habitat on bear predation, and age at maturity and sexual dimorphism of sockeye salmon populations. Canadian Journal of Zoology 79:1782-1793. – Rogers 1987. The regulation of age at maturity in Wood River sockeye salmon (Oncorhynchus nerka). Canadian Special Publication of Fisheries and Aquatic Sciences 96:78-89. – Ruggerone et al. 2000. Selective predation by brown bears (Ursus arctos) foraging on spawning sockeye salmon (Oncorhynchus nerka). Canadian Journal of Zoology 78:974-981.