Orca predation and salmon body size

A study examines how orca whales influence phenotypic traits of Chinook salmon. Large marine predators influence food webs and prey populations. However, the degree to which recovered predator populations affect the phenotypic traits of their prey is unclear. To examine how growing orca populations influence the body size of Chinook salmon, Jan Ohlberger and colleagues developed an age-based and size-based population model for Chinook salmon. The model simulations accounted for historical fishery harvest rates of salmon and the increase of fish-eating orca whales over the past 50 years. The authors found that increased orca populations that selectively feed on large salmon have influenced the body size and age at maturity of Chinook salmon to a greater extent than fisheries, given that fishery harvest rates have declined while predation has increased. Orca predation has contributed to more rapid growth, earlier maturation, and an approximate 9% decrease in body size in Chinook salmon. Orca predation has also disproportionately affected larger, older fish, which experienced the highest declines in average body size among the fish in Chinook salmon populations. The authors also determined that small marine predators, such as sea lions, did not affect the body size of Chinook salmon, given that they consume fewer adults compared with orcas. The findings suggest the need to reconcile marine mammal conservation efforts with management of Chinook salmon populations, according to the authors.

Article #19-10930: “Resurgence of an apex marine predator and the decline in prey body size,” by Jan Ohlberger, Daniel E. Schindler, Eric J. Ward, Timothy E. Walsworth, and Timothy E. Essington.

MEDIA CONTACT: Jan Ohlberger, University of Washington, Seattle, WA; tel: 206-999-0955; email: <

[email protected]

>; Daniel Schindler, University of Washington, Seattle, WA; tel: 206-794-1743; email:

[email protected]

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