Bears control age of salmon

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The rate of predation by brown bears (Ursus arctos) on sockeye salmon (Oncorhynchus nerka) drives the aging rate of the salmon, a recent study by scientists from the University of Washington and McGill University found.

Stephanie Carlson, Ray Hilborn, Andrew Hendry and Thomas Quinn compared senescence (aging) rates among natural populations of sockeye salmon in southwest Alaska subject to varying degrees of predation by brown bears.

Sockeye salmon, like other Pacific salmon species, show marked senescence in the form of rapid physical deterioration from the time they start breeding until the time they die several weeks later.

There are two variables involved with regard to the predation on salmon by bears: predation rate and predator selectivity.

The authors write: With regard to predation rate, bears kill up to 89% of the salmon breeding in some creeks but only 10% of those breeding in other creeks...With regard to predator selectivity, bears generally prefer salmon showing little senescence because these fish have the highest energy density...The problem for bears is that these fresh fish are more vigorous and therefore more difficult for bears to catch.

As a result, bears in small streams, where salmon are easy to catch, tend to kill salmon showing little senescence (i.e., newly arrived, energy-rich salmon at the beginning of their breeding lives)...In contrast, bears in larger and more complex streams, where salmon are more difficult to catch, tend to kill salmon showing advanced senescence (i.e., energy-poor salmon at the end of their breeding lives).

The authors used a series of mathematical models to test if the variation in senescence rate in salmon is best explained by predation rate or by the predator selectivity.

They found that predator selectivity was the prime driving force behind the senescence rate of salmon.

In populations where older, more decrepit fish were selectively taken, the senescence rate of salmon was found to be slower (i.e. there is selection for individuals to show less senescence at any given age).

This finding goes against the coventional notion that senesence evolves in response to the number of individuals killed by predation and not to the condition of the individuals.

For more information, see the paper: Carlson SM, R Hilborn, AP Hendry and TP Quinn (2007) Predation by bears drives senescence in natural populations of salmon. PLoS ONE 2: e1286. doi:10.1371/journal.pone.0001286