Exercise 11.2

How Does Bear Predation Affect Senescence of Salmon?

(This exercise is based on Carlson, S.M., R. Hilborn, A. P. Hendry, T. P. Quinn. 2007. Predation by Bears Drives Senescence in Natural Populations of Salmon. PLoS ONE 2(12): e1286.)

(Note: The reference above links directly to the article on the journal’s website. In order to access the full text of the article, you may need to be on your institution’s network [or logged in remotely], so that you can use your institution’s access privileges.)


Classical evolutionary theory predicts that higher predation levels will lead to the evolution of faster rates of senescence. The reasoning for this comes from the fundamental foundation for the evolutionary theory of senescence: mutations that affect survival at an early age have a larger fitness consequence than those with similarly sized effects at later ages. With higher predation-associated mortality, the difference in fitness consequences of early- versus late-acting mutations is enhanced. Thus, senescence should proceed at earlier ages for populations that have evolved under high predation regimes.

A study by Stephanie Carlson and her colleagues of the effect of bear predation on salmon senescence demonstrates that this classical theory may be overly simplistic: while predation by bears does affect how salmon senesce, that senescence is determined by the nature of bear predation and not just by its intensity. Specifically, the average age of the salmon preyed on by the bears affects the rate by which the salmon senesce. Bears usually prefer to catch salmon that are young and healthy, because the flesh of these fish is more energy dense. Young, healthy, energy-dense fish, however, are more difficult to catch, especially in larger and more complex water systems.

Carlson’s group examined natural populations of sockeye salmon (Oncorhynchus nerka) and brown bears (Ursus arctos) in a series of creek systems in Alaska. Carlson’s group was able to assess whether salmon had died from bear predation (bear-killed), age-related deterioration (senescent), or other causes (other).


Use the information in Table 1 to answer questions 1 and 2. (Click on the table to enlarge it.)


Table 1 Some properties of the six streams and populations. The “other” mode of death includes individuals that died as a result of gull predation or that were stranded in areas of low water. The number of censored individuals in a given creek is equal to the number of individuals that did not die of senescence (i.e., the number of individuals killed by bears plus the number of individuals dying from other causes plus the number of fish still alive at the end of the study).


Question 1. Assuming that the streams are rectangular tubes, calculate the area (width × depth) of a cross-section of each creek at any given point along its length. Which stream is largest? Which is smallest?


Question 2. In which creek was the average age of fish killed by bears the highest? How does this correspond with the age of fish in this creek that died due to age-related deterioration?

Use the information in Table 2 to answer questions 3 through 5. (Click on the table to enlarge it.)



Table 2 Creek temperature with predation rates.  


Question 3. Which creeks have rates of bear predation above 75 percent?


Question 4. Which creeks have rates of bear predation below 40 percent?


Question 5. In which two creeks did bears show the greatest preference for newly arrived fish?

Use the information in Figure 1 to answer questions 6 and 7.


Figure 1 Population-specific predator selectivity for fish of different in-stream ages.


Question 6. The graphs show age of fish on the horizontal axis and bear predation rate on the vertical axis. In which creek(s) did bear predation rate decline with age?


Question 7. In terms of creek size, how do these creeks with declining predation compare with the others?

Use the information in Figure 2 to answer questions 8 through 11.


Figure 2 Rate of aging. The researchers calculated the rate at which death due to age-related deterioration (senescence) increased for each of the six creeks. They then plotted the rate of senescence against the absolute rate of bear predation and bear selectivity for young salmon.


Question 8. In which creek were the salmon with the highest rate of senescence? The lowest?


Question 9. Which seems to have a greater effect on the rate of salmon senescence, the absolute rate of bear senescence or the selectivity of bears for young salmon? Explain.


Question 10. How does the rate of senescence correspond with the creek size?


Question 11. There is still a concern that the responses in salmon senescence rates were not evolutionary changes but rather due to phenotypic plasticity. What type of test would be best for distinguishing between these possibilities?