Question

3. Winter-run steelhead trout live most of the year in the Pacific ocean and migrate up rivers to spawn, much like some salmon species. Winter-run steelhead trout that spawn in Hood River, Oregon, include individuals with multiple life history strategies. Some of these fish are semelparous and others are iteroparous. Iteroparous steelhead produce fewer offspring than semelparous ones in their first year, but can produce offspring for several years. Semelparous steelhead produce more offspring than iteroparous steelhead in a single year. Both of these life history strategies coexist in a more or less stable proportion. If the risk of adult mortality increases in the ocean, which life history (iteroparous or semelparous) would you expect to become more common in the steelhead population? Why?

Answer 1

The semelparous type of life cycle will be favoured when rate of adult mortality increases. It is because to nullify the mortality of adults more number of offsprings are required so that they can maintain the population .

Let's understand with an example:-

Consider the population of trout initially 100. The carrying capicity is around 150. Trouts reproduces and give rise to 200 offsprings and they themselves die. Now the population remains 200. Some of them will die during early phase let consider 10 die due to improper growth condition and birth defects.

Adult mortality is mainly due to predation . As the no. of predators increases the prey count decreases. Consider the adult mortality is around 60 . Then the final population count is around 130. This shows that population is expanding instead of predation and mortality . As the predation increases the trouts will increase the reproductive rate.