2. The semelparous fish species, Oncorhynchus bistwobe, is a new major target of

Question

2. The semelparous fish species, Oncorhynchus bistwobe, is a new major target of fishing. Most of these fish mature at five years of age and produce on average 1000 eggs when they spawn. Some individuals, however, mature at four years of age and produce on average 500 eggs when they spawn. If individuals are only harvested between ages 4 and 5, what level of harvest mortality will result in natural selection favoring the earlier-spawning individuals? Assume in the absence of fishing, all individuals of age 4 survive to age 5.

Explanation / Answer

As inviduals surviving 5 years lays 1000 eggs and individuals maturing at 4 years lays 500 eggs, theoretically greater than 50% mortality of these fish between ages 4-5 will result in natural selection favouring the earlier spawning individuals

Mathematically explained as

Let us assume each year is directly proportional to 50% mortality

Then start with 1000 indivuals 0-1 year -500 individuals survived

     1-2 year -250 individuals survived

      2-3 year -125 individuals survived

      3-4 year -62 individuals survived

      4-5 year -31 individuals survived

Now 4th year mature individuals produce 500 eggs each, 62x500 = 31000 eggs

Now 5th year mature individuals produce 1000 eggs each, 31x1000 = 31000 eggs

Despite of individual’s egg number is same hence > 50% mortality will favor earlier spawning fish (that is at nearly equal to 4 years)

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