Contains the full lesson along with a supporting toolkit, including teachers’ notes.
Balancing conflicting requirements
A population of fish will remain stable if the rate of harvesting and predation is equal to the rate of reproduction. This is true regardless of the size of the population; a small population can sustain only a low level of harvesting. It follows that the greatest rate of harvesting (i.e. the maximum sustainable yield) can be achieved when the population growth rate is a maximum.
When the population is small the growth rate is also small. As the population increases, so does its rate of growth. However, the limited supply of food reduces the growth rate. When the population is as big as the food supply can sustain, further growth leads to food shortage and a decrease in the population. At the maximum carrying capacity the growth rate is zero.
It turns out that the maximum growth rate typically occurs at 50% of the carrying capacity. This is the level which produces the maximum sustainable yield.
It is therefore important to keep fish populations as large as their environment can sustain in terms of food. Once the population falls below that level, the rate of harvesting must be reduced proportionately — otherwise the population will continue to decline, with no chance of recovery. In fact, harvesting should be reduced further to allow stocks to recover.
The wrong response
If the population of a particular species drops significantly then it becomes increasingly difficult to catch the amount of fish that was previously consumed. As the catch decreases the price tends to increase and this can encourage the fishing industry to expend more effort in order to increase the catch. This response only accelerates the decline of the fish stock. To restore the fish numbers the fishing effort must be reduced until the population returns to the maximum growth rate.
Increased ‘fishing effort’
As the world’s population increases so does the demand for food, including fish. In order to meet this growing need, the number and size of fishing vessels increased dramatically after 1950. In the EU the fishing effort is now ten times bigger than it was in 1950 but the catch is about five times bigger. In other words, the increase in the catch is only half as large as the increase in ‘fishing effort’. A rough, but simple, measure of fishing effort is the total amount of fuel used by the fishing vessels each year.