An ecologist wants to know whether the fish population of a large lake is increasing or declining over the years. Counting every fish in the lake is not possible.
Which of the following would serve the purpose?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → The number of fish caught per trap each year
A population is not always counted one by one. When it is huge and counting is impossible, ecologists use other measures. For some studies the absolute size is not needed at all; a relative density is enough, as long as it goes up and down with the real population. For fish in a lake, the book gives the number of fish caught per trap as a good enough measure.
Option (1) The number of fish species recorded in the lake each year — Incorrect. The number of species tells how many kinds of fish live in the lake. It does not show whether the number of fish of a population is rising or falling.
Option (2) The per cent cover of aquatic plants in the lake each year — Incorrect. Per cent cover is used for plants such as a huge banyan tree, where a count would mislead. Cover of aquatic plants says nothing about the fish population.
Option (3) The number of eggs laid by one female fish each year — Incorrect. Eggs laid by one female show her reproductive output, not the size of the whole fish population.
Option (4) The number of fish caught per trap each year — Correct. The catch per trap is a relative density. If it rises from year to year, the fish population is rising, and if it falls, the population is falling.
NCERT Extract:
Population Attributes – “… Total number is again not an easily adoptable measure if the population is huge and counting is impossible or very time-consuming. … Sometimes, for certain ecological investigations, there is no need to know the absolute population densities; relative densities serve the purpose equally well. For instance, the number of fish caught per trap is good enough measure of its total population density in the lake.”