Which change in a gene pool is occurring in the image below?

A large net is full of fish. Some of the fish are small and silver in color, and a majority are larger and darker in color.
gene mutation
gene flow
genetic drift
genetic resistance

Genetic drift is the change in the gene pool

Explanation:

Both micro and macro evolution depends on processes like natural selection, genetic drift, speciation etc.  

Genetic drift is the variations caused in the gene pool due to increase or decrease of alleles over time.

The variations in the phenotypes or traits of the fishes in the net are due to evolutionary adaptations developed over generations. The advantage of a dark color in differential reproduction and survival would have made to pass the dark color to the offsprings in each generation by making it the dominant allele.

Dark color helps the fishes to hide from their predators and also to attract their mates. Due to these reasons, the gene frequency of dark color becomes high and will cause an overall genetic drift in that species of fish.