According to the image, the fish underwent sympatric speciation. The new species of fish had mating seasons that were different from that of the original fish. Because of the differences in mating seasons, the fish underwent reproductive isolation. This mode of isolation would be temporal.
Sympatric speciation happens within a population of an organism that gets isolated reproductively due to differences in their mating periods. This time dependent isolation is called temporal isolation. Example, a fish population can split into two if some of the fishes start mating early in the spring while the rest mate late in the autumn. The spring-mating population will not become compatible to mate with the late-autumn-mating population.
