This can be explained in accordance with the General Relativity theory of Einstein's which says that the time dilation depends directly on the intensity of gravitational field (for an observer outside the field).
So, as the intensity of the field increases so does the time dilation.
The black hole has highly intensified gravitational field and as such the time dilation near the boundary of the black hole also increases immensely and stops at the event horizon.
Thus the gravity of a black hole is sufficient enough to slow down a clock as one gets nearer to it.
Due to a decrease in the supply of food and other resources for fish population their population will start to decline, some will migrates in the search for food while there may compete for the remaining resources. Impacting their population density and his may lead to the death of fishes, some of which may be eaten by other bigger fish.
