Older fish get caught first, and younger fish react by maturing faster. As these symptoms increase the number of fish caught remains the same, because the younger fish are reproducing at an earlier age. The number of fish caught remains the same until the population does not have any fish old enough to reproduce. When this finally happens, the population level drops dramatically. The depletion of one species affects the entire food chain.
Bigger fish depend on smaller fish for food.
Without a stable source of food, the larger fish suffer. Populations of smaller fish will increase, because they lack competition for the medium sized fish. Changes in those population can, in turn, effect other species all together. Over the past 10 years a number of fish stocks have collapsed. These include the Peruvian anchovy, Alaskan pollack, North Sea cod, South African anchovy, Alaska king crab, and California sardine.