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Why can fish stocks decline when fishing removes large sexually mature fish?

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A worker in dark clothing inspects large frozen tuna lined up on the floor at Tsukiji fish market in Tokyo, Japan, using a tool to check the fish. — commercial fishing Worker in the Tsukiji fish market, Tokyo.

Fish stocks decline when large, sexually mature fish are removed because these individuals are critical to the reproductive success and long-term sustainability of the population.

In many species, larger and older fish are more fecund, meaning they produce a greater number of eggs or higher-quality offspring compared to younger, smaller individuals. By disproportionately harvesting these mature fish, commercial fishing reduces the overall reproductive capacity of the stock. Furthermore, because many fish species rely on external fertilization—where eggs and sperm are released into the water—the removal of mature adults can disrupt the density and timing necessary for successful spawning.1

When fishing pressure exceeds the rate at which a population can replenish itself, it leads to overfishing and potential stock collapse. This is exacerbated by the fact that many modern fishing technologies are highly efficient at targeting specific sizes or aggregations of fish. The loss of these mature individuals not only diminishes the immediate spawning potential but can also alter the demographic structure of the population, making it less resilient to environmental changes and further threatening the health of marine food webs.2