Impacts of Overfishing

Every fish population has a maximum sustainable yield, the largest catch you can take year after year without shrinking the population. Push past that line and the stock starts shrinking faster than it can replace itself. Fishers often respond by fishing harder to maintain their catch, which removes even more of the breeding adults. Fewer breeders means fewer offspring, which means even smaller catches next year. Once the population drops low enough, the remaining fish may be too sparse to find mates or too few to rebound even if fishing stops completely. That is why the Grand Banks cod, closed to fishing in 1992, never recovered: the spawning stock had been gutted below the density it needed to bounce back.

The gear makes things worse. Trawling drags heavy nets across the seafloor, crushing coral and bottom habitat that took decades to form. Drift nets hang like curtains in open water and snag anything that swims into them, dolphins, sea turtles, sharks. Longlines bait thousands of hooks across miles of ocean and catch seabirds and juvenile fish along with the target species. This unintended catch is called bycatch, and it strips species out of the ecosystem that were never meant to be harvested at all.

Removing the biggest fish first hits food webs especially hard. On a reef, large predatory fish keep mid-level fish populations in check. Take the predators out and those mid-level fish explode in number, overgrazing the herbivores that normally eat algae off the coral. Algae takes over, coral smothers, and the whole reef community shifts. This chain reaction, a trophic cascade, connects overfishing directly to the broader theme of unit five: removing one resource at an unsustainable rate destabilizes everything built on top of it.