Fishing reshapes ocean ecosystems in ways most people never see, from the seafloor to the top of the food chain. About 35.5 percent of global fish stocks are now harvested at biologically unsustainable levels, according to the most recent FAO assessment. The environmental toll goes well beyond shrinking fish populations: fishing disrupts habitats, releases stored carbon, generates massive plastic pollution, and weakens the ocean’s ability to cope with climate change.
Overfishing and Declining Fish Populations
When fish are caught faster than they can reproduce, populations collapse. That 35.5 percent figure means roughly one in three monitored fish stocks worldwide is being overfished. The remaining 64.5 percent are fished within sustainable limits, but that ratio has been steadily worsening for decades. Some of the most commercially valuable species, including bluefin tuna, Atlantic cod, and several shark species, have experienced the steepest declines.
Sharks face particularly intense pressure. Global shark fishing mortality actually increased from at least 76 million to 80 million individual sharks between 2012 and 2019, roughly 25 million of which were threatened species. That rise happened even as governments around the world adopted new shark protections, highlighting a gap between regulation and what’s happening on the water. Coastal fisheries drove a 4 percent increase in shark deaths, while open-ocean fisheries saw a 7 percent decrease over the same period.
Bycatch: The Unintended Catch
Commercial fishing gear is not selective. Nets, longlines, and trawls routinely capture species that fishers weren’t targeting, a problem known as bycatch. Sea turtles, dolphins, seabirds, rays, and juvenile fish all get caught and often killed in the process. Bycatch is one of the leading threats to endangered marine species because it operates at an enormous scale across every ocean basin, day after day.
The problem doesn’t stop when gear is hauled in. Lost or abandoned fishing equipment, often called ghost gear, keeps killing long after it’s forgotten. Discarded nets, traps, and lines drift through the ocean for years, entangling anything that swims into them. An estimated 136,000 seals, sea lions, and large whales die from ghost gear every year. One U.S. study found that roughly 4,500 recovered ghost nets had killed upwards of 2.5 million marine invertebrates, 800,000 fish, and 20,000 seabirds during the time they sat in the water.
Plastic Pollution From Fishing Gear
Fishing is a major, often overlooked source of ocean plastic. In some parts of the ocean, fishing gear accounts for 50 to 100 percent of the plastic debris found. Modern nets, ropes, and traps are made from synthetic materials that don’t biodegrade. They fragment into microplastics over time, entering the food chain as small organisms ingest them. Unlike consumer plastic waste, which gets the most public attention, fishing-related plastic is designed to trap and entangle marine life, making it disproportionately harmful even before it breaks down.
Seafloor Damage and Carbon Release
Bottom trawling, where heavy nets are dragged across the ocean floor, is one of the most physically destructive fishing methods. It flattens complex seafloor habitats like coral reefs, sponge gardens, and seagrass beds that serve as nurseries and shelter for thousands of species. Recovery after intense trawling is slow, and some habitats never fully return to their original state.
There’s a climate dimension too. Ocean sediments store enormous amounts of organic carbon, accumulated over centuries. When trawl nets churn up these sediments, the stored carbon gets released back into the water and eventually the atmosphere. Research published in Nature Geoscience estimated that bottom trawling remineralizes 0.16 to 0.4 billion tons of sedimentary organic carbon globally each year. That translates to 0.58 to 1.47 billion tons of CO2 escaping from disturbed sediments, a footprint comparable to the annual emissions of some entire countries. Heavily trawled areas show a clear net loss of stored carbon, though the research also found that when trawling stops, carbon stocks can recover over time.
Food Web Disruption
Removing large predatory fish sets off chain reactions that ripple through entire ecosystems. Ecologists call these trophic cascades: when a top predator disappears, the species it used to eat explode in number, and that overpopulation suppresses species further down the chain.
The collapse of Atlantic cod off Canada’s Scotian Shelf is one of the best-documented examples. As cod vanished from overfishing, populations of their prey, including lobsters, snow crabs, shrimp, and herring, surged. The herring boom drove down zooplankton (the tiny animals that float in the water column), which in turn allowed phytoplankton (microscopic algae) to bloom unchecked. Meanwhile, without cod to control them, sea urchin populations exploded and grazed down kelp forests along the coast. One species removed at the top, and the entire structure of the ecosystem shifted.
These cascades don’t just rearrange species. They can fundamentally alter how an ecosystem functions, changing nutrient cycling, water clarity, and the productivity of the whole system in ways that are difficult to reverse.
Weakened Resilience to Climate Change
Overfishing doesn’t just harm ecosystems directly. It also makes them more vulnerable to warming oceans and acidification. Coral reefs illustrate this clearly. Healthy reefs depend on herbivorous fish like parrotfish and surgeonfish to graze algae and keep it from smothering coral. When those grazers are overfished, algae gains the upper hand.
Research modeling reef dynamics found that reducing grazing activity from 60 percent to 40 percent (simulating moderate overfishing of herbivores) significantly lowered coral resilience as CO2 levels rose. Dropping grazing to 30 percent exceeded the threshold for any coral dominance above 400 parts per million of atmospheric CO2, a level the planet has already passed. At high CO2 concentrations (700 to 1,000 ppm) combined with heavy overfishing of grazers, models projected near-complete coral loss with macroalgae covering up to 50 percent of the reef.
The takeaway is that warming, acidification, and overfishing all push reef ecosystems in the same direction. Reefs already stressed by fishing pressure are far less capable of withstanding the additional stress of climate change. This interaction works both ways: climate change makes overfished ecosystems less likely to recover, and overfishing makes climate impacts worse.
What Happens When Fishing Pressure Drops
The good news is that marine ecosystems can bounce back when given the chance. Fully protected marine areas, where no fishing is allowed, show an average 58.2 percent increase in fish biomass compared to unprotected waters. Even areas with partial protections that allow some regulated fishing still see meaningful gains, with about a 12.6 percent biomass increase on average.
Seafloor habitats also recover when bottom trawling ceases, though the timeline varies. Carbon stocks in sediments rebuild gradually, and bottom-dwelling communities repopulate, but these processes happen on different schedules. Benthic organisms may return within years, while full carbon recovery in sediments takes considerably longer. The evidence consistently shows that reducing fishing intensity, especially in the most heavily exploited areas, produces measurable environmental benefits. The ocean is resilient, but only if given enough space and time to heal.