Oceans, overfishing, and bycatch

The ocean is the part of the food system that most diets treat as a free resource. It occupies no farmland, appears on no deforestation map, and is often pitched as a lower-impact protein source than terrestrial meat. The empirical picture is narrower than that framing allows. Industrial fishing has reshaped marine ecosystems at continental scale over the past half-century, and its pressures compound with nutrient runoff and climate change in ways that no single policy lever is closing.

Stock depletion: the SOFIA trend line

The Food and Agriculture Organization’s biennial State of World Fisheries and Aquaculture (SOFIA) is the canonical global stocktake. The 2022 edition reported that 35.4% of assessed marine fish stocks were being fished at biologically unsustainable levels in 2019, up from 10% in 1974, while the share fished within biologically sustainable levels fell from 90% to 64.6% over the same period (FAO SOFIA, 2022). Of the stocks still classified as sustainable, the overwhelming majority are “maximally sustainably fished” — that is, operating at or near the edge rather than with headroom.

These figures rely on assessed stocks, which skew toward data-rich fisheries in wealthier jurisdictions. Pauly & Zeller (2016) reconstructed global catches using a country-by-country approach that included small-scale, subsistence, recreational, and discarded catches routinely missing from FAO submissions. Their reconstruction put peak global catch at around 130 million tonnes in 1996 — roughly 50% higher than the officially reported figure — and showed catches declining three times faster than FAO data implied. The gap is not a rounding error. It is the difference between a plateau and a drawdown.

Bottom trawling: the seabed as clearcut

Much of what is caught is caught by dragging heavy gear across the seafloor. Watling & Norse (1998) made the comparison that has stuck in the literature: mobile bottom fishing gear disturbs an area of seabed each year roughly 150 times larger than the area of forest clearcut globally, with impacts analogous to clearcutting on slow-recovering benthic communities. Coral gardens, sponge fields, cold-water reefs, and seagrass meadows — structurally complex, long-lived, and nursery-critical — are flattened by repeated passes of otter boards and rockhopper gear.

Sala et al. (2021) added a climate dimension to the seabed story. Global bottom trawling, they estimated, resuspends sedimentary organic carbon at a scale comparable to aviation emissions, releasing on the order of a gigaton of aqueous CO2 each year from previously undisturbed stores. Seabed sediments are the largest long-term carbon reservoir on Earth’s surface. Trawling is, in climate terms, a slow-motion unsequestration.

Bycatch: the non-target kill

Every targeted catch is accompanied by species the gear was not aimed at. Gilman et al. (2019) synthesized the global bycatch literature and showed that a large share of elasmobranchs (sharks, rays, skates), sea turtles, seabirds, and marine mammals taken at sea are incidental rather than targeted, and that single-species mitigation measures routinely shift the burden onto other taxa — a “robbing Peter to pay Paul” pattern that piecemeal management entrenches rather than resolves.

Oliver et al. (2015) focused on shark and ray bycatch in longline and gillnet fisheries and documented that these fisheries kill tens of millions of sharks per year, many of species already classified as threatened or near-threatened on the IUCN Red List. Longline tuna and swordfish fleets were the largest single source. Because sharks and rays grow slowly, mature late, and produce few young, even modest incidental mortality rates outstrip population replacement.

Marine mammals and seabirds follow a similar pattern. Globally, hundreds of thousands of cetaceans are estimated to die in gillnets and trawls each year, and albatross populations have been driven toward collapse in the Southern Ocean by longline interactions. Worm et al. (2006) projected that, on business-as-usual trajectories, the services provided by marine biodiversity — fish supply, water filtration, nursery habitat, coastal protection — would be severely compromised by mid-century.

Ghost gear: fishing that never stops

Not all fishing effort ends when a vessel leaves the grounds. Macfadyen, Huntington & Cappell (2009), in the FAO/UNEP reference study on abandoned, lost or otherwise discarded fishing gear (ALDFG), estimated that around 640,000 tonnes of fishing gear — approximately 10% of all marine debris by weight — enters the ocean each year and continues to entangle and kill non-target animals for years or decades. Lost gillnets and traps “ghost fish” most efficiently; lost longlines and trawl panels add to entanglement risk.

Ghost gear is a case where the externality and the solution are both structural. Gear marking, port-state reporting, buy-back programmes, and biodegradable panels all reduce the stock of ALDFG, but the underlying driver — fleet size and effort in poorly monitored waters — is the same variable that drives overfishing itself.

Dead zones: the terrestrial footprint at sea

The ocean also absorbs pressure that did not originate in it. Diaz & Rosenberg (2008) catalogued more than 400 hypoxic “dead zones” in coastal waters worldwide — a number that had roughly doubled each decade since the 1960s — driven mainly by nutrient loading from agriculture, sewage, and atmospheric deposition. The recurring Gulf of Mexico dead zone, fed by nitrogen runoff from the Mississippi basin (much of it from maize and soy grown for animal feed), typically spans an area the size of New Jersey each summer. The Baltic, Chesapeake, and East China Sea exhibit similar seasonal or persistent hypoxia.

Hypoxic waters exclude most mobile fauna, collapse benthic communities, and shift microbial cycling toward nitrous oxide and hydrogen sulfide production. The dead-zone map is, in effect, a downstream image of the global corn–soy–livestock complex.

Aquaculture: the footprint that moved, not shrank

Aquaculture now supplies more than half of the fish eaten by humans (FAO SOFIA, 2022). The common assumption is that this relieves pressure on wild stocks. For herbivorous species — carp, tilapia, bivalves, seaweeds — that is largely true. For carnivorous species, it is not. Salmon, shrimp, tuna, and many marine finfish are fed diets built around fishmeal and fish oil derived from wild-caught forage fish: anchoveta, menhaden, sardines, herring, krill.

Cashion et al. (2017) quantified the trade-off. Of the roughly 20 million tonnes of wild fish routed into fishmeal and fish-oil production each year, about 90% — by their analysis — is food-grade fish that could otherwise be eaten directly by humans. The fish-in/fish-out ratios for farmed salmon have improved over time, but the absolute demand for forage fish remains large, and it concentrates extraction on a small number of low-trophic-level stocks whose removal destabilizes the seabirds, marine mammals, and larger fish that depend on them.

Coastal aquaculture adds its own footprint: mangrove clearing for shrimp ponds across Southeast Asia and Latin America, nutrient and pharmaceutical effluent from open-net salmon pens, and sea-lice and pathogen pressure on wild stocks migrating past farms.

Climate interactions

Ocean warming, acidification, and deoxygenation are rearranging the baseline on which all of the above pressures operate. Stocks are shifting poleward, oxygen minimum zones are expanding, and the metabolic envelope of many commercial species is narrowing. Fisheries management systems calibrated to twentieth-century distributions are chasing moving targets, and extraction pressure applied to a climate-stressed stock is not equivalent to the same pressure applied to a pre-industrial one. The climate and fishing stressors are not additive; they are multiplicative.

What the picture implies

The dietary implication is narrower and harder than the marketing suggests. “Sustainable seafood” labels can distinguish between well-managed and badly-managed fisheries at the margin, but they do not resolve the system-level facts: a third of assessed stocks overfished, catches declining faster than reported, bottom habitats being flattened, bycatch removing slow-growing species faster than they can recover, dead zones expanding in coastal seas, and aquaculture’s carnivorous segment routing wild fish through an inefficient feed loop. Reducing or eliminating fish and other sea-animal products in a diet is the most direct way to lower one’s share of that pressure — and the one lever that does not depend on correctly reading an ecolabel.

The ocean has been treated, implicitly, as infinite. The last seventy years of SOFIA trend lines are the record of what happens when that assumption meets industrial capacity.