Scientists recently surveyed the bottom of the Gulf of Mexico around Louisiana and Texas and what they discovered was a larger-than-average area of oxygen-depleted water – a “dead zone” where nothing can live.

National Oceanographic and Atmospheric Administration (NOAA) scientists announced their findings this week: about 4m acres of habitat in the Gulf are unusable for fish and bottom-dwelling species. The researchers had estimated a smaller dead zone this year, predicting an average-sized area.

“The distribution of the low dissolved oxygen was unusual this summer,” Nancy Rabalais, the professor at Louisiana State University who led the study, said in a statement. “The low oxygen conditions were very close to shore with many observations showing an almost complete lack of oxygen.”

But the Gulf isn’t the only coastal region experiencing dead zones this summer.

The waters off Oregon have had hypoxic areas every year since 2002. But this was a record year in Oregon as well: the dead zone emerged earlier this year than in the past 35 years.

Dead zones develop when fertilizers and nutrients from farmland drain into oceans or lakes, creating an algae bonanza that eventually dies and decomposes. As the algae decomposes, it depletes the waters of oxygen, suffocating species that live in the area.

Studies show that fish in hypoxic waters change what they eat, which affects what people can catch. Dead zones also make commercially important species like shrimp less available in the Gulf and kills fish and crabs off the coast of the Pacific north-west.

The fertilizer pollution has caused an estimated $2.4bn in damage to fisheries and marine habitat every year since 1980, the Union of Concerned Scientists said in a study released last summer.

In Oregon, the global climate crisis is making the problem worse: warmer waters hold less oxygen than cold waters, encouraging the growth of dead zones. In addition, as more carbon is absorbed into the oceans, the waters become more acidic – in turn making it harder for creatures like shellfish and crabs to grow their shells.

All this amounts to “a double whammy from the atmosphere and the ocean”, NOAA researcher Richard Feely told the Washington Post.

This year, crab fishers have described finding the carcasses of hundreds of Dungeness crabs along the shores of Washington and Oregon.

In 2001, a taskforce of state and federal agencies set a goal of keeping the dead zone’s five-year average to no greater than 1,900 sq miles. This summer’s dead zone is about three times larger than that. NOAA has also created a tool – the runoff risk forecast – to help farmers apply fertilizer at optimum times to ensure it stays on fields, with the hopes of limiting nutrient runoff to the Gulf.

Some say these actions don’t go far enough. “Without a significant, concentrated effort to reduce nitrogen runoff from farms and livestock operations, Gulf Coast communities will continue to bear the costs of the dead zone,” said Rebecca Boehm, an economist with the Union of Concerned Scientists food and environment program in a statement.

“The dead zone has not meaningfully shrunk in the last 30 years, and we are no closer to the goals set by the Hypoxia Task Force. Policymakers need to rethink their strategy, or we will find ourselves back here next year with the same bad news.”