Dead fish washed up on Texas beaches in the tens of thousands over the weekend, creating a biohazard zone of rotting carcasses for over forty miles.

The phenomenon is known as a “fish kill,” and they’re becoming more common as sea conditions alter with climate change. This one, according to the Texas Parks and Wildlife Department, was due to low levels of dissolved oxygen in the water of the Gulf Coast. The dead fish drowned, in other words. And it’s common when temperatures increase sharply.

Dissolved oxygen levels increase with photosynthesis, the process by which plants transform sunlight, water and carbon dioxide into oxygen. When there is less sunlight, photosynthesis slows and at night it stops. But plants and animals in the water continue to consume oxygen at the same rate, decreasing the concentration, Texas Park and Wildlife officials said. When warming temperatures have caused an algae bloom, there are too many plants consuming oxygen to leave enough for the fish.

The parks department warned beachgoers away from all beaches in Brazoria County, south of Houston. That much rotting fish made the beaches a cesspit of bacteria and potential disease.

Clean-up efforts were put into effect quickly, and by Sunday evening, the county’s most popular beaches were as clear of dead fish as tractors and rakes could make them.

Texas has a Kills and Spills Team (KAST) which has investigated fish kills since 1958. Since 1970, they have seen a sharp increase in the number and scale of fish kills. According to their research, approximately 52% of fish kills are the result of human activities, and 37% more are due to severe weather events, which are being increased by human-influenced climate change.

Texas is currently heading into a heat wave which could see temperatures rise as high as 120 in southern counties like Brazoria.

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