This smells … fishy

Why dead fish follow a hurricane

If you live near the Texas coast, you may have seen dead fish following a hurricane. Lots and lots of dead, smelly fish lining the coast. Or maybe you saw them gulping for air at the surface of the water. Perhaps you wondered if it was pollution that caused these fish to die or struggle to breathe.

Most likely, though, the fish kills were naturally occurring from organic loading causing low dissolved oxygen in the water, simply meaning they can’t breathe. Following a hurricane, especially one with high wind and following a drought, organic material—such as wind-shredded grass, leaves, branches, and displaced stagnant water from swamps—will run off toward the coast. If the water isn’t flushed to the coast quickly, the organic material will begin to decompose. The decomposition depletes the dissolved oxygen, which makes it difficult, if not impossible, for fish to breathe.

Because the water turns very dark, is foamy, and has a sheen to it, people often assume what is killing the fish in the aftermath of a hurricane is oil or chemicals in the water. To determine if it is oil try using a stick to break up the sheen on the water and see how it reacts. If it breaks apart, much like shattered glass, it’s most likely from decomposing organic material, but if it comes back together quickly like oil, it could be pollution.

In addition to seeing how it reacts, TCEQ’s staff pulls a sample of the surface layer and observes it under a microscope to see if the material is from organic decomposition or composed of something else.

Ronnie Hebert and Scott Griffith, biologists from Region 10, first noticed what they call “hurricane effect fish kills” following Hurricane Rita in 2005. There were thousands of dead fish along the coast, ranging from sunfish, catfish, smallmouth buffalo, to the threatened paddlefish. After some research, it became apparent that fish kills are more likely when there is a drought prior to the storm, since more dried-up leaves have fallen to the ground and lower creek levels mean the debris build up in the watershed.

While Hurricane Ike in 2008 had some fish kills, what the team saw was mostly fish being stranded from tidal storm surge. Although the storm had winds similar to Rita, it was only three years later so there wasn’t as much organic material built up in the watershed. Following Hurricane Harvey in 2017 and Tropical Storm Imelda in 2019, there weren’t many fish kills because there was more rain and a lower storm surge that allowed the debris to be carried quickly to the coast. The storms also had lower winds that introduced less organic material to the watershed.

While low dissolved oxygen is the most likely reason for fish kills, a hurricane with a large storm surge can also cause fish and other animals such as snakes and alligators, to be displaced. If a freshwater fish flows into salty water or vice versa and can’t make its way back, the fish could die, which occurred following Hurricane Ike.

The point? Following a hurricane, there may be a surge in dead fish. A sign it’s from low dissolved oxygen is to see whether fish are gasping for air on the surface of the water—this shows there’s not a lot of oxygen in the water. Generally, if it’s from a lack of oxygen, the fish kills show up about a week after the storm, and recovery begins about three weeks later as oxygen begins to rise in the water.

So far, all TCEQ’s investigations into fish kills following hurricanes have pointed to natural causes, not pollution. If you see dead fish after a hurricane, leave them where they are and let nature do its work. It may not smell great, but decomposition in place is nature’s way of recycling.