Thousands of Dead Fish Are Washing Up Across Florida - Here's Why

Huge numbers of dead fish have washed up en-masse in Florida this week in two separate locations on either side of the state

Residents of Bayshore Boulevard in Tampa reported hundreds of dead fish floating in the waters of Hillsborough Bay on Tuesday, while on the east coast of the state, thousands of fish floated dead in the Indian River Lagoon and off the shores of Indialantic and Melbourne Beach.

The Florida Fish and Wildlife Conservation Commission tested the waters in Hillsborough Bay and found that the oxygen levels in the waters were lower than usual, and there was a non-toxic algae present. In Indian River Lagoon, an algal bloom was detected in the past week, which also lowered oxygen levels in the water, according to Florida Today.

Low oxygen levels in the water can lead to mass kills of fish and other marine creatures due to there simply not being enough oxygen dissolved to keep them all alive.

These low-oxygen events can be triggered by algal blooms (including red tides) which use up all the oxygen in the water, as well as release toxins that may harm or kill wildlife. Experts think that this is part of what drove the mass fish kill off Tampa.

"This will most certainly have been caused by the huge dead zone off that coastline," Ian Hendy, a senior scientific officer at the Institute of Marine Sciences of the University of Portsmouth, told Newsweek. "The dead zone sits off the coast where the Mississippi pumps out vast amounts of agricultural run-off, and nutrients—eutrophication. These excess nutrients can cause harmful algal blooms (HABs).

"This is a bust or boom scenario whereby blue-green algae, for example, will proliferate and utilize all of the available dissolved oxygen. This process creates very low oxygen levels in the water, making life for aquatic organisms such as fish and invertebrates very dangerous," he said.

It can cause "mass mortalities," Hendy added, also noting that the Gulf of Mexico dead zone is reportedly the size of New Jersey.

In the absence of an algal bloom, sometimes oxygen is sapped from an area due to organic matter being present.

"In a nutshell, these events are usually caused by quantities of organic matter being in the waterway. Organic matter can come from leaves and debris, following heavy rainfall, a spill, or from industrial or municipal discharge, including sewage discharge," Stuart Khan, a professor of environmental engineering at UNSW Sydney, told Newsweek.

"Bacteria consume ('biodegrade') the organic matter and, as they do that, they also consume oxygen from the water. Without that dissolved oxygen, the fish will suffocate. We sometimes give various names to these events, including a blackwater event, deoxygenation event, or anoxia."

This is what happened in the Darling River in Australia earlier this year, where millions of dead fish were found floating on the surface after receding floodwaters washed tons of dead organic matter into the waterway.

The exact causes of the mass fish kills on both sides of the Florida coast are yet to be determined, however, and are being investigated.

Algal blooms that drive low-oxygen events are heavily influenced by agricultural run-off, which will need to be reduced or prevented from entering the oceans if these events are to be avoided.

"To mitigate such impacts, we have to consider the whole seascape, and the impacts (pinch points) on the land, which can often be many hundreds of kilometers away. For example, there is no point in trying to improve the water quality in the dead zone if the Mississippi River continues to pump out extreme and toxic levels of nutrients," Hendy said.

"Thus, agricultural practices must be addressed (e.g. turn to regenerative sustainable farming) and improvements to buffer zones and creation of more inland wetlands and riparian zones. The regen farming will help reduce the current nutrient loading and levels, the buffer zones, wetlands and riparian systems will help bioremediate.

"Then, for example, greater care of the Gulf coastline to ensure rewilding of key habitats to include seagrass and mangroves—these will also help bioremediate."

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