In 2018, hurricane Michael churned through the Gulf of Mexico before making landfall near Florida’s Apalachicola River, leaving a trail of devastation in its wake. On land, the path was easy to follow, but debris and infrastructure failures lowered the river’s water quality and killed roughly half of the gulf sturgeon population. A study conducted in collaboration with the US Fish and Wildlife Service by researchers at the University of Georgia reveals new details about how a decrease in oxygen levels affected the river’s ability to sustain life in the days following the historic Category 5 storm.
The findings were published in the Transactions of the American Fisheries Society in December. The data supporting the findings was made possible by a long-term project monitoring gulf sturgeon in the Apalachicola River. Fish with special tags were still being tracked by equipment that survived the storm, while weather stations and tide gauges reported statistics like temperature, water flow, and oxygen levels.
As a result, scientists at the University of Georgia Warnell School of Forestry and Natural Resources have a precise timeline of what happened with the river’s oxygen levels, as well as the fate of sturgeon that were able to escape up the river or into the Gulf of Mexico.
“We had a couple different metrics for ‘before’ adult population estimates, and there’s also monitoring for other [sturgeon] in all the rivers nearby, so we would know if they went into other rivers,” said Adam Fox, an assistant research scientist at Warnell and co-author of the study. “So, all of that gave us information that we saw a decline of 36% to 60% of the adult fish compared to pre-storm estimates.”
This is obviously very concerning, especially because hurricanes are supposed to increase in frequency and intensity with climate change.
Adam Fox
The sturgeon is a federally protected species. The fish, which lives in both the Gulf of Mexico and nearby rivers, used to have a range that stretched from Tampa Bay to New Orleans. Today, only seven rivers have small populations, with the Suwanee and Apalachicola rivers having the largest.
On October 10, 2018, Hurricane Michael made landfall. Data collected in the following days revealed that fish that left the river within 60 hours – by Oct. 13 – survived. Normally, oxygen levels in the Apalachicola River range between 4 and 6 milligrams per liter, but in the aftermath of Hurricane Michael, the level dropped to near zero and remained there for nearly a month.
When debris, sewage or other nutrients increase in a waterway, it can reduce the amount of oxygen that’s dissolved in the water and available to aquatic life. Previous studies have documented drops in water oxygen levels due to hurricanes and the resulting fish kills but this is the first study to quantitatively assess how a hurricane affected gulf sturgeon.
With a population of around 1,000 adult fish, losing between 30% and 60% could be a huge setback, according to Fox, especially since hurricanes are expected to increase in number and strength due to climate change.
“This is obviously very concerning, especially because hurricanes are supposed to increase in frequency and intensity with climate change,” Fox said, noting that hurricanes have historically hit the western portions of the panhandle, where gulf sturgeon have struggled for generations. However, Fox and the study’s lead author, 2021 Master of Science graduate Brendan Dula, realized that another aspect of the sturgeon population was still missing.
“Because if the adults were dying, we thought we were probably going to lose an entire juvenile year class,” he added. After hatching further upstream, juvenile sturgeon spend the first couple years of their life in estuaries – not quite the gulf, but not quite the river.
When Michael hit, the team had only tagged five juveniles and all of them disappeared. The next year, when they set out to count the most recent juvenile fish to enter the estuaries, they were surprised by what they found.
“We had more ‘age 1’ fish than we had in the years preceding the storm. As a result, they sought refuge somewhere upriver. We believe the water was better oxygenated below the Jim Woodruff Lock and Dam “Fox stated. “Although Gulf sturgeon spawn in the spring, there is growing evidence that they also spawn in the fall. Spawning is also affected by flow, so a high river flow may have resulted in a successful fall spawn.”
Despite laying up to 400,000 eggs per year, only about 50 offspring survive to the age of one in the Apalachicola River each year. However, in the two years since Hurricane Michael, the number of juveniles each year has risen to more than 100.
