Hurricane Michael Water Conditions Killed HALF of Gulf Sturgeon
Eugene L. 12.09.22
Even 4 years later the effects of Hurricane Michael are still evident around the Panhandle of Florida and beyond. The Category 5 hurricane that made landfall near the Apalachicola River back in October of 2018 left a path of destruction behind it as it tore through the region. While the damage on land was easy to see by the flattened towns and forests, the destruction underwater was a bit more hidden. A study by researchers at the University of Georgia with the U.S. Fish and Wildlife Service has recently shown what extent the damage was to the river in the aftermath of Hurricane Michael. These results were published in the December issue of Transactions of the American Fisheries Society, showing the harm to gulf sturgeon.
Due to a long-term and ongoing project in the Apalachicola River in monitoring gulf sturgeon (Acipenser oxyrhynchus desotoi). Scientists with the UGA Warnell School of Forestry and Natural resources were able to make a timeline of what happened in the river, the oxygen levels, flow rate, and the fate of the sturgeons who either escaped up the river or into the Gulf of Mexico. All of this data was collected by special tags on the fish being tracked by equipment that survived the storm, and weather stations and tide gauges reporting the water conditions and flow.
“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.”
The sturgeons are a federally listed protected species, that once spanned from Tampa Bay to New Orleans. But now they only have small populations in only seven rivers, with the largest populations in the Suwanee and Apalachicola rivers. During normal conditions, the Apalachicola River usually has an oxygen range of 4 to 6 milligrams per liter. But right after Hurricane Michael the oxygen levels dropped to near zero and stayed that way for almost a month. The nutrient flush that occurred from all the debris and sewage reaching the river caused these anoxic water conditions.
There is a population estimate of around 1,000 adult fish in the river, and due to the water condition post-hurricane, the scientists estimate a loss of between 30% to 60% of the adult population.
“This is obviously very concerning, especially because with climate change, hurricanes are supposed to increase with frequency and intensity,” said Fox, noting that hurricanes historically have hit the western portions of the panhandle—areas where gulf sturgeon have struggled for generations.
But then Fox and the study’s lead author, 2021 Master of Science graduate Brendan Dula, realized another aspect of the sturgeon population still hadn’t been accounted for.
“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.
By the time of landfall of Hurricane Michael the team only had five tagged juvenile sturgeon, and all of them disappeared after the hurricane. But the next year surprisingly there were some good signs for the sturgeon population.
“We actually had more ‘age 1’ fish than we’d had in the years before the storm. And so somewhere upriver they found some sort of refuge. Probably, we think, below Jim Woodruff Lock and Dam where water may have been better oxygenated,” said Fox. “Gulf sturgeon spawn in the spring, but there’s growing evidence that sometimes they also spawn in the fall. Spawning is also related to flow, and so high flow from the river may have made for a successful fall spawn.”
Despite laying up to 400,000 eggs each year per fish, only about 50 offspring typically survive to age 1 each year in the Apalachicola River. But in the years since Hurricane Michael, the number of juveniles each year is now over 100.
“They’re very sensitive to a lot of things and have a slow maturation—between 8 and 15 years. So, if adults are harvested or killed by hurricanes, that has huge impacts down the road,” said Fox. “But as long as there’s some sort of refuge for them, the next generation may survive. It may take a while before they can spawn on their own, but at least they’re around.”