Biologists concerned about juvenile fish in Gulf of Mexico
Posted September 3
HOUSTON — Ryan Macias and Grant Lewis methodically unfurled a low-slung net stretched between two poles and began dragging it across the bottom of Matagorda Bay one cloudy morning in early May.
The Houston Chronicle reports the water was the color of chocolate milk, and shallow, only about waist-deep on Macias, a field technician for the Texas Parks & Wildlife Department.
The seine net dragged up a half-dozen sea creatures — a couple of brown shrimp, a pink one, and a tiny, translucent jellyfish.
But Leslie Hartman, TPWD's Matagorda Bay Ecosystem Leader who was overseeing the work, quickly saw what was not in the net: juvenile southern flounder.
She wasn't surprised.
"It's been warm, too warm, for southern flounder the past couple of years," she said. "You don't see the effect as much on the adults, but you do with juveniles. And in a couple of years, fishermen in Texas are going to start to notice."
This year, the number of juvenile flounder caught by the Texas Parks & Wildlife Department as part of its annual survey hit an all-time low, worrying fisheries biologists across the state.
For decades, the popular, albeit funny-looking fish has been declining in numbers off the Texas coast. Over-fishing is one reason, but a main factor is environmental.
Southern flounder, prized for its delicate meat, only spawn in cold water and prefer temperatures around 62 degrees Fahrenheit.
But water temperatures off the coast have been rising for years, especially the winter-time lows. This year, for the first time on record, the daily average surface water temperatures in the Gulf of Mexico never fell below 73 degrees, as Houston experienced its hottest winter on record.
Some fish species — like gray snapper — are thriving in warmer waters. Others, like the southern flounder, are struggling.
It's a trend happening worldwide, as climate change causes the oceans to heat up, either killing marine plants and animals or causing them to "shift" into habitats they find more suitable. Studies have found that roughly two-thirds of marine species that live in the northeastern part of the United States have extended their range due to warmer waters. Among them: black sea bass, lobster, yellow tail flounder and monkfish.
"If temperatures continue to rise, there might be some species that are not ultimately able to survive in certain places," said Rutgers University's Malin Pinsky, one of the nation's leading experts on the impact of climate change on fish populations.
In the Gulf of Mexico, several tropical fish have moved into coastal waters while warmer temperatures have thrown off the migration patterns of birds and fueled the spread of invasive plants.
In Texas, state leaders often acknowledge the impacts of climate change but frequently refuse to acknowledge the cause. For example, in the state's recently released coastal master plan, which runs 208 pages, climate change is never mentioned.
Hartman, however, doesn't mince words.
"It doesn't matter if you believe in climate change or not," she said. "The water is getting warmer. We have data that shows it. So our challenge becomes how do we manage this ecosystem that's in the middle of a change?"
Southern flounder is one of the most temperature-sensitive fish species found along the Texas coast. Not only does it dictate if they spawn or not, it also plays a major role in whether the fish become male or female.
Cold water tends to produce females, which are bigger than males and prized for their meat.
That trend was observed at the University of Texas Marine Science Institute in Port Aransas, which led the state's first efforts to get the finicky fish to spawn in captivity to start a restocking program.
"Eighteen degrees Celsius (about 64 degrees Fahrenheit) was the key," said Joan Holt, the former director of UTMSI's Fisheries and Mariculture Lab who oversaw the pioneering work. "At that temperature, you had a 50/50 split between males and females, which is what you want in a fishery. If you raised the temperature above that threshold, you'd get less females and more males."
Having a fish population that is seriously skewed toward one gender would obviously be detrimental to its long-term survival.
So are temperatures creating more male southern flounder in the wild?
"We don't know," Holt said. "But I would assume that would be happening."
It would take Hartman and her crew almost four hours to catch their first flounder recently on Matagorda Bay, the only one they would see that day.
During this year's spring gill net season — the 10-week period that the Texas Parks & Wildlife Department samples fish populations — crews observed more adult southern flounder than they had the year before.
Still, the numbers were low enough to be the fourth-worst on record, and State data shows southern flounder numbers dropping since the early 1990s.
"Long-term, they're showing a pretty steep and steady decline," said Mark Fisher, science director for the department's coastal fisheries division.
Any improvement in recent years likely has been due to stricter fishing regulations, Fisher said, and restocking efforts.
Along with the diminishing juvenile southern flounder population, another harbinger of the changes warming waters will bring along the Texas and Louisiana coasts is the increasingly ubiquitous presence of black mangrove, a plant that tolerates salt and thrives in the tropics.
"What has happened over the last 50 years, and we've got good aerial images that show this, is that cold snaps make mangroves retreat," said Steve Pennings, an ecologist with the University of Houston. "The problem is there haven't been many cold snaps."
Studies by Pennings and his colleagues project that black mangrove will replace 100 percent of all salt marshes in Texas, and 95 percent of those in Louisiana, should winter temperatures increase by two to four degrees.
The switch has serious ecological implications on coastal water quality, fish populations and bird habitat.
Black mangroves are virtually impenetrable because of their dense branches. They develop long, horizontal roots and root-like projections that poke through the ground.
Pennings and his team are studying a test plot of mangrove in Port Aransas, hoping to learn more about the ramifications of the plant's expansion in Texas.
One thing they know already is that wading birds try to avoid black mangrove, which is significant in Texas, where thousands of bird watchers flock to coastal communities each year.
And the changing climate is affecting bird migrations through the state, mostly in the winter.
A 2011 study conducted by scientists at the University of Texas Marine Science Institute showed a correlation between rising temperatures and the arrival and departure times of six species known to migrate through Port Aransas, including the double-crested cormorant, eared grebe and the herring gull.
Lee Fuiman, one of the study's authors, said it wasn't clear that climate change was exclusively to blame for the disruptions they documented since migratory birds face threats from other sources.
However, he said, there is mounting scientific evidence from around the globe that climate change is throwing off migrations for several species.
"The winters are getting warmer here. We know that. And when we look at the larger picture, you start seeing trends in seasonal patterns that cannot be ignored," said Fuiman, director of UTMSI's Fisheries and Mariculture Laboratory.
Two years ago, Shannon O'Leary pitched what sounded like a straightforward post-doctoral project to her advisers at Texas A&M University Corpus Christi: study the genetics of southern flounder to better understand its adaptation strategies.
She turned to the Texas Parks & Wildlife Department for help collecting specimens.
The first year, department employees in Galveston collected six juvenile flounder while Hartman managed to get two. By May this year, despite the department's efforts, none could be found.
"We decided to try to expand the project to Alabama and other states in the western Gulf," O'Leary said. "But they are telling us the same thing. "
During the spring sampling effort, not one of the 10 ecosystem teams was able to collect a juvenile flounder in the seine nets.
That might come as a surprise to many recreational fishermen, who still report great success in catching adult flounder, particularly during the fall spawning runs.
"The thing of it is, think about how fish spawn," Hartman said. "They're not like people who think, 'I'm going to have two kids and do real good by them.' They're like, 'I'm going to have a quarter million babies.' And here we are, and the best we can come up with is two juvenile flounder? That's not good."
Still, not everyone is convinced that warm water temperatures can be singled out as a threat to flounder.
Greg Stunz, director of sportfish conservation science at the Harte Research Institute at Texas A&M University Corpus Christi, said the fish population is affected by regulations and climate.
"I think it would be really hard to tease the two apart," he said.
To help southern flounder, the Texas Parks & Wildlife Department has banned gigging — which involves spearing the fish — during the fall.
It was a controversial move, but one state fisheries managers thought necessary.
Whether southern flounder ultimately adapts to its changing environment remains to be seen.
Studies suggest that coastal Texas may be on the edge of southern flounder's natural range, which means it wouldn't take much to force the population to shift elsewhere.
For "a lot of marine critters," Hartman said, the right temperature sets off hormonal cues.
"If they don't get those cues, they're just not going to spawn," she said. "Is that what's happening with flounder? It's hard to say. Biology is a messy, messy science."