Hungry For Answers
Why Were Black Drum Starving in Baffin Bay?
In South Texas there lies an estuary that is an underwater oasis for black drum. Baffin Bay has long supplied most of Texas’ commercial black drum and is a hot spot for recreational fishers, who catch more than 1.3 million pounds of black drum per year from Baffin Bay, according to the Texas Parks and Wildlife Department. It’s no wonder Baffin Bay has been described as the “jewel of black drum fishing.” But in 2012, there was trouble in paradise.
In the summer of 2012, recreational fishers in Baffin Bay began to catch small black drum with “jelly flesh” — translucent meat with a texture like gelatin. The fish were starving. Yet, nobody knew why. The mystery lead researchers at Texas A&M University – Corpus Christi (TAMUCC) to investigate this important, yet poorly understood, black drum habitat.
“When something happens to the black drum in this system, it is quite disturbing because Baffin Bay is a haven for these fish,” says Dr. Jennifer Pollack, chair for Coastal Conservation and Restoration at TAMUCC’s Harte Research Institute (HRI).
(Photo by Texas Parks and Wildlife Department)
A core sample of Baffin bay's benthos.
A core sample of Baffin bay's benthos. (Photo credit: Dr. Jennifer Pollack)
A sample of a serpulid reef formation found in Baffin Bay
A sample of a serpulid reef formation found in Baffin Bay (Photo credit: Dr. Jennifer Pollack)
PREY FOR CLUES
Fortunately, this conundrum happened at a time when many scientists were beginning to pull back the veil of mystery surrounding Baffin Bay. Scientists like Dr.Mike Wetz, chair for Coastal Ecosystem Processes at HRI, were beginning to study the most foundational aspects of Baffin Bay, namely the water quality
Wetz’s work led Pollack and her colleagues to investigate the next link in the ecological chain: the black drums’ prey. “We wanted to connect what was going on with the water quality in the bay with the fish,” she says. “To do this, we looked to the benthos— small critters that live in the sediment and form the prey base for the black drum.”
Baffin Bay is a peculiar place of extreme conditions. The bay has salinities that reach as high at 80 to 90 parts per thousand (ppt). For comparison, the salinity of the Gulf ofMexico is about 35 ppt. But, following a heavy rain or flood event, the salinity can drop all the way down to that of freshwater.
Despite knowing that such massive shifts in conditions occur, researchers didn’t know how these fluctuations affected the resident life of Baffin Bay.
“We wanted to know: In these incredibly hot, salty, stressful conditions that change rapidly, could changes in the prey base play a role in these emaciation events?”
Some black drum prey, such as clams and crabs, live in unique reef structures within Baffin Bay. These habitats were created thousands of years ago by serpulid worms, which form calcareous tubes that contain calcium carbonate. Much like oyster reefs, the innermost scaffold is the oldest part of the reef, which younger generations continuously built onto, eventually creating a rich outer veneer of living organisms.
In comparing the organisms living in the mud bottom, sea grass, and serpulid reefs of Baffin Bay, Tasha Breaux, research associate in Pollack’s lab, discovered that the reefs were teeming with life. In fact, there were hundreds more lifeforms on the reefs than in the mud bottoms. “Under a microscope, everywhere you look there is something moving and changing,” she says.
THE STRANGE EFFECTS OF SALINITY
After continued monitoring, Breaux and Pollack discovered that the community of creatures in Baffin Bay’s mud bottom habitats are sensitive to changes in salinity. As salinity increased, fewer prey were found in these areas, but when salinity decreased, there were more prey. While this finding was not surprising, what the researchers uncovered next was quite unexpected.
You’d be forgiven for thinking that a decrease in prey following high salinity conditions would cause black drum to venture beyond their normal hunting grounds to find other prey. Yet, the opposite is true.
Using stable isotope analysis, Breaux and Pollack were able to track down where black drum were getting their food from. What fish eat ends up leaving a chemical signature in their tissues, indicating whether black drum are eating prey from Baffin Bay or somewhere else.
“As salinity increases in the bay, and we see reduced food resources, we also see a strange phenomenon where fish were staying in the bay more and feeding on the limited resources. You might think 'fish can swim and leave,’ but that’s not what they are doing.”
The high salinity events appear to create a stressful environment for the drum, in which it becomes more ideal for them to conserve their resources by staying put rather than venturing out. “Just trying to survive and move becomes more costly to them under high salinity conditions,” Breaux says, “so checking out a new area to forage for food could be more energetically costly. It’s almost like they hunker down.”
However, in lower, less stressful salinity conditions, black drum have the ability to venture out into the Laguna Madre to look for food despite the fact that there is plenty of local food within Baffin Bay.
If this still seems counter-intuitive, imagine this scenario: It’s Friday evening. You’ve just got paid, and you want to go out to eat, even though you have a refrigerator full of food at home. You have a car and the money to go wherever you want for dinner. Compare that to a few weeks later. You don’t have as much spending money, and it’s been a while since you’ve grabbed some groceries. Plus, your car is in the shop. Although you’ve only got some pre-packaged meals in the freezer, you opt for that because it just isn’t worth it to go out. This is a similar situation to the one black drum find themselves in.
These findings were also confirmed by Texas Sea Grant funded-researcher at HRI Dr. Greg Stunz, who used acoustic telemetry to track the black drum. With this technique the black drum could be seen moving around more as the salinity decreased.
THE CAUSE OF JELLY FLESH?
The challenge with identifying the true cause of the black drum’s jelly flesh in 2012 is that nobody was studying these conditions in Baffin Bay at the time.“We haven’t seen another emaciation event while we’ve been taking these measurements. So, we haven’t really been able to identify the trigger,” says Pollack. “The likelihood is that there are a lot of things happening at once to cause an emaciation event.”
However, the work of Pollack and Breaux indicates that changes in the salinity do affect black drums’ prey and ultimately alter their behavior.
“We certainly see during the period that we’ve been monitoring that salinity changes affect the prey base. So, we could speculate that, at really high salinities, you would see more extreme effects to the prey base that would cascade to the fish, but we have not directly observed that in the period we’ve been measuring,” Pollack says.
To know the cause for sure, a similar event will need to happen again. However next time researchers will be equipped with a better understanding of the bay. “Now we are constantly taking the pulse of Baffin Bay, and that’s important,” Pollack says. “You need to know what things look like during normal conditions so you can have a reference for when things drastically change.”
Catching a black drum in Baffin Bay
Catching a black drum in Baffin Bay
NEXT STEPS
Pollack and her lab are still working to understand what lies beneath the surface of Baffin Bay. Now they are turning their attention to the unique reef structures that provide a home to so many of Baffin Bay’s species.
Serpulid reefs in Baffin Bay are declining in size and distribution compared to historic levels, and it still remains undetermined if the reefs can be restored. With support from the Coastal Conservation Association, Pollack’s lab is working on better understanding how to conserve and restore the reefs. This may mean building new habitats for the reef-resident organisms or it may mean creating habitat that supports sport fishing away from the reefs, which are often hit by boat bottoms.But, this is only the beginning.
“It’s just the tip of the iceberg: understanding a unique habitat type, in a unique estuary that we know virtually nothing about.” Pollack says. “I think that the Baffin Bay research and monitoring program will sustain itself for a long time because there is so much to learn, and because it’s such an exceptional system."
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