Tight lines: lowcountry fishing
Studies shed new light on cobia
by Matt Winter
Karl Brenkert/SCDNR
Scientists think cobia are spawning in St. Helena and Port Royal sounds
Discoveries raise new concerns about springtime fishery
Marine scientists in South Carolina have collected tantalizing new evidence that cobia do, in fact, use the Port Royal and St. Helena sounds as spawning grounds.
Though no immediate action is planned, this groundbreaking research may eventually support tighter regulations to protect the breeding stock.
During the past few decades, a popular recreational fishery has developed around the spring run of these prized fish. Though cobia are often caught at offshore and nearshore reefs in the summer and fall, high concentrations of adult fish move inshore to these two particular estuaries in April, May and June. Many charter captains now specialize in catching trophy- size cobia in these areas, and a number of tournaments have sprung up.
Over the years, anglers and marine scientists have come up with various theories on why cobia concentrate in these two estuaries. Some think the fish follow large schools of menhaden and threadfin herring into the sounds. Others think migrating cobia simply follow the shoreline into these large bodies of water as they move north from Georgia and Florida. Michael Denson, a Department of Natural Resources marine scientist who has been leading an ongoing, two-year study of cobia, says the evidence now points to spawning. Denson’s research teams have collected cobia specimens (280 in 2008 and 400 in 2007) and gathered biological information from catches made during tournaments. They recorded fish sizes and weights, collected fin clips for genetic sampling and examined cross-sections of the fishes’ ear bones, or otiliths, to determine age. Perhaps more importantly, they examined sexual organs and stomach contents to shed new light on breeding and feeding habits.
“The over-arching question is, ‘Why are they coming into these estuaries?’” Denson said during a September update of his research to the DNR’s Marine Resource Ad- visory Committee. “We can’t simply say they’re coming in to spawn without having the data. … Are they coming in to eat those threadfin her- ring that come in large numbers into the es- tuaries, or are they coming in to spawn?
“When we looked at the stomach contents, on almost all fish, we found blue crabs, mantis shrimp, sting rays, pipefish, toadfish, lady crabs and horseshoe crabs. Lots of benthic animals that we find in nearshore, offshore areas, that we may also find in the estuary. But what was absent in these stomachs were threadfin herring.
“So no fish caught were actually coming in to feed on these herring.”
Denson’s team also collected the reproductive organs of female cobia. Microscopic ex- amination indicated whether a particular fish was preparing to spawn, ready to spawn or had just spawned. Most of the female cobia collected in the Port Royal and St. Helena sound studies were preparing to spawn.
Matt Winter/Tideline Magazine
Fort Johnson laboratories
DNR scientists also used anchored nets to collect plankton being moved around by tides within the Port Royal and St. Helena sounds during the peak spring fishing months of April, May and June. Of 29 samples collected and processed in each estuary, 10 included cobia eggs and larvae.
This, Denson said, provides strong evidence that the fish being targeted by anglers are coming into these areas to spawn, and that those two sounds may be considered “as sort of an essential fish habitat to cobia.”
“We need to further determine if cobia are spawning offshore as well,” Denson said.
Though some offshore spawning is possible, cobia seem to favor Port Royal and St. Helena sounds for a variety of environmental factors. For example, the water in these estuaries is saltier than in other areas along the state’s coast. This higher salinity helps cobia eggs stay afloat in the water column.
The state’s ongoing studies may set the stage for the official designation of these sounds as “Essential Fish Habitats,” a move that would give fisheries managers the authority to restrict fishing based on time and location. Put simply, if further study proves that intense fishing damages the spawning stock of cobia, the state could put the brakes on the spring cobia fishery in Port Royal and St. Helena sounds.
But such a move is unlikely in the short term, and may not ever be needed, according to DNR officials at the September meeting.
Mel Bell, supervisor of the state’s Finfish Management Program, said Denson’s re- search helps managers build a better understanding of what is actually happening during the spring run of cobia, and gives the state “defendable options” to protect the resource if increased fishing pressure demands action.
According to Denson’s research, that doesn’t seem to be the case. “Snapshot” data indicate that about 18 percent of the state’s cobia are taken each year by anglers. With other popular saltwater species, catch can reach 30 percent or more. And though the state allows anglers to keep two cobia per day (33-inch minimum fork length), actually catching two can be a challenge, especially outside the spring run.
Denson also points out that many anglers targeting cobia in Port Royal and St. Helena sounds appear to be conservation-minded.
“They have done a really great job in the southern part of the state of sort of self-regulation, in terms of encouraging catch and release. A lot of the charter boat captains have self-imposed rules that are far more conservative than the two-fish-per-person-per-day limits.”
Karl Brenkert/SCDNR
Determining whether the spring run in southern South Carolina represents a spawning scenario is just one step in long-term management. Along with research into wild stocks, Denson’s team also studies growth rates and aquaculture potential of cobia at Fort Johnson on James Island and the Waddell Mariculture Center at Port Royal Sound. At Fort Johnson, DNR scientists maintain a captive brood stock of cobia, including some massive 80-plus pound females, in a large circular tank. Researchers manipulate light, salinity and other factors to induce them to spawn, then raise batches of cobia fry under tightly controlled experimental conditions. Researchers closely track growth rates while introducing variables in nutrition, salinity and other environmental factors.
Their research goals, aside from learning as much as possible about the cobia’s life cycle, include establishing optimum conditions for the captive breeding and raising of this fast- growing and tasty fish. Combined with fishery research, South Carolina’s experiments with captive cobia are steadily filling in the gaps of this poorly understood but growing fishery. “I think that we’ve probably come a lot fur- ther than anyone else has on the East Coast, and maybe even the Gulf Coast, in understanding the life history and behavior of this organism,” Denson said.
Reach Tideline Editor Matt Winter at 843-937-5568 or matt@tidelinemagazine.com
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