Researchers are gaining clues about the mysterious migration habits of cobia.
EDITOR’S NOTE: This is part two of a two-part series. Part one appeared in the August issue.Jim Franks of the University of Southern Mississippi’s Gulf Coast Research Laboratory in Ocean Springs, Miss., has devoted almost 20 years of his life to studying the cobia that swim in the shallow waters of the Gulf of Mexico during the spring and fall.
“When we started studying cobia in 1988, very little was known about these fish,” he said. “Their biology, age, spawning areas, food and migration patterns were a mystery. At that time, the cobia was being heavily fished and was unmanaged.
“Part of the reason we began the program was because we wanted to make sure we had healthy populations of cobia for future generations of Gulf Coast fishermen to catch.”
Prior to 1988, nobody else in the United States tagged cobia. As Franks studied and tried to understand the cobia, he learned just how much anglers didn’t know about these fierce-fighting brown fish.
Because of the tagging of 14,000 cobia since that time, researchers have gained a huge amount of knowledge. We’ve started learning about when and where the cobia’s spawning cycles take place, how long they live, how fast they grow and what they eat. We also have some new exciting data about cobia migration patterns.
Right now, researchers are trying to understand what different size cobia do at various times of the year and learn the answers to questions like:
• Whether young cobia migrate as far as older cobia do;
• Whether a non-migrating group of cobia lives in the northern Gulf of Mexico;
• How a cobia spends its day;
• How much time a cobia stays at the surface of the water;
• How much time a cobia spends on or near the bottom;
• Whether a group of cobia remains on the Upper Gulf Coast all winter long, and if they do, where they stay and whether they’re catchable;
For four years, Franks has had five cobia outfitted with satellite tags that have recorded the daily movement patterns of these fish. He just now has begun to analyze the data from these tags, and hopes to have his results finalized some time this month. However, while compiling these facts, he’s also learned some very-interesting new information about cobia.
Homegrown cobia
“Scientists are beginning to think that a population of cobia lives offshore in the northern Gulf of Mexico and stays in deep water all year,” Franks says.
While other cobia take part in an annual spring migration, Franks’ research tends to indicate that some homebody cobia will hold off the Upper Gulf Coast in deep water, which he defines as 200 to 300 feet deep. On many deep-water oil structures, divers have reported seeing cobia on the bottom of the ocean floor. Also, reports have come in of anglers catching cobia on or near the bottom during the winter months.
To test this theory, Franks and a group of fishermen traveled out to some of the deep-water platforms in the Gulf and fished for cobia in 200 to 300 feet of water.
“I’ve been out in the middle of the winter and caught several cobia right on the bottom near these deep-water oil platforms,” he said. “We try to send down live bait to attract the cobia to bite. But often in the winter, getting live bait to the bottom can be difficult because of sharks taking it on the way down. However, on the day I fished, we caught three mature cobia weighing from 25 to 45 pounds.”
Researchers found this catch significant because for many years, fisheries biologists believed most cobia lived off of Florida during the winter.
“One of the things we want to know and hope to learn is how big that population of cobia is that doesn’t migrate,” Franks says. “We have two options to learn more about this population of cobia. We can continue to go to the northern Gulf of Mexico and fish deep water during the winter months, which often can be rough, bad fishing. Perhaps a better option is to try to catch some cobia during the summer months and put satellite-tracking devices on them to learn where they go and what they do.”
Mixed population
Two separate populations of cobia may live on the Upper Gulf Coast — a migratory population and a stay-at-home population. Franks and his team don’t know for sure yet. Although some reports have come in of anglers catching 60- and 70-pound cobia during the winter months, we know very little about these cobia. Satellite-tracking devices can help solve this mystery.
“If there proves to be a population of cobia that stays at home and doesn’t migrate and a population of cobia that only migrates, this won’t be the first time that scientists have learned about two separate populations of the same type of fish,” Franks said. “For instance, the National Marine Fisheries Service has shown that there is likely a migratory and non-migratory population of king mackerel in the Gulf of Mexico.”
As such, anglers may have a better wintertime cobia fishery off the Gulf Coast than they ever before realized. If researchers can learn more about where cobia go during the winter months, anglers can have some outstanding fishing that they’ve never known about or enjoyed in the past.
East meets west
Fisheries biologists long have assumed that two populations of cobia migrate in the spring, with one moving up the east coast of Florida north along the Atlantic Coast and the other population swimming west along the coastline of the Gulf of Mexico. Inquiring minds want to know about how the cobia decide to go east or west. When do the cobia decide? How many of the western bands of cobia go up the eastern coast? How many of the eastern cobia decide to come west?
“We know there are some crossovers,” Franks said. “We know that some of the cobia we’ve tagged on the Upper Gulf Coast have been caught on the East Coast.
“We have no idea what causes cobia to go up the East Coast or the Gulf Coast. We’ve wondered if the water temperature at the Keys influences the cobia’s decisions to go east or west. We’ve speculated that perhaps the cobia have been chasing bait at the time of their migration and somehow that has influenced their route of travel.”
It’s in the stars
Although Franks and his team haven’t fully analyzed all the data yet, the satellite feeds for the past four years tend to indicate the cobia behave in much the way we’ve thought they do, based on our tagging data. However, there are some interesting bits of information that we never would have learned without the satellite tags.
In a regular tag-and-release program, we can learn where the fish has been tagged and recovered, but we don’t know what’s happened in between those two points.
The satellite tagging data tells us what these five individual cobia have done daily. We’re learning what water temperatures the cobia seem to prefer, what depths the cobia prefer throughout the day and which patterns or routes these cobia take on long migrations.
Franks has confirmed that cobia do make fairly-deep dives during the day, often going down to depths of 200 to 300 feet. Cobia also spend much of their time at the surface of the water.
“We’re attempting to learn the reasons cobia dive deep during the day and why they stay shallow at certain times during the day,” Franks says. “Perhaps the temperature changes in the water throughout the day influence this behavior, but we really don’t know yet.
“We’ve also learned that cobia go to the bottom and just rest, remaining motionless for some time, an unusual characteristic for pelagic fish, since they tend to stay in constant motion. For years, divers have reported seeing cobia lying still on the bottom, and when we’ve put cobia in aquaculture tanks, we’ve seen them display this same behavior. However, we still don’t understand why they do this.”
Cobia spawning spots
The satellite data will begin to tell researchers what temperatures the cobia prefer at different times of the year and information hopefully about the cobia’s spawning behavior. Throughout his years of observing cobia, Franks has tried to learn where cobia spawn. Many outdoorsmen have guessed they spawn at the mouth of the Mississippi River, along the barrier islands, 4 to 5 miles offshore and/or out in deep water.
“We want to know where the spawn occurs, what water temperature cobia need to trigger the spawn, what their behavior is during the spawn, and where they’re positioned in the water column before, during and after the spawn,” Franks said. “Based on our examination of the reproductive system of mature cobia and our efforts to collect cobia larvae in the northern Gulf of Mexico, all indications are that cobia spawn 20 to 50 miles offshore in deep water. This area is where the young larvae only a few days old have been collected.
“Although we’ve collected a large number of mature fish along the Gulf Coast, we haven’t seen any that are in imminent spawning condition. We have very little understanding of the young cobia in the wild, and know even less about the larvae in the wild.”
Anglers who would like to assist Jim Franks in his research can contact him at (228) 872-4202.
