BIG BLUE CATCH
International Popularity of Bluefin Tuna Has Managers Keeping Close Tabs On The Species
Story by Brett Falterman, LDWF Program Manager, Research and Assessment Section
Director, Fisheries Research Lab
Migration in fishes can follow many paths and seasons, but the annual migration of Atlantic bluefin tuna to the northern Gulf of Mexico off the Louisiana coast, occurring during the early spring, is noteworthy for many reasons.
These fish are the biggest of the big. Bluefin tuna are the largest of the tuna species and of the three bluefin species (Atlantic, Pacific and Southern), the Atlantic bluefin is the largest with adults reaching 1,500 pounds.
When it comes to annual migration, Atlantic bluefin cover more distance than almost any other fish, with mature adults making annual trips from Canadian waters of the North Atlantic to the Gulf of Mexico and back in just a few months.
When it comes to value in terms of price per pound, bluefin tuna is literally the most valuable fish in the world, with a single bluefin recently selling for $3.1 million.
And with all these superlatives comes drama. Hotly contested fisheries measures are the norm for this highly adapted marine predator cursed to exist as high quality sushi. The bluefin has been pursued by some of the most advanced fishing fleets across the world’s oceans and was even considered at one point as an additional to the Endangered Species List.
Biology of the Bluefin
Atlantic bluefin tuna get big, swim fast, and travel far. To do this they rely on a highly developed physiology that enables them to maintain an elevated internal temperature almost 10 degrees above the ambient temperature, tolerate intense pressure at depths over 1,000 meters, reach speeds of 50 miles an hour, and cover thousands of miles during their annual migration.
As a temperate tuna species, they prefer cooler habitats than the tropical tuna species which occur off the Louisiana coast throughout the year, like yellowfin, blackfin and skipjack tunas. Most of the Atlantic bluefin foraging grounds are at the latitude of North Carolina up to the Canadian Maritimes. However, despite their size as adults, they still start life as larvae that are less than 4 millimeters long. Larval growth is a function of temperature and larval survival depends on growth, so Atlantic bluefin have evolved to seek out some of the warmest waters their temperate physiology can tolerate to optimize larval growth, establishing their connection with the Gulf of Mexico.
Atlantic bluefin have a very broad range, which stretches from Brazil to Newfoundland and across the North Atlantic to Norway. These fish exhibit very high spawning site specificity and fidelity, which differentiates them from other Atlantic tunas.
There are two known spawning grounds for Atlantic bluefin, including the Gulf of Mexico and the Mediterranean Sea. Using chemical markers, genetics and tagging, scientists have shown that fish spawned in these areas return to them to breed when they reach maturity, which typically occurs at about eight years old. This difference has led managers to separate Atlantic bluefin into western and eastern stocks.
Compared to other Gulf tunas, the bluefin spawning season is protracted, or short, with western bluefin entering the Gulf in March, spawning in April and early May, and leaving the Gulf in late May or early June. Despite the short spawning season, bluefin tuna are batch spawners, which means that during the spawning season they can spawn multiple times (typically every few days). Contrast that with yellowfin tuna, which have been shown to spawn in the Gulf of Mexico from April to October.
The early spring spawning period enables bluefin to find waters along the northern Gulf of Mexico continental shelf edge at temperature conducive to spawning and larval growth, which seems to be about 24 degrees Celsius. The adults also need to leave the Gulf before the temperatures reach levels that would actually impede the cardiac function of these temperate tuna adults. Bluefin larvae and juveniles grow quickly and leave the Gulf within a few months, moving up the east coast to the mid-Atlantic foraging grounds.
The hunt for giant bluefin began in earnest after World War II. During a period of post-war commercial fishing expansion, far seas fishing fleets quickly identified the hotspots for these large tuna. These foreign fleets identified the Gulf of Mexico as a bluefin hotspot and heavy fishing pressure was exerted on the resource for almost three decades.
The Magnuson-Stevenson Fishery Conservation and Management Act of 1976 was enacted to protected national fishing rights by establishing Exclusive Economic Zones that extended 200 miles from the coast. It was the passage of this act that removed foreign fleets from the productive shelf water off Louisiana.
The most recent changes to bluefin regulations came in 2015 with Amendment 7 to the Highly Migratory Species Management Plan. This amendment was significant for both commercial and recreation interest in the Gulf. Amendment 7 established time-area closures intended to eliminate dead discards of bluefin tuna from the commercial pelagic longline catch in the Gulf. For recreational fisherman Amendment 7 established a specific category for incidental recreational catch in the Gulf.
Before 2015 the Gulf quota was part of one zone that extended from New Jersey south. Gulf recreational quota was not always guaranteed, as the giant bluefin (bluefin over 73 inches fork length) category was frequently already closed when the short season arrived in the Gulf. The Gulf allocation is based on poundage but equates to roughly five bluefin that can now be taken annually by Gulf recreational anglers. This is as incidental catch, meaning caught while fishing for something else.
The most recent stock assessment of western Atlantic bluefin tuna indicated that the stock was not currently undergoing overfishing.
LDWF Bluefin Case Study
In order to ensure effective management of the internationally managed tuna resource off the coast of Louisiana, LDWF biologists have been dock sampling offshore catches and many of the samples have helped to support research projects relevant to improved management.
When biologists at the Grand Isle Fisheries Research Lab received a request to certify a large bluefin as a potential state record, they also took some biologically relevant samples. The impressive catch was made by the fishing vessel WhoDat, with a youth angler landing an 833-pound bluefin tuna solo in under an hour.
The samples were impressive too. Biologists removed ear bones, known as otoliths, stomach, gonad and tissue samples from the fish. One ear bone was sectioned and a count of annual increments indicated an age of 25 years. The other ear bone was also sectioned and a small drill press known as a micro mill was used to remove the otolith material from the very center, which corresponds to the first few months of the fish’s life.
‘Natural tags’ are unique signatures of the water chemistry in which the young fish grew up, consisting of trace elements and isotopes of carbon and oxygen. These natural tags can be used to distinguish between bluefin spawned on the two different nursery areas, i.e. the Gulf of Mexico and Mediterranean Sea.
Based on the two ear bones, we now know that the WhoDat fish hatched in the Gulf of Mexico in 1992. Additionally, since most western Atlantic bluefin mature at about age 8, this one fish may have migrated back and forth between Canada and the Gulf of Mexico as many as 18 times.
LDWF biologists also sampled the gonad from this fish, which weighed 12.4 kilos and contained over 100 million eggs. The stomach and tissue samples were transferred to university and federal scientists.
Recreationally landing a giant bluefin is both a rare event and an incredible angling feat. To retain a bluefin tuna during the open recreational season in the Gulf of Mexico, vessels must have a HMS tuna permit, which can be either a private or charterboat/headboat permit. This permit allows for the take of one giant bluefin 73 inches fork length or larger per year during open season. Catch reporting within 24 hours of docking is mandatory.
Several enforcement cases have been made in Louisiana since 2012, most of which resulted from catches that happened shortly after the closing of the season.
Because the season status can change quickly, it is imperative that anglers who may incidentally encounter these fish and wish to retain them check the National Marine Fisheries Service permit shop website (https://hmspermits.noaa.gov/) or the newly launched NOAA Fish Rules app daily.
Illegal harvest of bluefin tuna can result in both criminal charges and civil restitution fines. Title 76 of the Louisiana Code sets the civil restitution values for illegally harvested game. Bluefin tuna has the highest fine per pound at $8.80 per pound, meaning that the civil restitution for an illegally harvested fish can exceed $5,000 for an average sized adult bluefin, which could be more than 500 pounds.
In 2018 the incidental recreational harvest in the Gulf of Mexico was closed in early May.
‘Grander’ is a term used by some big game fisherman to refer to a fish over 1000 pounds. Very few ‘granders’ have ever been landed by sport fisherman in the Gulf of Mexico. The largest fish caught on rod and reel in the Gulf to date is the current Louisiana state record bluefin, which weighed 1152# and was caught by Ron Roland aboard the Miss Cathy in 2003. A close second for biggest fish landed recreationally in the Gulf is the current Louisiana record for shortfin mako, at 1149.5#. Four other Gulf Granders have been recorded, all of which were blue marlin, including the current Louisiana state record blue marlin. That means that half the ‘granders’ caught in the Gulf of Mexico have been landed in Louisiana.
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