What magnificent warm-blooded creature moves as fast as a freight train, feeds voraciously in large groups, and is often enjoyed at the beach?
This would have to be the Atlantic Bluefin Tuna, thunnus thynnus, or “tunny” for short!
Incredible Adaptation
The body of the Atlantic bluefin tuna is football-shaped and strong, with a conical head and large mouth. The placement of their large eyes allows the bluefin tuna to navigate its surroundings easily. Their color is dark blue above and gray below, with a gold flashy brilliance covering the body and bright yellow fins on their underbelly.
Bluefin tuna can be distinguished from other family members by the relatively short length of their pectoral (side) fins. In addition, their head contains a pineal window (namely, a translucent patch of skin between their eyes) that helps the fish navigate over its multiple thousands of miles of range. Atlantic Bluefin Tuna have rows of upper and lower needle-like sharp teeth, and their livers are covered in tiny blood vessels.
The bluefin possesses enormous muscular strength, channeled through a pair of tendons to its tail fin for propulsion. In contrast to many other fish, the body stays rigid while the tail flicks back and forth, making for a very efficient stroke. Furthermore, the tuna’s fins fit into slots on the body that reduce drag while swimming, increasing its swimming efficiency.
Its circulatory system is also efficient, with one of the highest blood-hemoglobin concentrations among fish, facilitating oxygen delivery to its tissues that combines with an exceptionally thin blood-water barrier to ensure rapid oxygen uptake. Consequently, the design of their gills also implies that the faster they swim, the better oxygenated they get. In this respect, Atlantic bluefin tuna are ram ventilators, which means that they must keep moving to stay alive.
To keep its core muscles warm, for power and steady swimming, the Atlantic bluefin also enjoys a system that preserves its body heat by a unique thermoregulatory process in which the heat of the warm blood in its veins is transferred to the cooler oxygenated arterial blood. While all tuna are warm-blooded, this heat exchange system is much more highly developed in bluefin tuna than in any other tuna or fish, which allows them to function efficiently in the rich but chilly North Atlantic waters.
Atlantic bluefin tuna can grow as big as 1500 pounds. Fully mature adult specimens average 6-8 feet long and weigh around 500 pounds. The largest recorded specimen taken under International Game Fish Association (IGFA) rules was caught in 1979 off Nova Scotia and weighed 1,497 pounds and was 12 feet in length, though both the Smithsonian Institution and the U. S. National Marine Fisheries Service believe that this species is capable of achieving a gross weight of up to 2,000 pounds! Atlantic bluefin tuna reach maturity relatively quickly within about 15 years, but it is believed that the largest specimens can live for as long as 50 years.
Bluefins can dive to depths of over 3000 feet, while typically feeding on small fish such as sardines, herring, mackerel and menhaden as well as on invertebrates such as squid and crustaceans. Female bluefins can produce up to 30 million eggs, which eggs start to hatch in only 2 days. Because Atlantic bluefins group together in large concentrations to spawn, that makes them highly vulnerable to commercial exploitation during that time, most especially when spotted from the air by light aircraft.
Predation and Pressures
Atlantic bluefin tuna have a lot of value as a food fish, and are considered an apex predator due to their size, speed and power. Once matured, their biggest predatory enemies are sharks and whales. The larger fish are targeted for the Japanese raw-fish (sushi) market, for which all bluefin tuna species are highly prized.
Canned tuna is also a staple food source, especially consumed by young humans on beach picnics. Indeed, whenever I think of this species, I recall the seemingly endless series of beach picnics when I was a kid, where we always had tunafish sandwiches, and then had to wait a full hour before going into the water again. (I still wonder if this was just an old “wives’ tale” about the danger of cramps. None of my kids had that problem!)
Prior to the 1960s, Atlantic bluefin fisheries were relatively small scale, and populations were stable. However, in the 1960s, purse seining boats (ones that drag very large nets around big areas of fish) removed huge numbers of juvenile Western Atlantic bluefins in United States coastal waters, taking out several entire-year classes.
Consequently, because of their significant commercial value, Atlantic bluefin tuna are seriously overfished; their numbers have declined by over 70 percent during the last 50 years. Younger fish are also captured and raised in ocean farms until large, and then taken to market. This system denies those fish any opportunity to reproduce before they are harvested, a problem only compounded by this species’ slow growth and late sexual maturity.
The Mediterranean tuna fisheries have been poorly regulated and catches are regularly under-reported, further endangering this species. The fish’s migratory habits also complicate their management in a sustainable manner, because bluefins spend time in the national waters of multiple countries, as well as in the open ocean outside of any national jurisdiction. This species is also an object of an important recreational fishery, which exacerbates the pressure on them.
Conservation and Management Efforts
Regulatory agencies have not been very successful in regulating this species, especially due to excessive quotas supported by commercial fishermen. One of the problems is that bluefin is highly prized in Japan and elsewhere for raw fish dishes such as sushi and sashimi. For example, an Atlantic bluefin caught off eastern United States sold for US $247,000 at a Tokyo fish market in 2008, and in the past higher prices have been paid.
From an economic perspective, the dilemma arises from market processes: the scarcer the fish, the higher its price, so the profit incentive grows stronger for its increased exploitation when the species approaches depletion. Market signals are consequently counterproductive in this setting, working to exacerbate and not to reduce scarcity. As one might imagine, Atlantic bluefin tuna are listed as an “endangered” species of fish.
Our global appetite for fish is the predominant threat to Atlantic bluefin. Overfishing continues despite repeated warnings of their current precipitous decline. Bluefin aquaculture, which arose in response to shrinking wild stocks, has yet to achieve sustainability, in part because it relies on harvesting and ranching juveniles rather than captive breeding. Regulating the harvest of this species continues to be a major challenge, which is why it is currently listed as endangered in the North Atlantic region.
Perhaps we need to curb or better control our appetite for these fish in order to have them at all.
By Fred Jennings