bluefin life history
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Tunas are in the Scombridae family of teleost fishes and together with the mackerels make up the 50 species comprising this family. It is a small family of teleosts representing only a fraction of the 23,000 plus fish species in existence. Seven of the most important tuna from a commercial fisheries perspective are:
The term bluefin actually describes three regionally different fish; the Atlantic bluefin (Thunnus thynnus), the Southern bluefin (Thunnus maccoyii) and the Pacific bluefin (Thunnus thynnus orientalis). Atlantic bluefin are are found in the western Atlantic from Labrador to Brazil and in the Eastern Atlantic from the Lofoten Islands off Norway to the Northwest African coast. Pacific bluefin are found as far north as Alaska, but are typically found in waters off Baja and southern California. In the western Pacific, depending on the time of year, bluefin tuna are found in concentrations around Japan and the Philippines.
Age and Growth
Bluefin tuna are a fairly long-lived fish and may reach ages of 25 years or older. Determining the age and growth of any commercially exploited species is extremely important for management because it helps identify the health and size structure of the stock. Identifying rates of growth can determine the speed at which individuals may reach sexual maturity. This is important because removing individuals from a population before they can reproduce will result in stock depletion, sometimes referred to as "fishing down the stock." Unfortunately, there have been few reliable techniques to age bluefin tuna in all size classes with a high degree of certainty. Traditionally, bluefin were aged, as many other fish are aged, by examining otoliths (the inner ear bones that aid in hearing and balance), fin rays, scales and vertebrae. These only appear to be useful for younger fish i.e., <10 yrs. Graduate students at the Large Pelagics Research Center are currently using dorsal spine sections (as seen left) to determine age of bluefin tuna.
Bluefin tuna are voracious predators at all life stages. Their tremendous appetite, driven by their unique warm-bodied physiology (see description below) produces extremely high growth rates. For example, one year-old bluefin may achieve lengths approaching two feet and weights of close to ten pounds. Growth in western Atlantic bluefin does not appear to be uniform throughout the year, and is greatest from June through October. During this time adult fish may increase their body mass by 15-40% while feeding on high energy prey such as herring, mackerel and bluefish (picture on the left shows a bluefin stomach full of herring). However, bluefin will eat just about anything in their path. Their stomachs sometimes contain the unexpected, such as starfish, crabs, and sponges. Atlantic bluefin tuna are the largest of all tunas and individuals have been documented weighing over 1500 lbs and exceeding 10 feet in length. The IGFA All Tackle world record bluefin, weighing 1,496 lbs, was caught off Nova Scotia in 1979.
Basic Morphology and Physiology
Tunas are built for speed, their streamlined bodies (see photo to the right), retractable pectoral fins (see below left), and eyes that are flush with the body are only some of the adaptations that substantially reduce the amount of drag encountered while swimming. This, in conjunction with their unique physiology, results in extremely efficient swimming. Tunas are great long distance and burst swimmers able to attain speeds of 15-20 kts in just seconds, although they routinely swim at a more leisurely rate between 2 - 6 kts.
Bluefin tuna and many other epipelagic fish (including many species of sharks) are negatively buoyant. If they stop swimming they will sink. Utilizing the same lift properties as a bird or airplane wing, bluefin use their pectoral fins to generate lift, which reduces the overall costs of travel. The pectoral fins also help bluefin maneuver while chasing prey. Unlike many other fish that swim by bending most of their body from side to side, the tuna has a relatively stiff body. Powerful connective tissues channel most of the energy directly to the tail. Aiding the bluefin with its swimming is 
an extremely well designed and efficient lunate-shaped tail and caudal keel (shown below right). Small finlets, located along the top and bottom of the posterior margins of the tail, also reduce drag and improve swimming efficiency. Typically, bluefin tuna will cruise at about one body length per second.
Bluefin tuna experience a wide range of temperatures (42 to 78 degrees F.) through their vertical and horizontal movements. Rapid changes in temperature would ordinarily have a large impact on a fish's physiology. However, the "rete mirabile", a unique adaptation of the bluefin's circulatory system, reduces the cooling effects of the surrounding water by retaining as much as 95% of internal heat generated by metabolism. The rete consists of afferent and efferent arterioles and venules running side by side. This rete system and the bluefin's large size are two reasons core temperatures of bluefin tuna examined by early tuna researcher Frank Carey and colleagues, were more than 30 degrees F. warmer than the surrounding water. This endothermic lifestyle confers several advantages. Warm muscles operate more efficiently than cold ones, resulting in faster and more sustained swimming. Secondly, a warm body also supports a warm stomach (picture below), which also has heat-retaining circulatory features. This increases the efficiency of digestive enzymes and allows bluefin to convert prey into energy through rapid turnover. 
These are some of the characteristics that make the bluefin a formidable predator and long-distance traveler.
The bluefin's high metabolism requires large amounts of oxygen. However, it's rigid skull, a design that supports speed, does not allow active pumping of water over the gills as in some other species. Bluefin extract oxygen from the water using ram ventilation (forcing water over the gills while swimming). Their gills are very dense with capillaries, and have a surface area up to ten times greater than cold-blooded fish of similar size (see photo below).
Bluefin tuna have a specialized heart for delivering oxygen- rich blood from the gills to the tissues. The heart is 
pyramid shaped, and similar to other teleosts, consists of a sinus venosus, atrium and ventricle. The bluefin heart is highly muscular, and well suited for distributing large quantities of blood through a high-resistance circulatory system. Unlike the warm, inner muscle mass of the fish, the heart is close to the ambient temperature of surrounding water, and functions under large and rapid temperature changes. The bluefin heart can deliver large quantities of blood to the tissues by increasing stroke rate and/or stroke volume.
Identifying the spawning grounds of harvested fish is one of the most important factors for successful management. Spawning by bluefin tuna is thought to occur in two vastly different regions of the Atlantic. Historic research cruises utilizing plankton tows identified bluefin larvae and a spawning ground in Gulf of Mexico and the Straits of Florida, and in the eastern Atlantic, spawning occurs in areas of the Mediterranean Sea. However, this is not to say that only two spawning areas exist. Mature bluefin tuna are sometimes widely distributed across the Atlantic during the spring and summer, when they presumably reproduce, and there are other areas that could provide suitable conditions for spawning and larval development. Identifying whether other spawning areas exist is not an easy task. Trips to remote areas are expensive and difficult, especially in unpredictable weather. Also, bluefin spawn fractionally, that is, they release eggs over several days. Given their tremendous mobility, they have the potential to broadcast eggs over several hundred square miles within a few days. Another method used to identify spawning grounds is histological analysis of gonads (see left) for egg ripeness. Efforts are underway in our lab and elsewhere to determine the age of sexual maturity and accurate fecundity estimates for the stock.
