Ocean Fish
Atlantic Bluefin Tuna
Like many open ocean bony fishes, Atlantic bluefin tuna start out as extremely tiny larvae, no more than a few millimeters long and weighing only a few hundredths of a gram. Within three to five years, individuals reach lengths of three feet (1 m) and are sexually mature. Because Atlantic bluefin tunas undergo such an amazing transformation in size (from being nearly microscopic to being one of the largest open ocean predators), they eat a wide variety of prey throughout their lifetimes. At a young age, they eat tiny zooplankton, and their prey increases in size as they do. As adults, they eat fairly large bony fishes and invertebrates. Similarly, Atlantic bluefin tuna are eaten by a wide variety of predators. When they are newly hatched, they are eaten by other fishes that specialize on eating plankton. At that life stage, their numbers are reduced dramatically. Those that survive face a steady increase in the size of their predators. Adult Atlantic Bluefin are not eaten by anything other than the very largest billfishes, toothed whales, and some open ocean shark species.
Bluefin Tuna are known to be highly migratory, with individuals making long migrations every year. These migrations correspond with their spawning behavior and with their food needs. This species reproduces via broadcast spawning, where several females and several males release millions of eggs and sperm into the water column at the same time. This method increases the likelihood that the eggs will be fertilized and decreases the chances that they will be eaten by egg predators. There are at least two known populations of Atlantic bluefin tuna, one that reproduces in the Gulf of Mexico and one that reproduces in the Mediterranean Sea. Some researchers believe that the Mediterranean population actually represents two populations (one in the west and one in the east plus the Black Sea). This further division has some implications for fisheries management, because the fish in the eastern Mediterranean are generally considered more of a conservation concern than those in the western Mediterranean.
Though almost all fishes are cold blooded, Atlantic Bluefin have a specialized blood vessel structure – called a countercurrent exchanger – that allows them to maintain a body temperature that is higher than the surrounding water. This adaptation provides them with a major advantage when hunting in cold water, by allowing them to move more quickly and intelligently. The Atlantic bluefin tuna is one of the fastest swimmers in the ocean. Like some shark species, Atlantic Bluefin must constantly swim. In order to obtain oxygen from the water, fishes pass water over their gills. The tunas lack the ability to do so while stopped, so they must continuously swim forward with their mouths open to keep their blood oxygenated.
The Atlantic bluefin tuna is a highly prized food fish and is fished heavily throughout its range. Its value in the high-grade sushi market in Japan and elsewhere is high enough to cause fishers to target this species with almost unmatched effort. Scientists believe the species to be endangered (highly vulnerable to extinction), but fisheries managers continue to allow commercial fishing to target it. While the Atlantic Bluefin was formerly common in the Black Sea and off the coast of Brazil, it has not been observed in significant numbers in either of those places for several decades. As a result of these and other losses, the Atlantic bluefin tuna has experienced the largest range contraction of any open ocean species. Early indications imply that recent strong management regulations may be allowing populations to rebound, but continued conservation and management efforts are required to ensure that more subpopulations are not lost.
Note on closely related species: For many years, bluefin tunas around the northern hemisphere were assumed to all be one species. Scientists have recognized Southern Bluefin Tuna as a distinct species for quite some time, but the northern species was divided only in the last decade. Now, the Atlantic bluefin tuna and the Pacific Bluefin Tuna are generally considered to be distinct species.
Atlantic Cod
Physically, the Atlantic cod and its close relatives are noted for being the only group of fishes that have three distinct dorsal fins (along the back) and two distinct anal fins (along the ventral surface). Atlantic cod spend most of their time on or near the seafloor but may be observed feeding higher up in the water column at times. They eat a variety of prey, including several species of bony fishes, American lobsters, and other invertebrates. Adult Atlantic cod are only eaten by large sharks, but juveniles are eaten by a variety of medium-sized predators and are often even eaten by cannibalistic adults.
Atlantic cod reproduce through a behavior known as broadcast spawning, where females release eggs and males release sperm into the water column above the seafloor, at the same time. This method increases the likelihood that eggs will become successfully fertilized and that fertilized eggs will not be eaten by egg predators near the seafloor. Though they form large reproductive groups, researchers believe that dominant males gain exclusive (or nearly exclusive) rights to spawn with available females. The Atlantic cod is one of the most highly productive species of bony fish, and females may release hundreds of millions of eggs in their lifetimes. Very few of these survive to adulthood.
The Atlantic cod has been fished heavily for a thousand years. Northern European fishers following cod populations across the North Atlantic were the first Europeans to visit North America. For centuries, this species supported massive fisheries and drove the coastal economy of North America. Dried, salted Atlantic cod were also an important food source during the early colonization of the Caribbean Sea. This species was an important driver of New World civilization. Unfortunately, as fishing methods became more and more advanced, Atlantic cod populations decreased further and further, and in the late 1990s, the fishery collapsed. Even though individuals of this species produce an incredible number of eggs, the fishery was just too effective for the species to replace the numbers that were removed and eaten by humans. During the collapse, the fishery has been reduced dramatically (though not completely), but the species has struggled to rebound. Growth in numbers of other North Atlantic fishes and invertebrates may be partly to blame. Scientists agree that North Atlantic food webs have fundamentally changed as a result of the Atlantic cod collapse, and the species is currently considered vulnerable to extinction.
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