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Scombridae, Mackerels, Tunas family species, their habitats, characteristics, fishing methods.
Scombridae is the family of the mackerels, tunas, bonitos, the pilot fish, and the sword fish. Family includes some of the world’s most popular important and familiar food and sport fishes. Family is divided into two subfamilies, with 15 genera and 55 species. The single species, Butterfly kingfish is the sole member of subfamily Gasterochismatinae. The other subfamily, Scombrinae, is divided into five tribes: Scombrini (mackerels), with two genera and six species; Grammatorcynini, with one genus and two species; Scomberomorini (Spanish mackerels), with two genera and 19 species; Sardini (bonitos), with four genera and seven species; Thunnini (tunas), with five genera and 14 species.
The family Scombridae species length varies from the 20 cm length of the island mackerel to the 458 cm for the immense Atlantic Bluefin Tuna, one of the largest bony fishes. It also boasts the fastest-swimming fishes in the world; the Wahoo can attain speeds of 75 km per hour.
Description
Scombrids are medium to large-sized (to 3 m) species with elongate and fusiform body, moderately compressed in some genera, are highly adapted to continuous swimming in the open ocean. Their bodies are an ideal streamlined shape, with the thickest part of the body occurring two-fifths of the way back from the head. They have moderately large mouths with well-developed strong, moderate, or weak teeth and pointed snout. There are no true canines; palate and tongue may have teeth. Gill membranes are not attached to the isthmus.
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Scombrids have two dorsal fins that are normally separate and retracted into body grooves to reduce drag, and the deeply forked caudal fin with supporting caudal rays completely covering hypural plate. The first dorsal fin is composed of 9 to 27 densely packed rays, origin well behind the head. Caudal fin is strongly divided and stiff, with a slender, ridged sickle-shaped base for powerful propulsion. Anterior fin is usually short and separated from posterior fin. The 5 to 12 separate finlets behind the anal and second dorsal fins may allow the tail to push against less turbulence by preventing vortices from forming in water flowing toward the tail. Pectoral fins placed high on body. Pelvic fins moderate or small with 6 fin rays, placed below the pectoral fins.
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The slender caudal peduncle bears at least two small keels on each side that reduce drag and may accelerate water flowing over the tail. A larger keel is in between in many species. Body either uniformly covered with small to moderate cycloid scales (e.g. genus Scomber, genus Scomberomorus, mackerels) or a scaly corselet developed (area behind head and around pectoral fins covered with moderately large, thick scales) and rest of body naked (genus Auxis, genus Euthynnus, genus Katsuwonus), or covered with small tiny scales (Thunnus). Lateral line is simple. Vertebrae 31 to 66. Adipose eyelid sometimes present (genus Scomber, mackerels); premaxilla is beak-like, free from nasal bones which are separated by the ethmoid bone.
Scomber species are usually bluish metallic or greenish above with a pattern of wavy bands on upper sides and silvery below; Scomberomorus and Acanthocybium are blue-grey above and silvery below with dark vertical bars or spots on sides. Sarda, Atlantic bonito, has 5 to 11 stripes on back; genus Euthynnus has a striped pattern on back and several dark spots between pectoral and pelvic fins; genus of oceanic bonitos Katsuwonus has 4 to 6 conspicuous longitudinal stripes on belly; Auxis and Thunnus are deep blue-black above; most species of Thunnus have dusky bands and fins, bright yellow finlets with black borders. Spanish mackerels have golden yellow to bronze irregular spots above and below lateral line.
Habitats
Scombrids are, for the most part, pelagic (open-ocean) predators, and are found worldwide in tropical and cool temperate waters. Some members of the family, in particular the tunas, are notable for being endothermic (warm-blooded), a feature that also with a highly streamlined body and retractable fins helps them to maintain high speed and activity. Other adaptations include a large amount of red muscle, allowing them to maintain activity over long periods. They are capable of considerable speed, due to a highly streamlined body and retractable fins. Scombrids are continuous swimmers, and tunas in particular have a unique, efficient swimming style (called thunniform), in which the body remains stiff while the thin tail oscillates quickly. Some species of bonito (in the genus Sarda) habitually leap clear of the water when pursuing prey.
Most species of the family are schooling fishes, but some can be found singly. Some species make seasonal migrations of huge distances into cool temperate or cold waters. Some, especially the smaller mackerels, remain near coastlines, often live closer to shore, while many others roam deeper waters, often in wide migratory patterns. They are a marine family, although some groups occur in brackish water, and one normally marine species, Scomberomorus sinensis, has been found in fresh water 300 km up the Mekong River. Many groups within Scombridae tend to remain near the surface and over the continental shelf.
Scombrids are active predators that feed on a wide range of organisms. The diet of a single species may include crabs, shrimps, squids, and crustaceans, the larvae of fishes and invertebrates, and fishes several feet long. Some smaller species strain zooplankton through their gill rakers. Tunas feed on a variety of mid-water and surface fishes, with mackerel providing a favorite meal. Tunas’ ability to maintain elevated body temperatures enables them to swiftly pursue prey in the cold waters of deeper depths and higher latitudes. Migratory tunas have the fastest digestion rates and highest metabolic rates of any fish.
Mackerels (Scomber and Rastrelliger) filter plankton with their long gill rakers. Spanish mackerels, bonitos and tunas feed on larger prey, including small fishes, crustaceans and squids. Dioecious and most display little or no sexual dimorphism in structure or color pattern. Females of many species attain larger sizes than males. Batch spawning of most species takes place in tropical and subtropical waters, frequently inshore. Eggs are pelagic and hatch into planktonic larvae.
Most scombrids (tunas, mackerels, and bonitos) are important food, commercial, and sport fishes. In some parts of the world, i.e. the Mediterranean and Californian coasts, tunas have been fished locally for many years, but heavy commercial exploitation of open-ocean tunas has led in some cases to depletion of tuna populations. Much of the tuna catch is harvested for canning. Apparently the flesh of king mackerel has occasionally been toxic when eaten.
Scombrini (Mackerels)
Mackerels are having a slim, cylindrical shape (as opposed to the tunas which are deeper bodied), a spiny dorsal as well as a soft dorsal fin, several small finlets on the dorsal and ventral sides behind the dorsal and anal fins, a very slender caudal peduncle, a deeply forked or lunate caudal fin, a very shapely form tapering both to snout and to tail, and velvety skin with extremely small scales, if present. The scales are small, delicate, and smooth, the bones light, the tail slender, and gill covers unarmed; the first dorsal fin continuous, the rays of the second and of the anal detached, forming finlets, and with a large interval between the dorsals; the body is fusiform, the caudal fin powerful, the tail usually with a slight keel on the side, the vertical fins without scales; a row of small conical teeth in each jaw; branchiostegal rays seven; most of the species have no air bladder.
Mackarel are predaceous, swift swimmers, and powerfully muscled, may be found in all tropical and temperate seas and more or less migratory. Most live offshore in the oceanic environment, but a few, like the Spanish mackerel (Scomberomorus maculatus), enter bays and can be caught near bridges and piers. A female mackerel lays about 500,000 eggs at a time. Mackerel are prized (and are highly harvested) for their meat, which is often very oily. They are known for their fighting ability, and are an important recreational and commercial fishery.
The mackerel are usually seen in shoals just rippling the surface of the water, moving slowly in one direction, frequently to windward. They perform migrations almost as extensive as the herring; it probably inhabits almost every part of the European seas, and comes into shallow water at particular seasons to breed; were it not for these periodical visits, no effective fishery could be carried on, as it would be impracticable to follow the shoals over the ocean; great as is the number caught, it is very small compared with those which escape. It is caught on the shores from March to June, spawning in the latter month; as many as 500,000 eggs have been counted in a single female; the young, called shiners are 6 in. long by the end of August; in winter they retire to deep water. The mackerel is very voracious, feeding principally on the fry of other fish; it grows rapidly, and attains an average length of 15 in., and a weight of 2 lbs., though some considerably exceed this. It is considered better in May or June than earlier or later in the season; the flesh rapidly becomes soft, and must be eaten soon after being taken from the water; much of the flavor, however, is retained in the salted fish.
Thunnini (Tunas)
Thunnus is a genus of ocean-dwelling fish in the family Scombridae, all of which are tuna, although other tuna species are found in other genera. Their coloring, metallic blue on top and shimmering silver-white on the bottom helps camouflage them from above and below. They can grow to 15 feet long and weigh over 1,000 pounds. They can swim up to 50 miles per hour when pursuing prey.
Their bodies are formed to maximize swimming efficiency, and some groups: tunas are endothermic, have a vascular heat exchange system that allows maintaining a body temperature higher than that of the surrounding water to adapt a pelagic, nomadic existence for prolonged swimming in colder water. Endothermy also helps warm parts of the central nervous system, which stabilizes nervous system function in cold water. Butterfly mackerels keep brain and eye temperatures elevated using thermogenic (heat-producing) tissue.
Tunas are negatively buoyant and must swim continuously to avoid sinking. In addition, they require constant movement to ventilate the gills. Through a process called ram gill ventilation, swimming (at speeds no less than 65 cm per second) forces water over the gills. Tunas have numerous lamellae (gill membranes) and very thin lamellar walls, and are able to extract more oxygen from the water than any other fish. Tunas have large hearts and blood volumes. They also have a high proportion of the red muscle that permits sustained swimming, buried centrally along the spinal column to conserve heat. Other members of the family, such as the mackerels, also have red muscle, but located nearer the outside of the fish.
Tunas conserve heat produced by swimming muscles through an arrangement of blood vessels called a rete mirabile ("wonderful net") and act as a countercurrent heat-exchanger. When blood cycles through the gills to receive oxygen, it also is diverted to vessels near the outside of the body instead of traveling directly through the fish’s core. Before flowing inward, the cool, oxygenated blood passes through a network of small vessels, countercurrent to warm blood leaving the swimming muscles, and heat is transferred to the entering blood. In waters ranging from 7 to 30°C, Bluefin Tuna maintain muscle temperatures between 28 and 33°C. Others keep body temperatures 3 to 7°C warmer than the surrounding water. Some species, such as Bigeye Tuna, utilize the heat exchanger only when they enter colder water.
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