Biological characteristics of tuna
Tuna and tuna-like species are very important economically and a significant source of food, with the so-called principal market tuna species - skipjack, yellowfin, bigeye, albacore, Atlantic bluefin, Pacific bluefin (those two species previously considered belonging to the same species referred as northern bluefin) and southern bluefin tuna - being the most significant in terms of catch weight and trade. These pages are a collection of Fact Sheets providing detailed information on tuna and tuna-like species.
Table of Contents
Taxonomy and classification
[ Family: Scombridae ] : Scombrids
Upper systematics of tunas and tuna-like species
Morphology of larvae
It is often difficult or impossible to identify larvae and, in some cases, early juveniles by anatomical characteristics or colour patterns. Biochemical or genetic methods can be used to distinguished the larvae of the various species (Elliott and Ward, 1995, Chow et al., 2003). Diagnostic keys are available for larvae between 3 and 12 mm standard length. Larvae smaller than 3 mm are virtually indistinguishable (Nishikawa and Rimmer, 1987).
Morphology of juveniles and adults
Characteristics common to both scombrids and billfishes
Both scombrids and billfishes have two distinct dorsal fins, generally separated, the first one supported by spines and the second only by soft rays. The pelvic fins are inserted below the base of the pectoral fins. The caudal fin is deeply notched.
All scombrids and billfishes except swordfish have a pair of caudal keels on the middle of the caudal peduncle at the base of the caudal fin. The swordfish has only a large median caudal keel. The more advanced members of the Scombridae family also have a large median keel anterior to the pair of caudal keels. The bodies of all the Scombroidei are robust, elongate and streamlined. The first dorsal and first anal fins of all scombrids and billfishes, except swordfish, can fold down into grooves and the pectoral and pelvic fins into depressions when the fish is swimming rapidly.
The scombrids and billfishes, all have four gill arches on each side. The gill filaments are ossified as "Gill rays".
Tropical and temperate tunas
Because of different distributions due to their specific thermal tolerances and because of exploitation by different fisheries, a distinction is made between tropical and temperate tunas. Tropical tunas are found in waters with temperatures greater than 18° C (although they can dive in colder waters) whereas temperate tuna are found in waters as cold as 10°C, but can also be found in tropical waters (Brill, 1994).
Tropical and temperate tunas
Tropical tunas Temperate tunas
Tunas prefer oceanic waters, and 3 of the 8 species of Thunnus are found worldwide except in the Arctic Ocean. Most bonitos and little tunas (Euthynnus spp.) are primarily neretic, ie coastal fishes, but the distribution of individual species is often widespread. The frigate and bullet tunas (Auxis spp.) are probably both oceanic and coastal (Olson and Boggs, 1986). The slender tuna and the butterfly kingfish have circum-global distributions in the Southern Ocean. Most mackerels and seerfishes have restricted ranges of distribution. Exceptions are the Spanish mackerel and the wahoo which are found worldwide.
Oceanic and neritic tunas
Oceanic tunas Neritic tunas
Billfishes are widely distributed, at least, throughout the oceans in which they occur. The exception are the Mediterranean spearfish, which occurs only in the Mediterranean Sea, and perhaps the roundscale spearfish, which occur in the northeastern Atlantic Ocean around the Canary and Madeira Islands and in the western Mediterranean Sea. However, only the swordfish is cosmopolitan. All other Istiophoridae are being confined to the Atlantic Ocean or to the Indian and Pacific Oceans.
Habitat and biology
Tunas are pelagic marine fish, spending their entire lives relatively near the surface of tropical, subtropical and temperate oceans and seas. Scombrids and billfishes live primarily in the water layers above the thermocline, but are able to dive to depth of several hundred meters (see the Vertical distribution section). Tuna species attaining only small sizes and juveniles of those attaining large sizes are encountered in epipelagic waters (from the surface to the thermocline) whereas large tunas tend to be mesopelagic and are found also in deeper and cooler waters.
Seerfishes are generally restricted to coastal waters and enter estuaries to feed. One species, the Chinese seerfish moves long distances in freshwater up the Mekong River in China.
Distribution of tunas and tuna-like in the water
Tuna and their environment
Important environmental parameters for tuna are the sea surface temperature, the quantity of dissolved oxygen in the water and the salinity. Lower thermal boundaries vary between 10°C for temperate tunas and 18°C for tropical tunas (see above; Brill, 1994). The minimum oxygen requirement is estimated between 2 to 2.7 ml/l for principal market tuna species except for bigeye tuna which can tolerate oxygen concentrations as low as 0.6 ml/l (Sharp, 1978 ; Lowe, 2000). Most tunas tend to concentrate along thermal discontinuities such as oceanic fronts (Sund, 1981).
Vertical distribution constraints and diving behavior
The vertical distribution of most species of tunas is influenced by the thermal and oxygen structure of the water column. Tuna species attaining only small sizes and juveniles of those attaining large sizes tend to live near the surface, whereas adults of large species are found in deeper waters. The use of deep longlines showed that bigeye can be found at depths as great as 300 m (Suzuki et al., 1977). Albacore are also caught under FADs at depths to about 200 m (Bard et al., 1998). Acoustic telemetry has shown that billfishes are found near to the surface during the day, descending more frequently to greater depths at night (Block et al., 1992a).
Tunas use schooling to their advantage when they forage. Some tunas form parabolic-shaped schools to encircle their prey. Most tunas school according to size. Juveniles of tunas attaining large sizes are, therefore, often associated with tunas attaining only small sizes, such as skipjack or bonito. Schools of large adults consist of a few scattered individuals. Schooling offers protection for juvenile tunas by confusing predators and reducing the likelihood that any single fish will become a victim to a predator. Atlantic bluefin tuna can form giant schools spread over several nautical miles when migrating into the Mediterranean Sea to spawn during the summer. As is the case with the other fishes, the structure of tuna schools is maintained by the lateral line. Schools can gathered over 5000 individuals.
Migration and other movements
All tunas and tuna-like fishes move constantly to search for food and to keep water passing over their gills. Migrations are seasonal movements, often over long distances, for the purpose of feeding or reproduction. Temperate tunas, i.e. albacore, Atlantic bluefin and Pacific bluefin, migrate long distances between temperate waters, where they feed, and tropical waters, where they spawn without moving among different oceans. Southern bluefin tuna migrates among the southern parts of Atlantic, Indian and Pacific Oceans. Although, the distribution of the three species of bluefin is quite extended, their spawning is restricted to relative small areas of tropical waters. Tropical tunas, i.e. skipjack and yellowfin, are less migratory in terms of long-distance directional movements, although several tagged yellowfin released in the western Atlantic have been recaptured in the eastern Atlantic. Bigeye have some of the characteristics of both temperate and tropical tunas. They apparently do not make trans-oceanic migrations, but like the temperate tunas, they migrate back and forth between feeding grounds in temperate waters and their spawning grounds in tropical waters. When they are not making directional migration, tunas move nearly all the time in search of areas where the food is most abundant. Fishermen are sometimes able to predict on the basis of oceanic conditions where the fish are likely to appear and then, they can transfer their operations to those areas. Less is known of the movements of billfishes, but apparently, they make seasonal migrations between temperate waters, where they feed, and tropical waters, where they spawn. For instance, blue marlin display extensive trans-equatorial and inter-oceanic movements from the Atlantic into the Indian Ocean (Ortiz et al., 2003).
Tunas are excellent swimmers, and their bodies are designed for high performance at both sustainable and burst swimming speeds (Dickson, 1995). Tunas must swim constantly to satisfy their oxygen requirements and consequently stay alive. The direction of movements of some species, such as skipjack, seem to be dictated solely by the availability of food. The movement of other species, such as the three species of bluefin, seem to be influenced by both the distribution of food and the need to return to their ancestral spawning grounds at the proper time. Tunas can move up to 15 km per night in order to forage on organisms that swim upward from deeper waters at that time.
The net distances travelled by tunas and billfishes (shortest distances between the locations of release and recapture) exceed those of any other fish, as shown by the following records obtained from tagging studies (from Joseph et al., 1988 for tunas and Orbesen et al., 2008 for billfishes):
10,790 km for a Pacific bluefin tuna (from southeast of Japan to off Baja California)
Short-range, fast swimming
Scombrids and billfishes are adapted to fast swimming. The champions are, of course, the most highly evolved scombrids, the bonitos (Sardini) and the tuna (Thunnini) and the billfishes. They are able to exhibit startling bursts of speed, often exceeding one body length per second.The record (for all bony fishes) belongs to the black marlin (Makaira indica), which has been clocked at over 130 km/h.
Physiological aspects of swimming
In order to swim at high speeds for long periods, tunas are capable of taking in and utilizing large amounts of oxygen.
Scombrids and billfishes, like most fish, have two types of muscle, white and red. The white muscles function during short bursts of activity, while the red muscles, which have a relatively large mass, allow the fish to swim at high speeds (up to 45 km/h) for long periods without fatigue, as demonstrated by tagging studies with conventional and sonic tags (Joseph et al., 1988 ; Bushnell and Holland, 1997).
The proportion of red muscle is much greater for tunas than for other fish (Dickson, 1995) and their white muscles are capable of working in both aerobic and anaerobic conditions. Therefore, the increase in swimming speed can be portrayed as follows :
Heart and white muscle aerobic capacities are significantly greater in tunas than in billfishes and other scombrids.
Recovery from intense activities
Furthermore, tunas and billfishes are capable of recovering more quickly than other fish after intense activities, such as that involved in capture of prey. For some tunas, the rates of removal of lactate from the blood and white muscle tissue approximate the rates measured in mammals, which allows the tuna to recover within two hours (Dickson, 1995).
Thermoregulation in tuna
As a consequence of swimming constantly to maintain hydrostatic equilibrium (Magnuson, 1973) and oxygenate the blood (Roberts, 1978), muscular metabolism continuously generates heat as a byproduct. Tunas get rid of this excess, but, on the other hand, the heat can be used by the tuna to enable them to forage in cold waters.
Metabolic mechanism for thermoregulation
Among all bony fish, the Thunnini are unique in their ability to regulate their body temperatures, due to a complex counter-current heat exhanger system, also called the rete mirabile (miraculous network) (Stevens and Neil, 1978). The only other fishes with this system are some sharks of the family Lamnidae (Collette, 1978).
Rete mirabile (from Weinheimer, 2003)
The tuna maintain their body temperatures above that of the ambient water by passing arterial blood through vascular countercurrent heat exchangers. All species of tuna have a lateral rete, consisting of small arteries branching from the lateral subcutaneous arteries and small veins emptying into the lateral veins (Graham et al., 1983). In addition, many species of tuna also have a central rete within the vertebral haemal canal, consisting of arteries from the dorsal aorta and veins to the posterior cardinal veins (Stevens and Neil, 1978). The arterial blood is, then, warmed by the venous blood that flows through the red swimming muscles (Holland et al., 1992).
Behavioural mechanisms for thermoregulation
Combined with the physiological mechanisms, movements into cooler water will facilitate heat dissipation (Bushnell and Holland, 1997).
Advantages of thermoregulation
Thermoregulation allows the tunas to sustain high swimming speeds for long periods and to recover quickly after prolonged exertion (Carey et al., 1971), because most biochemical reactions proceed more rapidly at higher temperatures. Therefore, according to Bushnell and Holland, 1997, elevated body temperatures allow:
red muscle to contract more quickly, approaching the contraction rate of white muscle and consequently, contributing to high-speed swimming resulting from white muscle contractions
Trophic relations and growth
The following three stages can be distinguished:
larvae (recently hatched individuals which are considerably different in appearance from juveniles or adults)
Trophic position of larvae
Larvae of tunas and tuna-like fishes live in warm surface waters and feed primarily on the zooplankton including small crustaceans and larvae of crustaceans, fishes, molluscs and jelly-fish. Larvae of tunas and tuna-like species are preyed upon by zooplankton foragers, such as larger larvae and early juveniles of pelagic fishes. Cannibalism is, therefore, an important cause of mortality for tuna larvae.
Trophic position of juveniles and adults
Tunas and tuna-like fishes in the oceanic food web
Tunas and billfishes prey on fish, squid and crustaceans. The larger individuals (wahoo, bonitos, tunas and billfishes), which feed on pelagic fishes, are positioned at the top of the trophic web. The smaller individuals (juvenile tunas and billfishes, mackerels and seerfishes) prey on zooplankton (mainly crustaceans) and constitute part of the ration of large scombroids, sharks, cetaceans and seabirds. Analyses of stomach contents of yellowfin and skipjack tuna indicate that they feed on small epipelagic fishes between 1 and 10 cm in length (Roger, 1994). Since these preys of yellowfin and skipjack feed directly on zooplankton (mainly copepods), it seems that the tunas are at the top of a short food web, which is probably very efficient from the point of view of energetics.
Position of tunas and tuna-like fishes in the oceanic food web
Tunas and billfishes are opportunistic feeders. At the species level, they do not have strong preferences for certain types of prey. However, on a regional scale and at a given time, a few species may represent almost all of the food of fish of a specific age group (Cayré et al., 1988). Tunas and billfishes prey on pelagic or epipelagic fishes (including juveniles of tunas), crustaceans and cephalopods (squids). Yellowfin and bigeye tunas as well as swordfish feed on mesopelagic fishes (Ménard et al., 2000, Allain, 2005). Coastal tunas feed on neritic and epipelagic prey (Olson and Boggs, 1986). Larger tunas feed on small pelagic fishes such as mackerels, small tunas, carangids or flying fishes.
Example of preys that are often found in the stomach of tunas
Foraging behavior of juveniles and adults
Tunas and billfishes are predators that locate their prey visually. To satisfy their food requirements tunas and billfishes have to swim long distances. Their type of locomotion is, therefore, particularly adapted to the search for prey in a large volume of water with the least expenditure of energy. However, they appear less effective than transient predators, such as esocids, in actually capturing the prey (Webb, 1984). To compensate for this, tunas tend to break up schools of prey, producing disorientation and straggling, and/or search for prey in schools (Webb, 1984; Partridge, 1982). Tunas can detect minute traces of scents of oils, proteins and amino-acids of the mucus layer produced by their prey. When prey is detected, some tunas show changes in their behavior consisting of a general increase of activity (also called frenzy): increase in swimming speed, change in swimming pattern, jaw snapping and display of dark stripes on the flanks. Tropical tunas and swordfish often dive down at significant depths below the thermocline (commonly at 500 m) to feed on mesopelagic fishes (Holland et al., 1992). It is commonly believed that tunas feed during the day. However, sonic tracking experiments show that some tunas feed also at dusk, when mesopelagic micronecton migrate toward the surface (Bard et al., 1998).
Growth of juveniles and adults
Growth ratesMost scombrids grow rapidly and reach their adult sizes in a few years. Average growth rates vary according to the species, the age and the location. In general, larger tunas grow to about 40 to 55 cm the first year, then the annual growth rate ranges between 20 to 30 cm per year decreasing with age. Tuna species attaining only small sizes grow to 20 to 35 cm in the first year and their annual length increments rapidly decrease to less than 10 cm. In the Atlantic and Indian Oceans, several studies have shown that yellowfin grow rapidly during the first year, slowing their growth during the next one or two years and then having again a fast growth before gradually slowing down as the maximum size is approached. Seerfishes and mackerels have also a fast growth during their first years of life. Sizes of 35 to 45 cm at age 1 year are common.
Billfishes can grow to more than 80 cm during their first year of life. After this very fast juvenile growth, adult growth rates are comparable to those of tunas.
Weights and lengths rangesThe maximum weights attained by tunas range from about 1 to 2 kg for bullet and frigate tunas to more than 600 kg for Atlantic bluefin tuna. The maximum lengths attained by tunas range from about 50 cm for bullet and frigate tunas to more than 300 cm for Atlantic bluefin tuna.
Seerfishes, mackerels and bonitos are relatively small (less than 1 meter in length), except for some species of seerfishes such as the king mackerel or the narrow-barred king mackerel which grow to more than 240 cm, for 70 kg.
The smallest billfish is the Mediterranean spearfish, which reaches a maximum length of a little more than 180 cm. The largest billfishes are the black marlin and the Indo-Pacific blue marlin, which reach lengths of more than 4 m and weights of more than 600 kg.
Relative sizes of tuna main market species
* Sarda australis, S. chiliensis, S. orientalis, S. sarda
Source: IGFA (1995), note that IGFA records are not anymore on public domains and records might have changed since 1995.
Sexual dimorphism is observed with billfishes. For example, Atlantic Blue marlin exhibits sexually dimorphic growth patterns: somatic growth of male slows at about 100 kg round weight and males do not exceed 150 kg, while females can reach up to 910 kg (Wilson et al., 1991). Similarly, swordfish exhibit a sexual dimorphism of growth: males grow more slowly and reach a lower asymptotic length than females.
Longevities of tunas vary from a few years for the smaller tunas to 12 to 15 years for the larger tunas. The longevity record for tunas is about 20 years for the Atlantic bluefin tuna (Cort, 1990) or 25 years for the southern bluefin tuna (Gunn et al., 2008). Longevities of 15 to 27 years (Pacific blue marlin) or 28 years (Indo-Pacific blue marlin) have been estimated for billfishes and for swordfish. Longevities of seerfishes and mackerels are moderate with some records at 16 years for the Spanish mackerel.
For larger tunas and billfishes, adult natural mortalities range from 0.2 to 0.6. Juvenile natural mortalities are higher. Little is known on natural mortalities of seerfishes and mackerels.
Tuna spawn in open water close to the surface. Eggs are released by females in several batches. For example, yellowfin tuna in the Pacific spawn nearly every day. However, for some species like the bluefin species, spawning is more seasonal.
Spawning areas and seasons
Tunas spawn in areas where the survival of their larvae is greatest. Most species of tunas spawn only in waters where the surface temperatures are greater than 24°C. Tropical tunas appear to spawn in equatorial areas all year around and at higher latitudes during the warm seasons. Albacore and bigeye appear to migrate annually from temperate feeding areas to tropical spawning areas. Bigeye larvae are less abundant than those of other tropical tunas, and are found mainly in equatorial waters in which the temperatures are greater than 28°C (Collette and Nauen, 1983). Atlantic bluefin, Pacific bluefin and southern bluefin tuna exhibit a homing behavior when they mature, and return to restricted areas in the Atlantic, Pacific and Indian Oceans to spawn. It is commonly accepted that there is a homing behavior, but to a lesser extent, in yellowfin in the Atlantic Ocean. Billfishes appear to spawn seasonally in warm tropical and subtropical waters.
Maturity and fecundity
With the exception of bluefin tunas (Thunnus thynnus, T. orientalis and T. maccoyii), most tunas, seerfishes, mackerels and billfishes reach their age of maturity between 2 and 5 years of age. Due to their sexual dimorphism of growth, male billfishes are mature at a smaller size than female billfishes.
Maturity of tunas and billfishes
The batch fecundities of most species of tunas range from 2 to 70 million eggs, the lowest fecundity being for albacore and the highest for skipjack tuna and other small-sized tunas. Known batch fecundities of mackerels range from 300 000 to 1 500 000 eggs. Little is known on fecundities of seerfishes. Fecundity of wahoo has been estimated to 6 million eggs. Less is known on the reproductive biology of billfishes, but batch fecundity is estimated to range between 1 and 7 millions of ovocytes. Swordfish batch fecundity was estimated to 3.9 millions eggs in the Atlantic.
It has been shown that for yellowfin, bigeye and albacore, the sex-ratio changes with the age of the fish with a predominance of males for the larger sizes. A predominance of females has also been observed for medium-sized Atlantic bluefin tuna. For skipjack, differences in the numbers of males and females have been observed locally. Predominance of females at older ages is observed for several species of billfishes.
AcknowledgmentMichel Goujon and Jacek Majkowski, who compiled the information contained in this presentation, are grateful to:
- Aureliano Gentile for editing and incorporating information into FIGIS and
- Ignacio de Leiva, Marcella Pesce, Yves Jaques and A. Gentile for their contribution to the former version of this document.
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