Chapter 7 tuna introduction




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CHAPTER 7

TUNA
Introduction
Tuna are collectively among the more valuable of marine finfish, and because of their tendency to undertake long horizontal migrations, their management as a fisheries resource requires international collaboration. Tuna belong to the family Scombridae, which includes the mackerels and bonitos in addition to tuna. Tuna are assigned to the tribe Thunnini, which includes four genera and thirteen species. The seven species primarily involved in the commercial fishery are listed in Table 7.1.


Table 7.1. Principal species of tuna involved in commercial fishing

Common name

Scientific name

Albacore

Thunnus alalunga

Bigeye

Thunnus obesus

Skipjack

Katsuwonus pelamis

Yellowfin

Thunnus aobacares

Atlantic bluefin

Thunnus thynnus

Pacific bluefin

Thunnus orientalis

Southern bluefin

Thunnus maccoyii

It is fair to say that tuna represent one of the peaks of fish evolution (Bardach and Ridings, 1985). They are voracious predators with a torpedo-like body shape and an ability to elevate their body temperature above that of the surrounding water. Although strictly speaking tuna are not warm-blooded, their ability to regulate their body temperature to some extent probably explains the fact that they can swim faster than most other fish and are able to recover rapidly after rigorous exercise. Their so-called thermoconserving ability arises from adaptations of their circulatory system and musculature. These adaptations are associated with high metabolic costs, and as a result the distribution of tuna is somewhat restricted with respect to temperature and oxygen concentrations. Skipjack, the smallest of the tuna species, have the largest surface-to-volume ratio and hence the most difficult job of thermoconserving. They are confined to tropical and subtropical waters. Skipjack are also unique among the tunas in that adults have no swim bladder. While this characteristic allows them to undertake very rapid vertical swimming bursts, it also forces them to expend more energy in routine swimming than is the case for the other more buoyant tuna species, which have swim bladders. As a result skipjack have a higher oxygen demand than the other tunas, and this fact restricts their vertical distribution.

The high metabolic rate of tunas is associated with an unusually high food requirement compared to cold-blooded fish. Daily food consumption, especially for juvenile tuna, can be as high as 25-30% of their body weight. For comparison, other juvenile fish typically consume about 15% of their body weight per day, and for adult fish daily food consumption is 3-8% of body weight (Bardach and Ridings, 1985). Despite their high metabolic requirements, tuna are able to go for up to two weeks without food, an ability that serves them well as the travel across the open ocean, where the distribution of food can be very patchy.

Overall the distribution of tuna is primarily regulated by temperature and oxygen concentrations. Because the thermocline as well as the oxygen minimum zone is shallower on the eastern side of ocean basins (Chapter 1), the depth range of tuna is generally shallower on the eastern side of ocean basins. Historically this fact has facilitated the use of purse seines to catch tuna in places such as the eastern tropical Pacific (Chapter 7). Not surprisingly, the distribution of tuna is also influenced by the availability of food, which in turn is controlled by factors such as upwelling and turbulent mixing that impact the delivery of nutrients to the euphotic zone.

Certainly one very interesting characteristic of tuna is their tendency to undertake very long migrations. Knowledge concerning these migrations has come from the recapture of tagged fish. Based on such studies, we have learned that albacore and skipjack will migrate across the entire breadth of the Pacific, and bluefin tuna have been shown to undertake north-south migrations that may extend over 60 degrees of latitude (Bardach and Ridings, 1985). How tuna navigate across the vast expanse of the open ocean is not entirely clear, but there is good evidence that they have a geomagnetic sense. Skipjack, for example, have a high concentration of magnetite crystals in their ethmoid bone1 complex, with no occurrence of magnetite in any other organ or structure, and live skipjack can be trained to respond to a change in the magnetic field surrounding them (Walker, 1982). Although tuna may also navigate by other means, e.g., simply following currents, it is therefore possible that they share the ability of animals such as homing pigeons and salmon in being able to orient their migrations relative to variations in the Earth’s magnetic field.

Some relevant information on size and age of tuna are summarized in Table 7.2. Bluefin are the largest of the tuna species, with adult weights of several hundred kilograms and lifespans of several decades. The other species attain weights of typically 10-100 kilograms and have lifespans of roughly 10 years.




Table 7.2. Pertinent information on commercially important tuna species

Species


Length

(cm)


Weight

(kg)


Age of sexual maturity

(years)


Lifespan

(years)


Albacore

60-90

10-20

5

10

Bigeye

80-180

15-20

4

10

Skipjack

30-80

8-10

2

12

Yellowfin

40-180

5-20

3

10

Atlantic bluefin

45-450

135-680

4-8

15-30

Pacific bluefin

150-300

300-555

6

30

Southern bluefin

200

200

8-12

40

The commercial catch of skipjack, yellowfin, bigeye, and albacore tuna are summarized in Fig. 7.1. All of these catches have increased more-or-less steadily since 1950. Of the four species, only the skipjack is currently considered to be underfished. The others stocks are felt to be fully exploited.


Skipjack
Skipjack account for more of the commercial tuna catch by weight than any other species. Virtually all the catch occurs in the Pacific and Indian oceans. The Pacific contributes about 70% of the catch and the Indian Ocean 24%. The west central Pacific alone accounts for 50% of the catch.

Figure 7.1. Commercial catches of skipjack, yellowfin, bigeye, and albacore tuna.


Skipjack are an epipelagic species that occur in waters with a temperature range from 20 to 30oC. Adults are sometimes found in waters as cold as 15oC. Skipjack are found in all tropical and subtropical waters with the exception of the Mediterranean Sea and Black Sea. They spend the day near the surface but may descend to depths of as much as 250 meters at night. Skipjack have a definite tendency to school, frequently under floating objects, but they may also associate with other animals, including dolphins, sharks, and whales. Skipjack schools tend to contain tuna within a fairly narrow size range, perhaps because the small fish find it difficult to keep up with the large fish. Small skipjack may school while foraging. Large skipjack (length greater than about 20 cm) tend to feed alone. Their diet includes clupeids, crustaceans, and mollusks, but may also include other skipjack, i.e., cannibalism is quite common. Peaks in foraging occur around dawn and dusk. Predators on skipjack tuna include sharks, billfish, wahoo, yellowfin tuna, and, in the case of small skipjack, even seabirds.

Like other tuna, skipjack are oviparous2, and a typical adult female may produce 80,000 to 2 million eggs per year. In tropical waters skipjack spawn throughout the year. At higher latitudes spawning is limited to the warmer months of the year. Spawning occurs over a much narrower range of latitudes than the area otherwise occupied by the adult fish. In the western Pacific the spawning area is substantially larger than in the eastern Pacific, but this is not the case in the Atlantic and Indian oceans. The migrations of skipjack suggest that they tend to move toward the west with the equatorial current systems to their preferred spawning grounds. They then continue to drift with the flow of the subtropical gyres toward higher latitudes and then to the east, eventually reaching the productive waters of the eastern boundary current upwelling systems.



Most skipjack tuna are caught with purse seines, although the use of hook-and-line is still common in artisanal fisheries. The use of fish aggregation devices (FADs) has been a major impetus to the use of purse seines for catching skipjack in the western Pacific. Some skipjack are also taken as part of the bycatch in longline fisheries. The major nations involved in the skipjack fishery are summarized in Table 7.3. The principal market for skipjack is canned tuna. Skipjack, along with yellowfin tuna, are considered to be “light meat” tuna and are so-designated in canned tuna. There is also a market for fresh skipjack in raw fish dishes, where it is sold as “katsuo” for finger sushi and sashimi. In Japan, smoked skipjack is marketed as “katsuo-bushi”. Skipjack are also marketed as frozen loins, frozen fillets, and fresh whole fish. The principal markets for skipjack tuna are Japan (sashimi), western Europe, and the United States.
Yellowfin
As their name suggests, yellowfin tuna have bright yellow fins. Their geographical distribution is similar to that of skipjack, roughly 40oN to 40oS latitude, as is their preferred temperature range, 20-30oC. They will occasionally move into colder water, apparently for feeding (Bardach and Ridings, 1985). Their spawning area covers a much narrower range of latitudes and occurs throughout the year near the equator. At higher latitudes spawning is restricted to the summer months. Like skipjack, yellowfin feed visually during daylight hours. They are more-or-less indiscriminate foragers, the size of their prey being limited only by the gaps between their gill rakers and the distensibility of their esophagus (Bardach and Ridings, 1985). As noted above, yellowfin tuna are known to forage on skipjack tuna.


Table 7.3. Principal nations contributing to the catch of skipjack tuna in 2002.

Country

Principal fishing area(s)

% of catch

Japan

Northwest and west central Pacific

13.6

Indonesia

West central Pacific and eastern Indian

11.7

Taiwan

West central Pacific

11.5

South Korea

West central Pacific

8.6

Spain

Western Indian and east central Atlantic

6.9

Maldives

Western Indian

5.7

Philippines

West central Pacific

5.4

United States

West central Pacific

4.4

Yellowfin tuna tend to associate with dolphins more than any other tuna species. As noted in Chapter 7, the reason for this association is poorly understood. One possibility is that the association helps both species find prey. Both tuna and dolphins have excellent vision. Tuna also have a keen sense of smell, whereas dolphins have no olefactory sense at all. On the other hand, dolphins are able to track prey using ultrasonic echolocation. Whether a school of prey is smelled by the tuna or “heard” by the dolphins, each species could take advantage of the sensory strengths of the other to home in on a meal.

About 67% of the commercial catch comes from the Pacific and 22% from the Indian Ocean. The central Pacific accounts for 57% of the catch. Most of the yellowfin catch is accounted for by purse seining, although yellowfin are also taken by longlining and to a much lesser extent by trolling and hand lining. The primary market form is canned tuna. Like the skipjack, yellowfin tuna is classified as “light meat” tuna. There are also markets for fresh fillets (sashimi), fresh whole fish, frozen loins, and smoked fish.

Stock assessments indicate that there is little or no prospect for sustainable expansion of the yellowfin tuna catch. The principal nations involved in the fishery at the present time are listed in Table 7.4. Unlike the skipjack fishery, the western and eastern central Pacific make roughly equal contributions to the overall catch, 32% and 25%, respectively. Mexico and Venezuela are the principal yellowfin fishing nations in the east central Pacific. Indonesia, the Philippines, Japan, and Taiwan account for most of the catch in the west central Pacific. Indonesia, Japan, Taiwan, Spain, and France share most of the catch in the Indian Ocean. The principal markets for yellowfin tuna are Japan, western Europe, and the United States.




Table 7.4. Principal nations contributing to the catch of yellowfin tuna in 2002.

Country

Principal fishing area(s)

% of catch

Mexico

East central Pacific

11.1

Indonesia

West central Pacific and eastern Indian Ocean

10.6

Venezuela

East central Pacific and southeastern Pacific

9.6

Philippines

West central Pacific

7.5

Japan

West central and northwest Pacific, western Indian Ocean

7.4

Taiwan

West central Pacific and western Indian Ocean

7.2

Spain

Western Indian Ocean and east central Atlantic

6.7

France

Western Indian Ocean and east central Atlantic

5.1



Bigeye
Bigeye tuna are sometimes hard to distinguish from yellowfin, but they swim at greater depths than either skipjack or yellowfin tuna. Because of this fact they have a higher fat content (for insulation), and this characteristic makes them particularly attractive for the Japanese sashimi market. They are found at latitudes ranging from 47oN to 40oS. The Pacific, Indian, and Atlantic oceans account for roughly 50%, 30%, and 20%, respectively, of the commercial catch. Small bigeye are caught as bycatch of purse seining for skipjack and yellowfin. The larger and more valuable bigeye are taken with long lines. The meat of bigeye tuna turns light grey and sometimes darkish after cooking or grilling, a characteristic that makes it less suited for canning than skipjack, yellowfin, or albacore tuna. By far the most popular market forms for bigeye tuna are fresh fillets (sashimi) and fresh whole fish.

Not a great deal is known about the ecology of bigeye tuna, and scientists do not agree as to whether the bigeye resource is being fully exploited. The bycatch of small bigeye associated with the skipjack and yellowfin purse seining fisheries is a concern, since it could adversely affect reproduction.



The principal nations involved in the bigeye tuna fishery are summarized in Table 7.5. Unlike the catch of skipjack and yellowfin, this fishery is dominated by two nations, Taiwan and Japan, which together account for almost half the catch. The principal market is Japan, where bigeye sashimi is in high demand.



Table 7.5. Principal nations contributing to the catch of bigeye tuna in 2002.

Country

Principal fishing area(s)

% of catch

Taiwan

Indian Ocean, west central Pacific, southeastern Atlantic

24.2

Japan

Indian Ocean, central and southeastern Pacific

23.7

South Korea

East central Pacific

7.4

Spain

Western Indian Ocean, east central Atlantic

7.0

Ecuador

Southeastern Pacific

4.9

Indonesia

Eastern Indian Ocean

4.9

China

East central Pacific

4.3

USA

East central Pacific

2.9



Albacore
Albacore are basically a temperate water species of tuna. In the northeastern Pacific, for example, 19oC seems to be their preferred upper temperature (Bardach and Ridings, 1985). The principal fishing areas are in the western and central Pacific. Together these areas account for 58% of the total albacore catch. The north and south Pacific albacore stocks are known to be disjoint and should be managed as separate stocks. Table 7.6 lists the principal nations involved in the albacore fishery. As is the case with bigeye tuna, Japan and Taiwan dominate the catch. The Japanese catch is taken primarily from the western and northwestern Pacific. The Taiwanese fishery is much more global, but with a significant bias toward the southern hemisphere. The albacore stock in the northern Pacific is moderately exploited. Other stocks appear to be fully exploited or over exploited at the present time.

Due to its white appearance, albacore tuna meat is sometimes referred to as “chicken of the sea”. It is the only canned tuna marketed as “white tuna”. Due to its unusual bone structure and the soft consistency of its meat, albacore cannot be filleted. Fresh or frozen albacore is available only as loins/steaks. There is no purse seine fishery for albacore. Virtually all albacore are caught either with pole-and-line, surface trolling, or long lines. The former methods target younger albacore. Long lines take the older, deep-swimming fish. Albacore are marketed primarily as canned tuna. The major markets are the United States, Canada, Japan, and Spain.





Table 7.6. Principal nations contributing to the catch of albacore tuna in 2002.

Country

Principal fishing area(s)

% of catch

Japan

Northwest and west central Pacific

28.8

Taiwan

Southwest and central Pacific, southwest and west central Atlantic, western Indian Ocean

24.6

USA

Northeast and southwest Pacific

7.4

Spain

Northeast Atlantic

4.5

Fiji

West central Pacific

3.4

South Africa

Southeast Atlantic

2.8

American Samoa

East central Pacific

2.5

New Zealand

Southwest Pacific

2.4

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