Availability of “seed” for capture-based aquaculture

Bluefin tuna shoal by size, sometimes together with other species (e.g. yellowfin, albacore or skipjack). Natural stocks are known to fluctuate considerably year-by-year, and these fluctuations are attributed to either food availability and/or predation at some period of their early life history – the so-called “critical period” (Cushing 1974, 1990). “Shoalfishes” normally adopt ecologically antipredatory behaviour (Masuda and Tsukamoto 1999).

This mechanism is of vital interest to fishermen, because it largely determines the success or failure of fishing techniques and tactics and, due to this behaviour, bluefin tuna schools have been heavily exploited. Tuna prey on other schooling fish e.g. anchovies, and the effect of their capturing activities are visible on the sea surface. This predatory behaviour allows fishermen to easily detect bluefin tuna schools.
As bluefin tuna are widely distributed and migrate for thousands of kilometres, a general understanding of tuna movement and migrations is necessary for a more accurate analysis of bluefin tuna size availability for capture-based aquaculture. In the northeastern Pacific Thunnus thynnus tend to migrate northward along the coast of Baja California and California, from June to September. Off the Pacific coast of Japan, they migrate northward in summer and southward during winter. Adults may enter the Sea of Japan from the south in early summer and move as far north as the Okhotsk Sea; most leave the Sea of Japan through the Tsugara Strait, north of Honshu (www.fao.org/fi/sidp/species/th_th_ht.htm).

There are only two confirmed spawning locations: the Gulf of Mexico in the Western Atlantic and the Mediterranean Sea in the Eastern Atlantic. Spawning in the Gulf of Mexico occurs between mid-April and mid-June when females (typically 8 years old) each release approximately 30 million eggs. According to Buck (1995), sexual maturity is reached at the age of 5 to 8 years, depending on the stock. Some fishery biologists believe that eastern Atlantic bluefin tuna reach sexual maturity several years earlier than those in the western Atlantic (possibly as young as ages 4 or 5), a claim disputed by some other biologists. All the bluefin tuna collected during the spawning season in the Balearic Islands in 2001 were between 19 and 34 kg and had fully matured (spawning) ovaries – a fact that would apparently confirm that the eastern stock of Atlantic bluefin tuna are able to spawn from 3 years old (Abascal, Megina and Medina 2003). Further studies of bluefin tuna biology are clearly necessary. In 2002, a captive-raised bluefin tuna (145.6 cm total length, 35.2 kg body mass) was found to be a hermaphrodite, the first record of hermaphroditism in this species (Sawada et al. 2002). The highest density of bluefin larvae, the primary indicator of spawning, occurs in the northern Gulf of Mexico, with lesser larval concentrations appearing off the Texas coast and in the Straits of Florida (NRC 1994).
In the Eastern Atlantic, spawning occurs exclusively in the Mediterranean: usually from May to July in the Balearic Sea, the South Tyrrhenian Sea, and in the southern Mediterranean. Aggregations of juveniles have been reported in the eastern Aegean sea, in the Southern Adriatic Sea, in the Tyrrhenian Sea, in the Ligurian Sea and in the Balearic area, sometimes close to the coast (De la Serna et al. 2003). The larval abundance of tuna around the Balearic Islands was determined between June 15 and July 10, 2001, as part of a Spanish research project. The highest abundance was observed south of Minorca and along the midsection of the Mallorca channel (Garcia-Gomez et al. 2003).
Bluefin in the South Atlantic belong to a distinct southern population, the southern bluefin tuna (Figure 83), with known spawning areas south of Java, Indonesia. The aggregations of southern bluefin tuna juveniles swim near the surface during their migration between the spawning areas and western Australia, where individuals of class O are concentrated to the dispersion areas, close to Tasmania and New Zealand.

Figure 83. Southern bluefin tuna (Thunnus maccoyii) (Photo: L. Mittiga)

Large size, slow growth, late age-at-maturity, short spawning duration and long life spans are characteristic of temperate seas bluefin tuna. The turnover of these species is slow and has characteristics similar to those of the “k-selected” species (Odum 1998). Atlantic bluefin tuna and southern bluefin tuna live in temperate to cold waters but only reproduce in warm waters. Reproduction could be the key ecological factor for the fish from temperate and cold seas, because of the higher risk of starvation for larvae and young juveniles. It could be argued that tunas have partially resolved this difficulty, by means of yearly reproductive migrations into warmer and more stable environments. However, spawning duration and frequency are much shorter for temperate than tropical sea tunas (Fromentin and Fonteneau 2001).
The Mediterranean fishery for bluefin tunas is based on migration patterns. At the beginning of the season, from May to July, those that arrive in the Mediterranean Sea are in the “genetic phase” (large pre-spawned fish); at the end of the season, from mid July to September, they are on their way out of the Mediterranean, moving into the Atlantic Ocean for feeding (the “trophic phase” or large post-spawned fish) (Doumenge 1999). The first commercial tuna capture-based farm, established in Ceuta (Spain), was based on the capture of large post-spawned tuna (mostly over 150 kg). In these locations, catches have shown that the average weight of bluefin tuna leaving the Mediterranean is in the range 185-215 cm TL (from 106 to 150 kg), which is 18% less than the specimens of the same class entering the Mediterranean. This is the effect of post spawning weight loss.
Since 1997, most of the tunas caught for capture-based aquaculture are moved from the catching area, inside cages, to the on-growing cages. In Croatia, the capture of juveniles for capture-based aquaculture occurs at the end of the spring and in early summer. The individuals captured range from extremely small fish (less than 10 kg, sometimes undersized or just within the legal minimum landing size set by ICCAT of 6.4 kg) to somewhat larger fish (20-80 kg). They are caught by Italian and Croatian purse seiners in the Adriatic Sea (Miyake et al. 2003; Tudela 2002b). Spanish, French and Italian seiners supply most of the fish for the farms in Murcia (Spain). These fish are caught in the Western Mediterranean, particularly near the Balearic Islands; they range from small (20-80 kg) to medium size (80-120 kg) and are mostly immature, but also include some matured fish of larger sizes (Miyake et al. 2003). Bluefin tuna for the Maltese farms are mainly caught in international waters, the main fishing period being from May to July. The fish size ranged from 80-250 kg in 2000, and from 50 kg to 620 kg in 2001 (Miyake et al. 2003).
Fishing for northern bluefin tuna (Thunnus thynnus orientalis subspecies) in Mexico for capture based aquaculture is more difficult than in any other part of the world, due to problems with the depth at which the fish are able to swim there. The main fishing period is from July to late August, but depending on fishing locations, the season can extend into November. The catches occur mainly at night and rely on the presence of bioluminescence to locate bluefin tuna schools. Individuals range from 15-45 kg, with smaller fish being caught in southern areas and larger ones in northern regions (Sylvia, Belle and Smart 2003).
In contrast to the practice in other regions, juveniles of 150-500 g body weight are caught off the coastal areas of Japan during spring and summer (Ikeda 2003). Fishing activities in Australia have centred in South Australia, mostly established in Port Lincoln, which is situated in the South Australia Gulf. The fishing fleet consists of purse seiners, and a “rod fishing” flotilla. Specimens have a range from 20-25 kg, although some schools of 50 kg specimens are sometimes encountered. The fishing season lasts from December to May.