Thunnus thynnus

Thunnus thynnus (Linnaeus, 1785) –

Bluefin tuna, ϗόϗϗινος τόνος/όρϗυνος

It is the emblematic tuna, the largest fish in the Mediterranean. Bluefin tuna is a cosmopolitan, highly migratory, a schooling fish, able to tolerate a wide range of environmental conditions (Arrizavalaga et al. 2015). Its presence, feeding and reproduction is strongly influenced by these environmental conditions, such as the temperature and salinity of the water, and for this reason its migration trajectories fluctuate from year to year (Druon et al. 2011; Fromentin et al. 2014). Its current absence from the Black Sea, which on literary and historical evidence appears as a rich fishing ground and also possibly a reproduction area in the past, is the result of such changing conditions (including industrial pollution) since the 1970s (Mackenzie & Mariani 2012).

The maximum reported length of bluefin tuna exceeds 4 m and its maximum reported weight is 726 kg, (although there exist unverified reports by fishermen for individuals of 900 kg; Mather et al. 1995). Bluefin tuna in the Mediterranean reach maturity when approximately four years old (at 110-120 cm 2, 25- 30 kg; Fromentin 2006) and that is the age/size at which we expect them to perform their first reproduction migration. Tuna exhibit a rapid growth in the first years, but they keep growing all through their life, which may reach 30 years. There is a standard correlation between their age and length/weight (Arena et al. 1980). A bluefin tuna has a high metabolic rate, which allows it to maintain its body temperature in a wide range of environments and also to achieve very high swimming speeds. As a result, its blood is copious and bright red, being rich in oxygen.

Spawning of bluefin tuna in the Mediterranean, especially in the eastern basin, has been a much-debated issue. It is generally agreed that it takes place in the warm waters (> 24°C) of specific and restricted locations: around the Balearic Islands, Sicily, Malta, Cyprus (Fromentin 2006), all well-known tuna-fishing areas in antiquity (Curtis 1991: 116-118, 129), as well as the Black Sea in the past (Piccinetti & PiccinettiMarfin 1993). It usually occurs in May-June (Heinisch et al. 2008; Damalas & Megalofonou 2012). The exact spawning grounds, i.e., the locations towards which the reproduction migrations head, are still not well known. Bluefin tuna form dense schools on the reproductive leg of their migration and less dense ones after spawning and on their return trip to their feeding grounds. Young individuals feed mostly on zooplankton and older ones prey on schools of small pelagic fish and on cephalopods, such as squid (Sarà & Sarà 2007). Both juveniles and adults move through the water column; the older bluefin tuna can reach as deep as 500-1000 m (e.g., Brill et al. 2002). It is agreed that bluefin tuna tend to aggregate and feed along ocean fronts, where food availability is highest (Druon et al. 2011).

2. Length figures refer to “fork length”, from the tip of the nose to the point where the tail divides into two parts.

The frequency, timing and movements of bluefin tuna in the Aegean are reported from several sources, of different dates. Often there is no correspondence between their findings (Ninni 1922; Athanasopoulos 1923, 1924, 1926; Belloc 1961; Lefkaditou et al. 1988). Modern data (post-1980s) on bluefin tuna populations in the Aegean are relatively few compared to other Mediterranean areas. According to the most recent of these reports, bluefin tuna in the Aegean tend to appear in larger numbers at certain areas on their migrating route(s) (e.g., the Chalkidiki peninsula, northern gulf of Euboea, Sporades), but there is a diffused presence of tuna in other zones, such as Dodecanese or Lesvos (Lefkaditou et al. 1988). However, Ninni (1922) reported that tuna migrated north towards the Black Sea in two groups. The largest of them skirted the coasts of Asia Minor and the adjacent islands, including the Dodecanese, and the smaller group crossed the channel between Euboea and the mainland to enter the Pagasetic Gulf. The bluefin tuna schools were denser just off the Bosporus straits and along the Marmara Sea (Di Natale 2015 and references therein). The exact timing of the fishing for bluefin tuna in various locations in the Aegean varies considerably, but it roughly occurs in spring (spawning migration) and in autumn (feeding migration).

Most of the available reports are based on data provided by modern fishing vessels, which use fishing gear that is either very large (e.g., large nets several hundred meters long, and long lines several kilometers long) or recently introduced in the area (e.g., Japanese pole and line fishing for bluefin tuna in the Kavala Gulf after the 1980s, Lefkaditou et al. 1988). These vessels have access both to spawning and to feeding bluefin tuna. Although relevant data map the timing and geographic distribution of bluefin tuna in the Aegean, not all of them can be used as a predictive tool in research on tuna fishing in antiquity, where the available technology posed certain restrictions as to which fishing grounds and resources could be accessed. Certain observations, however, are useful. Medium-sized bluefin tuna (30-100 kg), for instance, are found off many of the Aegean coasts throughout the year, while large tuna (over 150 kg) are abundant only from April to September (Mather et al. 1995: 66, 67). It seems likely that the first category represent fish that feed in the area, while the second category are fish on the reproductive run, visiting the area to spawn.

Besides the time of the year and the inshore or offshore location of fishing operations, the lunar phase appears to be another important factor in tuna fishing. It has been observed that the probability of catching bluefin tuna exhibits a periodicity that coincides with the lunar circle and is linked to their predatory behavior. Fishermen in the Aegean refer to certain rich catches as the “full moon of May tuna” (Damalas & Megalofonou 2012). Additionally, Greek and Turkish fishermen at the beginning of 20th century, echoing Aristotle (HA 598b), reported that bluefin tuna migrate keeping their right eye towards the coast (Ninni 1923), so that their schools move anti-clockwise along the coasts. This observation is crucial to the prediction of their occurrence in different locations, and also to the construction of tuna fishing gear (see “Implications of Scombridae physiology to their fisheries in antiquity ”). Also relevant to the nature of tuna fisheries in the area (in terms of location and fishing gear) is the observation that, if a tuna school is encountered in the shallow coastal areas, it is more likely to be a large one (Damalas & Megalofonou 2012).