Big Eye Tuna – Thunnus obesus

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The Bigeye is close family of the Yellow n tuna and equally ferocious – but bigger! They are perhaps not as popular in South Africa simply because we don’t see them as frequently as the Yellowfin or Longfin, but they are still very much a prized species by anglers and restauranteurs alike.

Bigeye tuna is a schooling, pelagic and seasonally migratory species found in warm temperate waters of the Atlantic, Pacific and Indian Oceans. In South Africa they can be found in the same areas as other tuna, mostly concentrated in the deep waters off Cape Point in the Western Cape.

Many anglers struggle to distinguish the Bigeye from the Yellowfin tuna and there have been many catches of Bigeye gone unreported plainly because the angler thought he had caught a Yellowfin. In fact, at one time the Bigeye wasn’t recognised as a separate species but considered a variation of the Yellowfin. They are both typically ‘tuna-shaped’ in that rugbyball-like build, but the Bigeye grows to be even bigger than their yellow-finned cousins.

The Bigeye also differs in that their second dorsal and anal fins never grow as long as those of the Yellowfin. Also, in the Bigeye tuna the margin of the liver is striated and the right lobe is about the same size as the left lobe, whereas in the Yellowfin tuna the liver is smooth and the right lobe is clearly longer than either the left or the middle lobe.

They are large streamlined fish that can grow to as large as two metres and sometimes even longer! They have large heads and big eyes – hence the name. Their first dorsal is a deep yellow while the second dorsal and anal fins are a brownish yellow and may be edged in black. The pectoral fins are quite long and may reach the second dorsal fin but the second dorsal fins and anal fins never grow quite as long as those of the Yellowfin tuna.

Bigeye grow very large. The IGFA All Tackle record for a Bigeye caught in the Atlantic Ocean is a 197.31 kg monster caught off Cabo Blanco in Peru in 1957 by Dr Russel Lee. The biggest Pacific Bigeye ever caught according to IGFA standards was a 178 kg fish caught off Gran Canaria, Spain in 1996 by Dieter Vogel.

Bigeye tuna has a lifespan of up to 12 years and reach sexual maturity at four. Unlike the Yellowfin, the Bigeye is able to forage in cold, oxygen-poor conditions as well as surface waters. It is reported that Yellowfin tuna will not descend to depths that differ in temperature by more than 8° C from the surface temperature. The Bigeye, on the other hand, will swim down to waters 20° C colder than surface waters. Not only is the Bigeye less sensitive to colder temperatures than the Yelllowfin, but they are able to extract oxygen in very oxygenpoor conditions, their vision is much better in low light conditions, and their hearts function better in subsurface waters. Some cases have recorded Bigeye tuna at depths of 500 metres and temperatures colder than 5° C.

Nevertheless, they must return to the surface periodically to recuperate. They will typically spend large portions of the day in deep water and will return to surface waters during the night to depths of around 16 m on average. It is said that the Bigeye travels down to deep waters in order to secure prey without competition from Yellowfin, as they can’t dive as deep.

Bigeye, like all tuna, are carnivorous fish. They typically prey on other smaller fishes, crustaceans and cephalopods. Because the Bigeye and Yellowfin share similar ranges and sometimes even school together, they share a similar diet that includes squid, mullet, sardines, mackerel and so forth. But Bigeye also include some deep water species to their diet since they are able to dive deeper than other tuna.


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