A couple of weeks ago someone WhatsApped me a video that was doing the rounds on social media. It was of a 20-odd kilo tuna caught by the Bartho brothers on the KwaZulu-Natal coast in South Africa.
In the video they suggest that the fish is a southern bluefin tuna (SBT) Thunnus maccoyii. Several SBT are caught in SA each year, but they are typically caught about 600 km further south in the waters south of Cape Point by the Tuna Pole fleet or further off-shore in the pelagic longline sector. I couldn’t recall ever hearing of one from the warm waters of KZN, especially not near-shore, in 20 m where they were targeting yellowfin tuna. So my interest was immediately piqued. I scrolled back and forth through the video. There was something odd about this fish. I have never seen a small SBT in the flesh, but have caught several of their northern cousins Thunnus thunnus in the Mediterranean, and had only a few weeks prior caught my first SBT off the Cape. This tuna just didn’t look right. Firstly, it had black finlets or ‘scoots’ (the series of small fins between the second dorsal and anal fin to the tail) which seemed very weird to me. The second dorsal and anal fins, which make up the sickles, were silver with black edges, which also struck me a weird. SBT typically have yellow finlets and a slightly yellow second dorsal, much like yellowfin T. albacares and bigeye tuna T. obesus, which can often make them hard to distinguish.
There are not many tuna in the world that have similar proportions with those colours. I started running through the possibilities in my head. Longtail tuna, T. tonggol, they have black finlets and dark second dorsal and anal fins, but are largely restricted to the northern parts of the Indian Ocean and Indo-West Pacific and are seldom found south of the equator in the western Indian Ocean. I’d seen a few on the tuna purse seine vessels during my years in the Seychelles. While the colours were close, the proportions were way off. Blackfin tuna, T. atlanticus, as the Latin name suggests are found exclusively in the Atlantic, generally restricted to the western half, so it would be very lost, and very big too, as they average only about 5kgs. Slender tuna, Allothunnus fallai, they have a dark second dorsal, it is within their distribution, but have a tiny pectoral, the proportions were all wrong, this was not a slender tuna. I had a hunch, but would need to see more angles to be convinced. So I contacted Daryl Bartho and he sent me a few more photos of the fish.
As it turns out they didn’t help my hunch much. I was still quite uncertain. The main thing that was bothering me was the length of the pectoral fin, in the various photos and video it just looked too long to be an SBT.
I decided to reach out to an old friend and colleague in Hawaii, David Itano. If there was anyone that would know it would be him. Dave has worked on commercial vessels catching or tagging tuna for most of his life and has authored several identification guides for tunas of various sizes and states (fresh or frozen) for observers and processors in industrial tuna fisheries. I WhatsApped him the video and photos late one night and must have caught him as he was getting up on the other side of the planet because he replied immediately. Technology!
I said “Dave check out this tuna caught near Durban in SA, thought to be an SBT, I’m not convinced.” I intentionally left out my hunch to avoid influencing his judgment. He replied “looks like and oddly colored yellowfin”, “we saw a yellowfin here with a silver second dorsal a few years ago”. And a few minutes later again, “That’s definitely not an SBT. Pectoral fin way too long. Eye is too large. All morphology says yellowfin that I can see except pigmentation.”
Now I had more confidence in my hunch! I had never heard of pigment variation in a yellowfin or any tuna for that matter, apart from rare cases of albinism. Moments later I got an email from Dave with the old photos of the fish form Hawaii. There it was! This one was quite a bit bigger and had much longer sickles, which is typical of the yellowfin in Hawaii, but unmistakably a yellowfin.
And a little while later he sent a couple more from a silver-finned yellowfin caught in the Cooke Islands that had also been sent his way some years back.
I was convinced. This was not an SBT but rather a strangely pigmented yellowfin. I suspect this condition is extremely rare. Millions of yellowfin tuna are caught around the globe each year, yet there is no mention of any pigment variation in the literature. I sent the Hawaii pics on to Daryl and he agreed completely. Mystery solved.