Category: High Risk

FOR many anglers the capture of a yellowfin tuna is their first encounter with a true bluewater gamefish. Memories of the powerful runs of a decent sized yellowfin will live on in their mind, often forever. The sheer power that a big yellowfin, with its "afterburners" glowing, can generate once it hits a lure has to be seen to be believed.

I still recall a pair of barrel-gutted yellowfin hitting trolling lures we had only just run out at the Rottnest Trench. We had just finished a filming session with Steve Starling for Channel 7, and dropped everybody off on a larger vessel to continue filming for the day. The boat they were on was still in view when the scream of our 50Ws jolted me into gear.

After their characteristic rolling strikes this pair of big `fin would have taken 300 metres-plus in the first run alone! The ratchet noise was so loud that I thought the reels were going to explode. But they didn't and both fish shook the hooks, so we never got to find out just how big they really were.

In Hawaii yellowfin tuna are called ahi meaning fire. That name says it all. In Australia they are often referred to simply as `fin. It wasn't too many years back that a captured yellowfin tuna was just bait. Not so these days. As soon as a decent yellowfin hits the deck it's iki jimi spiked, cut properly for bleeding and on ice in no time. Lightly marinated, chargrilled and served with a Greek salad and a cold beer - way to go. It doesn't get much better than that.

Identification
A yellowfin tuna over 40 kilos is not too difficult to identify with its long yellow sickle-like second dorsal and anal fins. The pectoral fins on yellowfin extend back to the beginning of the second dorsal. However, fish between 10 and 40 kilos are a bit more challenging because the bigeye tuna has a similar appearance and pectoral fins of a similar length.

But the bigeye doesn't actually have the noticeably bigger eye that its name suggests, and though generally it lacks the long sickle fins of a yellowfin, colour variations can make identification difficult.

Without a detailed autopsy the best way to tell them apart is probably by looking at differences in the tail. On a yellowfin it has a yellow/golden tinge and the middle part of the trailing edge is indented into a distinct "V" with two raised ridges on either side. The bigeye tuna's tail is black after death with little if any yellow and no indent or ridges are evident. Yellowfin under 10 kilos often carry about 20 vertical whitish bars on their lower sides but at this size can be extremely difficult to distinguish from a bigeye tuna.

Size
The largest yellowfin caught on rod and reel in Australian waters weighed 124kg and was taken off Bermagui in NSW. I'm fairly sure the largest capture recorded in Western Australia was around 83 kilos and the fish came from the Rottnest Trench. Trolling inshore will produce school yellowfin to 20 kilos but the really big specimens in our waters are well offshore. Yellowfin tend to school by size so if you encounter a pack of hungry 12-kilo fish you are unlikely to pull a monster out of that particular school. A good fish in WA waters is 30 to 40 kilos.

Distribution
Yellowfin tuna are found along the entire WA coastline and indeed around the entire Australian coastline. In fact with the exception of the Mediterranean Sea, these tuna inhabit all tropical and sub-tropical seas, where they predominantly live in the warmer upper layers above the thermo cline.

Breeding and migration
The Exmouth Plateau is a likely major breeding ground of yellowfin in WA, with large aggregations occurring in the summer months in sea surface temperatures just above 26C. It is likely that yellowfin reach maturity at around 45 kilos and three years of age. Female yellowfin are prolific egg layers capable of spawning every day, or second day, for several months. Their eggs are epipelagic which means they float on the surface.
Growth rates are fast with a two-year-old fish weighing 20 kilos. That age/weight rate is constant up to at least five years, by which time they reach 50 kilos. Yellowfin are highly migratory and capable of travelling more than 1,000km in 12 months. No clearly identified migratory pattern has been established off the WA coast, but it is assumed there is a seasonal north-south migration along the Continental Shelf with a subsequent south-north migration farther offshore.

Threats
With increasing commercial exploitation of various tuna species in the Indian Ocean, it is the breeding habits of yellowfin that in fact allow stocks to be maintained at reasonable levels. However, there is concern on our east coast about depletion of inshore stocks of yellowfin by longliners, and therefore it is increasingly important that we carefully monitor recreational catches on the west coast. The principal threat to yellowfin is rapidly increasing longline activity, which has grown from just about zero four years ago to 7 million hooks (and counting).
To put that in perspective, the Japanese longline effort in the 70s and 80s was around 5 million hooks and gamefishermen are aware what that amount of effort did to our offshore fishery!

Yellowfin probably have the most robust stocks of all the bigger tunas, but it must be feared that if it's possible for bigeye tuna to become as rare as southern bluefin, the days will be numbered for yellowfin. Clearly a case for potential sequential stock depletion.

Tackle and bait
Cubing for yellowfin used to be popular with east coast anglers but it appears to be less so these days, probably due to a big drop in numbers of large yellowfin being caught that way. That method has never really taken off over here. Yellowfin will take a well-presented cube of fresh fish or mulie in the right circumstances. That is to say, if you can find the `fin staying in one place for a while. More likely than not they will be moving quickly as they feed or travel, which means that lures are a better option.
Inshore school yellowfin will chase bibbed and bibless minnows, and a tuna trolling feather will also pull a few fish. In these waters 10-kilo gear is fine but make sure your reel holds 200m-plus of line because, as I said earlier, when the "afterburners" cut in - look out. Offshore I would not go lighter than 15-kilo tackle and I suggest stepping up to 24-kilo. Out wide, switch to skirted trolling lures and big bibless.

Fishing methods
I will assume that most of our readers want to pursue big yellowfin. We don't have any significant actual structure to hold yellowfin in depths of 100 metres or more, so the structure we must find is thermal. We have the FADs, of course, but as yet we haven't had much regular success with `fin around these man-made structures. Yellowfin travel along vertical thermal gradients where warm water pushes against the edge of colder water. It is in these areas in that bait aggregates and in turn the `fin come looking for a feed.
Birds are one sure indicator to anglers of a big school of feeding yellowfin. Dolphins are also frequent among feeding `fin, and even occasional marlin show up. It appears that when `fin eat, everybody eats! Don't drive through a feeding school - try trolling around the edge so as not to put the school down. These feeding orgies occur mainly throughout summer and autumn in Perth waters in depths from around 100m out to 180m. I run a pattern of lures ranging from four down to two, depending on how many crew we have on the boat. Yellowfin tuna fishing is synonymous with multiple hookups! Smaller skirted trolling lures in the size range 20 to 25cm are ideal, but I have had some of my biggest tuna on 35cm marlin lures.
When they're on, they're on big time. I favour Hawaiian jetheads by lure makers such as Joe Yee, but there are some great Australian lures too. Go for pink as a main colour in at least one lure, but that's about all you need to know about colours. When the `fin are ready to play they are nowhere near as fussy about colours as billfish. It's a good idea to have a deep-running lure in short, right in the prop wash, between the skirted lures. Lures such as Giant Trembler, Rapala and Laser Pro 190 are ideal for this position. If you hook a yellowfin don't be impatient. They fight doggedly and characteristically down deep, so too much pressure may pull hooks.
And if you are successful, be sure to look after such a prize catch properly. Don't leave it unattended on the deck and carry on fishing because it will cook slowly in its own elevated body heat. Cool the `fin down with seawater and get it on ice quickly. Enjoy your sashimi and tuna steaks.

References
Kailola, Williams, Stewart, Reichelt, McNee and Grieve Australian Fisheries Resources 1993. Hutchins and Swainston Sea Fishes of Southern Australia 1986.