Butterfly Tuna – Gasterochisma melampus – off Fiordland coast during summer months
Butterfly Tuna – Gasterochisma melampus
by Gary Wilson
Butterfly Tuna Description
Butterfly tuna colouration is metallic blue, black on the back with intense silver flanks and belly. Silver pectoral fins whilst the other fins are black and slightly transparent.
One of the more unusual tuna, this species changes quite considerably its appearance at it matures. While small it is relativity elongated in shape with a slightly pointed snout although the most distinguishing feature of the juveniles is their large fan like pelvic fins. These fins in proportion to the rest of the fish are greatly oversized and loosely resemble the wings of a butterfly in shape hence the name butterﬂy tuna. When not in use these wings tuck neatly away in small reassesses along the fish’s belly.
Once the tuna matures its head becomes blunter. Its first dorsal fin shrinks and separates from the 2nd dorsal fin. (See below). The pelvic fins also shrink and they become a size more in keeping with the fish’s bulk.
The other major distinguishing feature of the butterfly tuna is its scales which really have to be seen to be believed. On this species, they are extremely large and rough and give the impression that the tuna is enclosed in a fish-net stocking with perfectly aligned rows of scales crisscrossing the fishes entire body. It is these scales that are the principle reason for the butterfly tuna’s other common name – scaly tuna.
Little is known about the habits of this species and its movements around our coasts are a bit of an enigma. From the little information that is available, we can ascertain that it is a widely cosmopolitan deep-water species that can be caught worldwide in waters from the tropics right down to around 45 deg south. Here in New Zealand catches range from areas as geographically diverse as Dusky Sound in Fiordland to Bushett Shoals in Canterbury and then right up the East Coast to the north of Gisborne in the North Island. Although by far the majority of specimens caught by recreational anglers have been in the productive waters off the Fiordland coast. These fish have been predominately small juvenile tuna up to 5kg in weight although every now and then the occasional mature individual is landed and these fish have been known to top a massive 35kg in weight. Unfortunately, these larger specimens are generally taken by the commercial sector – what one of these adult tuna would be like on the end of a rod one can only wonder but if they show only a portion of the power and stamina of their smaller brethren they would be a real handful.
As a sports fish, the butterfly tuna would have to be an excellent candidate for light tackle. Trolled lures, flys or small spinners presented on lines of 10kg and below would really enable these fish to show their stuff. The real problem at this point is finding enough fish on a regular basis to cast a lure at because as things stand at the moment their movements in Fiordland waters are not well understood and most of the captures to this point have come as bonuses from anglers targeting other species.
These tuna appear off Fiordland’s coast throughout the summer months and those few that have been caught have been taken during the brief period between December and May which incidentally coincides with the period of most angler activity in this remote area. They may be present in other months of the year but as angling pressure is almost non-existent over the winter months we have yet to conclusively add that piece to the puzzle.
From examining stomach contents of captured fish we can determine that this species has a diet primarily consisting of anchovies and other such small pelagic fish and it is whilst in pursuit of such prey that they are occasionally sighted feeding on the surface, which immediately makes them venerable to a well-presented spinner. One would have to say casting lures into the surface feeding fish would have to be one of the most exciting ways to target these tuna but unfortunately, more research will have to be done before this fish becomes a regular catch with recreational anglers as catches to this point have been relatively spasmodic.