The good oil – Fish findings on the sea floor

The good oil – Fish findings on the sea floor

Assistant Curator Natural History Andrew Stewart describes a recent specimen processed by the Fishes team – one of the more striking deep-water predators and relative to the barracouta (a.k.a. the Fisherman’s curse), a large oilfish called Ruvettus pretiosus
Weighing the latest specimen (NMNZ P.061916) with (L-R) Carl Struthers, Salme Kortet, Jeremy Barker, and Andrew Stewart. Photo by Carl Struthers. Te Papa

This specimen was only half grown, but still an impressive 177 cm long and 50.5 kg. Fresh oilfish are an attractive violet sheen over a dark brown, but quickly fade to a flat brown on death. This one also had scars from encounters with cookiecutter sharks.

Cookiecutter shark bite scars. Photo by Carl Struthers. Te Papa

Scaling up

What makes this fish so unusual are its scales, which are unique in the fish world. They are deeply embedded with rows of large erect spinules and holes leading to spaces just under the skin.

Bioengineers believe that the oilfish is a wait-and-lunge predator, and when it accelerates into an attack, water enters the spaces and creates vortexes over the skin, thereby decreasing friction. Acceleration is extremely fast, in the region of 5–7 m/sec2.

Although the massive jaws are armed with banks of teeth, the oilfish has been observed from submersibles bending its body into a slight S-shape then body-slamming fish and squid before seizing and swallowing them.

The rough scales lacerate the unfortunate prey, shocking it, so it’s incapable of escape. The large eyes also have a silvery mirror at the rear of the socket, which enhances vision in dim light.

The silvery back of the eye enhances vision in low light. Photo by Carl Struthers. Te Papa

Observations from submersibles also show oilfish hover just off the bottom, usually solitary, occasionally in pairs. They lack a swim bladder but the skin, flesh, and skeleton is richly supplied with oils and waxy esters for buoyancy. This oil content can be as high as 25% of the total weight of the fish.

So, is it edible?

The oilfish has been historically targeted in the Pacific Islands for food. Boats ventured past the edge of the atoll at night and lines were lowered deep down several hundred metres. The stone sinker was then released by a sharp tug, and the specialised wooden hook floated horizontally in the water. Landing a fully-grown oilfish on a handline would have taken considerable effort; they can reach 3 m long, and weigh about 100 kg.

Yes, but can I eat it?

Oilfish do have very large, dense blocks of muscle, however before breaking out the fileting knife, there are a few things you need to know. Those oils and esters mentioned above (called gempylotoxin) are indigestible and should give you a reason to reconsider. Filleting too close to the skin, including any bones, or eating too much will expose you to the risks of headaches, abdominal cramps, vomiting, and explosive oily orange diarrhoea (Keriorrhoea), or leakage, that may smell of mineral oil. Symptoms can last up to two days.

Supermarkets overseas have been heavily fined for selling this fish without warnings, often under unhelpful generic names like ‘cod fish’, ‘butterfish’, and even ‘white tuna’. Those still brave enough to eat it though have reported that it is fine textured, with a delicious buttery flavour.

If you simply must try it, a recommendation is a single portion of no more than 170 g. A Rarotongan staff member once told me that they baked oilfish in earth ovens with lots of limes to counteract the oils, and that children were not allowed to eat it. The best news is that, while dramatic, the oils and wax esters are not toxic.

What’s in a name?

Apart from its challenging culinary properties, the oilfish also presents an interesting puzzle for fish taxonomists. Although it is currently in the same family as gemfish and barracoutas (Gempylidae), it also has features of tunas (Scombridae). There is some thought that it is unique enough to belong in a completely separate family. The size of this specimen, and the number we hold, means that this individual has been cleaned up for skeletonizing.

Andrew Stewart in the process of skeletonising the oilfish. Photo by Jeremy Barker. Te Papa

1 Comment

  1. Thank you, Andrew, for solving an 18-year-old mystery. Back in 2005 and again in 2008, we ate at the restaurant of Musket Cove on Malolo Lailai while sailing from Nadi to Vanuatu. On both occasions, my wife subsequently experienced “explosive oily orange diarrhoea then leakage, that smelt of mineral oil”. We thought the source was contaminated groundwater, but my wife had eaten the fish dish while I chose otherwise both times. Malolo Lailai is very close to really deep water so oilfish would be quite accessible.

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