Cornish blackfish, Centrolophus medusophagus, caught off Lihou Island on 19 June 2010

June 25th, 2010 by Richard Lord

The Cornish blackfish, Centrolophus medusophagus, is a poorly known fish.  It is not uncommon at a depth of over 200 to 300 metres.  It is caught by trawlers working out of the west coast of Ireland, but to find one in a gill net set in 20 feet of water to the south of Lihou Island, off Guernsey’s west coast is a very unusual occurrence.

According to Fishbase the Cornish blackfish is distributed in the Eastern Atlantic from Iceland and Ireland to Morocco.  It is found also around the Azores and around Madeira, and in the western Mediterranean.  In the Western Atlantic it lives off the Grand Banks of Canada and all the way south to the state of North Carolina in the USA.

So it must have been a surprise when Stuart Exall and Casey Brehaut hauled in their gill net at 9 am on 19 June and found a chocolate brown fish they had never seen before.  They took the fish to Len Le Page, author of “Angling in the Bailiwick of Guernsey.”  He had never seen such a fish in a lifetime of fishing Guernsey waters.

Cornish blackfish, Schedophilus medusophagus, caught in a gill net by Stuart Exall and Casey Brehaut south of Lihou Island, off Guernsey's west coast on 19 June 2010

The fish was identified using the “Fishes of the North-eastern Atlantic and the Mediterranean Volume III” published by UNESCO in 1986.  The fish belongs to the family centrolophidae, which includes the ruffs, barrelfishes, and blackfishes.

Professor of Fisheries Biology emeritus, Richard Haedrich, from the Department of Biology at Memorial University, Canada, who authored the chapter on centrolophidae, wrote that these fish are “oceanic, or mesopelagic, the young tending to occur shallower than adults. The young to 20 cm commonly hover with medusae (jellyfish) in surface layers.”  He wrote that these fish feed on jellyfish tentacles and gonads, and also on small crustaceans.

In the days proceeding the capture of this fish, small blue jellyfish, Cyanea lamarckii, were washing up on L’Ancresse beach on Guernsey’s north coast.

The fish weighed 1 lb. 7 oz. and 11 drams on the official Guernsey Angling Record Committee scale and shortly afterward, 641 grams on an electronic digital scale.  The fish had a total length of 44.5 cm and a fork length of 39.3 cm.

Close-up of the head showing the soft spines on the preoperculum

The Fishes of the British Isles published by Frederick Warne & Co. Ltd in 1925 states that the “Cornish blackfish is known from a single specimen which came ashore in Polperro, Cornwall in 1859, and is now in the British Museum.

At that time the Portrush barrelfish was thought to be a distinct species, but it is now known to be the juvenile of the Cornish blackfish.  The Portrush barrelfish was given the name Lirus medusophagus (medusophagus meaning ‘jellyfish eater’) because of its association with jellyfish.  Juveniles associate with jellyfish but it is not known if adults do.

In Alwyne Wheeler’s “The Fishes of the British Isles and North-West Europe”, published in 1969, he writes “the Cornish blackfish is a rare vagrant inshore in our area.”

“Occasionally it is taken by commercial trawlers in deep water on the edge of the continental shelf where it is relatively common, at least in the south of the area in the summer.  These trawled specimens have been taken in depths of 200 to 500 fathoms (365 to 914 metres) but there is no certainty that they were not caught in mid-water.  The blackfish grows to a length of 58 cm.  All records of its occurrence are noteworthy.”

 

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