Atlantic Bonito Sarda sarda
A 1.3 kg (3 lb) Atlantic Bonito was caught by an angler whilst feathering for mackerel at the West Hook Ledges (SM 764094) of the Marloes peninsula headland, south Wales, on 1 July 1996. This capture caused a flutter of excitement and speculation because the fish was not recognised immediately. Obviously, this streamlined fish was a member of the Scombridae family of Mackerels and Tunnies, and the first suggestion was that the fish was a Frigate Mackerel, Auxis rochei, but the shape of the first dorsal fin removed this possibility.
The identification was confirmed by the Natural History Museum at Cardiff. This is only the eighth confirmed record from Wales since the last century. The Atlantic Bonito can reach a weight of 7 kg, but anything over 4 kg is unusual, so this is a bigger fish than the Mackerel, Scomber scombrus, which rarely exceeds 2 kg.
This fish is well known from the Mediterranean and Black Seas and is found on both sides of the Atlantic Ocean. Its preferred temperature is 22 degrees C and is only occasionally found below 15 degrees C. The temperature of the sea when caught was 13 degrees C. The seas in the south-west of Britain and English Channel reach 19 degrees C in August and these are highest of the year.
The fish is steel blue to olive with oblique lines running from the base of the dorsal fins forward on the top half of its body. It has two dorsal fins that are almost joined. It is known to feed on mackerel and similar fishes.
A tuna-like fish has been recorded by divers swimming with the mackerel shoals off the coast of Sussex. It could be this fish or one of the other species of larger tunny recorded in British seas.
Report by Kate Lock, Skomer Marine Nature Reserve.
Big-eyed Thresher Shark Alopias superciliosus
On the 22nd August 1995, a Bigeye Thresher shark was landed at Newlyn fish market in Cornwall. It had been caught in an area known as the Porcupine Sea-Bight (51 N 13 W) where it had become trapped in a drift net. The drift net had been set at 5 metres in waters more than 2000 metres deep to catch tuna.
The Bigeye Thresher is a widespread species; it typically inhabits oceanic and coastal waters from the surface to depths of 500 metres and prefers warm-temperate to tropical waters.
This was the most northerly confirmed recording of a Bigeye Thresher in the north east Atlantic and coincided with above average sea surface temperatures in the summer of 1995. It was a female weighing 381 kg (gutted) with a total length of 484 cm which makes it the largest and heaviest record to date for this species.
Research conducted in the north west Atlantic has shown that Bigeye Threshers tend to be caught in waters where the sea surface temperature ranges from 16 degrees C-25 degrees C with a minimum of 14oC at 75 metres.
The Big-eyed Thresher is not one of the 21 species of sharks regularly found in the shallow seas (to a depth of 200 metres) around the British Isles. These 21 species belong to 12 different families. The closely related rays and skates, and electric rays, number at least 19 British species in 3 families with the common species featuring in excellent displays in the larger Public Aquaria and Sea Life Centres.
Common Thresher Sharks, Alopias vulpinus, occur widely throughout the north east Atlantic and are commonly found in coastal waters around the British Isles. Common Thresher Sharks are sometimes seen in the English Channel in the late summer and are readily noticed because of their long tail fin. The biggest of the carnivorous British sharks are the Mako, Isurus oxyrinchus, and the Porbeagle, Lamna nasus.
Information from Teresa Thorpe. Full report in the Vernal/Summer 1996 Glaucus.
Swordfish Xiphias gladius
Andrew Johnson, from the University of Glasgow, spotted a two metres long Swordfish, Xiphias gladius, five miles out of the Firth of Lorne in September of this year. On a beautifully calm day with the Atlantic like a millpond, he spotted something rising out of the water and coming straight towards the boat. He could not believe his eyes, but with a long sword about a third the length of its body, he identified it as a species of swordfish. To his surprise, it rose a further seven times in full view of the divers on the charter boat heading for a site west of the Garvellachs.
The Swordfish is the only member of the family Xiphiidae. It has a long pointed snout which it uses to stun fish on which it will then feed. This fish is a widespread and solitary fish throughout the warmer oceans, but it is an extremely rare vagrant in British seas. One specimen was seen off the south of the Isle of Wight a couple of years ago.
The Swordfish, Xiphias gladius, has a tuna-like first dorsal fin and attains a length of 4.9 metres. Specimens are usually between 2 and 3 metres in length. It feeds on shoaling fishes and squids. Its colour on the most often seen upper surface is a blue-grey, paler underneath.
The only other fish recorded in British seas with a pointed snout is the Sailfish, Istiophorous platypterus, which is distinguished by a huge dorsal fin. This latter species has only been recorded once stranded on a Devon coast in 1928 (A. Wheeler). This fish belongs to the family Istiophoridae, which includes at least three species of marlins and spearfishes with pointed snouts.
The Firth of Lorne is the six mile wide passage between the mainland and the Isle of Mull and there is an open stretch of water to the Atlantic.
Send marine wildlife reports to:
Shore Watch, British Marine Life Study Society, Glaucus House, 14 Corbyn Crescent, Shoreham-by-Sea, Sussex. BN43 6PQ.