The biology of sharks occuring off the Yorkshire Coast – some recent examinations

A porbeagle shark being winched out of the fish hold on the scarborough trawler Independence in 1999.

A porbeagle shark being winched out of the fish hold on the scarborough trawler Independence in 1999.

D. E. WHITTAKER

 

A number of sharks are taken by fishing gear as a by catch off the Yorkshire coast and are invariably brought back to the fish market, where they always attract interest.

Only one shark species occurring locally has formed the subject of a specific, targeted fishery by Yorkshire commercial boats, although Porbeagle sharks have, in the past, been targeted along this coastline by Danish fishermen, setting sub-surface long lines.

There is very little biological information relating to most previous catches of shark occurring off the Yorkshire coast, because until recently, the unnecessary tradition of selling sharks in a whole and un-gutted condition has usually precluded anything other than a cursory examination.  Over the years a small amount of information has been gleaned from local catches of Basking shark and, very rarely, of the porbeagle.

In recent years, however, more thorough examination has been made of three species through the help and permission of a number of fishing skippers and fish salesmen; porbeagle sharks and a thresher have been dissected either before or after sale, and their stomach contents, gut parasites, and reproductive organs have been examined. Extraction of vertebrae, which are used for ageing studies of sharks, is however, not possible for examinations carried out when the fish are still in the middle of the marketing and transport process.

A rare opportunity to examine a small example of the basking shark, now protected from exploitation, was obtained when the skipper of a Scarborough trawler, Shaun Crowe, kindly brought back for examination a specimen found to be dead on hauling the net, instead of merely dumping the dead fish back onto the fishing grounds without any information being obtained from it.

Sharks Recorded From The Yorkshire Coast And Central North Sea

A fine blue shark stranded on a Yorkshire beach.

A fine blue shark stranded on a Yorkshire beach.

A large number of shark species inhabit the deep water both on and off the continental shelf to the west and north of the British Isles, including the deep northern north sea, beyond the 200 metre depth contour. The greater North Sea area to the south of this becomes increasingly shallow, thus effectively restricting their distribution further south. Twelve species of shark may be regarded as having been recorded from the survey area of the central North sea, but only two of these deep water demersal species, the large Greenland shark, Somniosus microcephalus and the curious bramble shark, Echinorhinus brucus, have, with any frequency, penetrated further south into this shallow area.

The Greenland shark was caught on a number of occasions in Victorian times and in the early 20th century, but has been extremely rare since then. There have been three remarkable captures of small juvenile fish only during the present survey, one of which had been feeding on a cetacean corpse immediately before capture.

Twelve species of shark have been recorded from the Yorkshire coast, including four epipelagic, open water species and seven demersal species while another, the wide ranging and cosmopolitan spurdog, is the only species to have been targeted specifically by Yorkshire fishermen.

Two female porbeagle shark of similar size on Scarborough fish market. One is immature, the other mature.

Two female porbeagle shark of similar size on Scarborough fish market. One is immature, the other mature.

Isuridae

Lamna nasus (Bonnaterre, 1788) Porbeagle Shark

Cetorhinidae

Cetorhinus maximus (Gunnerus, 1765) Basking Shark

Alopiidae

Alopias vulpinus (Bonnaterre, 1788) Thresher or Fox Shark

Scyliorhinidae

Scyliorhinus canicula (Linnnaeus, 1758) Lesser spotted dogfish, locally Nurse

Scyliorhinus stellaris (Linnaeus, 1758) Nursehound, Greater spotted Dogfish

Carcharinidae

Ovaries of immature and mature porbeagle sharks shown above on Scarborough fish market.

Ovaries of immature and mature porbeagle sharks shown above on Scarborough fish market.

Prionace glauca (Linnaeus, 1758) Blue Shark

Triakidae

Mustelus asterias Cloquet, 1821 Starry Smooth Hound

Galeorhinus galeus (Linnaeus, 1758)  Tope

Squalidae

Squalus acanthias Linnaeus, 1758  Spiny or Spur dogfish

Echinorhinus bruchus (Bonnaterre, 1788) Spinous or Bramble Shark

Somniosus microcephalus (Bloch & Schneider, 1801) Greenland Shark

Squatinidae

Squatina squatina (Linnaeus, 1758) Fiddle fish, Monk fish

A NOTE ON THE SPECIES OF MUSTELUS IN THE NORTH SEA

Two species of the common small ground sharks known as smooth hounds are known from european waters, Mustelus asterias, the starry smoothhound, and Mustelus mustelus, the plain smoothhound, and both species are frequently stated in reference works to occur throughout the North Sea.

Ovary detail of a mature porbeagle shark.

Ovary detail of a mature porbeagle shark.

M. asterias frequently has a scattering of white specks over the body, but it is extremely variable in this respect, and therefore this marking must not be relied upon to be a specific character; off the Yorkshire coast, many fish are totally devoid of this white speckling and therefore resemble M. mustelus.

Positive identification must be made by examination of the dermal denticles, which in M. asterias are ridged almost to the tips, while those of M. mustelus are ridged only at their bases, with smooth tips.

All specimens of Mustelus examined during the survey, whatever their external markings, have proved to be Mustelus asterias, and M mustelus is therefore not regarded as being present in the central North sea.

Changes In The Status Of Some Central North Sea Sharks

There have been changes in the status and distribution of several of the species formerly recorded from this sea area. As noted above, the Greenland shark, Somniosus, is now extremely rare and has been so for several decades. The bramble shark, Echinorhinus, a sluggish demersal species from deep water of the continental shelf, was occasionally caught in the North Sea during the 19th century, but has not been recorded since the last example was caught, rolled up in Whitby fishermen’s lines, in the 1890’s.

Dissection of the basking shark at Scarborough.

Dissection of the basking shark at Scarborough.

Two ground sharks, the angel shark, Squatina, formerly known to Yorkshire fishermen as the fiddle fish, and the greater-spotted dogfish or bull-huss, Scyliorhinus stellaris, are now extinct in the area; Squatina has not been seen since the late 1950’s, and the bull huss was last seen in the early 1970’s.

The thresher, Alopias vulpinus, was also frequently reported in the 19th century, particularly around the Whitby area, where it was regularly reported by fishermen to be chasing salmon and trout off the harbour; individuals were caught into the early 20th century, since when records have ceased until recently. A young, recently born pup was reported from a salmon net in Bridlington Bay, and released, while a female  caught in Filey bay in 2007, was  believed, on dissection, to have liberated young just a short time before capture.

The Most Recent Examination; A Female Porbeagle Shark From Whitby, Caught By Richard Brewer, Nov. 2009.

A large female porbeagle was landed by Richard Brewer on the Whitby fish market on November 13th 2009 and was bought by Alliance fish . The shark was brought through to their premises at Scarborough, where the fish was dissected before final transport to their customer; we are grateful to Mr Bob Scarborough for permission to dissect the shark, and to his staff for assistance.

The shark was one of the largest seen on the local markets, and also one of the most interesting. It had eaten a number of fish, but we were surprised to find its most recent meal had been of a large salmon, full of roe, that was only partially digested.

This shark was a gravid female, with huge ovary and swollen uteri, each of which contained numerous unfertilised eggs in capsules, among which were two tiny pups.

Gravid porbeagle at Whitby , Nov. 09; two pups among capsules full of yolk eggs that they will slowly devour.

Gravid porbeagle at Whitby , Nov. 09; two pups among capsules full of yolk eggs that they will slowly devour.

Porbeagle pups from a shark landed by Richard Brewer at Whitby, nov. 09. Note the enlarged branchial area and the stomach bloated with yolk eggs devoured while in the uteri.

Porbeagle pups from a shark landed by Richard Brewer at Whitby, nov. 09. Note the enlarged branchial area and the stomach bloated with yolk eggs devoured while in the uteri.

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