Sometimes called the ‘poor man’s monster’ because of its habits of venturing close inshore to be caught by shore anglers, the porbeagle provides genuine big game fishing for everyone.
Sharks belong to the group of fishes which have a skeleton made of car-tilage or calcified cartilage instead of bone, as is the case with most other fish. As such they share a common feature with such species as skates, rays, monkfish (or angel-shark) and sawfish; this group is known as the Elasmobranchii. Modern representatives first ap-peared at least 70 million years ago and fossil evidence suggests that there has been little or no change in that time.
The porbeagle, Lamna nasus, is a member of the mackerel shark family, the Isuridae. Two other members of this family are found in the coastal waters of Europe: the mako, Isurus oxyrinchus, and the great white shark, Carcharodon car-charias, of Jaws fame. The great white shark seldom wanders northwards of the Portuguese coast and can be discounted as a quarry for the British angler.
Unlike other sharks, water temperatures seem to have little effect on the porbeagle, which has been taken all around our coasts in every season of the year. They become more common during the summer months, particularly off the west and southwest coasts, the Isle of Wight and the Channel Islands. On some steep, rocky coasts it comes very close inshore. On the County Clare coast of Ireland, for example, large specimens have been taken in just four fathoms of water by anglers casting from the rocks. The porbeagle has sometimes been called the ‘poor man’s monster’ for this reason, since in these circumstances shore anglers stand almost as much chance of cat-ching one of these sharks as does the shark-fishing devotee in an expensively chartered boat.
The porbeagle has a large, thick-set body with a rounded snout and large eyes. It has distinct keels present on each side of the tail stalk and smaller ones on the tail fin. Its colouring ranges from dark grey to grey-blue on the back, shading to white underneath. It grows to enormous sizes, attaining a length of over 10ft and a weight of over 700lb. Its teeth are large and triangular in shape, not firmly fixed to the jaw, but embedded in the gums. As old teeth lose their effectiveness, they are constantly replaced by new ones throughout the life of the fish, the size of the teeth gradually increasing as the fish grows.
The porbeagle is often confused with its close relative, the mako, which grows even larger—13ft specimens weighing over 1,000lb have been caught off the American coasts. The teeth provide the most obvious difference: in the mako they are long and pointed with no lateral cusps; in the porbeagle triangular with a single cusp on each side.
Like all sharks, porbeagles have a distinctive internal structure. The cartilaginous skeleton is relatively simple, consisting of fin-supports, backbone and head elements. Of these the skull is a single structure, not made up of separate elements joined together by complex structures. The upper jaw is not joined or fused with the skull, but is merely loosely connected. Articulation with the lower jaw is equally simple. The backbone, while consisting of individual vertebrae, is unconnected to the paired fins by pectoral and pelvic girdles. There are no ribs.
At first glance, the intestines appear to be a short, thick structure, stretching from the stomach to the anus. This is not a simple tube as in other fish and animals, but a spiral within an external covering—the spiral valve of all cartilaginous fishes. Digested food is in contact with an extremely large area of intestinal wall so that absorption of nutrient is very efficient. ‘Skin teeth’
Porbeagles have fairly rough skin produced by dermal denticles or ‘skin teeth’. These scales, quite unlike the scales of bony fish, are structurally very similar to their teeth, consisting of dentine covered with a layer of enamel-like material. The whole body of the shark is covered by these scales, as well as the fins and even parts of the inside of the mouth.
The scales are very small, giving the skin a smooth, silky appearance, and do not increase greatly with increasing size. This can be deceptive, for the porbeagle’s skin is extremely rough, especially from tail to head, as the scales have backwarddirected spines. Because of their structural similarity to teeth, the scales are extremely hard and can cause great damage to any object coming into contact with them. This also applies to parts of the animal itself, so that there is always an area without scales immediately behind the pectoral fins which prevents them from suffering damage as they move up and down.
All sharks reproduce by way of internal fertilization of the ova. Because of this there are obvious external differences between male and female fish, the males bearing a pair of claspers. These are developed from the inner edge of the pelvic fins from skeletal elements of the latter, and are present, though extremely small, at birth. These claspers only develop fully within a very short time as the male reaches maturity.
Porbeagles are ovoviviparous: the internally-fertilized eggs hatch within the oviduct of the female. There is no placental connection after the first few weeks and the liberated embryo receives nourishment from the original yolk-sac, together with nutrient fluid released from the wall of the oviduct, absorbed directly into the yolk-sac. In some instances the young quickly absorb the yolk-sac, and then proceed to feed on the unfertilized eggs around them to get further sustenance for development. Such young have grossly swollen abdomens or yolk-stomachs. With ovoviviparous reproduction the number of young at birth is very small—porbeagle have up to four young, but they may be up to 20in long.
The porbeagle’s ovoviviparous method of reproduction cannot produce the large number of young that can potentially be produced by the egg-laying reproductive method of bony fishes. There is low mortality during early life, however, since in every case the dangerous stage of embryonic development is spent within the safety of the female, the young at birth being able to swim and fend for themselves.
The function controlled by the pores found around and especially on the underside of the snout of sharks is little understood. Various abilities have been attributed to these pores. All are flask-shaped, and have free nerve ending at their base, the whole of the pore being filled with an electrically conductive jelly. Sensing of temperature differences or differences of pressure may be one function, but the sensing of electrical impulses has been generally accepted as the prime function as it is considered to be the most useful. Any shark in the final moment of taking its prey has restricted vision, as it not only tends to close the eye with the second eyelid or nictitating membrane, but also because the position of the eye on the side of the head prevents it from seeing the space in front of the mouth. The prey, in moving, produces a weak electrical field, due to the activity of its muscles, so that its exact position can be defined if the electrical field can be detected.
Like all fish, sharks have a well- developed lateral-line sense system, although this is not as apparent as in many bony fish. The function of the lateral-line sense system is to detect low frequency vibrations, such as are produced by the swimming motion of other fish or the movement of animals in the water.
Sight plays a greatly varying part in the life of sharks. This is obvious from the differences in the size of eyes when comparing different types of shark. Some species, like the porbeagle, have extremely large eyes that may be able to see over great distances, while other species have very inconspicuous ones that may only function over a short distance. These are the species which have a well-developed sense of smell, the olfactory areas of the brain being much larger than the visual centres of the brain. Furthermore, in these species the actual olfactory organs are much larger comparatively than in the ‘visual’ species. The organs of smell are housed in two pits on the side of the head in front of the mouth and just on the underside. There appear to be two openings for each pit in such a way that there is an obvious forwarddirected opening and a se-cond rearwarddirected one. Water continuously flows into and out of the pit as the fish swims forward. On lifting the flap, a greatly folded, soft structure, which contains the olfactory nerve endings, can be seen. Experiments have shown that some species of shark can detect dilutions of less than one part per million of certain substances, indicating the extent to which the sense of smell has been developed.
Porbeagles are strong swimmers and prey particularly on shoaling fish such as herring, mackerel and pilchards. They are also partial to cod, pouting, whiting, hake, flat-fishes and squid. It is evident that they are not selective feeders, and anglers often catch them with herring, mackerel or pilchard baits.
It is often contended that in British waters true big-game fishing is confined to fishing for sharks: porbeagle, thresher, mako and blue.
The world record porbeagle was a fish of 465lb caught off Padstow in Cornwall. 350lb specimens are not uncommon. Porbeagle are extremely powerful fighters, which make frequent irresistible plunges to the bottom. One method of fishing for them is to drift with a whole deadbait beneath a large float set at a depth of 15-20ft. A ‘rubby-dubby’ ground-bait trail should be laid, and 300 yards of 100lb b.s. Line, specially-tempered steel hooks and a 15ft wire trace, are necessary.
The porbeagle is a typical shark in the broadest sense of the name as applied to round-bodied elasmo-branch fishes. It prefers temperate to coldish waters in the North as well as South Atlantic. Like many sharks it is a live bearer. Not a gregarious species, loose groups of individuals sometimes appear off our coasts probably because feeding is plentiful in these places at that time or because they gather there for mating. It is essentially a ‘visual’ species, finding its prey, initially through its lateral-line organs, at closer quarters through sight. Only the blue shark is more abundant in the waters around the British Isles.