They’re smaller and less exotic than some of their monstrous relatives, but thornbacks are so abundant offshore that a sea angler is sure to encounter one sooner or later.
The skates and rays cannot be confused with any other fishes. Their outline and general structure are distinct from the roundfishes; the only characteristic shared with the flatfishes is a flattened body, but even this is different, being a vertical flattening in the skates and rays and a lateral compression in the true flatfishes.
The only fish that might be confused with the skates and rays is the monkfish, a species that bridges the gap between the group and the sharks, tope and dogfish families. Its squat body is similar to a ray’s, but fin and gill slit arrangements as well as the general structure are more shark-like.
The rays may be separated from the three species of skates on size alone: no British ray in any way challenges the massive weight of mature skate. It is often thought that skates are biologically different from the rays but in fact there is little or no difference between them except in size.
In other parts of the world there is no recognized division between the two groups: some of the bigger rays—the mantas and eagles—are far bigger than even the huge British white skate.
The thornback or roker, Raja clauata, is the most important British ray, widely distributed throughout inshore waters and ap- pears to have no significant migratory pattern. It is° not a deep-water species but generally prefers shallows and moderately deep water. Many of the thornbacks hooked off the East Anglian coast, a major thornback area, are caught in less than 30ft of water.
During the height of the season —May to July—many fishes venture close inshore and are taken by the beach fishermen. But as a rule charterboats and inshore dinghies take most of them.
The nature of the seabed is more important than anglers seem to realize. Thornbacks congregate where the seabed is broken up by stones and weeds, where mud, sand and shingle intermix into a rough carpet. Undulating ground, with hills and gullies swept by the tide, hold the skate’s attention far more than does flat, open sand.
The habit of massing on relatively small areas of seabed explains why just one or two boats in a fleet of dinghies take dozens of rays when everyone else fails to get one bite. It also leads to ‘harvesting’ by the commercial boats: once a shoal is located it is easy to sweep the bottom with a beam trawl to take almost every fish. In some places round the coast the thornback has been virtually eliminated from its once traditional haunts.
Like all our native rays, the thorn-back is a predator whose diet includes worms, small fish, crabs and shellfish. The dentition and jaw structure provide the clue to its mode of feeding because, like the huge skates, the ray has a grinding mill of flat, interlocked teeth powered by strong muscles. A small ray can crunch the hard shells of crustaceans and molluscs and might even wear away the angler’s hook and trace unless the tackle is suitably reinforced.
Seabed vacuum cleaner
Unlike the midwater predators that are adapted to snatching their prey and whose eyes aid the final assault on their food, the ray is rather like a seabed vacuum cleaner. Its eyesight is poor and, when related to its jaw position and structure, suggests that vision plays only a minor role in feeding habits.
The exact locatory mechanism is open to doubt but its sensitivity to vibration and electromagnetism is high. A thornback can detect and home-in on the tiny amount of electricity generated by the rhythmic movements of a plaice’s gill covers.
How rays detect forces of this nature is poorly understood but is almost certainly linked to the electricity generators that rays themselves possess. Perhaps the mechanism is triggered by interaction between the force-fields of both rays and victims—a kind of radar. On the other hand, there is no doubt that taste and smell play the major role in food location. The olfactory organs are well developed, as are the nervous system and those areas of the brain devoted to smell.
Rough ground with crannies, weeds and rocks in and around which food creatures thrive, provides a much better haunt for rays than open stretches of sand and shingle where marine life is deeply buried, scarce or virtually unobtainable.
Food supply and general preference apart, thornback and other rays need rough ground for breeding. The ova are fertilized inside the female, in marked contrast to most of the higher fishes where ova and milt are normally shed directly into the sea.
Nor is spawning a brief interlude in the yearly cycle: rays produce their eggs on a conveyor-belt system in which mature, fertilized eggs are laid at intervals over a long period. The egg is encased in a tough capsule—the ‘mermaid’s purse’—and attached to weeds and stones by tendrils, hair-like projections and stickly secretions. Rough seabed is essential for firm anchorage.
The incubation period is long—4 to 5 1/2 months in thornbacks—and the newly hatched rays are fully formed. The young fish is only a few inches long but it is ready for an independent life and does not rely on the yolk sac which nourishes most young fishes. Growth is rapid, but mortality is high because the young fish themselves fall victims to tope, dogfish and mature rays.
An adult thornback is never a huge fish. An exceptional weight is 30lb and anything over 20lb is very good. Rod-caught (boat and shore) thornbacks average between 5lb and 15lb and the bigger fishes to 30lb are usually females.
The mature thornback is usually white underneath and dark green to brown on top with yellowish mottling. Coloration and patterning is variable and even two fish caught from the same ground might be entirely different from each other. The back is dotted with pale patches which are sometimes encircled by dark speckles, and the body is covered with thorns and spikes.
The presence of thorns does not confirm identification because most rays have at least a few on the wing-tips, tail and mid-line. The characteristic thornback spines are those on the mid-wing areas. Total numbers vary: some rays are moderately armed while other seem more akin to porcupines.
Most of the other rays are similar to thornbacks in habit, feeding and general characteristics. Identification is more of a problem and it is perhaps fortunate that most of the other species are seldom caught on rod and line, or are so restricted in distribution that they become a feature of local fishing and therefore always half-expected by the angler.