MARINE NATURE NOTES

Otago Daily Times, Issue 22504, 23 February 1935, Page 4

MARINE NATURE NOTES

Written for the Otago Daily Times. By David H. Graham, F.R.M.S., F.Z.S. THRESHER SHARK. ■ The thresher shark is easily identified by. its extraordinarily long tail, which is nearly equal in length to the whole body,' and into which the vertebral column extends. Of this tail it makes great use: as a weapon, striking the water or its foes with great force. So much is it,, dreaded by other marine animals and fish that it is not uncommon, for a whole herd of dolphins to take to flight at the first splash of the tail of a .thresher shark., It is from the use it makes of its tail in threshing the water that it has acquired the name of “ thresher.” It is known in some parts of the world as the “ foxshark” or “ sea-fox,” from the fact of its finding the whereabouts of suitable pj-ey, either to kill by itself or to work in conjunction with the swordfish. These two dreaded sea fish attack whales, either: conjointly or in packs. When a whale’s back appears above the water the threshers, springing several feet into the air, descend with great violence upon the object of their onslaught, and inflict upon him the most severe slaps with their long tails, the sound of which resembles the reports of guns fired at a distance. The swordfish in their turn attack the -distressed whale, stabbing’ from below, and thus, beset on all sides, and bleeding, from numerous wounds, the hug 4, animal, though dealing the most dreadful blows with his enormous tail and lashing the crimsoned water into foam succumbs in the end. ' r The teeth of the thresher shark are equal in both jaws, with a third tooth on each side 'of the upper jaw much smaller than the others. They are of moderate size, flat, triangular, and not serrated as in many other sharks. The Snout is short .and conical, ,the spout holes 'are very small, and the mouth is not so large as that of the white, the tiger, or the nurse shark, nor are the teeth so formidable, but the thresher is extremely, bold and voracious, readily attacking grampuses and dolphins much larger than itself. It is found in all temperate and tropical seas —England, Mediterranean, Atlantic, and Cape of Good Hope, During the ’ warm weather, and especially in' February and March, these sharks are most numerous both off and inshore, roving singly or in company; and pursuing the shoals. of sprats as they migrate northwards to warmer waters. It does not, so far as I am aware of,, manifest any desire for humans as‘'food. Neither does it turn itself round in the water to seize' the hbject, be it bait or moving fish, its mouth being placed in tjie lower part of its head., ■„ •> • The maximum, size caught off Otago: Heads is 18 feet, but the average is about 10 feet. Several caught and brought to me by fishermen proved that they consume a great many pilchards and spiats. One Specimen caught off the Heads measured from tip of snout to tip of tail Oft Gin; the‘body was 4ft 9in and j the tail exactly the same length.- The side, o r pectoral fins; were 17 inches in length by 9J inches wide; the back fin was 9in by 7|in, while the anal fin was 8i by--7* inches. The circumference of the fish, was 26 inches and the .stomach .was crammed with pilchards and sprats. . Fishermen catch thresher sharks whilefishing for groper and kingfisn at the North Reef, and many a line is lost because of them, as they take the bait with a rush and swim away so rapidly that Tt is almost impossible to play them with a line like a. groper line. Usually they make a decided rush at the bait and then tarn upwards for a short distance, when all of a sudden the line slackens, giving the fishermen the impression that the shark- has got away. Then, again, the hooked shark suddenly hurls, itself out of the water, and should the descending fish be near enough to reach the boat with its tail there is grave danger of damage being done. At one time Mr Sid Broadley hooke.d a thresher shark, and managed to manipulate it until he had it lying stretched, on the water alongside his-whaleboat. , It had been his intention to kill the creature, and he was .'about to do so when, without a second’s warning, the fish leapt out . of the water, bringing its tail down across the boat and engine doghouse within a few inches of his face. A few inches nearer, the blow would have killed him. As it was, the boat creaked and rocked under the impact. At one time fishermen who were attempting to catch groper in Blueskin Bay were continually having, their* hand lines taken by thresher sharks, their bait, hooks, sinkers, and lines being taken completely away. Day after day one ■ fisher--man or another would complain that he had lost his lines. One day two fishermen in, a boat looked over the side ahd were amazed to see a huge thresher shark swimming' along below them with a number- of lines streaming behind it. Without any. warning, and unknown to one of these fishermen,, his line caught in one of the sinkers attached to the shark. When the fish felt something holding, it began to surge ahead, .taking the fisherman’g line with it. As fathom after fathom rail out,’the fisherman, turning to his mate, - said, ,“ Be ready to cut the line,” when the line suddenly slackened, and, on pulling it in, he was surprised to-find, not only his line complete, but all those lines they had seen attached to the shark, and which other fishermen had lost some days before. His Hook had caught‘the .sinker attached to the hook in the shark’s'body, and, when the creature surged ahead, had pulled the ’ hook out of the shark’s mouth.

There are two sub-classes of fish—the gristly and the bony fish. In the former, which comprises all sharks, elephant fish, and skate, the skeleton is composed in the main -of gristle. This in' turn is strengthened by the’ disposition of lime salts in various places such as- in the teeth and scales and not infrequently in the vertebrae and jaws. The easiest way of distinguishing the two sub-classes is to look at the gill slits. In the gristly fish fire or more gill slits can be seen as separate openings, while in the bony fish they are covered by a flap of bone; the gills cover and open by a single opening. Those who have witnessed these sharks feeding say that one or more swim round and round a shoal of small fish, threshing the watfer with their long tails and thus driving the prospective victims into a compact mass, when they form an easy, prey., ■ ■' ;, .