VIGNETTES FROM NATURE.

Otago Witness, Issue 3333, 30 January 1918, Page 55

VIGNETTES FROM NATURE.

By G. M. Thomson, F.L.S. XX.—EELS AND ELVERS. When walking along the Brighton Beach one calm morning at low water I picked up a singularly interesting fish which the ripples from the outlying surf had washed ashore. It had a compressed, ribbon-like body about 6in long and nearly lin deep, which was so nearly transparent that the bony structure and the internal organs could be readily seen. The head was disproportionately small as compared with the rest of the body. The fish was too much knocked about in the surf to survive long, though I tried to keep it alive in a jar of sea-warer. Such specimens are not common, though they are met with every now and again. They were classed by the older naturalists as belonging to the genus Leptocephalus, meaning thin or narrow skulled. This curious little creature, which seems, when in the water, to be made of flexible glass, has a rather interesting and remarkable life-history, which is bjing found out little by little. No one has ever bred a Leptocephalus from the eggs of any known fish, and no one till recently has reared one from the glasslike stage to any other form. So for very long the relationship and development of this curious fish remained a mystery, but of late years they are becoming more fully known.

The American ichthyologist, Gill, in 1864, on purely anatomical grounds, came to the conclusion that Leptocephali were the young stages of eels, and that L. Morrisi, a form which had been first discovered in Wales in 1763, and which is not uncommon on the coast of Britain, was the larval form of the conger 'eel. Then Dr Gunther, of the British Museum,' considered that h e could prove that they were not normal stages of development, but larva, the development of Avhich had been arrested at a certain stage, but which continued to grow in size. One reason was that young conger eels, which, however, were fully-developed fish as far as the form of their organs was concerned, were often smaller than the Leptocephali. In 1866 M. Yves Delage, working at the Boilogical Station at Koscoff, on the coast of Normandy, observed the transformation of a living Leptocephalus into a younoconger.

Several so-called species of Leptocephalus had been noticed and described at various times, but their identification with any known species of eel, except the conger, was not accomplished until the season of 1891-2. In that year Messrs Grassi and Calandruccio, working at Catania and Messina, on the coast of Sicily, obtained Leptocephali from the Straits of Messina, placed them in aquaria, and were able to follow their development to a very considerable extent. The strong currents and whirlpools to which the Straits are subject—which gave rise to the old fables of Scylla and Charybdis. —and the deep water areas in the vicinity of the straits, appear to be favourable to the occurrence of these curious larvse. Though these naturalists were not able to follow the whole process of development in the case of the common eel, they succeeded in satisfying themselves that the form known as Leptocephalus brevirostris. which had hitherto only been met with in the neighbourhood oi the straits, was the larva of the common fresh-water eel. It is a small form, scarcely exceeding 3in in length, and fin in height. The fully-transformed young eel was never less than Sin In height. 'Besides the larvae, they also obtained specimens of the adult eels with reproductive organs advanced in development, and found that these were distinguished from the ordinary specimens taken in fresh water by having larger eyes and by their silvery colour. Judging from the Increased size of the eyes and from the fact that eels and deep-sea fish were taken together, they inferred that the eels spawned' at depths of not less than 200 fathoms. If this were true of eels in

tho Mediterranean it was probably true of all eelsj hence it was not altogether a surprise when certain Danish naturalists discovered that eels are hatched beyond tho 600-fathom line to the west of Ireland and in the Bay of Biscay. This discovery, which was worked out by Johannes Schmidt, was not due to accident, but to systematic and reasoned investigation. In the year 1904 the Danish steamer Thor was employed in collecting the young stages of edible fish in the neighbourhood of the Faroes and of Iceland, in connection with the International Fisheries Investigations. Mr Schmidt attempted to obtain specimens of eel-larvre, using smallmeshcd nets at various depths down to the bottom, but he got only a single Leptocephalus brevirostris. This was taken near the surface, at a point west of the Faroes, where the depth was about 650 fathoms. "If the young eels Avhich reach the coast in April or earlier were tho offspring of adults* which migrated to the sea in tho precedincr autumn, it would follow that Leptooephali would occur only in the winter months ; but the Italian investigations showed that they occurred throughout the summer. It might also be that the_ larva? were not free-swimming, but buried themselves in the sea-bottom: but the fact of the single specimen being taken near the surface made this improbable, and it was concluded that the larvce must be sought in more southern latitudes. Accordingly, in 1905, from May to the end of June, the Thor made a voyage from the north southward along the west of Scotland and Ireland, following the 500-fathom line, and Leptocephali were taken in increasing numbers till positions were reached off the entrance to the English Channel, where they were found to be most abundant, as many as 80, being taken in a two-hour haul. Further south again, above similar depths in the Bay of Biscay, few were taken. The larvae were found to be most abundant at a depth of about 50 fathoms from the surface, but were nearer the surface at night than in the daytime. These results proved that the greater number of eels spawn in the deep ocean off the mouth of the English Channel, and indicate that the adults, in their migration from the coasts bordering the North Sea, pass chiefly -through the Channel to reach the ocean.

In September a few specimens were taken to the west of St. Kilda (west of the Hebrides), and these were found to be in progress of metamorphosis. The young eels, after the transformation ie

complete, are still quite transparent, and they make their way to the coasts, and enter the rivers. The ascent takes place earlier in places near to the Atlantic, and also the young eels in such places are more numerous; thus Ave have the explanation of the elver fishery in the Severn and other rivers entering the Bristol Channel. Similar ascents and similar fisheries occur on the Atlantic coast of France, becoming earlier as Ave pass from north to south, while, on the other hand, in more northern countries, as Holland and. Denmark, the ascent is later, taking place chiefly in April, May, and June. In 1906 the Thor made another voyage along the borders of the Atlantic basin to the west of Europe, and found the Leptocephali in considerable numbers farther to the south in the Bay of Biscay; but here, as the sea-bottom slopes more rapidly, they were captured at a shorter distance from the coast; this was especially the case along the north coast of Spain, where the 500-fathom line is quite close to the coast. In August and; September larger numbers of more advanced specimens were obtained, and it was proved that these occur somewhat nearer' to the coast than the earlier stages. The occurrence of eel larvae has naturally not been so completely followed in the winter months; but the fact that perfectly transparent cylindrical forms occur in the sea a little earlier than the time at which the elvers ascend rivers, has been fully established. In the Channel, off Cape Grisnez, Professor Gilson, at the beginning of February, captured from 100 to 2CO at a single haul, and in March and April thev have been taken in numbers in the Skager Rack, north of Denmark. From the large number of specimens which have been captured by Schmidt it is easy to trace the course of the metamorphosis, the essential points of which were already known from the work of Grassi and Oalandruccio. The overage length of the first stage is 7.5 cm., or 3in; the average length of the fullydeveloped young eel taken in fresh water is 6.6 cm., or 2 l-3in. No reduction in length takes place until the curious change in the form of the body marked bj r the reduction in height has been very nearly completed. One of the most extraordinary facts about this metamorphosis is that the larva takes no food whatever during the course of it—a period of eight or nine months. In the glass-eel stage food has occasionally been found in the intestine, and some of these young eels may feed on their way to the more distant coasts; but the fact that the perfect pigmented stage is smaller than the transparent stage shows that no growth ha,s taken placo, but a continued reduction. Schmidt made a comparison between the average weight of the Leptocephali in the first stage and of the glasseels, and found that during the transformation the Leptocephalus lost over 77 per cent, of its weight. It might be that this was merely a loss of water, and was due to the consolidation of the tissues; but comparison of the weights of the dried specimens showed that the loss of dry weight was over 32 per cent., so that, although the reduction in size Is partly duo to contraction of the tissues, there is air© an actual consumption of material as the necessary result of so long a period of fasting. On the average 691 Leptooephali in the first stage weigh one kilogram, or a little over 21b, while it takes 3112 glass-eels to make the same weight. Those interesting facts apply to the eels found in Europe; hut though no definite information is available about eels in this country, the species are sufficiently closely allied to the European species to

warrant us in believing that tficir development is somewhat similar. The following may be considered to a PPty to tho eels of tho Australasian region. They never breed in fresh water, but in the deep sea at a depth of from 3000 ft to 4500 ft. The mature eels go down to the sea (if they can), the mam run seawards being in autumn. The only record I have of this migration in New Zealand is by the late Captain Button, who stated that in the Waikato River Anguilla aucklandii, the commonest eel in the North Island, "goes down to the sea in great numbers during tho months of February and March to breed." After breeding tliey never seem to return, and it is believed they die; but this is not certain. The young eels probably remain in the sea one year, slowly migrating to the mouths of the rivers. They como up the rivers in countless numbers in spring and early summer, according to the weather. The movements of the young eels are called "eel-fares" in England, and havo been known for centuries. In Australia they are seen yearly in Victoria, in such rivers as the Barwon and the Murray. In our own country they are found in 6uch rivers as the Mataura, where the falls offer a natural obstacle, and where the young fish may be seen in myriads about the beginning of January. It is quite probable that, as deep water occurs at no great distance from the coast in most parts of New Zealand, our eels and conger-eels do not need to go far to their breeding grounds; and when our biological stations are properly equipped for deep-sea work it will be possible to follow the development of these and other species of fish through all their stages. The little glass-eel found by me on the Brighton beach must have got out of its bearing and come too close inshore in the calm weather. Once it got within range of the breakers it shared the fate of other pelagic organisms, and got washed up on the sand.