In the early days of New Zealand much blood was shed over the rights to this or that eel water, so highly was the fish prized by the Maoris. It is not surprising therefore to find that much of the literature available on the eels deals with Maori methods of fishing for it and Maori interpretation of the various phenomena in its life history. The eel in New Zealand has also attracted the interest of the pakeha but little extensive or intensive study has been undertaken until recently. In an earlier series of papers the writer set out some preliminary studies on the biology of the two species (Cairns 1941 and 1942 a and b) of fresh water eel in New Zealand. The present paper summarizes some of the results obtained over a period of ten years and indicates some of the problems still requiring solution. If you have pondered over such questions as the age of eels, their breeding and feeding habits and why some eels grow to giant sizes, then you will find the answers in the pages that follow.

Taxonomy

Despite many claims by Maoris and others to the contrary, fisheries biologists now generally agree that only two species of eels exist in fresh-water in this country. Schmidt 1927 and Griffin 1936, are the two prnicipal authorities in this connection, the latter establishing the names Anguila dieffenbachii Gray for the commonly called “long-finned” eel and Anguilla australis schmidtii Phillips for the “short-finned” eel. The common terms “long” and “short” refer to the length of the portion of the dorsal fin projecting forward beyond the position of the vent on the ventral surface.

Fig. 1—Long-finned eel. (a) Outline of eel showing position of dorsal fin, thick lips and position of eyes; (b) upper jaw. (Note central or vomerine band of teeth.)

Fig. 2—Short-finned eel. (a) Outline of eel showing position of the dorsal fin and eye; (b) upper jaw. (Note the central or vomerine band of teeth and the thin lips.)

The most reliable method of separating the two species is based on the differences in length of the dorsal and ventral fins in relationship to the total length of the body. The distance between the anterior edge of the dorsal fin and the anterior edge of the ventral fin is measured along the lateral line of the specimen and this length is expressed as a percentage of the whole length. Measurements over a range of many thousands of eels have given the following figures—long-finned eels, 6% to 19%; short-finned eels 1% to plus 5%. (In the case of minus readings the dorsal fin is shorter than the ventral.)

No other character is as constant as the fin measurements and it is possible, without error, to separate all specimens on this character alone. Only fragmentary material representing the males of the short-finned species has, however, been examined.

Age and Growth

Scales first form along the lateral line in the posterior third of the body in the seventh year of life. They injure easily and regenerate freely, consisting of a series of concentric platelets representing years of growth. They are not reliable when used alone.

Otoliths (ear bones) contained in small capsules at the side of the brain are used in conjunction with scales and calculate the age of eels. After removal, otoliths are subjected to a long process of grinding until they are thin enough, when cleared in aniline oil, to reveal the annual “rings” of growth. During periods of active feeding in the spring and summer months of the year a substantial increment of loosely packed bony growth is added to the otolith whereas during the autumn and winter months when little food is taken only a narrow zone of tightly packed material is added. Viewed under the microscope the zones appear clear (spring and summer) and dark (autumn and winter). A count of the total clear or dark zones gives the age of the fish. In the early years of life when growth is rapid the zones are clearly marked but the age of eels over 12 years cannot be read with great accuracy since both zones become narrow in extent and crowded on the edge of the otolith.

Age determinations with otoliths are based on the assumption that the nucleus of the otolith is complete when the elvers (young eels) reach fresh water and that it represents a larval period of two years.

As far as New Zealand eels are concerned only indirect evidence can be brought to bear to confirm these two assumptions.

In the first ten years of life N.Z. eels grow to an average size of 47.8 cm. (short-finned eels) and 39.0 cm. (long-finned eels), which is fairly close to lengths for eels in Europe. Sex organs are not differentiated at this size. In the next five years of life, N.Z. long-finned eels fifteen years old range between 75.1 and 88.9 cm., but British eels reach only 48 cm. Short-finned eels in N.Z. over the age of fourteen years have not been found—at this age, however, the range of size is 75.0—92.5 cm. The shirt-finned eel of N.Z. grows more rapidly and matures quicker than the long-finned eel. Specimens of the latter over twenty years of age have been recorded. Both species in N.Z. outstrip in size the recorded figures for British eels.

Migrations

Little is known of the sea life of the larval eels on their way from their breeding grounds to the rivers of New Zealand. An occasional Leptocephalus has been recorded from the coasts—this being the name commonly given to the leaf shaped larval form of the eel captured from the sea. Metamorphosis normally occurs some distance from the coast and the young needle-like elvers move into the estuaries of rivers and streams.

Three distinct migrations have been observed in New Zealand.

The first occurs from October to December each year as indicated above when elvers (2 years old) move from the sea into fresh water. Many of these “runs” consist of millions of young eels. Moving at night or during floods they are not frequently observed. This migration ends as the young eels find suitable cover and feeding grounds in the lower reaches of streams and rivers. Here they remain for the next 4 or 5 years.

Shortly after the arrival of the seaward migration in this habitat an upstream migration of eels occur. These are 4-5 year olds. This upward migration has been trapped by the writer at certain points on a number of New Zealand rivers and it is now known that it is an annual event and takes place at the end of January each year.

Fig. 3—Metamorphosis of the eel larvae from the Leptocephalus stage to an elver (after Schmidt).

Normally about 12% to 18% of upstream eel populations show signs of maturing and preparing for the downstream run (see full details of anatomical and physiological changes later) by mid-summer. By April only 1.3% of the eels which would normally be considered migratory (from gonad development) are left upstream. These individuals are temporarily delayed in development (causes not known) or are permanently sterile. These latter are the “giants” in the eel population so well known to most New Zealanders.

Eels move seawards in a definite sequence. First the upstream females move to brackish waters where the join the males. The migration into the sea is first made by the males of the short-finned species, followed by the females of this species. The males of the long-finned eel then move seaward and the last movement is made by the females of this species.

Distribution

By systematic trapping over most of New Zealand and by obtaining access to records kindly made available by fishermen and biologists of the Marine Department, the writer has established a fairly definite pattern for the distribution of the two species in New Zealand waters.

Schmidt (1927) in a brief report following a visit to New Zealand attempted to work out the distribution from rather meagre material—at least half of his 1,400 specimens were captured during migrations. Migratory eels must be eliminated for the sample on the basis that the point of capture does not indicate their true habitat. He can b brought to bear to confirm these two assumptions. considered that the short-finned eels were confined chiefly to the north and east and the long-finned eels to the south and west of New Zealand.

The widespread sampling of the writer has revealed the following distribution: The long-finned eel inhabits all types of water from tiny streams to the largest rivers, coastal and inland lakes, brackish estuaries and lagoons in both Islands. In the colder waters it is generally the predominant species.

The short-finned eel on the other hand is not distributed throughout all the fresh-water in this country. In the South Island it remains principally in the tidal waters of streams and rivers, in the coastal lakes and in some low altitude inland lakes. In the North Island it is much more widely distributed being the predominant species in the lake fed rivers of the Auckland Province. It appears to favour warmer water than the long-finned eel and particularly areas where rapid temperature changes do not occur. The exact environmental requirements need further study.

All eels captured in New Zealand waters above tidal reaches have been female eels. The significance of this most interesting observation is dealt with in a later note on the development of sex in eels. The page 48 location of nothing but female eels in upstream waters has been confirmed recently in other countries. Male eels normally inhabit estuarine waters and coastal lakes subject to tidal influence. The population of estuarine areas is not, however, exclusively male.

Barriers to Migrations causing Anomalous Distribution

The problem of eel access to certain waters has exercised the minds of many observant people. There appears to be two divisions to the problem. (1) How do eels gain access to certain lakes and ponds cut off from streams or rivers? (2) Why are some rivers and/or lakes without any eel population at all?

With regard to the first question the answer in most instances is fairly simple. Eels migrate across country from streams and rivers during wet or dewy nights and will travel fair distances. In the case of one or two lakes in New Zealand, however, it is likely that eels gain access through a subterranean stream when in the “needle-like” stage. In at least one case (Lake Virginia, Wanganui) no exit is available for mature migrants which reach gigantic sizes and die in the lake.

The second problem—that of rivers and lakes without eel population is of great interest. Some of the best known areas are Lakes Rotorua and Rotoiti, Lake Taupo and the upper part of the Waikato River to the Huka Falls, also the upper part of the Waiai-ua River in Marlborough and the Waihou River above Ohoroire. There is ample evidence that young eels can traverse vertical rock walls provided they are damp and thus by pass the steepest waterfalls out of the main rush of water. The “barrier” to eels in each of the examples cited above consists of a short stretch of river through which the water rushes turbulently and at great speed, the smooth rock walls forming no shelter for fish. While trout and salmon (if they were present) can negotiate such “barriers” eels will not. The waters above the “barriers” are thus free of eels. Attempts to acclimatize eels above these sections of water (principally by the Maoris) have always failed because the mature eels will migrate downstream past the “barrier” on their journey to the sea and further eels can not migrate upstream past the “barrier” to replenish the stock.

These eel-less waters present interesting evidence on the effect of the absence of eels on trout populations. This is discussed in more detail later in this paper.

Hibernation

There is a definite correlation between water temperatures and feeding velocity in the eel. Low water temperatures as winter approaches result in fewer eels being observed actively feeding at night. With the onset of winter eels hibernate to deep mud in backwaters, swampy areas or drains where they lie dormant until spring. Hibernation has been amply confirmed by trapping and observation in the winter months in page 49 rivers and streams and by recovery of the dormant eels during swamp excavations. Eels recovered by this method are curled up tightly and encased in a gelatinous slimy cover. Physiological studies particularly oxygen consumption of the blood might prove interesting.

In the warmer lake controlled or spring controlled rivers and streams hibernation of the whole population of eels never takes place. Even in mid-winter some eels can be seen actively moving around and feeding at night.

Food of Eels

Over 10,000 eels have been examined and their gut contents noted.

For the purpose of presenting this information three rather arbitrary groups have been established. The first consists of eels up to 40 cms. in length—these normally live a semi-subterannean existence, seldom, if ever, feeding in open water. The second group comprises eels from 41 cms. to 75 cms., feeding actively in the open but restricted somewhat in choice of food by the size of the mouth gape. The third series, eels 76 cms. and more are not restricted in any particular way in the choice of food.

Both long- and short-finned eels in the first groups were found to live on much the same diet consisting of the following organisms (principal genera only given): Crustacea (Water fleas); Paracalliope; Daphnia; Oligochaetes (Worms); Lumbricus; Ephemeroptera (May-fly larvae); Ameletus; Deleatidium; Atalophlebia; Trichoptera (Caddis-fly larvae); Pyenocentria; Olinga; Hydropsyche; also a small number of Molluscs and Coleoptera larvae. The habitat occupied by eels in this group is quite clearly reflected in the food consumed.

The next group 41 cms. to 75 cms. are more diverse feeders in open water. Principal items in the diet of the long-finned eels are as follows: Ephemeroptera (May-fly larvae); Trichoptera (Caddis-fly larvae); Mollusca (Fresh-water snail and mussel); Oligochaeta (Worms). Subsidiary groups identified and of some importance included: Diptera (two-winged flies); Crustacea (Crayfish, crab and shrimp); Fish (various including bully, inanga, Retropinna or smelt and trout); Coleoptera (Beetle larvae and adults).

In this group the short-finned eels consumed very much the same types of food but the order of preference shows a distinct change and no Ephemeroptera are recorded. The principal items in order of preference are as follows: Mollusca; Oligochaeta; Diptera; Crustacea; Trichoptera.

Main genera noted were: Ephemeroptera (except in short-finned samples); Ameletus; Deleatidium; Ameletopsis; Coloburiscus Trichoptera, Olinga; Pycnocentria, Hydropsyche; Hydrobiosis; Crustacea; Paranephrops; Xiphocaris; Daphnia; Boeckella; Mollusca; Potamopyrgus; Isidera, Myxas; Diptera; Chironomus; Austrosimulum; Calliphora; Bombylius; Oligochaeta; Lumbricus; Coleoptera; Odontria; Pyronota.

In the largest size group of eels 76 cms. and over, the very diverse nature of the diet is again notable. The preference for certain types of food changes again. The order for the long-finned eel is as follows: Salmonidae and other fish; Crustacea; Mollusca; Ephemeroptera; Trichoptera; Coleptera; Oligochaeta; miscellaneous groups.

For the short-finned eel quite a different pattern is observed. Trichoptera tatke preference followed by Mollusca, Oligochaeta and Crustacea.

Food habits alter, however, with the season of the year and the habitat of the eel. Detailed samples have been recorded (Cairns, 1942a) illustrating the food of eels in many different environments in New Zealand.

Eel-trout interrelationship

An analysis of the trout contained in long-finned eel stomachs indicates that this eel is a predator on trout fly and fingerlings after a certain stage has been reached in its growth. Later it includes large trout in its diet. Many trout bear distinct eel scars indicating that they have succeeded in escaping capture—the proportion of scarred trout in some streams is as high as 15% of the legally takeable stock.

It has also been observed that in those waters where eels are absent, very heavy stocks of trout are carried, some of the lakes and rivers providing first class fishing.

The most important factor, however, in the eel-trout interrelationship is the continual competition for food between the two fish stocks. A comprehensive study of the gut contents of eels and trout from the same habitats has revealed that the two are direct competitors for the same types of food. The heavy stocks of endemic eels in our rivers and streams therefore impose a limit on the population of trout (an acclimatized fish) which can be supported. As erosion and pollution reduces the food supply it seems most likely that trout populations will suffer in greater degree than the more diverse and easily adaptable eel particularly since it is a native of our waters.

Development of gonads

The Dutch worker, Tesch (1928) asserted that all eels under a certain size were not sexually defined and that determination of sex was dependent on environment. He considered that male eels developed in brackish and salt water—females in fresh running water. Much evidence has been accumulated to support this contention both by overseas workers and the writer. Certainly no male eels are to be found in New Zealand rivers beyond tidal influence. Transplanting of stocks of young eels (prior to gonad development) from tidal areas to an upstream habitat consistently results in the development of females. In the original environment the remainder of the sample develop en- page 51 tirely as males. Further studies are necessary here and interesting histological and genetical material should be available.

The male sex organ consists of a considerable number of flat petal-like smooth lobes situated in a band on either side of the intestine. Within the separate lobes tubes are developed for the transference of sperms to a common duct which passes back to open near the vent. It seems probable that the opening of the vent is used for the extrusion of the sperms.

The female sex organ is situated in the body cavity in a position similar to that of the male gonad. When immature it is a delicate pinkish band on either side of the gut, one edge of which is attached to the body wall and one free. As maturity approaches, the bands widen and become creamy-yellow in colour. When the maximum development in fresh-water is attained they are yolk-yellow and in the form of an extensively convoluted frill. (The outer edge of the ovary is much longer than the inner attached edge.) The eggs (ova) are situated on the lamellae on the outer side of the ovary and may number up to four or five million. They are probably liberated in the body cavity and passed out of the vent.

Other phenomena noted during the development of the gonads as maturity approaches are as follows: General loosening of pigment in the melanophones, increased size and clarity of the eye, atrophy of the gut, general prominence of the lateral line cells.

The atrophy of the gut in the migrant eels has, of course, interesting repercussions from the fishing angle. The Maori knows well that the downstream migrant eel on its way to the sea to breed cannot be caught on a baited hook. They are captured but only in unbaited pots set in the path of the shoals of migrants. When the eel reaches salt water the gut consists of only a narrow strand of tissue—the gonads almost fill the abdominal cavity.

Physiology of the Eel

Many problems in physiology have been studied particularly in the migrant eel. Further studies are required.

The following important changes are worth noting, (1) colour changes due to expansion and contraction of the melanophones and xanthophones; (2) eye changes which may be regarded as an exophthalmic condition due to a thyrotrophic stimulation; (3) development of sexual organs due to a gonadotrophic stimulation; (4) various anatomical changes including atrophy of the gut, gross morphological changes in the head and stimulation of the lateral line glands.

Many of these changes must, in addition to be associated with the development of the gonads, be related to the eels journey to salt water and ultimate spawning at great depths under extreme pressure.

The colour changes have on preliminary examination appeared to result from activity of the posterior lobe of the pituitary. The silvery colour of the eel on embarkation into the sea indicates that all pigment has been released.

The eye changes appear to result from the activity of an anterior pituitary hormone on the thyroid gland. The eye enlarges and the pupil becomes more prominent. Evans (1940) who has also made a preliminary study of this subject discusses the connection of external conditions with the brain through the lobes inferiores of the infundi-bulum and thence by two ganglia to the pituitary. The sensitivity of eels to thunder indicates that connections may be of importance in the transfer of stimuli.

In the last two months before migration there is a great increase in size of both the anterior and the posterior lobes of the pituitary, there is a remarkable activity in the cells of the posterior lobe and a pronounced change in the cell contents of the anterior lobe.

Attention has already been drawn to the fact that a small percentage of eels remain sexually immature (all females in New Zealand) and these exhibit the phenomenon of gigantism.

It is probable that eels 20lb.-50lb. in weight and approaching six feet in length would fall into this category. Populations of eels imprisoned in ponds by special conditions may exhibit a high proportion of giants. In normal river populations only a small percentage fail to migrate and develop as giants. The evidence suggests that if the eel is prevented from going to sea or normal sexual development does not take place the gonadotrophic hormone is replaced by the growth hormone and gigantism follows. A similar pathological condition may occur in children resulting in gigantism and in adults acromegaly.