The Nematode Parasites of Lake Ellesmere Trout
[Read before the Canterbury Institute, August 7, 1935; received by the Editor, October 29, 1935; issued separately, June, 1936.]
Life History and Distribution.
Effects of Parasitism.
The identity of the various worms dealt with in this paper was determined by Dr. Baylis, of the British Museum (Natural History), to whom a collection of fish parasites was forwarded by the Canterbury Museum. Duplicates of these were retained by the writer, who, through the kindness of Professor Speight, has had the additional advantage of access to paratypes and named specimens returned to the Canterbury Museum, together with reports upon the whole collection.
It was found necessary, when investigating the effects of parasites upon trout, to restrict the procedure to comparisons between the parasited and the unaffected fish of the same locality, in order to avoid the introduction of various outside influences that are by no means constant in all waters. The trout of Lake Ellesmere are heavily, though not universally, parasited, and comparative material is usually obtainable, but one serious complication exists which should be noted here. Lake Ellesmere is a brackish coastal lake, about 45,000 acres in extent, normally separated from the sea by a narrow shingle-spit through which an opening is annually effected for the purpose of drainage. The principal tributary of the lake is the Selwyn River, which rises in the foothills of west Canterbury, and is normally dry on the open plains, but reappears as permanent water about 12 miles from the lake. About 3 miles from its mouth the river abandons its rapid shingly character and becomes wide, deep and sluggish. The Selwyn is joined on the south by the Hororata, a shingly stream of considerable size, and on the north by the courses of the Hawkins and the Waianiwaniwa, the lower sections of which are merely storm-water channels. All of these streams converge at a point where the Selwyn is normally dry, and the water of the Hororata usually disappears in the shingle after a few miles, thus isolating this stream from the lower permanent waters for the greater part of the year.
The material used when investigating the effects of parasitism was obtained from the mouth of the Selwyn River, the mouth of L.2 Creek, and the portion of the lake in the vicinity of these streams. Upwards of 100 specimens were collected at various times from 1929 to 1931, but the material obtained in the 1932 season was characterised by certain features that necessarily disqualify it for the present purpose. At the opening of this season the lower Selwyn obtained a good deal of notoriety on account of the poor condition of the trout taken by anglers. The portion of the lake adjacent to the Selwyn, and the deep section of the river, together with all deep pools as far upstream as permanent water extended, contained large emaciated trout, and the presence of dead fish in all post mortem stages ranging from specimens that had died within 48 hours to others that had been reduced to mere skeletons, showed that these trout had been dying off for several months.
A group of 20 specimens captured for examination had an average length of 18.2 inches, and an average condition factor of 31. All were mature Lake Ellesmere trout and all except two were females. These invariably contained a few loose eggs in the body-cavity, in some instances lodged between the liver and the diaphragm, and all showed some degree of injury to the ovarian membranes. There was frequent rupture of the peritoneum and occasional rupture of the lower intestinal mesentery, while in one specimen the posterior portion of the spleen, amounting to about half of the total bulk, was almost completely severed from the remainder. Severe inflammation usually existed in the ventral region, and where this was associated with peritoneal injury, there was considerable disintegration of tissue. Such fish could not have recovered, but a few specimens, notably the two males, showed only slight traces of recent inflammation, and appeared to be on the point of recovery.
The food of the 20 fish consisted of 116 insects, principally Trichopterous larvae, and 7 smelts.
The disinclination of these trout to return to the lake in the customary manner is fully accounted for by their pathological condition, and the abnormal absence of floods during the winter of 1932 can be regarded only as instrumental in revealing, to general observation, conditions that were by no means peculiar to this season. The injuries described in no way resemble the effects of parasitism, but have obviously been inflicted in the process of extruding the eggs when the fish were being artificially spawned for cultural purposes. The incidents of a lake trout's existence include none that is capable of causing disruption of internal structure without disclosing some evidence of it in the form of scars or missing scales, but pressure applied through some non-rigid agency, such as human hands, could cause all of the injuries described, without leaving an outward mark. It is impossible to explain the presence of eggs between the liver and the diaphragm except as the result of severe abdominal pressure, or of the fish being held head-downward when out of the water, under which circumstances the mesovarium of a ripe specimen could be expected to burst anteriorly, and the eggs gravitate to the position mentioned.
As this material was useless for the present investigation, further collecting was deferred, but similar conditions existed at the opening of the 1933 season, and in 1934, although no injured fish were to be found in the river in consequence of their having been washed away by winter floods, many of those taken in the lake revealed evidence of recent injury to the mesovarium, or had fully developed eggs imbedded in the liver. It is noteworthy that such fish, which apparently represent the survivors of the winter's stripping, form a much greater part of anglers' catches than they did prior to 1930, and although the number of eggs collected each year has increased greatly since that date, the increase has not been proportionate to the increase in the percentage of injured fish. It would thus appear that trout are becoming fewer, and a greater percentage of the lake population is now being dealt with.
It is necessary to add that the Lake Ellesmere material was heavily culled, and all specimens showing evidence of past injury were rejected, as were those which, by scale examination, proved to be not of the typical lake stock, but more or less interlopers.
The material employed in the investigation of effects was thus reduced to 66 specimens. Other material considered when dealing with distribution consists of 26 adult lake trout taken from the Selwyn at Meadowbank, 34 immature fish from the upper limit of the deep-section of the river, and 65 stationary trout from the Hororata. The whole of the fish mentioned above are brown trout, but data from 32 rainbow trout taken at Lake Coleridge are also included.
Particulars of the average length, food and parasitic infestation of these groups are given in an appendix together with a table based on the examination of 17 shags taken at Lake Ellesmere.
Life History and Distribution.
1. Hedruris spinigera.
Hedruris spinigera is a small white worm which exists, in the adult state, in the stomachs of fishes inhabiting brackish water. Its maximum length and thickness are 12 mm. and 0.6 mm. respectively, but these dimensions are attained only by females, the corresponding measurements for males being 7.3 mm. and 0.24 mm. The sexes differ so greatly in one structural feature as to give the impression, on first acquaintance, of two forms by no means closely related. The posterior of the female is furnished with a hinged hook and sheath, by means of which the animals attach themselves to the stomach-lining of the host, while the males are entirely without this development, and are dependent for support upon the females, around which they are usually coiled. This structure has been described and figured by Baylis (1931), but as the specimens available to him had been fixed with the hook in the closed position, and the writer has since succeeded in fixing a specimen with the hook extended, a micrograph of this is shown in Fig. 1. Fig. 2 shows the interior of a trout stomach with the worms attached to the lining
as in life. This worm is the most common parasite of Lake Ellesmere trout, and occurs also in a number of other fishes, some of which belong to widely separated zoological groups.
The following natural hosts have been collected at Lake Ellesmere:—Salmo trutta, Oncorhynchus tschawytscha, Retropinna retropinna, Galaxias attenuatus, Agonostomus forsteri, Rhombosolea retiaria, Rhombosolea plebia, Rhombosolea tapirina, Anguilla aucklandii.
It is usual for worms of this class to have an existence in two different animals, the first of which is taken as food by the host in which the parasite attains maturity, but no intermediate host of Hedruris spinigera is known, and none has been definitely recorded of any species of this genus occurring in other parts of the world. The most consistently infested of the indigenous fishes are the smelt, Retropinna retropinna, and the mullet, Agonostomus forsteri, and if the life history of this worm conforms to that of other closely related forms, it is among the animals forming the food of these fishes that its intermediate host could most profitably be sought. It was found that young smelts of up to 40 mm. in length, taken at Lake Ellesmere, had fed exclusively on copepods and were free from Hedruris, but somewhat larger specimens in which this diet had been supplemented by a few specimens of Tenagomysis novae–zealandiae contained occasional immature worms. The food of adult smelts taken in the lake consisted of Tenagomysis, Paracalliope fluviatilis and small Dipterons, while adult mullets contained Paracalliope, Tenagomysis and gastropods (Potamopyrgus) represented quantitatively in the order mentioned. Of the five food-animals thus associated with the occurrence of the worm, Tenagomysis appears to be the most probable intermediate host, as it is the only one that has been found in all natural hosts except those in which the presence of the parasite may be explained as the result of direct transference from infected food-fishes, and it is, moreover, perfectly satisfactory as regards size. One hundred and seventy of these crustaceans were collected from the lake-bottom and dissected, but all were free from Hedruris, although twenty-four proved to be parasited by trematodes. The remaining food-animals, with the exception of the copepod, which may be definitely ruled out on account of its size, were also subjected to investigation, one hundred specimens of each being dissected, but the only worms discovered were trematodes of somewhat peculiar structure which occurred in several specimens of Potamopyrgus. Several animals that have not been found in the fishes under consideration were also investigated with similar results.
Notwithstanding the failure of these investigations to discover any intermediate host of Hedruris spinigera, there is ample evidence that some preliminary existence is necessary to this worm. The maximum dimensions of the eggs contained in mature females are given by Baylis (1931) as 0.042 mm. by 0.017 mm., and the embryos visible in advanced specimens appear to the present writer to be coiled only once, or, more correctly, doubled. The length of the larva on emergence from the egg can scarcely exceed 0.1 mm., and as the smallest worms found in fishes are approximately 4 mm. in length, there is obviously a considerable gap in the present knowledge
of this parasite's life history. The possibility exists that the larva is free-living during this stage, but no evidence of this has been detected in nettings from the water or in mud and detritus from the lake-bottom.
Table I gives particulars of the food and average length of 66 adult Lake Ellesmere trout classified according to the nature of parasitic infestation. The fish composing the first group were free from parasites of any kind, the second ground contains specimens infested with Hedruris spinigera, and the third those in which the parasite Eustrongylides sp. was present in addition to Hedruris. A consideration of the data presented in this and the following tables suggests that trout become infested with Hedruris only when in the same waters as smelts, and that the parasite is merely transferred to the former fish when the latter is taken as food. There is no material difference in the food of the two parasited groups listed in Table I, each of which contains a considerable proportion of smelts, but it is to be noted that the food of the unaffected group includes only a single specimen of this fish. It is possible that this difference is, to some extent, accidental, and that the food contained by the unaffected group at the time of capture may not be truly representative of the normal diet, but if it is assumed that these trout had recently fed freely on smelts, the absence of Hedruris is difficult to explain. The majority of the adult smelts of Lake Ellesmere are parasited, and in view of the ready adaptability to trout shown by this worm, it is scarcely conceivable that smelts could be taken, even in moderate quantities, without some degree of infestation resulting. It seems more probable that certain trout, either from choice or owing to peculiarities of individual habitat, adopt feeding habits differing from those of the majority, and that freedom from infestation is maintained so long as the natural hosts of Hedruris are excluded from the diet.
The 34 immature fish detailed in Table 2 were taken in early summer, about the upper limit of the deep section of the Selwyn, and appear never to have been lower than this point. The scales of these fish show perfectly open edges, which indicates that growth was proceeding freely at the time of capture, and the uniformity of the structure formed since the winter supports the view that the same locality had been inhabited during the last five or six months. The presence of Hedruris in ten of these fish is quite consistent with the explanation that the parasite is obtained from smelts taken as food, as the summer range of the latter fish extends to the locality under discussion, and specimens taken at this point were found to be almost universally infested. The difference between the food of these trout and that detailed in Table I appears to be primarily one of environment. At the point where the present specimens were taken the country is for the greater part grassed and heavily infested with brown beetles, Odontria zealandica, conditions that do not exist on the lake-flats near the mouth of the river. During summer evenings these beetles resort to the willow trees bordering the stream at this point, and many are precipitated into the water, where they are freely taken by trout. Considerable numbers of caddis larvae are also taken from the nearby shingle reaches.
The group of adult Lake Ellesmere trout detailed in Table 3 consists of fish that had completed their season's growth in the autumn and moved upstream in anticipation of spawning. Such fish discontinue normal feeding when they leave the deep water, and during their upstream residence take very little food, the greater part of which consists of insect-larvae. The present group was taken during February, March and April, about five miles above the highest point at which smelts have been observed, and had existed in this locality for periods ranging from one month to three months. Only three specimens contained Hedruris, but as all of the fish are from Lake Ellesmere, it is reasonable to suppose that the rate of infestation normal to this water had recently existed, and that the majority of the parasites possessed by the fish at the time of migration from the lake had been lost, possibly owing to natural death after reproduction. A residence of three months in a locality where facilities for reinfection are absent would thus appear to be sufficient to rid trout of this worm.
It has not been possible to obtain a group of trout that had spent the whole of their lives in the shingly section of the Selwyn, as such fish are now extremely rare in this water, but particulars of a group from the Hororata, which still retains a scanty stock, are given in Table 4. This group is composed entirely of stationary river trout, and is noteworthy for the complete absence of parasites. The food of these fish consists principally of insect-larvae, and agrees substantially with that of river-dwelling trout from all waters with which the writer is acquainted. Trichopterous larvae were found in 63 specimens and occurred indiscriminately in the smallest and largest fish in the group. The next most evenly distributed foods are Plectoptera and Coleoptera, which were found in 41 and 31 stomachs respectively; fishes, Gobiomorphus gobioides, were present in two specimens, and gastropods in fourteen.
A similar absence of Hedruris was noted in the rainbow trout of Lake Coleridge, particulars of which are given in Table 5, and also in the rainbow and the brown trout from other alpine lakes. This worm has never been found in fresh-water quinnat salmon from Lake Coleridge and Lake Wanaka nor in lake-dwelling Salmo salar of Lake Te Anau; neither does it occur in the various species of Galaxias existing permanently in upland waters. Its occurrence is obviously dependent on conditions that are present only in a brackish-water habitat, and while it is highly probable that these are associated with its intermediate existence, the precise circumstances controlling the primary infection of fishes are unknown. The various species of Hedruris recorded from other parts of the world occur only in fresh-water, amphibious and terrestrial hosts, and it is somewhat surprising to find that the New Zealand representative of the genus is restricted to brackish-water fishes.
2. Eustrongylides sp.
The intermediate stage of this worm is passed in the body-cavity and the flesh of fishes, and maturity is reached in predatory birds. The immature worms, which are red in colour and attain a maximum length of approximately 60 mm., are usually coiled in cysts attached
to various organs within the body-cavity of the fish, but most frequently to the outer surface of the stomach, as shown in Fig. 3. The exact stage of development at which the parasite enters the body of the fish is unknown, but it appears that entry is effected by way of the mouth, either as an egg or a recently-hatched larva, and that the worm penetrates the stomach-wall at an early age, as specimens measuring 9 mm. in length have been found completely encysted on the stomachs of Galaxias attenuatus. The period during which the worm remains in the cyst is evidently of considerable duration, but it may be inferred from the fact that free specimens found in the abdominal cavity and the flesh are always at the limit of development observed in fishes, that the period of encystment in terminated when a transference to the natural host becomes imperative. At this stage the worm apparently leaves the cyst, and ranges freely in the abdominal cavity, penetrating the vital organs and also the flesh, but never leaving the body of the fish during the lifetime of the latter. The injuries inflicted by the worm during this active period bring about a debilitated condition of the host, which eventually becomes an easy prey to the predatory bird in which the parasite attains maturity.
The following intermediate hosts have been collected at Lake Ellesmere:—Salmo trutta, Gobiomorphus gobioides, Galaxias attenuatus, Rhombosolea, retiaria.
Two natural hosts of this worm have been collected at Lake Ellesmere, the Black Shag, Phalacrocorax carbo, and the Spotted Shag, P. punctatus. The extent to which these birds are infested is indicated in Table 6, which also gives particulars of the food and other parasites; but it must be pointed out that Hedrius spinigera is not parasitic in birds, and the specimens of this worm included in the list represent part of the stomach-contents of fishes that had recently been taken as food. As none of the fishes listed in Table 6 is known to harbour Eustrongylides, it appears that the diet of shags includes other forms, possibly Gobiomorphus gobioides, which, apart from the trout, is the most consistently infested fish that has been observed. It is also probable that other natural hosts exist, but no opportunity has occurred for the examination of various other birds that might conceivably be infested. The most probable of these apart from species of shags other than those mentioned above, is the bittern Botaurus poeciloptilus,* but it is possible that the worm will be found in other by no means closely related forms, as most species of Eustrongylides appear to possess remarkable adaptability.
The following natural hosts of the genus are recorded by Jägerskiöld (1908) :—Nucifraga caryocatactes, Ardea goliath, Ardea cocoi, Herodias egretta, Botaurus pinnatus, Pelecanus rufescens, Phalacrocorax pygmaeus, Phalacrocorax carbo, Colymbus septentrionalis, Colymbus minor, Colymbus articus, Alca torda, Uria troile, Anhinga rufa, Plotus anhinga, Harelda glacialis, Podiceps cristatus, Mergus merganser, Mergus albellus.
[Footnote] * Since the above was written a specimen of Eustrongylides has been found in a hiltern that had been killed by coming into contact with the electric powerline in the Lake Ellesmere district.
The adult existence of this worm is remarkable, as it combines the fixed location of an encysted parasite with the feeding and reproductive habits of an intestinal one. The greater part of the body is enclosed in a cyst or sinuous tube attached to the outer surface of the pyloric branch of the stomach, or continued within the thickness of the wall, which is fleshy at this point, but both ends of the worm protrude into the gut. It is obvious that the immature worm, upon being released in the alimentary tract of the natural host by the digestion of an infested fish, penetrates the wall at the outlet of the stomach, and, after withdrawing the greater part of its body into the abdominal cavity, returns its head to the gut by means of a second perforation. The body of the worm is naked during the early stages of existence in birds, and the enclosing tube is a subsequent development of true cystic character. When the tube is complete the worm appears to be incapable of leaving it, by reason of its body being distended medially, but communication with the gut of the host is maintained for the purposes of feeding and copulation. The eggs of the female are discharged into the gut and are ultimately voided with the excrement into the water, there to form the source of infection of fishes.
The specific identity of adult specimens collected at Lake Ellesmere has not been determined authoritatively, but so far as the writer's observations have gone, there is not satisfactory agreement with any species described by Jägerskiöld (1908). The papillae on the head agree in number with those of E. ignotus, but differ some-what in both form and disposition. More important differences occur in the posterior of the females, particularly in the form of the oviduct, which appears to be spiralled. The eggs measure 0.074 mm. by 0.045 mm. and are covered with minute papillae which do not differ greatly in disposition from those of several recognised species.
A micrograph of the posterior portion of a mature female is shown in Fig. 4.
Reference to the tables already quoted when dealing with Hedruris spinigera will show that the occurrence of Eustrongylides in trout is associated with a lake existence, and that river-dwelling fish are normally free from infestation.
The 23 adult trout taken from the Selwyn River at Meadowbank (Table 3) included 9 specimens in which this worm was present, but the group is composed entirely of Lake Ellesmere fish, and the occurrence of a normal rate of infestation merely shows that the parasite is not lost by migratory trout during a temporary upstream residence.
A few specimens of this worm were observed in trout from the Hororata River some years before the material listed in Table 4 was collected, but it is not known whether these fish had become infected in the locality in which they were captured, or were Lake Ellesmere trout that had obtained access to the Hororata at a time when the upward migration coincided with the existence of connection between the tributary and the lower permanent water. As all of the worms observed were in the flesh, it is evident that the fish had been infested for a considerable period. The present Hororata group contains no specimen of Eustrongylides, a distinction that
it shares with the group of rainbow trout from Lake Coleridge, particulars of which are given in Table 5. It is, however, probable that some degree of infestation exists in the latter water, as a group of young fish ranging in length from 4 inches to 8 inches, taken at or near the mouths of various tributaries, included an 8 inch specimen that contained two cysts. This fish was less than two years old, and is the youngest trout in which the worm has been found; its food consisted of one Chironomid and one larva of Procordulia smithii.
The food of the Lake Coleridge fish listed in Table 5 consists largely of Trichopterous larvae, and in this feature differs from that of all other lake-dwelling trout with which the writer is acquainted. A freedom from infestation with Eustrongylides, and a diet in which Trichopterous larvae predominate, are conditions that, conjointly, are typical of river-dwelling trout, and the recurrence of this association in a lake on which one natural host of the worm is present is of considerable interest. It is by no means clear whether the absence of this worm from the Lake Coleridge group is due to a preventative influence exercised by the particular kind of food upon which these fish had subsisted, or to the absence of some agent that has a positive influence. The heaviest infestation yet observed occurred in a group of 16 rainbow trout from Lake Taupo, 8 of which were parasited and contained a total of 73 specimens of Eustrongylides. Such food as was present in these fish consisted principally of vegetable matter, but the majority of the stomachs were almost empty, and it would be unsafe to regard this as representative of the normal diet. Phillipps (1924) found that the food of 28 rainbow trout taken at Lake Taupo consisted of 137 Gobiomorphus, 14 Galaxias, 8 insects, 6 gastropods, 1 spider and a quantity of vegetable matter, and, if, as appears probable, these particulars give a reasonably accurate indication of the usual food of Taupo trout, there exists the same association of parasitism and a diet in which bullies predominate, as will be noted in the Lake Ellesmere table. In view of the frequent occurrence of the worm in the bullies of both waters there is at least a suggestion of direct transference of the parasite from the food-fish to the predatory one. Free worms are found occasionally in the stomachs of trout, together with remains of bullies, and the possibility that they would have penetrated the stomach wall and re-encysted themselves cannot be disregarded. As the instinct to leave the alimentary tract is present when the worm first occurs in fishes, and still persists when access is gained to the natural host, it might reasonably be expected to be operative in partly developed specimens obtained from infested food-fishes. It is of course possible that the worm is acquired by trout in the same manner as by bullies, and that a similar habitat is favourable to the infection of each, but this suggestion does not account for the occurrence in the indigenous fishes of much smaller worms than have been found in trout. This latter circumstance cannot be explained on the hypothesis of adaptation to the size of the host, as the ultimate size attained by the worm is the same in each, but suggests a partial development of the parasite in other fishes before its acquisition by trout.
It is necessary to draw attention here to the occurrence in the Lake Coleridge group of another larval nematode, Contracaecum sp., which was found in the stomachs of four trout. This worm occurs in the body-cavity, and occasionally in the alimentary tract, of Gobiomorphus gobioides inhabiting upland waters; upon this fish being eaten by trout the parasite transfers itself to the latter, in which, however, its existence is usually only temporary. The adult occurs in the alimentary tract of various species of shags (see Table 6) and has also been found in Podiceps cristatus. Its restriction to the stomach of the former birds and to the intestine of the latter presents something of a problem in adaptation.
Effects of Parasitism.
1. Condition factor.
The effects of parasitism, if detrimental to trout, should be reflected in the condition factor and, more particularly when infestation is of long standing, in the growth rate and the ultimate size attained. For reasons already stated, the existence and extent of these effects can be determined only by comparisons between the parasited and the unaffected fish from the same locality, and the present consideration is necessarily restricted to the 66 specimens listed in Table I, additional data of which are given below. The following table gives the average condition factors of the three groups already differentiated:—
|Class||Number of specimens.||Average condition factor.|
|Infested with Hedruris||27||45.8|
|Infested with Hedruris and Eustrongylides||20||43.8|
The superiority of the unparasited group appears to be sufficiently definite to exclude the possibility of accident and to justify the conclusion that both species of worm are injurious to trout, but the association of Hedruris with Eustrongylides in the third group renders the position of the latter worm somewhat obscure. The condition factors for the two parasited groups are not unduly low, and are actually higher than the average for some waters in which parasitism does not exist, but their relative values are in no way affected by this consideration, and they can be logically compared only with the average for the unparasited fish from Lake Ellesmere. The inclusion in the parasited group of all fish containing worms, irrespective of the number present or the duration of infestation, has also resulted in the averages being materially influenced by specimens in which the effects of parasitism were not yet visible. The presence of worms in several trout of extremely high condition factor indicates that infection is in no way dependent on a debilitated condition of the potential host, but that the healthiest and best-conditioned fish are susceptible.
2. Growth rate.
The growth rates of the three groups are shown graphically in Fig. 5, the light line representing the unaffected fish, the heavy line those parasited by Hedruris alone, and the broken line the group in which both species of worm were present. The average lengths attained by the fish at the completion of the first, second and third years have been obtained by measuring the scales of immature fish in the usual manner, while the figures for the later years are based on the average total lengths of the various age classes, the completed portion of the year of capture being averaged in each class and the curves drawn to the corresponding point beyond the verticals.
There is no difference of any importance in the three curves till after the 4-year line, but thereafter a divergence occurs, the unparasited curve continuing at a slightly reduced inclination, the Eustrongylides curve becoming almost horizontal, while that for the group infested with Hedruris alone occupies an intermediate position.
It may be inferred from the falling off of the latter part of the Eustrongylides curve that this worm is retained by trout for a considerable period, but the reason for the absence of definite evidence of ill effects before the age of 4 years is not clear. If Table 2 is referred to it will be found that the fish listed therein are young specimens that had apparently never left the river, and the absence of Eustrongylides from this group suggests that infection normally occurs in the lake. If these fish represent a definite stage in the lives of all Lake Ellesmere trout a tolerably satisfactory explanation is provided, as their average age is over 2 years, and if they had entered the lake at this age it is probable that, by the time they had become infested and the influence of the parasite had become appreciable, their ages would have approached that at which the curve flattens out. It is, however, at least questionable if the whole of the smolts that descend the river each year make a temporary residence in the deep section before entering the lake.
The fish listed in Table 2 include a higher proportion of specimens from the Hororata, and the Whitecliffs section of the main stream, than occurred in the Lake Ellesmere material as collected, and although there can be little doubt that these would ultimately have become lake-dwelling, it is conceivable that such individuals would make a more gradual downward progress than the early migrating smolts of the typical Lake Ellesmere fish. Many of the latter should be in the lake before the completion of their first year of life, and if, as already suggested, Eustrongylides is merely transferred from an infested food-fish to the trout, the age at which the infection of trout becomes possible will coincide with that at which such food-fishes are capable of being taken. The writer has found bullies in 7-inch trout from other waters, and as this length would represent an age of about 18 months in many Lake Ellesmere fish, there should be opportunity for infection much before any falling off is visible in the growth curve. It should, however, be noted that the point at which the curve flattens out is roughly coincident with the attainment of maturity, and it is possible that the influence of the parasite is incapable of seriously affecting the growth of vigorous immature fish, but is sufficient to prevent complete recuperation when vitality has been lowered by the strain of reproduction. The curve for the unparasited group indicates a type of growth that has not hitherto been associated with any definite circumstance. The maintenance of a high growth rate after maturity has occurred is unusual in New Zealand, but it appears from the present data that migratory trout existing under conditions of abundant food and freedom from parasitic infestation are capable of regularly resuming vigorous growth after the annual event of spawning. It is also evident that if no distinction were made between parasited and unaffected fish the gradient of the latter part
of the growth curve would be dependent on the relative numbers of each class in the collection, and the data obtained from such un-differentiated material would not be representative of any class of fish existing in the lake.
3. Ultimate size.
The effect of parasites upon the ultimate size attained by trout can more accurately be judged from the number of fish exceeding a certain limit of length in each class than by comparisons of maximum lengths, although in the present instance, the same placing of the groups is obtained by either method. The exact limit of length that should be observed in making such a test as that suggested above is somewhat arbitrary, but it was noted by Godby (1918) that the majority of the Lake Ellesmere trout attained a maximum length of 23 inches, although a few individuals, not necessarily very old fish, considerably exceeded this length, and this figure will be accepted as a basis for dividing the present material. The individual lengths of all specimens exceeding 23 inches are given in the following table:—
[The section below cannot be correctly rendered as it contains complex formatting. See the image of the page for a more accurate rendering.]
|Unparasited||Parasited by Hedruris.||Parasited by Hedruris and Eustrongylitdes|
|25||23 ½||23 ½|
|26||27 ½||23 ½|
Of the 19 fish composing the unparasited group, 6 exceed 23 inches in length, and yield an average of 27 ½ inches; the Hedruris group of 27 fish includes 2 specimens exceeding the limit, with an average of 25 ½ inches, while the figures for the 20 fish in which both species of worm were present are 2 fish averaging 23 ½ inches. The unparasited fish thus possess a definite advantage both as regards the percentage of individuals exceeding 23 inches in length, and the greater average size attained. The failure of any of the fish containing Eustrongylides to exceed the prescribed limit by more than half an inch suggests that the growth of these specimens has been comparatively poor for several years, and supports the inference from the growth curve that this worm inhabits trout for a considerable period.
It is somewhat surprising that the group infested with Hedruris alone does not contain a greater percentage of large fish. When it is considered that this parasite becomes lost by trout after a few months' sojourn in fresh water, and that the present groups have been differentiated solely on conditions existing at the time of capture, it seems probable that the worm would have existed previously in some specimens that were free from infestation when examined, and that the group position occupied by individuals would be, in a great measure, accidental.
The majority of the Lake Ellesmere trout delay their spawning migration till winter and it is probable that these fish are not in the river long enough to rid themselves of the parasite, but even if this habit is constant in individuals it is not of itself sufficient to account for a condition of permanent infestation. The life of the parasite appears to be short, and frequent reinfection seems necessary if infestation is to be maintained. As the worm is apparently communicated to trout by infected food-fishes some degree of selective or restricted feeding is suggested, and this explanation is supported, to some extent, by the difference in the food of the parasited and the unaffected fish listed in Table I.
It may be deduced from the consistent inferiority of the parasited groups in all tests to which the present data have been submitted that both species of worm are injurious to trout, but in the absence of any records of the quantity of fish produced each year by Lake Ellesmere the effect upon the fishery cannot be estimated. The annual output of such a lake could be expected to be large, but so far as personal observations have shown there is nothing to suggest that it is comparable with that of really productive waters in Great Britain. According to particulars given by Dahl (1930) the annual output of Blagdon lake in Wales is 5.3 pounds per acre, and of Loch Leven in Scotland, 7.6 pounds. As Lake Ellesmere has an area of approximately 45,000 acres, an annual output in excess of 100 tons would be necessary if its productivity equalled that of Blagdon, or 150 tons computed at the rate of Loch Leven. Having regard to the number of anglers fishing Lake Ellesmere, their average fishing time and average catches together with the operations of commercial netters, and taking into consideration the possibility of a large number of adult trout being destroyed each year by artificial spawning, the writer can find no justification for the belief that the actual yearly output exceeds one-tenth of the smaller amount.
Baylis, H. A., 1931. A species of the Nematode genus Hedruris occurring in the trout in New Zealand, Annals and Magazine of Natural History, series 10, vol. vii, pp. 105–114.
Dahl, K., 1930. A visit to British Reservoirs, Salmon and Trout Magazine, no. 58, pp. 15–22.
Godby, M. H., 1918. A Preliminary Investigation of the Age and Manner of Growth of Brown Trout in Canterbury as shown by a Microscopic Examination of their Scales, Trans. N.Z. Inst., vol. 47, pp. 42–67.
Jagerskiold, L. A., 1908. Zur Kenntnis der Nematoden-Gattungen Eustrongylides und Hystrichis, Nova Acta Regiae Societatis Scientiarum. Upsaliensis, ser. iv., vol. 2, no. 3
Phillipps, W. J., 1924. Food-supply and Deterioration of Trout in the Thermal Lakes District, North Island, New Zealand, Trans. N.Z. Inst., vol. 55, pp. 381–391.
[The section below cannot be correctly rendered as it contains complex formatting. See the image of the page for a more accurate rendering.]
|Number of specimens||Average length (inches)||Gobiomorphus gobioides||Retropinna retropinna||Galaxias attenuatus||Larvae | Trichoptera||Imagines||Larvae | Perlaria||Gastropods||Vegetable matter||Hedruris spinigera||Eustrongylides sp.|
|Parasited by Hedruris spinigera||27||20.8||67||29||1||–||2509|
|Parasited by Hedruris and Eustrongylides||20||21.4||51||11||1||–||1805||36|
[The section below cannot be correctly rendered as it contains complex formatting. See the image of the page for a more accurate rendering.]
|Number of specimens||Average length (inches)||Gobiomorphus gobioides||Retropinna retropinna||Imagines | Lepidoptera||Larva | Trichoptera||Imagines||Larvae | Diptera||Imagines | Coleoptera||Larvae | Plectoptera||Spiders||Worms||Pebbles||Hedruris spinigera|
|Parasited by Hedruris spinigera||10||12.4||3||1||18||7||88||1||473|
|Average length (inches)||Larvae | Trichoptera||Imagines||Imagines | Diptera||Imagines | Coleoptera||Imagines | Hemiptera||Larvae | Plectoptera||Hedruris spinigera||Eustrongylides sp.|
|Average Length (inches)||Gobiomorphus gobioides||Imagines | Lepidoptera||Larvae | Trichoptera||Imagines||Larvae | Diptera||Imagines||Laravae | Neuroptera||Imagines | Hymenoptera||Larvae | Coleoptera||Imagines||Imagines | Hemiptera||Larvae | Perlaria||Imagines||Larvae | Odonata||Imagines||Larvae | Plectoptera||Imagines||Spiders||Gastrods||Worms||Small quantity vegetable matter and pebbles|
|Average lenth (inches)||Gobiomorphus gobioides||Galaxias sp.||Larvae | Lepidoptera||Larvae||Pupae | Trichoptera||Imagines||Larvae||Pupae | Diptera||Imagines||Imagines | Hymenoptera||Larvae||Imagines | Coleptera||Imagines | Hemiptera||Larvae||Imagines | Perlaria||Imagines | Orthoptera||Larvae | Odonata||Larvae||Imagines | Plectoptera||Gastropods||Gordius||Seeds, roots, bark, wood, leaves, moss, and other vegetable matter. Pebbles, sand, and feathers.||Contracaecum sp. Larvae|
|Agonostomus forsteri||Retropinna retropinna||Sardinia neopilchardus||Anguilla sp.||Gastropods||Insects||Waterweed||Grit and sand||Eustrongylides sp.||Contracaecum spiculigerum||Contracaecum sp. (Larval)||Unidentified Ascarids||Hedruris spinigera|
|Selwyn mouth||28/7/34||P. carbo||1||8||67|