The Ill-conditioned Trout Present in the Lower Selwyn During the Spring of 1932.
[Read before the Canterbury Branch, April 6, 1938; received by the Editor, May 4, 1938; issued separately, December, 1938.]
The conditions existing in 1932–33–34.
The cause of the injuries.
The effects of the artificial stripping of trout as practised at Lake Ellesmere by the North Canterbury Acclimatisation Society are discussed by Professor Percival (1937) in a paper criticising certain brief statements on this subject made in the introduction to a paper by the present author (1936), and as it is fitting that some account other than that of a member of the body concerned should be placed on record the following particulars are submitted.
The fish concerned are migratory trout from Lake Ellesmere, which are distinguished from the permanent residents of the shingly part of the Selwyn River by the microscopic structure of their scales and by their dentition. The scales of lake-dwelling fish show comparatively poor definition between the summer zones and the winter zones in the section formed between the parr stage and the attainment of maturity, and also disclose a much more rapid growth rate than those of river-dwelling fish. Figure 1 shows a micrograph of a Lake Ellesmere scale and Figure 2 one from a river-dwelling fish, both photographed at the same magnification. The river scale shows the structure sharply contracted at the winter bands followed by wider spacing of the ridges representing the parts of the scale formed in the summer or growing periods. In the lake scale the structure is much more uniform; the spacing of the ridges in the winter band, the edge of which is marked W.B. in the photograph, is only slightly closer than in the summer zones that precede and follow it. It is further to be noted that erosion consequent on sexual maturity is quite common in Lake Ellesmere scales, but in those of permanent residents of the river it can seldom be detected.
The second character by which migratory trout may usually be distinguished from permanent residents of the river is the vomerine teeth. In Lake Ellesmere trout there is a definite tendency for these teeth to be lost from the rear in adult fish. Some specimens have only one or two teeth left on the head of the vomer, while in others there are only one or two missing at the rear, and in extreme examples
the row is complete. It follows, therefore, that the existence of a complete row of teeth on the vomer does not necessarily identify a fish as river-dwelling, but a fish with a vomer partly or completely denuded of teeth may definitely be regarded as migratory. Figures 3 and 4 show the dentition of the two classes of fish.
The Conditions Existing in 1932–33–34.
The winter and spring of 1932 were remarkable for the complete absence of floods in the Selwyn River. At the opening of the fishing season on October 1st the river was unseasonably low with the lower water extending upstream only to Hadstock, a few miles above which the course became entirely dry and continued so to within a short distance of the mouth of the Hororata. The deeper pools throughout the shingly section of the lower permanent water contained large ill-conditioned trout of a size much greater that that of the permanent residents of this locality, but agreeing with that of the fish composing the spawning run from Lake Ellesmere. In some pools only a few fish were visible, while in others such as the large pool just below Meadowbank upwards of 50 were counted. Many dead fish, skeletons, skulls and vertebrae, all agreeing in size with the large ill-conditioned fish in the pools, were found stranded at the water's edge, and others were obtained from the water by dragging. A considerable stock of young trout of up to about 5 inches in length was present, and there was a scattering of fish of from half a pound to a pound or more in weight.
A group of twenty of the ill-conditioned fish was collected for investigation, seventeen being taken by the writer and a companion by angling in pools in the shingly part of the river, and three being obtained from anglers fishing the deep section near the mouth. The fish ranged in length from 17 inches to 22½ inches and showed an average for the group of 18.2 inches. By the methods indicated above all were identified as Lake Ellesmere trout. The condition factor as determined by Corbet's calculator, which is based on the formula Weight in pounds X 100,000/Length in inches cubed, showed the low average of 31. The suggestion put forward by Professor Percival (loc. cit., p. 348) that the failure of these fish to regain condition in the river was due to lack of a stimulus to feed in the form of a flood or fresh is not only purely speculative but discloses a lack of acquaintance with the habits of Lake Ellesmere trout. These fish never regain condition while upstream on their spawning migration, and their failure to do so in the present instance requires no explanation. The onset of maturity is accompanied by a discontinuance of normal feeding (see tables 1 and 3 in the original paper), and during the reproductive period the fish exist on bodily reserves, condition being regained only after return to the lake. It sometimes happens that lake fish become land-locked in the permanent water above Coalgate, where they may exist for a year or more before return to the lake becomes possible, and during this enforced upstream residence they adopt, to some extent, the feeding habits of river-dwelling fish, but either through insufficiency or unsuitability of the food available or disinclination of the fish to seek it in the shallower and more rapid
parts of the stream their condition remains much lower than that of the small permanent residents of the river. The remarkable thing about the ill-conditioned trout of 1932 is not that they failed to regain condition in the river, but that they remained upstream so long after the spawning season when there was no obstacle to their downward passage, and it was for the purpose of determining the cause of this abnormal behaviour that specimens were collected and examined.
Upon dissection the group proved to be composed of eighteen females and two males, the latter being among the specimens taken upstream. The presence of fully-developed eggs, ranging in number from nine to one hundred and forty-six, in all females examined indicated that all had reached maturity and that most of the eggs developed by each fish had been parted with. The majority of the eggs present were lying free in the body cavity, but in some specimens a few eggs were retained within the ovaries; others had eggs lodged between the liver and the structure that is referred to by Gunther (1880) as the diaphrama, by Kendall (1920) as the diaphram, and by Percival (loc. cit., p. 346) as the abdomino-pericardial wall, and in four there were eggs embedded in the liver. The latter organ was, in one specimen, much broken up, with eggs present in the interstices of the mass. More than half the specimens examined showed discolouration of parts of the liver, usually associated with a roughening of the surface and a shrinkage of the parts affected. These parts when sectioned, revealed considerable breaking down of structure. The ovaries of all females examined were damaged in a greater or less degree. In one specimen the left mesovarium was parted from its anterior and dorsal anchorage for about three-quarters of its length and was turned back with its anterior end to the rear of the body cavity, the partly detached membrane showing several rents and openings. The right mesovarium, which had retained its position, had five rents extending transversely from the free edge, two of these rents having formed a flap which had turned inwards and appeared to be in the process of forming an egg-retaining pocket such as is frequently observed in Lake Ellesmere trout. The least damaged specimen had one ovary uninjured and only a slight transverse rent toward the rear of the other. Between this extreme and the specimen first described there was complete intergradation, the injuries being identical in character and differing only in severity. The genital organs of the two males examined showed no definite evidence of injury.
In three specimens the lower intestinal mesentery disclosed injuries consisting of holes through the membrane, partings from the point of anchorage and the tearing back of the vertical face, and in two the upper intestinal mesentery was injured in a similar manner. These injuries were usually almost healed and had obviously been inflicted some time previously. Eight specimens revealed one or more ruptures of the peritoneum, most of which were in process of healing, but in two the structure of a considerable area of peritoneum together with that of much of the adjacent muscle was completely broken down. The continuance in life of these fish must necessarily have been brief.
In most specimens the muscular structure of the abdominal wall showed considerable inflammation, this being most severe in the region between the ventral fins and the genital outlet. Three specimens, including the two males, showed only slight inflammation, which would probably have passed away in a short time. One specimen had a large portion of the spleen almost severed from the remainder while in two others small portions had been detached at the porterior extremity. In a few instances the intestinal wall showed breaking down of structure.
Externally the fish showed no trace of recent injury. Two specimens possessed old scars, but the scale covering had been completely replaced on the parts affected, and by examination of the replacement scales the date of injury was determined to be over two years previously in one specimen and nearly two years in the other, the date of injury in each instance preceding the attainment of maturity. A similar percentage of scarred fish is normal in all collections of mature Lake Ellesmere trout. With the exception of one specimen taken near the river mouth, which contained seven smelts, the stomachs of the fish examined were almosh empty.
The ill-conditioned fish remained upstream in the pools and continued to die off until October 25 when a flood occurred and they were washed away, their places being taken later by stragglers from the Hororata and the upper permanent water of the main stream. No further flood occurred during the summer, autumn and winter, and in the spring of 1933 conditions were indentical with those of the previous year. It is stated on page 348 of the paper under discussion that in September, 1933, there were many large ill-conditioned fish lying in pools about ten miles from the river mouth, and that these fish would not leave the pool although there was free egress; and it is further stated that at the end of Sepember a slight freshet occurred and the fish disappeared. The latter statement is not correct. There was no freshet, rise or discolouration of the water in September, 1933, and there was no noticeable movement of fish in this month or the following month. At the opening of the fishing season on October 1st the river was very low with an abnormal development of algae, and the pools in the locality indicated contained a greater number of large ill-conditioned fish than had been present in the previous year. Reference to the writer's diary shows that on October 1st, 1933, over thirty large fish were visible in Butterfield's Pool, about twenty in Plunket's Pool, nineteen in the Meadowbank Pool, and over seventy in the Clay Block Pool. This pool affords considerable facilities for the concealment of fish, and it is probable that double the number counted was present. The Aquarium carried a large shoal, the full extent of which could not be determined on account of the deep shade, there were eleven fish in the Moonlight Pool, about twenty-five in the Swirl, between thirty and forty in McGregor's Pool and a small shoal in the Secrecy Pool. The intervening sections of the river carried many individual specimens and small groups. The fish gradually dwindled away during the first two months of the season, some dying and many being taken by anglers. Specimens were procured by the writer on the 2nd and 5th of October and, on examination, were found to be identical with
those of previous year; all were mature Lake Ellesmere trout, and all possessed internal injuries of the nature described but were free from external marks. The fish examined this season were invariably females.
In the winter of 1934 several floods occurred, and, at the opening of the fishing season, no injured fish were found in the shingly part of the river, but the majority of those taken by anglers fishing in the lake off the river mouth and by flounder fishermen operating in the same locality were in poor condition and injured in the manner described above. Of thirty-nine specimens taken from the lake only four were well conditioned, three of these being immature and the fourth a large fish that had spawned three times, in none of which was any internal derangement visible. It may further be briefly stated that injured fish were not peculiar to the three seasons mentioned, that they were present in various numbers in many previous years, and that specimens have been observed in subsequent ones.
It will thus be seen that the existence of large ill-conditioned fish in the spring is not controlled by weather conditions and the consequent state of the river, but that the precise locality in which they are to be found is so. The absence of winter or spring floods allows the damaged fish to lie quietly in upstream pools where their presence is easily detected, but when floods occur they are washed bodily into the lake where they die or recover out of sight.
The Cause of the Injuries.
The injuries described form so complete an explanation of the abnormal behaviour of these fish in remaining upstream instead of returning to the lake in the natural manner that the matter resolves into the question of how the injuries were received.
Professor Percival, who, incidentally, has publicly admitted that he did not examine a single specimen, suggests two explanations. The first explanation, that the injuries were really not injuries at all but merely the effects of post-mortem changes, would require, on the present author's part, an ignorance of the universally known fact that fish undergo rapid decomposition after death. The second speculation, that the injuries were caused by dynamiting, is consistent with neither the effects produced by the use of explosives nor the conditions existing on the river at the time. In view of the fact that dynamited fish are killed by concussion, and that death is usually brought about without visible external or internal injury being caused, it is impossible to believe that a fish could survive a shock capable of causing serious disruption of internal structure, even if the latter result were mechanically possible. There was, moreover, none of the conditions that accompany the use of dynamite in such a stream. The most noticeable feature of all dynamited pools that have come under the writer's observation is the presence of small dead fish, of from four to six inches in length, which either show conspicuously from the bottom or drift down and become stranded on the shallows below. The complete absence of such dead fish from the dozen or more pools examined, together with the failure to discover bones of corresponding size, is satisfactory evidence that no dynamiting had occurred for a considerable period.
A further suggestion made on page 348 of the paper under discussion, that injury may have been suggested by the somewhat ragged appearance of the ovaries, which, it is stated, normally remain flabby for many weeks after the eggs are shed, is irreconcilable with the statement made on page 345 that shrinkage of the ovary occurs before the eggs are discharged from the body, and cannot be considered.
In addition to these suggested explanations of the injuries present in the ill-conditioned trout of 1932, evidence is presented in opposition to the present writer's conclusions that stripping was responsible. It is implied on pages 347 and 350 of the paper under discussion that trout do not proceed upstream after being stripped, and that injuries present in fish collected some miles above the trap could not have been caused by stripping.
Discussing the artificial spawning of trout, Mr W. H. Armistead, proprietor of the Solway fishery, and one of the leading fish-culturists in Great Britain, states (1908):—
Spawned fish should be put into the stream above the trap, if there is no fear of poachers. Most of them will journey up to the spawning beds and deliberately go through the whole performance of making redds, etc., as though they were quite unaware of the fact that their eggs had been taken from them. Speaking on the same subject during the discussion on Kendall's (1920) anatomical paper, Mr J. W. Titcombe, fish culturist, Conservation Commission, Albany, New York, says:—
In the case of wild fish held in pens and stripped from day to day, I have found that if in stripping we leave one or two eggs the trout will stay around the spawning bed until it gets rid of those eggs. We find they are very persistent. We have penned fish two miles from the spawning beds and taken what we believed to be all their eggs, and within twenty-four hours we have found those same fish over on the spawning beds two miles away. I inferred that these fish were there to get rid of the two or three eggs we had left behind in the stripping process.
This statement is referred to by Percival (loc. cit., p. 347) as follows:—
It is probable that Titcombe's phrase “one or two eggs” means perhaps half stripped or thereabouts.
As the grounds upon which this assumption is based are not stated it is unnecessary to attempt any reconciliation between it and Titcombe's high standing as a fish-culturist.*
[Footnote] * Since the above was written an experiment has been conducted for the purpose of determining the post-stripping behaviour of migratory trout in local waters. With the permission of the North Canterbury Acclimatisation Society, to which thanks are proffered, the writer liberated 80 stripped fish above the Selwyn trap after they had been rendered sufficiently conspicuous for detection in moderately deep water by the attachment of a white celluloid tag to the dorsal fin. The tagging was performed on June 27, 1938, at a trap erected at Boyles ford, about 8 miles above the usual scene of stripping operations. Both sexes were handled, about half the fish being stripped on the same day as tagging was performed, while the remainder had been stripped the previous day and retained in the pen. The river had recently been in flood, but was perfectly clear, although a large stream was flowing. Upon release above the trap after being tagged, active fish immediately proceeded up and across stream, usually becoming lost to sight after proceeding about three chains. Sick fish sheltered for some time alongside weed banks in the vicinity of the point of liberation or alongside the wire netting of the pens, four fish being visible in such positions when observation ceased in the evening. The remainder had disappeared into deep water upstream where their presence could not be detected. On the two following days the curator, Mr Claringbold, examined the trap for tagged fish, but failed to find a single specimen, and on the third day the river again rose, washing out the trap and preventing further observation. There was, consequently, no opportunity of determining how far upstream the fish travelled, but the experiment has at least shown that Lake Ellesmere trout, if allowed their liberty, will proceed upstream after being stripped and will not return within three days. The results of the experiment, therefore, agree with the statements of Armistead and Titcombe quoted above.
A section of the paper under discussion is devoted to a consideration of the ovarian anatomy of trout, and the conclusion is reached therefrom that “it is not clear how eggs could be forced forward in the body cavity when a ripe fish is held upsidedown preparatory to stripping.” As no such suggestion was made by the present writer, the words used being “head downward” (Stokell, 1936, p. 81), no further comment is required.
The suggestion is made (Percival, loc. cit., p. 350) that since the act of stripping females may be regarded as more vigorous than with males, there should be shown some reflection on the sex ratio of fish handed in successive years if stripping were injurious Without presuming to express an opinion on the first contention the present writer submits that, if it is a valid one, a test could more logically be applied to the fish in which injuries existed than to those taken each year in the traps, many of which are necessarily first spawners and cannot have come under the influence suggested. Actually the percentage of females in the injured fish examined, including those from the lake, was 94.
It is further suggested on pages 349 and 351 of the paper under discussion that if stripping were injurious to trout the loss of a number of mature fish would tend to affect the higher age groups, thus causing a reduction in the average size of fish handled at the traps in successive years, and it is contended that as no appreciable reduction in average size is revealed by the table of weights and lengths presented on page 349 the population has remained constant. It is not clear how the extent of a trout population may be judged in this manner when only an inconstant fraction of the spawning run is dealt with and the ages of the fish are unknown. A reduction in the population could be expected to render an improved food supply available to the remaining fish, and this could conceivably be reflected in an improved growth rate and consequently a higher average size. There are also other influences operating at Lake Ellesmere which tend to disqualify any inference of a numerically constant population that may be drawn from a constant average size. The standard mesh of the flounder nets, which permits the passage of small mature trout but retains and kills larger individuals, must be considered as an influence in regulating the average size of adult trout, and the same may be said of the effects of parasitism, which have been found to become progressively more potent in the higher age groups. Quite apart from these considerations the table of average sizes presented in the paper under discussion invalidates itself as a means of disclosing serious loss of mature fish. As it is certain that in 1932 and 1933 fish of the size dealt with in the table died in considerable numbers it is equally certain from the failure of the table to reveal their loss that this method of detection is unsound.
With regard to the contention that egg production has remained constant in successive years the figures given on page 349 of the paper under discussion are so obviously questionable that it would be unsafe to draw any inference from them. If the year 1932 is considered, which is the only one for which particulars of both egg yield and size of fish are given, it will be found that trout stated to average 2.9 pounds in weight are credited with an average production of only 1119 eggs, which works out at approximately 385 eggs per pound of fish. In April, 1937, the present writer counted the eggs of a small but representative group of Lake Ellesmere trout which, though taken at random, had an average weight almost identical with that of the fish just referred to. The average yield of eggs per pound of fish proved low for brown trout, and the individual yields showed considerable differences which, however, appear to be attributable to differences in the sex history of the fish. The most productive specimen was approaching its first maturity while the specimen showing the lowest comparative yield had matured twice previously and revealed evidence of past ovarian injury of the kind described in the second section of this paper. It would thus appear that the low average egg yield of Lake Ellesmere trout is contributed to by the development in individuals of partial sterility consequent on injury by stripping.
The data obtained from the group are given below.
|Weight (lbs.).||Number of Eggs.|
Average number of eggs per lb. 751.
Maximum individual number per lb. 960.
Minimum individual number per lb. 587.
It will thus be seen that the average yield of eggs per fish is 2200, or nearly double the number (1119) recorded by Percival (loc. cit., p. 349) as being obtained by stripping fish of approximately similar size, and that the yield per lb. of fish is 751 as compared with 385, an almost identical ratio. As the figures submitted in the paper under discussion deal with only about half of the eggs produced by Lake Ellesmere trout as they now exist, and as no attempt has been made to ascertain what becomes of the balance, they must be regarded as fractions of unknown constancy from which no deductions can be made. The most probable explanation of the difference between the egg counts and the stripping figures is that many fish
are imperfectly stripped, and, being liberated below the trap, they re-enter and are again stripped and recorded. If this is so it would appear, from the retention of so large a number of eggs as to render the fish indistinguishable as having been stripped, that, in the first instance, stripping is performed before ripeness is complete.
It is now necessary to consider the positive evidence leading up to the conclusion that the injuries were caused by stripping. The absence of injuries from immature Lake Ellesmere fish examined for comparison and from mature river-dwelling fish taken in the vicinity of the pools from which many of the injured fish were taken restricts the injuries to the class of fish handled at the traps, namely, mature lake fish. The presence of stripped fish in pools several miles above the point where the stripping trap is operated is found to be consistent with the behaviour of trout after being subjected to the unnatural process of stripping. The next consideration is that the injuries present in the specimens examined were restricted to the part of the body that is operated upon by strippers. In stripping, the lower part of the body is squeezed from the pectoral fins to the vent, that is to say, the liver, the reproductive organs, the greater part of the alimentary tract, the spleen, the mesenteries, the peritoneum and the muscular structure of the flanks are subjected to pressure. With the exception of the anterior portion of the alimentary tract these are the very structures in which injury existed. The absence of any external injuries such as scars or noticeable derangement of scales disqualifies any explanations based on the grounds of poaching or attacks by predators. Contact with poaching implements, such as spears, hoopiron or snares, if sufficiently severe to cause serious internal injuries, must leave obvious evidence in the form of external marks, and the same applies to attacks by predatory animals, such as shags and large eels. In explaining the association of internal injuries with the absence of external injury it is necessary to postulate a causative agent if a non-rigid character yet capable of exerting considerable pressure—a specification with which human hands completely agree. The further circumstances of time, place and class of fish to which the injuries were restricted leave no alternative to the conclusion that stripping was responsible, and the matter is placed beyond all question by the reproduction of the whole of the conditions described, execept those that are of a secondary nature, in the manner suggested. The simple test of stripping trout with various degrees of pressure and immediately dissecting them showed that serious internal injury could be inflicted without leaving an outward mark. It was unusual to dislodge more than one or two scales. The organs that were found to be the most easily damaged are the ovaries and the liver; then come the mesenteries and the peritoneum. The spleen proved less easily damaged, by reason of its being protected to some extent by the pelvic bones,* but it was found that by exerting sufficient pressure it could be injured in the manner described without showing any outward mark on the fish. The stomach and duodenum withstood the greatest
[Footnote] * Further experiment suggests that while the pelvic bones protect the spleen from moderate pressure, it is actually the anterior extremities of these structures that inflict the injuries when the pressure is severe.
Fig. 5.—Brown trout showing injuries inflicted during outine stripping R.M.: Rupture of upper intestinal mesentery with filamentous remnants of the structure extending across the opening. B.S.: Complete break in spleen. L.: Position of injury to liver. Injury not visible in photograph.
pressure applied, but it was possible to damage the structure of the intestinal wall near the anus in a manner that would probably have been fatal. These results produced experimentally were found to be identical with the effects of routine stripping as carried out by official fish-culturists. The fish shown in figure 5 was killed and opened about four hours after it had been officially stripped, the only selection exercised being in the direction of securing a first spawner for the purpose of excluding any complication that might be associated with a previous maturity. Upon the body cavity being opened, evidence of serious hemorrhage was revealed, no part of the peritoneum and viscera being free from blood. Loose eggs were present in the body cavity, and there were injuries to the liver, dorsal mesentery and spleen as indicated in the caption of fig. 5. The fish had been preserved in formalin solution before being photographed.
It may finally be stated that injuries similar to those described in the second section of this paper are not restricted to brown trout but have been observed in rainbow trout from Lake Taupo and in freshwater-dwelling Salmo salar from Lake Te Anau, both of which are heavily trapped for stripping. A photograph of a Te Anau salmon showing the unnatural form of the ovary after recovery from moderate injury inflicted at a previous maturity is shown in fig. 6.
(1) That the failure of the ill-conditioned trout present in the lower Selwyn during the spring of 1932 and of 1933 to return to the lake in the normal manner was due to their being incapacitated by internal injuries.
(2) That these injuries were caused by stripping.
(3) That the destruction of mature fish by stripping causes a serious drain on the fishery stock, and that there is no evidence that this is compensated by commensurate gain resulting from artificial culture.
Armistead, W. H., 1920. Trout Waters, A. and C. Black, London, p. 113.
Gunther, A., 1880. The Study of Fishes, A. and C. Black, Edinburgh.
Kendall, W. C., 1920. Some Previously Unrecognised Anatomical Facts, and their Relation to Fish Cultural Practices, Trans. American Fisheries Soc., vol. 1, pp. 43–56.
Percival, E., 1937. Remarks on the Brown Trout Stock of Lake Ellesmere, Trans. Roy. Soc. N.Z., vol 67, pp. 341–351.
Stokell, G., 1936. The Nematode Parasites of Lake Ellesmere Trout, Trans. Roy. Soc. N.Z., vol. 66, pp. 80–96.