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.