WHAT SPECIES?

Evening Post, Volume CXII, Issue 119, 16 November 1926, Page 9

WHAT SPECIES?

TROUT OR SALMON? A CATCH IDENTIFIED. Some weeks ago fishermen off the Wanganui lliver caught what was considered to be an Atlantic salmon. The fish was handed over to Mr. H. J. Duigan, who sent it to Mr. Ayson, fishery expert, for identification. He, after a hurried examination, thought that by appearances the fish was an Atlantic salmon, Mr. Heffoi'd was under the same impression when he first saw it, but after a very minute examination and study has come to the conclusion that it is a searun trout, and not an Atlantic salmon. It will be understood that it is difiicult in, the early stages of acclimatisation to come to a definite conclusion on one specimen, and a number of specimens are desirable for purposes of checking, particularly in view of the possibility that, owing to change of habitat, there may be some change of characteristics. Mr. A. K. Hefford's report on the specimen is here given and contains some interesting information about the characteristics of trout and salmon. "The specimen measured on the 26th October, after being kept in cold storage, had a total length of 18& inches. The' possibility of its being a quinnat salmon is precluded by its having only 10 anal fin rays, and it cannot bo a sea-run rainbow (or steel head), because, it has no spots on its caudal fin. "It can only be one of two possible species—Atlantic salmon (S. salar) or trout (S. troutis)) unless we include the possibility, which is rather remote but not altogether out of the question, that it is a hybrid between these two. When I saw it it had suffered to some extent by having been frozen and its normal outline had been marred by the shrinking of the abdomen. "On a first view of the fish as 'a whole it seemed more than anything else to resemble a grilse (that is, v salmon which has spent more than one year but less than two years in the sea). The grilse character is conveyed by its generally slender shape, especially the fineness of the- tail just in f:i nt of the tail fin, and by the crescentic shape of the free edge of the tail fin, which in a trout of the same size one would expect to be more nearly straight, and by its relatively small upper jaw; also by its markings—the dark spots on its body do not seem to. be quite so numerous as they would be on a seatrout, although there are more than are usually seen on European salmon, and this .is. especially the case with regard to the spots on the dorssal fin. "With regard to all these characters, however, both species show considerable variation. On reading a description of the external characters of salmon or sen trout one constantly finds, various points (nullified by the term "usually." Anothor diii ficulty is that these characters vary ac cording to the age of the fish. "To arrive at a definite identification it is necessary to examine the more permanent anatomical characters. There are two tests which are generally considered to be conclusive. One of these is the number of scales counted along the line which runs 'obliquly forward from immediately behind the adipose fin to the lateral line. In salmon this number is 10. to 12 (exceptionally 13)'. In trout the number of scales in this line is 13 to 16 (exceptionally only 12). So even in this there is a convergence since it is possible for either salmon or trout to have 12 or ]3 scales in this line. If the count is definitely below 13 the fish is clearly a salmon. If about 13 clearly a trout. The scales in this series, in this- particular specimen numbered 14. They appear to have been subjected to some disturbance, so it is possible that the number may be 13, but it is certainly no lower than this. On this criterion, therefore, the fish might be either a salmon or a trout, with a bias towards trout. "There is still another permanent anatomical character which is recognised _as decisive. This is the number of gillrakers (the spiny processes on the inside edge of the bony arch which bears the gill). The first of these gill arches bears 18 to 22 gill-rakers in the salmon and to 16 to 18 in the trout. The number of gill-rakers on the first arch of the specimen examined was 10. This_ appears to decide the verdict in favour of its being a trout and not a iialnion." Referring later to a couple of trout caught in tidal waters in the Horokiwi Creek (Paluiutanui), both silvery sea-run fish, though smaller than the Wanganui specimen, with fourteen 'scales in tho series from back to adipose fin to lateral line, and 16 gill-rakers on tho first arch, Mr. Hefford says:—"ln general external characters they closely resemble tho Wanganui specimen, though one of them was somewhat deeper in. general shape. The comparison enables me to discount the partial superficial resemblance of the Wanganui specimen to a grille, and confirm its identification as a sea-trout, and not l&lmo salar."