Breeding habits of Windermere charr, Salvelinus willughbii (Günther), and their bearing on speciation of these fish

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Abstract
In Windermere charr, Salvelinus willughbii, are either (1) autumn spawners, main breeding period November, most spawning on the lake shore, in shallow water, some in the main inflowing stream; or (2) spring spawners, main breeding period February-March, spawning in the lake only and in deep water. The comparison of these two is mainly based on the autumn spawners netted on the lake spawning grounds at Low Wray Bay and Red Nab, and on a river spawning ground in Brathay, at Purdom's Dub, and on the spring spawners netted on the spawning ground at Holbeck Point. Information on their breeding habits was obtained by rearing fish in hatchery ponds and on their spawning behaviour by observation in the field and in aquaria. This paper, which deals with the breeding habits of the two types of spawners and the implications arising from them, is divided into two parts, with the Discussion in between. Part I describes the breeding habits of the autumn and spring spawners and shows how these separate the two types from each other. It is concerned with the question of whether these autumn and spring spawners so isolated represent distinct populations. (There is a note on the charr from other English Lake District waters.) The Discussion comes at the end of Part I. Part II gives further details of breeding habits of autumn and spring spawners and describes early stages in the life history of the charr. Aspects of the reproductive life of autumn and spring spawners are compared between themselves and also with other Salmonidae. Some of the information given expands that mentioned in Part I. Part I The difference in spawning times of autumn and spring spawners cannot be explained by reference to the light penetration and temperature conditions during the spawning periods, but day-length may be a factor associated with spawning time. Although the spawning places of autumn and spring spawners differ markedly in depth, both are characterized by a stony substratum, an essential feature of the breeding site. Autumn and spring spawners tagged on their breeding grounds were all recovered in subsequent seasons on their previous breeding grounds. Furthermore among autumn spawners the consistent return to a particular spawning place (tested by displacement experiments) further emphasizes the constancy of the spawning habits of individual Windermere charr. Thus Windermere charr 'home' in the sense of repeatedly returning to the same place to spawn and to this extent autumn and spring spawners keep separate. There is some indirect evidence, from experimental planting of eggs and fry, that spawners home in the sense of returning to spawn in their natal stream. Of characteristics in which autumn and spring spawners differ two are of particular significance: the pattern of early scale growth and the mean number of gill rakers. The difference in the pattern of scale growth and in the number of gill rakers may be associated with environmental conditions but they are good evidence that autumn spawners breed autumn spawners and spring spawners breed spring spawners. Thus it is highly probable that there are two distinct self-perpetuating populations of charr in Windermere: the distinction may be imposed on each generation or be genetical. Evidence from rearing experiments in hatchery ponds shows that the time of spawning is not genetically fixed and there is no barrier to cross-fertilization. Experiments on homing (return of the adult to spawn in the natal stream) suggest that the difference in spawning sites of autumn and spring spawners is not genetic but that without genetical aid the two spawning populations can be kept separate. The difference in depth of autumn and spring spawning sites is explained by the selective effect of temperature on the developing eggs. Although the possibility of some genetical difference cannot be ruled out, the evidence so far suggests that imposition and imprinting are sufficient to explain the division of Windermere charr into autumn and spring breeding populations. The Discussion considers other examples of situations comparable to that in Windermere in Europe and in the British Isles in view of which the taxonomic status of the Windermere charr is discussed. Theories are suggested of the possible origin of the autumn and spring populations. It is noted that in none of the other Lake District waters have two populations been found. Part II Comparative information is given on frequency of return of spawners to the breeding ground, time spent there, proportions of the sexes, estimated number of spawners and estimated survival rates. The spawning behaviour (based on observations in field and aquaria) is briefly described. Comparison of the female's weaving (undulating) and cutting actions with similar activities in the genus Salmo leads to the suggestion that in Salvelinus weaving is replacing the cutting-to-cover activity of Salmo and that cutting of any kind is becoming vestigial in Salvelinus. On the evidence from rearing and field observation, the eggs of autumn spawners hatch about the first week in March and those of spring spawners about the first week in May. Therefore the former have 2 months longer growing time which may account for their being larger than the spring-spawned fish at the end of their first year of life. It also seems likely that there is an association between the hatching of eggs in May and the habit of spawning in deep water. Observations on alevins and fry indicate that the latter are not territorial. Predation, which is on both adult charr and their eggs, is much greater on the autumn than on the spring spawners.

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