Order:	Salmoniformes
Family:	Salmonidae
Genus and species:	Thymallus thymallus

William Yarrell (1836) in "A History of British Fishes":

THE GRAYLING.

Thymallus	vulgaris,	Cuvier, Rčgne An. t. ii. p. 306.
"	"	Willughby, p. 187, N. 8.
Salmo	thymallus	Linnęus. Bloch, pt. i. pl. 24.
"	"	Grayling	Penn. Brit. Zool. vol. iii. p. 414, pl. 72.
"	"	"	Don. Brit. Fish. pl. 88.
Coregonus	"	"	Flem. Brit. An. p. 181, sp. 49.

Generic Characters. - Head and body elongated ; the sides marked with longitudinal bands; two dorsal fins, the first much longer than high, with numerous rays ; the second small, adipose, without rays: the mouth small, the orifice square; the teeth very small ; branchiostegous rays 7 or 8.

THE GRAYLING, though abundant in some streams, is yet a very local fish. Similar in many respects to the Trout in its habits and wants, there are numbers of rivers abounding with Trout that do not produce Grayling. In the southern counties of Hampshire and Wiltshire, the Grayling is found in the Test and both the Avons. In Herefordshire, in the Dove, the Lug, the Wye and the Irvon. In Shropshire, in the Teme and the Clun. In Staffordshire, in the Hodder, the Trent, the Dove, and the Wye. In Derbyshire, in the Dove. In Merionethshire, in the Dee, between Curwen and Bala. In Lancashire, in the Ribble. In Yorkshire, in the Derwent, the Ure, the Wharfe, and the Wiske, near Northallerton. Dr. Heysham says it is occasionally taken in the Eden and the Esk in Cumberland. It is not found, that I am aware, either in Ireland or Scotland; Mr. Low, however, includes this fish in his Fauna Orcadensis, and it is known to be plentiful in Sweden, Norway, and Lapland. The peculiarity of the local distribution in this country gave rise to the supposition that the Grayling had been originally introduced by the monks, as a fish worth cultivating; many of the rivers containing the Grayling being near the remains of great monasteries. But two circumstances affect this solution : it would be very difficult to bring this fish alive from the Continent to this country ; and it is not found in the rivers of Kent, Dorsetshire, Devonshire or Cornwall, where monastic establishments were formerly numerous.
The Grayling thrives best in rivers with rocky or gravelly bottoms, and seems to require an alternation of stream and pool. According to Sir Humphrey Davy, who has given a good history of the Grayling in his "Salmonia," this fish was introduced into the Test, in Hampshire, from the Avon ; and the former river, in particular parts, appears to suit it the better of the two. Large Grayling are, however, occasionally taken in both these waters, which are particularly resorted to by the southern anglers. Three Graylings, weighing together twelve pounds, were caught by Thomas Lister Parker, Esq. in the Avon, near Ringwood. A Grayling of four and a half pounds' weight has been killed in the Test, and one of five pounds is recorded to have been caught near Shrewsbury.
However fastidious in the quality of the water or the choice of situation in the stream the Grayling is known to be, experiment has proved that this fish will live in ponds that have been newly made in hard soil, or in such as have been very recently and carefully cleaned out ; but in these situations. the Grayling does not breed, and they will not continue to live in old muddy ponds. The ova of this fish are numerous, large, and of a deep orange colour: the spawning season is in April, or the beginning of May ; in this respect differing from the other Salmonidę, most, if not all, of which spawn towards the end of the year, and generally in cold weather. The Grayling, however, is in the finest condition in October and November, when Trout are out of season, not having then recovered the effects of their recent spawning while the young Grayling of that year are about seven inches in length.
The food of the Grayling, as ascertained by examination, besides the various flies - imitations of which are successfully used by anglers, - consists also of the larvę of Phryganea Ephemera and Libellula ; the remains of the cases of the former and the tough skins of all of them, being frequently found in their stomachs. I have found also several small shells, examples of the genus Physa, and Neritina fluviatilis. Dead shells and small pebbles are also found ; but whether these last are taken up by the fish to serve any useful purpose, as in the stomachs of gallinaceous birds, or have only formed part of the cases of the Phryganea, may be questioned.
Some English authors have considered the Grayling a migratory fish, passing the winter in the sea, and the summer in fresh water. " Early in spring," says Mr. Donovan, "they ascend the rivers, where they remain till autumn, and then return to their former element." This may apply to Grayling on some parts of the European continent,* but is not the case certainly with our fish in this country, in the rivers of which it is found in the most perfect condition, and in consequence most eagerly sought after, in October and November. The finest specimens I ever saw were taken in November; and Sir H. Davy states in his "Salmonia," he had proved that the Grayling of England would not bear even a brackish water without dying.
The term Thymallus is said to have been bestowed upon this fish on account of the peculiar odour it emits when fresh from the water, which is said to resemble that of thyme ; and from its agreeable colour as well as smell, St. Ambrose is recorded to have called the Grayling the flower of fishes. To be eaten in perfection, it cannot be dressed too soon. The name Grayling is supposed to be a modification of the words gray-lines, in reference to the dusky longitudinal bars along the body.
It has been considered that the large dorsal fin of the Grayling enabled it to rise and sink rapidly in deep pools ; but this power would rather seem to be afforded by the large size of the swimming-bladder. The very large dorsal fin, compared to the small size of all the other fins, renders the Grayling unable to stem rapid currents : they are much more prone to go down stream than up, and are never seen leaping at a fall, like Trout.
In a Grayling of ten inches long, the length of the head is to the body alone as one to four; the depth of the body rather more than equal to the length of the head : from the point of the nose to the commencement of the dorsal fin is equal to one-third of the length of the whole fish to the end of the fleshy portion of the tail ; the posterior edge of the dorsal fin half-way between the point of the nose and the end of the longest caudal rays ; the adipose fin rather nearer the dorsal fin than the end of the tail : the height of the dorsal fin equal to half the height of the body, the first ray short, the next five increasing gradually in length ; the sixth ray nearly as long as the seventh, and, as well as the five anterior rays, articulated and simple ; the seventh ray and all the rays behind it articulated, branched, and nearly of the same height ; the length of the base of the fin not equal to twice the length of its longest ray : the pectoral fin small, narrow, and pointed: the ventral fins commencing in a vertical line under the middle of the dorsal fin ; the ventral axillary scale one-fourth of the length of the fin : the anal fin commences half-way between the origin of the ventral fin and the end of the fleshy portion of the tail, and ends on the same plane as the adipose fin above it; the longest ray but little longer than the base of the fin : the tail forked ; the middle rays rather more than half as long as the longest. The fin-rays in number are -

D. 20 : P. 15 : V. 10 : A. 13 : C. 20. Vertebrę 58.

The head is small and pointed, flattened at the top : the breadth of the eye equal to one-fourth of the length of the whole head; irides golden yellow, pupil blue, pear-shaped, the apex directed forward: the opening of the mouth, when viewed in front, square ; the teeth small, incurved, numerous ; none on the tongue, and only a few on the most anterior part of the vomer : behind the head, the nape and back rise suddenly ; the body deepest at the commencement of the dorsal fin, then tapering off to the tail abdominal line but slightly convex ; the scales rather large the lateral line in the middle of the body not very conspicuous, with seven rows of scales on an oblique line above it, and seven rows below it ; the sides marked with about fifteen dusky longitudinal bands. The general colour of the body light yellow brown, beautifully varied with golden, copper, green, and blue reflections when viewed in different lights, with a few decided dark spots: the head brown; on the cheeks and gill-covers a tinge of blue - all the fins somewhat darker than the colour of the body; the dorsal fin varied with square dusky spots on the membrane between the rays, the upper part of the fin spotted and streaked with reddish brown The Grayling appears to become darker by age, and the pectoral fins are reddish about spawning time, with small black spots.
The vignette represents two states of the Stone-fly of anglers.

* Bloch says the Grayling descends to the Baltic in autumn.

Alwyne Wheeler (1969) in "The Fishes of the British Isles and North West Europe":

Grayling

Thymallus thymallus (Linnaeus, 1758)

NAMES Fr. Ombre; Du. Vlagzalm; Ge. Äsche; Da. Stalling; Nor. Harr.

BIOLOGY The grayling prefers clear, swiftrunning waters, although it is found as well in mountain or arctic lakes. It is a gregarious fish, forming small, sometimes large, shoals which are usually very active. In the spring-time, however, the shoals break up and pairs of fish are to be found on the spawning grounds. Spawning occurs from March to May, on gravelly shallows about 6 in (15 cm) deep. The female cuts a shallow bed in the bottom and the male lies beside her with his high dorsal fin wrapped over her back. The eggs are buried by the female; they take from eighteen to twenty days to hatch at a temperature of 10°C (50°F) and the fry are 10 mm long on hatching.
Growth rates vary greatly from stream to stream. In good waters it may be rapid for the first five to six years: fish of two winters are 8-9 in (20-3 cm); at three winters they are 9½-11½ in (24-9 cm); at four winters 12-14 in (31-5 cm), and at five winters 14-15½ in (36-9 cm). Males, which are believed to grow slightly faster than females, attain maturity at two or three years in southern waters and at five or six in the north. The females are believed to mature slightly later.
The food of the grayling is composed predominantly of bottom fauna, except that in rivers with an impoverished fauna a considerable amount of its food will consist of non-aquatic organisms. Grayling are unselective, omnivorous feeders and the variety of their food is enormous. Amphipods, mainly Gammarus (the freshwater shrimp), molluscs, particularly Lymnaea and Ancylus, ephemeroptera nymphs, Trichoptera and Simulium larvae are the most important food items. Terrestrial animals (spiders, insects, woodlice, and exceptionally shrews) also feature in the diet. Details of diet vary considerably between populations.
The grayling is alleged to compete with trout in fishing waters, because in some areas where it has been introduced the trout population has declined; this may be due as much to predation on trout eggs and fry as to competition. In other waters, however, the two species successfully coexist, and it seems that normally the grayling occupies a different ecological niche; where this is not present in a certain water, competition will result. In poor trout waters grayling offer better sport prospects, as they have on average a greater annual growth increment.
The grayling has been widely introduced both in the British Isles and Europe to rivers which did not originally contain a stock. It is a good angling fish, although too often despised in favour of trout; it also is a well-flavoured food fish. It grows to a weight of 5 lb (2.25 kg).

DISTRIBUTION

Throughout northern Europe to the Urals.