Order:	Cypriniformes
Family:	Cyprinidae
Genus and species:	Gobio gobio

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

THE GUDGEON.

Gobio	fluviatilis,	Willughby, p. 264, Q. 8, fig. 4.
"	"	Gudgeon.	Flem. Brit. An. p. 186, sp. 60.
"	"	Cuvier, Règne An. t. ii. p. 273.
Cyprinus	Gobio,	Linnæus. Bloch, Pt. i. pl. 8, fig. 2.
"	"	Gudgeon	Penn. Brit. Zool. vol. iii. p. 476.
"	"	"	Don. Brit. Fish. pl. 71.

Generic Characters. - The species of this subgenus have, like those of the last, the dorsal and anal fins short ; are furnished with barbules or cirri about the mouth, but have no strong, bony, serrated ray at the commencement of either the dorsal or anal fins. In other respects like Cyprinus.

THE GUDGEON is found in many streams that in their course flow over gravelly soils : it appears to delight in slow rivers that have shallow scours over which the current of the water is increased. The Thames, Mersey, Colne, Kennet, and the Avon, produce abundance of the finest Gudgeons. Daniel, in his Rural Sports, says they thrive well in ponds that are supplied with fresh water from brooks running into them. Gudgeons swim together in shoals, feeding on worms, aquatic insects and their larvæ, small molluscous animals, ova, and fry, affording excellent amusement to those anglers who are satisfied with numbers rather than weight; the Gudgeon rarely exceeding eight inches in length, and being seldom so large. When angling for them it is usual to scratch the gravel at the bottom of the water with an iron rake, the Gudgeons resorting to the newly moved ground for the sake of the small living animals that are exposed by this disturbing operation. They bite freely, even to a proverb, at a small portion of a red worm ; and many dozens may be caught, in some situations, in a few hours.
The Gudgeon spawns in May, generally among stones in shallow water, in which situation the deposit is exposed to the vivifying influence of the sun's rays ; the fry are about an inch long by the beginning of August. The Thames fishermen take them in shallow water with a casting net, keeping them in their well-boats till wanted. The London fishmongers are also able to keep Gudgeons alive several weeks in leaden or stone tanks, which are constantly supplied with fresh cold water ; and Colonel Montagu says that a very considerable quantity of these fishes are taken with the casting net in the Avon near Bath, long famous for its Gudgeons, which are exposed for sale alive in shallow tabs of water, and are thus obtained in the highest perfection for invalids, being considered easy of digestion.
The length of the head is, to the whole length of the head, body, and tail, as one to five ; the depth of the body, which is greatest at the commencement of the dorsal fin not equal to the length of the head ; the lower jaw broad, shorter than the upper ; the mouth wide, with a barbule at the angle on each side ; the nostrils in a circular depression ; the eye placed high up on the side of the head, and about half-way between the point of the nose and the free edge of the operculum ; the dorsal fin commencing on a vertical line rather before the ventrals, the rays slender, the connecting membrane thin and transparent, the base of the fin one-third shorter than its longest ray ; the distance from the point of the nose to the origin of the pectoral fin, from thence to the origin of' the ventral fin, again to the anal fin, and from thence to the end of the fleshy portion of the tail, are four very nearly equal distances ; the tall deeply forked, the outer rays nearly as long again as those of the centre; all the fins rather long, the rays slender, the connecting membrane thin and transparent ; the lateral line straight from the middle of the base of the tail forward till near the operculum, then suddenly rising to its upper edge ; the scales of the body moderate in size, about ten rows, completing the oblique line of their arrangement between the base of the dorsal and the origin of the ventral fins. The fin-rays in number are

D. 9 : P. 15 : V. 8 : A. 8 : C. 19. Upper half 10.

The colour of the upper part of the head, back, and sides, olive brown spotted with black ; irides orange red, pupil large and dark ; gill-covers greenish white ; all the under surface of the body white ; pectoral, ventral, and anal fins early white, tinged with brown ; dorsal fin and tail pale spotted with darker brown.

Frank Buckland (1880) in "Natural History of British Fishes":

GUDGEON.

(Gobeo fluviatilis. Cyprinus gobeo.)

Local name: Googen, Gogone. German : Gründling, Gressling, Krebsfisch. French: Le Goujon. Dutch: Grondell. Italian: Gobione.

To the fishermen living on the banks of the Thames the Gudgeon is one of the most valuable of our British fishes, inasmuch as fishing for the brave little fellow, which gives such excellent sport, is an excuse for many a pleasant outing.
When in the 2nd Life Guards at Windsor I used to devote much of my spare time to gudgeon-fishing in the Thames, and have now in my room a water-colour drawing of the two brothers Reid, the Riding-master and the Adjutant, who were my constant companions in gudgeon-fishing. Gudgeon are a true bottom fish, and always swim in shoals. They feed among the weed at the bottom of the river, and more especially on the worms and other insects that lay among the gravel. If good sport is desired in gudgeon-fishing, it is necessary, as old Finmore, the fisherman, used to say, to “keep scratching their back,” that is to say, a heavy iron rake is thrown out of the punt and the bottom of the river below the swim is well raked. If there are any gudgeons about, they will soon be brought up to the swim by the clouds of mud which float down the river. It frequently happens that one catches all the gudgeons which arrive in answer to the first raking operation. It is therefore necessary, then, to “scratch their backs“ again, which, being continued, brings up fresh arrivals. The gudgeons bite, as shown by the float, is a merry one. All on a sudden you see the float dance again and then disappear under water. At that moment strike, and you are nearly sure of your fish. You must be very careful to strike very smartly every time the float arrives at the end of the swim, as sometimes the gudgeon coquettes with the bait, and if you are not quick enough when he is making up his mind you lose him altogether. The gudgeon fisherman has to serve an apprenticeship as to taking the proper depth with the plummet. Unless the worm, which is the best bait, swims exactly right, and the line is shot into the exact depth, you will catch no gudgeons, although there may be thousands in the swim. The fisherman who goes out in the punt must not be deprived of his glass of ale for his instructions in these matters.
When out gudgeon-fishing on the Thames, be sure and take a frying-pan, as gudgeons taken out of the water and immediately fried are delicious. Clean, wipe, and flour, then well fry in boiling fat, or, better, in oil, till they are crisp and of a light brown colour. Such a fish dinner is always a great feature in a pic-nic on a fine day.
0 The habits of the gudgeon are little understood, as some years they are very abundant and other years very scarce ; and it not unfrequently happens that they disappear from localities where they have been plentiful for years. The cause of this is unknown, and I leave it for our friends, to whom we fish culturists would be exceedingly obliged if the cause of these appearances and disappearances of the gudgeon can be explained. The gudgeons are said to spawn in the months of April and May, and the French authorities say they require a month to hatch out.
The best months for gudgeon-fishing are August, September, October, and even November. The angler will observe that among his catch some may be very little fish, while others are of three times the size. Here, then, is another difficulty. What is the age of these fish, and to which hatchings do they belong? In the winter the gudgeons go into very deep holes. Favourite spots for them when in the biting humour about Windsor are the deep holes dredged out of the bed of the Thames by the dredging, locally called ballast, barges, from which, by the way, I have often procured, when out gudgeon-fishing, many fine specimens of what the bargees call “water-bones,” i.e., antlers and other bones of the ancient British red deer and roebuck and where, every now and then, I have procured specimens of Roman swords and other antiquities.
To lawyers the poor innocent gudgeon may be a very serviceable fish, for he may be brought as a witness into court to prove that pollutions are not injurious to salmon or trout fisheries. A live gudgeon is placed, in the presence of the judge and jury, in the polluted water and he does not “turn up,” therefore the counsel argues the manufactures or the town sewer is not in fault, as the water allowed to flow into the river is not injurious to fish life. If an expert in fishery matters does not happen to be present, this gudgeons argument will go down, as the judge and jury probably are not aware that gudgeon are very fond of living in sewer-water as long as it is just running, whereas the same water would be almost immediately fatal to a trout or young salmon. The largest gudgeon I ever caught were in a sewer which ran along the east side of the college meads at Winchester.

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

Gudgeon

Gobio gobio (Linnaeus, 1758)

NAMES Fr. Goujon; Du. Grondel; Ge. Grund-ling; Da. Grundling; Sw. Sandkrypare.

IDENTIFICATION The body is long, in cross--section round in front, compressed laterally towards the tail. The head is heavy but short, the snout is rather long and rounded, with a barbel at each corner of the mouth. Both anal and dorsal fins are short-based. It grows to a maximum length (usually in large lakes) of 8 in (20 cm), but usually less than 6 in (15 cm). The British record fish weighed 4 oz 4 dr (120 g).
The gudgeon is greeny brown dorsally with small darkish spots, shading to greeny yellow on the sides, and silvery yellow on the belly. There is a row of large, indistinct, rounded dark marks on the side, and the dorsal and tail fins are heavily spotted.
D. III/5-7; A. III/6-7; lateral line scales 38-44; pharyngeal teeth in two rows 5 + 3 (or 2) each side.

BIOLOGY The gudgeon is a widely distributed fish, usually found in shoals, and as commonly in deep swiftly running water as in ponds, lakes or marshes. It is particularly common in clear flowing rivers with sandy or gravel beds. It is always on, or close to, the bottom. Spawning takes place from mid-April to the end of July, but is at its most intense in May and June. The sticky eggs are shed at night, and adhere to plants and stones. They are bluish in colour, 2 mm in diameter and hatch from two to four weeks after spawning. The males have white tubercles during the spawning season; they are also longer, age for age than the females, although the largest fish, 5½ in (13.5 cm) and above, are always females, for the males die younger. Males mature in the third year at an average length of 4 in (10 cm). Some females mature in their second year, all by their third, at an average length of 3 in (8 cm). Growth in the first four years is rapid, then slows down. An average growth rate would be 2½ in (6.5 cm) at one year, 3¾ in (9.5 cm) at two years, 4½ in (11.5 cm) at three years and 5 in (12.5 cm) at four years. Six- and seven-yearold fish are known.
The gudgeon's food is primarily animal. Insects are the most important food, particularly in spring and summer; while molluscs and crustaceans are the primary foods in winter. Of the insects eaten the majority are dipteran larvae, mostly chironomids, then ephemeropteran and trichopteran larvae. The chief crustacean food of larger fish is the freshwater shrimp (Gammarus), and among the molluscs the gastropods Bithynia and Theodoxus. The majority of the gudgeon's food items in rivers are bottom-living animals, whilst in still waters the young fish eat a considerable quantity of planktonic crustacea (Chydorus sp., copepods and ostracods generally).
The gudgeon's chief value to anglers is as a bait for larger predators. Its flesh is tasty, but its small size reduces its value as food.

DISTRIBUTION

Eastward to the U.S.S.R. A related species G. uranoscopus (Agassiz, 1828) in the Danube basin.

Gudgeon

Izaak Walton (1653) in "The Compleat Anlgler":

"Observations of the Gudgeon, the Ruffe, and the Bleak; and how to fish for them"

Chapter XV; [Fourth day]