The bleak and the gudgeon, although very different in size, appearance and habits, are both members of the carp family and play a vital part in the ecology of rivers in which they live
The bleak, Alburnus alburnus, is a freshwater fish which gives few problems of identification simply because when well grown, there is nothing quite like it in our rivers. Its conspicuously silver sides and silvery-white belly are just that little bit whiter than those of any other of our native fishes. Its scientific name, Alburnus, recognizes this coloration, the root of the word being the Latin for ‘white’, albus. The back and upper sides are clear yellowish green, the tail fin is yellowish grey, while the pectoral, pelvic and anal fins are only faintly yellow – the result of clear fin membranes and yellowish, bony fin rays.
Bleak related to the carp
The other features which set the bleak apart from other fish are the slenderness of the body, which is both elongated and compressed so that the belly forms a narrow keel, relatively large eyes, an oblique mouth with its opening at the extreme upper edge of the head, and a long, many-rayed anal fin. These features suggest that the bleak belongs to the group of carp-like fishes which includes the bream and the silver bream. These species, while related, are not closely so, and one has to look to Europe and western Asia for the immediate relatives of the bleak. Here, in the basins of the Black and Caspian
Seas, there are several species which bridge the gap between the bream and the bleak, and its nearest relatives in northern Italy and the Adriatic countries.
Distribution in Britain
The bleak is well distributed in lowland areas, but is virtually confined to England, only just penetrating into eastern Wales. With the silver bream, the ruffe, and the burbot, it is one of the fishes which, until recent times, were confined to the rivers of eastern England between Yorkshire and the Thames. However, it has spread rather more than those species, helped perhaps by anglers using it for livebait and then releasing surplus fish into new waters. On the other hand, it is an adaptable little fish that has colonized the canal system very successfully, and it is equally likely that its presence in the Bristol Avon and the Severn, and in other rivers, is due to their canal connections with the Trent, the Thames, and other rivers in which the species is native.
The bleak is very much a fish of the surface layers of the water – indeed almost its whole life is spent in the uppermost 3ft of its environment. Its coloration is typical of the ‘counter-shading’ found in surface-living marine fishes such as herring, mackerel and blue shark. While these, like the bleak, are conspicuous when taken out of the water, in their natural habitat they are well concealed. From above, the greenish back merges well with the background hue of lowland river water; from below the silvery belly is lost in the mirror-like shine of the surface. Predators thus find a school of bleak hard to distinguish against the constant flickering of the sun on the surface. Only in dim twilight would the colouring of the pale bleak make it at all evident from below, although this is, of course, when many predators are most active. In the absence of observations in natural conditions, it can only be suggested that the bleak tends to become inactive at such times and to move away from the surface layers.
Spawning takes place mainly in May and June, although exceptionally it has been observed in April and July. The spawning site is usually gravel-bottomed shallows and the nearby vegetation, the eggs sticking to the plant leaves and stems as well as to stones. Spawning is not completed in a single operation – females usually spawn two or three times, at intervals of one to three days. The number of eggs produced by each is quite large for such a small fish and varies with age and size. Fish in their third year of life produce around 4,300 eggs, in their sixth year, 6,400 eggs, and in their | eighth year around 8,200.
As all bleak are sexually mature in their second year, and produce a relatively large number of eggs, it is not surprising that where conditions
Surface-living insect diet
The bleak’s diet is in keeping with its life-style. Insects form a very large proportion of its food in summer, especially surface-living aquatic insects and those which venture over the water from the land. Much of its food comprises midges, gnats, and mosquitos, and it eats a considerable quantity of midge larvae as well. In the first year its diet contains a large amount of small copepod crustaceans. However, the bleak is not so specialized a feeder that it cannot accept other foods when available in quantity.
A study in the ‘Electricity Cut’ at Peter-borough showed that during the angling season a substantial amount of their food consisted of maggots and groundbait, both, presumably, intended by anglers for rather larger fish. Here, in a man-made habitat in which natural food was somewhat restricted, the bleak was also found to feed quite extensively on plant matter, which illustrates its adaptability with regard to diet. Plants, especially the filamentous algae, as well as diatoms, have been found in their gut in other investigations, as have fish eggs.
Relatively unspecialized spawning habits, and, possibly, the species’ abundance, mean that the bleak tends to hybridize with other carp family members. In Europe, bleak/roach, bleak/rudd, and bleak/dace are said to have been identified. The latter two cases are open to some doubt, as it seems unlikely that rudd or dace are likely to be spawning in the same place and time as the bleak. In Britain, the only hybrid to have been identified with certainty is that with the chub, which seems to be moderately common. Unfortunately, it does look very much like the bleak, so that there is a great danger of misidentification. Over the years, several claimants to the bleak record have proved to be this hybrid, whose growth potential is, not surprisingly, greater than that of the smaller species. It can be distinguished by the rather broad head, small eye, and less compressed belly, but these are characteristics which need to be compared with those of a true bleak before one can be certain. The one in-dicator which clinches the matter is the number of anal fin rays. If a ‘bleak’ has fewer than 14, it is almost certainly a hybrid. Moreover, the anal fin looks shorter-based than in the true bleak.
Primarily a surface living fish, the bleak, as it feeds, adopts a swimming angle to the surface in which the long anal fin provides much of the swimming power, while the outspread pectoral fins provide lift with the upturned mouth picking small items of food out of the surface film.
There could hardly be a greater contrast to the bleak than that offered by the gudgeon. Instead of living at the surface in active shoals, it frequents the bottom in small groups, and about the only features the species have in common are that they are small, and they are both easily recognizable.
The body of the gudgeon is basically cylindrical, flattened on the belly and only slightly compressed from side to side from the anal fin to the tail. The snout is bluntly rounded, the eyes sited high on the head, while the mouth is placed on the extreme lower side of this. A short barbule, about as long as the diameter of the eye, is situated at the extreme end of the upper lip on each side. These barbules, the upper and lower lips, and much of the lower jaw, are richly supplied with sensory cells which continually monitor the mud on the bottom of the river or lake. As one might guess from its general shape, the gudgeon is rather sluggish.
Possibly because it spawns in very shallow water, the gudgeon has not been reported as hybridizing. It is also very likely that the eggs or sperm are not compatible with those of other native species so that any attempts at inter-breeding will not be fruitful. The gudgeon is of a rather distinct group of the family Cyprinidae and is only distantly related to our freshwater fishes.
Gudgeon live for up to seven years, at which time they are around 6in long. Older, larger fish are always females – the males mature earlier and grow faster, but most die in their fourth or fifth years.