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Abramis bjoerkna (Linnaeus, 1758)
Blicca bjoerkna is distributed across most of Europe and in the adjacent Western Asia. The natural distribution however excludes peripheral areas including northern Sweden, northern Finland and Norway, and most parts of the British Isles (except Southern England) as well as the Iberian and Italian peninsulas. Introduced populations occur also in Spain and Italy, for instance. The Asian distribution is in the Caspian Sea and Aral Sea basins and in Anatolian Black Sea drainages.
Small silver bream are very similar in overall appearance to the immature 'common', or 'bronze' bream, Abramis brama, but can be distinguished by the larger scales. Counting the scale rows from the front of the dorsal fin to the lateral line and including the lateral line scale, is the most reliable first step in determining the species. Bronze bream have 13 scales or more, Silver bream 9-11. The Lateral line scale count for silver bream is 44-49, whilst for bronze bream it is 49 and above, and more usually well above 50.
Larger specimens are far easier to discern, because of marked differences in color and body shape. By the time both species attain sexual maturity they become relatively easy to tell apart from one another, the silver bream's scales remaining a bright, highly reflective, silver colour all their lives, whilst the scales of the bronze bream begin to take on a variety of hues, from dark brown to a light ochre yellow, however, some bronze bream do remain a silvery colour all their lives, depending upon habitat. There is confusion when sexually mature silver bream and sexually immature bronze bream are cursorily compared in the field, because bronze bream grow to the same size as sexually mature silver bream very quickly whilst still very young, and when they are almost invariably a silver colour themselves.
The maximum weight a silver bream can reach is determined by habitat of course, but give the optimum conditions for fast, healthy growth, they have an absolute maximum upper limit approaching, or possibly just exceeding, 1.6 kilos, or 3.5 pounds. Fish that do not live in such rich environments may attain a weight of around 0.9 kilos, or 2 pounds, but most silver bream never exceed a weight of 0.45 kilos, or 1 pound, and in small ponds, may never achieve even .3 kilos, or 0.8 pounds.
The body shape in males and females is subtly different. The female is less compressed than the male, has a rounded form by comparison, and is a little deeper overall, often with a pronounced bulge of the chest. The male is slimmer, and far more compressed. The bulge in the chest is absent, and he may only be two thirds the width of a comparative female of the same length, and consequently weigh far less. The males head is quite pointed and the snout appears slightly upturned, whilst the females head is rounder and with a snub nose. In the breeding season males become covered in tubercules and often display a reddish flush on their bellies and intense vermilion pectoral and ventral fin colouration, whilst all other fins can become very dark and opaque. The females at the same time, can become very rotund and deep set.
The eye of a silver bream is very large by comparison with its head and is one of the defining characteristics that sets it apart from bronze bream, and all other European cyprinids apart from bleak. It is round, protruding and a with yellowish cornea and black iris. The size relationship of the eye to the head is approximately 4 times eye width for head length from tip of snout to far extent of gill plate, and 2.5 times for head depth, and this relationship does not change greatly over its lifetime. The relationship of eye to head in bronze bream, however, does change considerably over the lifetime of the fish, and though it begins life with similar ratios as silver bream, by the time it is full grown the ratios can be as much as 6-7 times eye width to head length and 4-5 times for depth. These measurements are taken along lines bisecting the eye in both directions. The eye is also set very close the end of the snout, and close to the top of the head, but in bronze bream this is far less the case.
Silver bream have light pink to vermilion pectoral and ventral fins. The anal fin is transparent grey to dark grey, as is the deeply forked caudal fin and the dorsal fin. The anal fin has 21-23 branched rays.
Silver bream rarely have mucous on their bodies, and if they ever do it is only a very small and insignificant amount. Bronze bream on the other hand, are often very slimy indeed, especially when young.