Smaller than its close relatives, the common and king carp, the crucian is nevertheless a hardier fish which will often survive in conditions where other species would die.
Among the freshwater fish of the British Isles there are many fine sporting species with devotees dedicated to their capture and study. There are also many small species, like minnows, which the angler regards only as prey for big-ger fish or as bait. Between the two extremes lie a number of species with some appeal to the angler but which few attempt to catch. Prob-ably the most common of these is the crucian carp, Carassius carassius.
The crucian carp is not a well-known species to the angler, perhaps because of its poor distribution. Originally thought to be a native of the east of England, roughly the region covered by Yorkshire rivers, the Trent, the Great Ouse, and the Thames systems, today, by the intervention of man, it is found well outside these river systems. There is even some doubt as to whether it is native to the Thames basin at all, its distribution being very patchy in that area. Whether this is due to sporadic introduction by man or the destruction of so many suitable natural habitats in this area is uncertain. Whatever the truth of this, the crucian is not found in many parts of the British Isles. It is absent from Scotland and Ireland, and Wales has only isolated populations as do the extreme southwest and the north of England.
Freshwater migration The reason for this sparse distribution owes nothing to the crucian’s need for special habitat (for it will survive where most other species would die) but in the prehistory of these islands. Following the last Ice Age, obliterating all the native freshwater fish in Britain, a land connection between England and the Continent persisted for two to !&MB when the North Sea was finally I formed.
Adaptation to change
The crucian carp thrives in the kind of swampy region that best resembles the prehistoric North Sea landbridge. It is found in overgrown, weedy pools and lakes choked with vegetation, both growing and decaying on the bottom. An extremely hardy fish, it is able to tolerate a wide range of temperature and oxy-gen variations. So tolerant is it, in fact, that it can survive where few other fish will. Very low levels of dissolved oxygen such as often exist in densely weeded pools, especially at night when the plants are producing deadly carbon dioxide rather than oxygen, or in winter when the vegetation has died down and is rotting to the detriment of dissolved oxygen, fail to kill the crucian carp. Able to face long frozen periods, low oxygen levels beneath the ice and near zero temperatures, high sum-mer temperatures only make the crucian more active.
Given such adaptability, the cru-cian is often put into waters in which it will survive and breed—but never grow to any great size. Its natural habitat, the lowland pools and mar-shes in flood plains of rivers, where plant growth is rich and suitable food abundant, no longer exists in England today. As a result, places where the crucian now lives are frequently less than ideal and are often man-made lakes and pools. Small woodland pools, the result of some long-forgotten landscape gardener’s work, with a thick residue of rotting leaves from nearby trees and too few plants, and old claypits made in the days when clay was dug by hand, are frequently its habitat.
Two types of crucian
There are two types of crucian carp: one big-headed and flat-bellied, the other solid, dumpy with a small head. The slenderbodied, big-headed mini-crucians of so many of these pools are the result of their fooddeprived environment. In Europe these fish are often referred to as the humilis form of the crucian (a name proposed in 1840 by a scien-tist who thought the variation from the normal crucian shape so extreme that it must be another species). In fact, they are simply half-starved crucians. From aquarium experiments it seems that given abundant food they will grow to resemble the full-bodied fish, if they were not too old when captured.
The plump, full-bodied crucian found in the larger, richer lakes is richly coloured, with the back a deep bronze-green, the sides bronze or yellow, the ventral fins tinged with red and the tail and dorsal fin dark. Its small head, humped back, and solid body show a fish that has never been short of food. They feed when young on the minute crustaceans, including water fleas and copepods, abundant in rich, still freshwaters, graduating to insect larvae (especially bloodworms and mosquito larvae), larger crustaceans, and even small water snails as they grow. When well-grown there are few invertebrates that the crucian carp will not tackle. Remains of plants, especially the finer leaves of the pondweeds, are often found in crucians’ stomachs, although it is possible that the plant is eaten for the sake of some animal, such as snails, or perhaps their eggs, living on it.
In East Germany and Poland some fish farmers grow crucian carp for the food market, mainly because the common carp, Cyprinus carpio, pro-ves to be rather delicate and slow-growing in their cold continental climate. Here, by selective breeding and of course, food feeding, they have produced deep-bodied farm fish known as Spechthausen which mature at two years when they reach a length of 5-6in. Clearly, such selection could be practised on a small scale by fish farmers in this country to produce fast growing, large fish for the angling market. In the course of a generation or two, however, such fish may revert to the normal-growing, often stunted population of so many lakes, due to spawning.
Spawning takes place from May to June—sometimes in cool summers, as late as July. The eggs are golden to pale red in colour and stick to the leaves of plants. They hatch in five to seven days depending on the temperature, and even as long as 10 days in cool weather although this results in high mortality. The fry hatch out at about 4.5mm in length and hang on the leaf by a special sucker on the head for a further two days while they absorb the last of the egg’s yolk which at first forms a bulge on the belly. After this their survival and growth depend on the amount of food available. The female crucian carp lays between 150,000 and 300,000 eggs in several spawning sessions. This number is not particularly large—only one-quarter to one-third the number produced by a 10lb common carp—and the abundance of crucians cannot be due to high egg production. Where the crucian has the advantage over such fish as the common carp, with which it is frequently found, is in its adaptability and toughness, and the much higher rate of survival of the newly hatched young. While this, no doubt, has advantages for the sur-vival of the species, it often tends to be a disadvantage to the individual population. All too often one finds large numbers of small crucians in a lake stunted in their growth simply carp are often spoken of as if they are close relatives. In fact, the relationship is not especially close. The crucian is more closely related to the goldfish, Carassius auratus, and when young it is sometimes difficult to tell the two apart. This, however, is not likely to cause many anglers problems, although it can be a pro-blem for fish breeders as the two will readily breed together to produce a because there are too many mouths for the available food.
Annual netting to remove some of the excess population is possibly the best solution to this problem. If this proves impractical, a club rule that all crucians caught have to be killed is the solution. It sounds unkind, but in the interest of future angling it is often necessary.
Crucian carp and the common cruciangoldfish hybrid. The pro-blem for the angler is more likely to be difficulty in distinguishing the crucian carp from the cruciancommon carp hybrid. This is not always very difficult, although sometimes the odd hybrid can give the angler identification problems.
Aids to identification
First, remember that crucians have no barbules round the mouth and the shape of the dorsal fin is smoothly curved outwards (convex). Second, the common carp has a bar-bule at each corner of the mouth and its dorsal fin is curved inwards (concave). Hybrids usually have one bar-bule, either on the left or the right side. Sometimes they have two tiny barbules, not much more than pimples, on each side. Very rarely it may be that they have no barbules. The dorsal fin shape is literally in-between the parent species, usually high in front, but with only a shallow dip in the outline, but often straight edged. Other features, such as the number of pharyngeal teeth, tend to be intermediate between the parent species but in the really difficult cases examination of the pharyngeal teeth is necessary.