There is a widespread misunderstanding that crucian carp are not native to Britain. The reasons for this are uncertain, but may be because of confusion between the names crucian and Prussian and the known introduction of true carp.
Nevertheless, there is no reason to believe that crucians are an introduced species. Their native range is very similar to several other native freshwater fish such as the bleak, silver bream, spined loach, ruffe and burbot. All are – or were – common in eastern England, roughly confined to the area bounded by the Thames and the many Yorkshire rivers flowing into the Humber estuary. From this region some have been redistributed by man or through man’s interference with nature – cutting canals for example.
Even today – after several centuries of rivers changing course – crucians are still more common in eastern counties and the south of England. They are apparently absent from Scotland, most of Wales and the whole of Ireland. ‘Tale and hardy
They are most accommodating little fish, able to survive in the smallest of pools. They thrive in densely overgrown, swampy waters with a soft, muddy bed – often places where no other fish can exist.
Being remarkably hardy, they have the ability to lie dormant through the severest winters and worst droughts. Pools and rivers which have virtually dried up during periods of drought have been known to hold live crucian carp among the roots of reed mace. During winter they can survive being frozen under ice, buried deep in the mud or in shallow swamps; as soon as the ice melts they ‘wake up’ again.
Crucian carp can also live for extensive periods in water with very little oxygen. In these conditions their growth is stunted and they can only reach about 10-12cm (4-5in) in length. But in large lakes where there is plenty of food crucians become very deep-bodied and can grow to an exceptional 46cm (18in), with a weight of 7 lb (3.2kg).
Follow the feeding
They are not at all fussy about what they eat, except when very young. At this stage they feed only on planktonic crustaceans. As crucian carp grow their diet widens and eventually they eat most insect larvae, molluscs, crustaceans (such as water slaters) and a lot of plants. Such unfussy feeding habits are an advantage when living in small ponds where potential food sources can be limited.
Mature at three to four years old, crucian carp spawn in May-June in the margins of ponds and lakes. The beautiful, golden-coloured eggs stick to fine-leaved weeds (or, in poor conditions, thin tree roots growing in the water).
The eggs hatch in a week but the young fish stay attached to the plants for a two or three days more to use up the rest of their yolk sac, before swimming off.
For the first weeks of life crucian carp form small schools, living at the edges of weed beds and under lily pads. With the onset of winter these groups break up.
Crucian carp form hybrids with common carp and also with their close relative the goldfish. The former are fairly easy to identify because the hybrids have a small head -like crucians — but are equipped with one or two, and rarely three, small barbels like the common. For some reason, nearly all common/crucian hybrids are male.
Anglers know that many coarse fish form shoals. If you catch one bream, others are likely to follow. While freshwater fish don’t shoal in the colossal numbers of some sea fish such as certain tuna species, shoals of several hundred are not uncommon. This shoaling behaviour is related to the survival instinct, which suggests it is an inherited characteristic.
As a general rule small browsing and plankton feeding fish, such as roach and bleak, tend to form shoals. Larger browsing fish, for example carp and tench, form small groups and large predators such as pike are usually solitary hunters.
Mass protection The main reason small fish form shoals is for protection from predators — most fry group together in the shallows. There are plenty of advantages in forming shoals. Safety in numbers There are many eyes and other sense organs in a shoal, making it hard for a predator to sneak up undetected. A shoal fish can respond to danger by following its fellows, though it hadn’t noticed the threat itself. Whether members of a shoal have different tasks isn’t clear, but it does seem that some fish act as look-outs, or that the fish at the edge automatically adopt this behaviour.
Confusing the issue It is harder for predators to single out a fish because they are constantly changing their position within the shoal. When a predator attacks, the shoal splinters, making it even more difficult to select a victim. Statistical safety A single bleak is almost as likely to encounter a pike as a group of 50 bleak. So 50 solo bleak will meet about 50 times as many pike as they will if they move as a shoal.
The most dramatic coming together of fish is just before, and during, spawning. In rivers, shallow feeder streams may become packed with fish for a few days each year as large numbers of fish move as one to their spawning grounds.
On the River Erne in Northern Ireland, the huge shoals of roach are so determined to move up river to spawn that they have actually flattened nets set in their path!
Other reasons to shoal
Some fish, such as perch and perhaps zander, form shoals so they can hunt together.
It seems that the predators may actually cooperate to drive their prey into a tight bunch, and then attack as a group.
Some shoaling is apparently accidental. Pike, for example, are generally solitary fish, which sometimes form loose groups. Rather than actually shoaling, it is likely that they are attracted to a feature or an area by a concentration of bait fish.
Many species of the carp family shoal over winter. They gather in compact groups, usually in the warmest, deepest water. However, these may be shoals of convenience – all the fish head for the warmer water independently.
Carp and perch, among others, often shoal by size or year class. This may explain why they form large shoals when young, but live in smaller groups as they mature. As members of a year class die off, smaller shoals of bigger fish result.
Or it may be that a big shoal of large fish quickly exhausts the food of an area, forcing them to move in search of fresh pastures. Small groups can feed in one place for longer. You’ll notice you often catch carp or tench in twos or threes.
It may explain why bream shoals patrol: they must keep moving – or run out of food. If you can discover the patrol route of a bream shoal – you’re in for some top sport. ;