In order to survive long enough to reproduce, many of Britain’s small freshwater fish have adopted a variety of strategies to avoid being eaten by such predators as pike. The best option open to them is to be cunning because their swimming speed is restricted by their size.
The dark bars on the perch’s body serve as camouflage, enabling it to hide among weed stems when it is hunting or being hunted.
By constantly moving forward and within the shoal itself, fish like these young perch confuse the predators that try to target a particular individual.The strong spines that make up the dorsal fin on species like this ruffe prove a useful deterrent when predators such as small pike, perch or catfish try to swallow it. By erecting its prickly pins it can make itself rather too big a mouthful for some fish to cope with. Fish such as these minnows swim in shoals to ensure safety in numbers. The more fish in the shoal, the more eyes, ‘ears’ and nostrils there are to sense any danger. Shoaling also greatly reduces the odds of an individual fish being caught.
One of the most obvious devices is the possession of strong spines. When approached by a predator the fish erects its spines and makes itself inedible, or at least difficult to catch.
The three-spined stickleback – with its strong dorsal and pelvic spines – is a classic example. Experiments with small pike have shown that a captured stickleback Often avoids being swallowed by locking its spines into an outward pointing position.
While this works as a defence against small pike, it is not very effective when it comes to large pike or otters — which don’t mind chomping them up spines and all.
Crafty kingfishers have also found a way around this prickly customer – with a few licks of the head they knock the spines off ay bashing the fish against a branch.
Camouflage and shoaling
The perch also deters predators with its quite formidable spines, but its additional defences are far more subtle and effective. Its camouflage colouring — broad vertical bars along its olive green body – enables it to hide among the straight stems of water plants like reed mace or rushes.
The life-style of other species, such as the bleak, dace and roach, also plays an important role in their defence. When young, these fish form shoals numbering as many as two hundred, which means for a small fish, living near the surface, there are many more ‘ears’ and nostrils alert to danger.
Most predators select a single fish to attack, often aiming for its conspicuous eyes. The presence of other fish of the same size close by makes aiming difficult.
Most fish, mainly those that live in open water, are dark coloured on the back and white or silvery on the belly. This is known as counter shading. It makes the fish less visible to predators whether they are in the air or deeper in the water below.
The fish’s silvery white belly merges with the light sky and the water’s surface – when seen from below it appears white and is hard to identify. The dark back seen from above — by a heron perhaps — merges with the murky bottom of the river bed.
Fish that live near the river bed also use their colouring to remain inconspicuous. The greenish-brown colour of the tench and the dark brown of the bream are good ways of staying unseen against a dark muddy background. Both species also burrow right into soft mud.
Despite the great variety of methods used by these fish to avoid capture, no prey fish species has managed to upset the balance of nature and develop a perfect defence strategy. This is a somewhat reassuring thought when you consider what the alternative is likely to be… waters full of minnows and nothing else. fish is kept off the bottom and away from the weed. Once coaxed into open water and away from trouble, it is played out under the rod.