Know your fish – inside and out

Know your fish – inside and out

For land-based animals like ourselves, the surface of the water is a barrier to another world. The problems involved in living underwater are quite different from those encountered on land and fish have a number of special adaptions to deal with them. Understanding how fish are adapted to their aquatic environment is the first step to predicting their feeding and living habits and so to being a better angler.

How fish float

Water is denser than air and so supports fish against the pull of gravity much more than air would. Thus fishes’ limbs’ (fins) are not required for support and can be used entirely for propulsion and steering.

However, the bodies of fish are still somewhat denser than water and tend to sink. If they couldn’t regulate the density of their bodies, they would need to use a lot of energy to keep themselves afloat. Most species of fish do this with a swim bladder containing gases, and with which they can suspend themselves at any depth.

Clever fish

Fish have very small brains indeed and most of their behaviour consists of inborn responses to objects or events such as food, light or vibration. To talk of carp being ‘clever’ is something of an exaggeration. However, repeated exposure to a particular bait, coupled with a highly stressful ‘punishment’ (which is what being caught must be, after all) can lead to a form of basic learning, similar to a phobia for that bait. This is why, with fish that grow large and are caught many times, it is often helpful to try an unusual bait if you aren’t getting any runs.

The rules of fish form

If two fish species living in similar waters were to feed and five in the same places, one would be squeezed out in the competition for food and living space. Roach and rudd live in many of the same waters, grow to much the same size and look very similar. But they are not in competition because the rudd has adapted to feed mainly at the surface whereas the roach feeds off the bottom.

Bottom feeders, such as roach, have an upper lip which overlaps the lower, giving them downward-pointing mouths, while the opposite is true for surface feeders, like rudd. Bottom feeders also have extendible lips for rooting around in the mud, and often have barbels – which are feelers packed with taste-buds, allowing them to taste the mud as they go.

Fish from fast-flowing rivers – for example barbel, trout, grayling and dace – are slim-bodied since streamlining helps them fight the flow. Stillwater fish, such as tench and bream, are deeper bodied because a slim shape is of no advantage. Indeed, perhaps a deeper shape is better for food storage over the winter months.

The body design of predatory fish is very different, reflecting the difference in lifestyle. Pike and zander ambush prey, so they are streamlined with their fins grouped towards the tail. This enables them to accelerate quickly to attack speed. Perch harry their prey over a longer distance and are built to cruise, not sprint. They all have large mouths filled with sharp teeth, and very flexible jaws, allowing them to gulp down large meals quickly. This means that energy is not wasted in catching fish too big to swallow. They are also well camouflaged, giving them a better chance of surprising their prey.

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