Understanding fish shapes and fin positions

The freshwater fish in our lakes, ponds and rivers come in all sorts of shapes and sizes, but this diversity is not the result of random chance.

Body shape and fin location have evolved to allow the fish to perform and operate effectively in the environments they live.

Living in water is not at all like living in air. There are many specific differences such as the fact that water is 800 times denser than air, so moving in water takes a lot more effort. This particular factor means that fish have to optimise their bodies to allow movement. This situation becomes a lot worse when you consider that in moving rivers and streams, just maintaining position consumes energy.

The main powerhouse for movement comes from a fish’s tail, which acts like a propeller at the back of the fish. Different species have different tail shapes and this is also linked to how and where a fish is moving.

Riverine fish tend to have deep, sharply forked tails, which reduce the ‘drag’ which would otherwise hold the fish back when it moved. The forked nature means that the surface area of the tail is relatively low so there is less surface for the water to stick to when the fish moves at speed. This means a riverine fish in moving water can efficiently beat its tail for relatively low energy input because the drag on the tail is low. However, forked tails generally provide less power per sweep so need to be moved more rapidly to achieve movement.

Shallow forked tails generate more energy per sweep of the tail but create more drag, so fish that generally move around in stillwaters have less forked tails. This allows them to accelerate for short periods with lots of power, but they don’t maintain these top speeds for long because it uses too much energy.

The torpedo-shaped body of a barbel makes it a highly efficient swimmer

The body shape of the fish also gives a clue as to where it lives. Torpedo-like body shapes are highly efficient in terms of moving in water, so fish that live in fast flowing rivers have this characteristic. Fish like barbel and chub epitomise this body shape and they have evolved to live in rivers and streams. They have low-drag bodies and allow the fish to have high muscle content in their bodies which actually generates the power.

This torpedo shape can be even more accentuated in a pike. These fish rely on their ability to rapidly accelerate to capture other fish. The cylindrical body shape allows efficient movement in water and most of the fins are positioned at the back to provide propulsion for rapid acceleration.

The broader, deeper body shapes such as bream and roach are less suited to moving in fast-flowing rivers but they have other advantages, one of which is camouflage. Their broad scaly flanks actually reflect the light around them, which means their bodies act as mirrors so the fish blends in with its surroundings. This principle, that fish have evolved millions of years ago, is now the same that has been used to design camouflage suits for the military! So the fish has balanced the problem of drag and slower movement against the need to avoid being eaten.

The type of scales and the levels of slime on fish also reflect the types of environment they live in. Tench have small scales with lots of slime and spend most of their time near the bottom and around snags which means they tend to bump into stuff more often.

The small scales are less easy to knock off and the extra slime protects the fish’s skin against damage. Larger scales such as those on roach and rudd are more sensitive to damage but are more efficient as mirrors and therefore give better camouflage in open water.

The lateral line is very obvious on most fish and is part of the fish’s sensory system which allows it to detect movement around it, and to hear low frequency sound.

The level of development of this system is often related to need, so in a fish like carp it clearly runs down either side of the body. However, in a pike, the system also extends to the head.

This ensures that pike are able to detect water movement directly ahead of them and can focus to pinpoint the exact location of their prey.