How Fish Cope With Floods ?

When we see TV reports of land and communities devastated by flooding it is often hard to believe that anything can survive the churning maelstrom that is a flooded river. Yet, coarse fish, even juveniles, can survive even the worst flood events thanks to their survival instinct.



Floods are a natural phenomenon that actually help to sculpture our rivers and replenish the land surrounding them. The churning water can shift millions of tons of silt, gravel and even boulders in a river, creating new fast-flowing, shallow riles interspersed by deeper pools and features such as undercut banks, bends and shallow margins. Along the banks of a river the silt deposited during high water levels helps to keep the land fertile by replenishing the nutrients essential for plants.

A natural river and the land surrounding it form a complex landscape that means that the water will spread outwards almostas quickly as the level rises. The water in the main channel will flow faster and may become inhospitable, especially for young fish but, as the level rises, new slow-flowing refuges are formed as margins flood.

As the level of a river rises and the flow increases, the natural instinct of fish is to find refuge. This leads them to move towards the margins, where the flow is usually slower. This is especially true if the margins are filled with vegetation that will reduce flow and raise the water level, causing the river to rise and widen.



The level of a flooded river may rise several metres and in a natural channel this can lead to the river expanding massively. The flood plain is often ten times the width of the river under normal flows, and with the shallow angle of the banks the flood plain will quickly fill up giving the fish further refuge. In fact, in a natural river there may actually be more habitat available for fish during times of flood than when the river is at a normal level. As the water level slowly recedes the fish can return to the main river channel without becoming trapped.

Some species make use of the flooded margins to move around and for spawning. Species such as tench and crucian carp naturally dwell in the small pools that line the flood plain of a river. During floods these pools are replenished with fresh water and connected to the main river, enabling the fish to spread out.

Species such as pike and carp may use periods of high river level in the spring to gain access to spawning sites alongside the main river. Here their tiny eggs and fry will be safe from most of their predators. The tiny fish make their way back to the river as the water level falls slowly in late spring.

How fish manage to find their way in flooded rivers is still very much open to conjecture. What we do know is that often the water close to the riverbed can be much clearer than it appears at the surface.
Sight is not the only sense that fish use to find their way around and so a combination of the lateral line, the senses of touch and perhaps also smell come into play to help the fish navigate in poor conditions.

In a natural river there is always some habitat for even young fish, but in our highly managed waterways this is often not the case. As a consequence of trying to control floods, our rivers have often been straightened and sometimes dredged to enable them to carry more water out to sea faster. Much of the flood plain has been lost as the banks have been built up so that the adjacent land is not flooded. Instead of the river rising slowly and widening, many modern rivers just increase in level with a rapidly increasing current speed. In extreme cases this can lead to a massive reduction in the amount of habitat suitable for fish and even lead to young fish being washed downstream, perhaps eventually out to sea, where they will not survive. Weed removal can make this situation even worse, as the slow flowing refuges among the beds of plants are removed.

Flooding in heavily-managed rivers can be even worse in extreme cases when the banks are finally overtopped. Now the water runs out across the land, and often the fish will follow suit. As the level recedes
fish can become trapped on the wrong side of the flood banks and either are left stranded or are easy prey to predators.

Fortunately, fish populations are extremely resilient and we can mitigate some of the worst efects of floods in even the most highly managed rivers by incorporating some simple refuges. Backwaters, dykes and marinas can all make brilliant habitats for fish during floods and often the smaller fish will be in incredible densities in these refuges.

Careful cutting of the marginal plants can also create useful safe zones when the water level is high. Re-wilding, the return of simplified managed environments to their more complex semi-natural state, is another management practice that is growing in popularity and that can benefit the whole ecosystem, not just the fish.

We are slowly learning by our mistakes and with more frequent flood events on the horizon we can make sure our fish populations remain safe.