Below the surface

Exactly what are the consequences of floods on our fisheries?

The effects of flooding on our fisheries

ONE of the likely impacts of global warming on the United Kingdom is that rainfall will become more erratic, with a trend towards high winter rainfall and drier summers.

The last few years have seen major flooding on many river catchments, as a result of high rainfall, with many rivers experiencing some of their highest levels ever recorded. But do these floods necessarily have an impact on river fisheries? After all, floods are a natural part of the water cycle and fish have evolved to deal with changes in flows.

Even in highly channelised rivers floods probably do not alter the numbers of adult fish present. Fish tend to seek areas of slower flows, particularly close to the banks, during the peak of big floods. Here they will enjoy some respite, especially if the banks are lined with trees or rooted plants that will deflect some of the water.

Even out in the main river the current speed is likely to be much lower close to the bottom compared to what we see on the surface. Often the flow will be only half the speed near the deck as it is up top. Perhaps more of a problem during big floods is the amount of debris coming downstream. In the murky water fish will need to try to dodge this to hold position.

Upstream movement

Species such as barbel may even move upstream in floods, as this gives them the ideal opportunity to overcome both natural and man- made barriers.

Moving upstream is an imperative for riverine fish, as this compensates for the drift downstream of their young. Often the upstream passage can be blocked by shallow riffles, yet in times of flood there will be enough depth for the fish to pass by.

The same applies to many weirs, where the only time that radio- tracked fish are found to move upstream is when the obstruction is overtopped by a flood. Now, the fish are able to move over or around the obstruction.

Young- of- the- year fish, because of their small size, are relatively poor swimmers compared to adults, and so areas of slack water are much more important for them.

There is scant hard evidence that young fish are swept downstream in large numbers during floods, because of the difficulty of sampling fish when conditions are so extreme. The indirect evidence on the effect of floods on young fish is variable, although a general movement of the downstream is probably likely.

The bigger the young fish the better their chances of survival, which is influenced more by the length of the summer and the availability of food than it is by floods.

Spawning

Spring floods, when the water temperature is rising and the fish are starting to think about spawning, can often lead to large numbers of fish moving out of the main river channel and on to the floodplain.

In natural rivers, where the banks are gently sloping and the fish can easily find their way back into the main river it makes a lot of sense to spawn on the shallow, warm, weedy water of the floodplain. Many rivers, however, lack this connectivity, thanks to channelisation, so fish that move on to the floodplain can often become trapped in pools as the level recedes. These fish will be cut- off and stranded, eventually dying when the pools either deoxygenate, or dry out. Fish have an uncanny knack of moving up even the tiniest ditch, so it is important ensure that these are kept clear so that the fish can return as the water level drops.

Pollution and disease risks

Rarely is flood water comprised of rainwater. Very often it is mixed with runoff from the land, along with the contents of our sewers, which can lead to a lethal cocktail of pollutants entering our river systems. Most of the time these will be relatively dilute, thanks to the huge volume of water, but some pollutants may become trapped in the system, particularly in silt, leading to a build- up over time.

One aspect of flooding that is rarely talked about, but which can be incredibly serious, is the effect of an influx of water on adjacent stillwaters. A great many gravel pits have been dug along the flood plains of rivers precisely to gain access to the rich beds of sand and gravel deposited by the river over many millennia. With many pits being low- lying flooding can be a regular occurrence and in some instances is even used to increase the water storage of the catchment.

Yet, a sudden inundation of water can lead to the transfer of disease and parasites into a fishery. This is particularly likely when other fisheries further upstream already have these pathogens. Not just the intermingling of fish stocks, but just the mixing of water can lead to significant fish deaths.

River water may also have a different chemistry to the lake water and also bring in significant amounts of silt, which can alter the lake’s habitat for months and years to come. Anecdotally, there is often a period after pits flood when the fishing can be poor, although the exact reasons for this are unknown. While the impact of floods can be many and varied, from the devastating to the beneficial, there can be little doubt that they are here to stay and likely to become more severe in the future. Managing our rivers, for fish and for all wildlife, should be a major consideration in how we deal with the challenges that lay ahead.