The effect of floods on trout

The recent and current floods are concerning for all those affected. Having your house or business flooded is an utterly miserable experience, and farmers staring at flooded fields wondering when they will be able to sow or harvest a crop must be close to despair. 

So perhaps the fate of trout in floods will not be at the forefront of everyone’s mind, but it is a question that we get asked a lot. Along with the effect of drought on trout, of course! 

WTT Conservation Officer Professor Jonathan Grey looked at the science and wrote this fascinating article for our journal, Salmo Trutta, this time last year.

Here is a summary of the points about floods and trout – but do please read the whole article, as the impact of drought is generally worse than floods. And, disruptive and destructive as floods and drought can be, the ​‘perturbation’ of ecosystems can be a good thing; for example, floods can redistribute gravels and woody material and create new habitat to be colonised by plants, invertebrates and fish. 

• The magnitude and duration of the flood, and the nature of the river channel and floodplain, will determine how organisms respond. 
• Most people associate winter floods with wash-out of gravel/redds/eggs/swim-up fry, and extreme winter flooding can wash out all or most gravel and eggs.
• Trout compensate for this risk by ​‘over producing’ eggs so that at least some may survive. 
• Lost production in one year effectively creates a cohort gap within a population and leaves resources (food/​shelter) which can be exploited by the production of previous or subsequent years; less competition = faster and/​or greater growth. 
• Loss of one or two years of production across a five to eight year period would be barely detectable in a wider population. 
• Loss of three or more years of production over the same time period, especially consecutively, would probably start to take a toll on trout populations.
• Adult wild trout may be washed downstream or onto the flood plain. They are well adapted to find any available slacker water, and to head back to the main river as floods recede. This is where connectivity within the river and to the floodplain is critical. Any barriers (such as weirs) or bunds prevent trout from finding their way home when floods recede.