By Fran Southgate
Wetlands Landscapes Officer
Quite a lot of people talk about ‘Fish passage easements’ these days. It conjures up images of colonic irrigation style holistic health treatments for ailing aquarium pets but in fact is something with somewhat less eye watering implications! In this case we are mostly talking about removing or bypassing intentional blockages placed in rivers and streams by humans, such as bridges, culverts and weirs.
Whilst installing man-made structures, little thought was historically given to the need for most fish species to move around - upstream and downstream to find food, shelter and other fish to breed with. Some fish like eels and sea trout need to be able to make epic annual migrations between the sea and the smallest headstreams of a river in order to sustain their populations. Populations of eels are thought to have crashed by over 99%, so we need to start unblocking their passageways fairly promptly!
Man-made barriers prove difficult for almost all of our native fish to navigate, especially the ones which can’t jump. The journey of a fish from spawn to tiddler to adulthood is peppered with hazards, many of which are now man made. In many cases these weirs and structures don’t even serve a purpose any more, other to turn the river into a ponded canal, reducing natural flow patterns (which would otherwise help to aerate gravels that fish breed in), reducing natural habitat niches (which would help fish to hide from predators, or to avoid getting swept out to sea in a flood), and increasing the accumulation of silts, nutrients and debris (which is detrimental to most flow loving river life).
This is where the fish passage easements come in. There are now range of ways in which fish can be given a leg up (or down, or around) some of the weirs and structures that we have put in their way. These can be as simple as putting a special fish ladder on the structure, replacing a culvert with a clear span bridge, creating a bypass channel which flows around it, or even removing the structures altogether. There is often a moment of fear when a concrete structure is taken out of a river – fear that we will flood more, or have more droughts. These fears are increasingly being shown to be unfounded, and their removal is generally having a positive effect on river ecology and flood storage, as well as stimulating huge increases in the range and numbers of fish found further up into our rivers.
There is some fantastic work being done to help fish commute up and down our rivers at the moment. By doing this and by increasing fish stocks these projects are also helping support local angling groups, local wildlife and local people. Where we can we’ve been helping the Rivers Trust, Environment Agency and the Wild Trout Trust with easing fish passage across Sussex, and we hope to continue to do so into the future.