Biomanipulation – A Fishy business!

‘Biomanipulation’ is THE key technique that we are using to improve the water quality of Hoveton Great Broad. It’s been used successfully at several other sites in The Broads and involves closing off the Broad from the river and removing most of the fish. But how does it work?

“It’s not like it used to be!”

For much of the year Hoveton Great Broad is a horrible cloudy brown colour, but it wasn’t always like this. In the past, the water was crystal clear and the Broad supported an abundance of water plants. Aerial photographs from the late 1940s show that nearly half of the surface of the broad was covered with emergent or floating vegetation – plants like water lilies, reed and lesser bulrush.

In the 60s and 70s, as the local population grew, increased discharge from sewage treatment works began to make the water more polluted. This allowed tiny floating algae to thrive, changing the water quality to the brown murk that we see today. Since then, the water companies have spent £millions to clean up the sewage and the river water quality is getting better and better. However, Hoveton Great Broad – and many others like it – is still dominated by algae. Very little light can penetrate this murky water and therefore few plants can grow.

When the Broad was healthy, most of the algae was eaten by water fleas (zooplankton). These tiny golden-coloured creatures are themselves food for many fish – particularly roach and bream. When plants were plentiful, the water fleas had somewhere to hide from the fish. However, now that the plants are gone, they have nowhere to hide and most of them are eaten by the fish, so they don’t get a chance to eat much of the algae.

The solution

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  1. By removing the fish, we will give the water fleas a chance to thrive in the broad.
  2. They will then feed on the algae, clearing the water.
  3. Clear water will allow light to reach the bottom of the broad, allowing plants to germinate and grow.
  4. While the plants are growing, it is vital to keep the fish out, so that the water fleas continue to thrive and keep the water clear.
  5. After 5 – 10 years the plants will have re-established themselves and the fish will be allowed back in.

When the biomanipulation succeeds, there will be enough plants to provide hiding places for the water fleas. The fish will benefit too, as the plants will provide better spawning and nursery habitat for young fish.

The clever bit

We won’t need to catch all the fish to remove them. We have been using sonar cameras to monitor the natural movements of fish in and out of the broad. By carefully choosing the right season and the right time of day to close the broad off from the river, we will lock out a lot of the fish  as they move into the river by themselves. Clever eh?!

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