The entrances to the dock basins were permanently blocked in the 1980s, sealing the water inside off from the rest of the Manchester Ship Canal and allowing aquatic scientists to begin the long and successful job of restoring life to them.
In 1988, as scientists confirmed that the water was becoming clean enough for fish to survive, over 1,000 bream, carp, roach, rudd and tench were released into the basins. The following year 12,000 chub, dace and perch joined them. More fish were stocked into the quays at regular intervals over the next decade.
Heather Webb, principal scientist at APEM, explained: “The dock basins are effectively like giant fish tanks and we work very closely with Salford City Council to manage the ecology within them.
“We keep a close eye on the different fish species in the docks and especially the perch.
“Female perch can lay up to 45,000 eggs per kilogram of body weight and we also know that the perch spawn earlier than the other species in the docks. The juvenile perch will often then feed on the eggs or very small young of the other species once they hatch later in the year.
“The danger is that the perch would soon become the dominant species. So when the fish spawn in spring and early summer it’s our chance to help keep everything in balance.”
In common with many other fish species, perch like to lay their eggs in among the protection of plants and twigs – like, for example, bundles of old Christmas trees.
For several years scientists from APEM have been carefully placing several dozen of these spawning bundles just beneath the surface of the water.
Once the perch have laid their eggs among them, the bundles are retrieved from the water. Then, with the perch eggs removed, they are placed back in the water for other species to use.
Peter Dennis, head of APEM’s field scientists, said: “We’re not trying to eradicate the perch, just keep their numbers down a bit. They lay so many eggs that we can never remove them all anyway, so there will always be some young perch hatching out each year.
“The signs are that the Christmas tree bundles are helping us to restore a better balance of fish species to the docks.”
If you have any queries, please contact Heather Webb, principal aquatic scientist.
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