Originally there weren’t going to be any fish in the lake. But then swarms of midges began to bite concert-goers enjoying their Pimms between acts. So carp were introduced to control the midges. In that they were a success, but they created a lot of mess in the water.
It seems that carp are just about the dirtiest fish there is. ‘Carp’ is quite possibly a misspelling. They were defecating in such quantity that they were poisoning the other fish, killing off the plants, and quickly filling up the meagre half meter depth of the lake with silt (my pleasant euphemism).
Carp were a short term solution, but a longer-term disaster. Carp are ‘bottom feeders’ so they stir up the silt. This makes the water full of nutrients, which encourages algae to reproduce like crazy, causing ‘algae bloom’ – which makes the lake look a queasy shade of green. When the algae die, the bacteria which eat them absorb oxygen, and then the fish start to suffocate. The ecosystem becomes increasingly unbalanced, spiralling out of control, till there’s nothing left but dirty water and a very nasty smell.
So they restocked the lake with 1,000 Golden Orfe and Golden Rudd, which are more dainty surface feeders. Unfortunately they tend to fall prey to a visiting heron which occasionally take up residence on St Giles’ church roof. If it becomes a serious problem, then they will have to buy bigger fish. Really, that’s the plan.
An earlier, and rather dramatic, solution to the carp problem was employed by some unknown person who put a pike in the lake many years ago. A few months later there was one fat pike and no carp.
When they drained the lake in 2004, they had to catch all the carp. Apparently they removed 3,500 carp. (They say they were all re-housed, but I suspect that boys at fairgrounds around the country won some very large goldfish that year.)
Incidentally, on the highly important question of whether the plural of ‘carp’ is ‘carps’ or ‘carp’, I remember a letter in the Times objecting to the use of ‘two carps’ in an article. The letter read: “Two carps? One carps: ‘two carp’.” I liked that.