Originally there weren’t going to be any fish in the lake. One story was that the lake attracted swarms of midges which bit concert-goers enjoying their Pimms between acts in the theatre and so carp were introduced to control the midges.

That is not true – although it might have been a useful side benefit. The real story was that the water in the lake performed a dual function as part of the Barbican Centre’s cooling system, which was a very clever idea, but in practice algae growing in the lake was gumming up the equipment. The Chief Biologist for Metropolitan Water Services of the Thames Water Authority was brought in. He offered six options – the one the City adopted was to introduce algae grazing carp into the lake. The Superintendent of Parks and Gardens was authorised to spend £1000 on importing a stock of carp.

The carp 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 the City 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 takes 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 the City drained the lake in 2004, they had to catch all the carp. Apparently they removed 3,500 carp. (The City insisted 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’.” You can’t carp at that.