Or does it? Usually a sound equation for breeding Bitterns is good supply of fish, especially Eels, added to a wet reedbed. As the Bittern booming season gets going, with the remarkable early news of up to 30 boomers in Somerset, a very few sites stand out as being stuck in the recession. The most noteworthy is the former Bittern stronghold of Leighton Moss, where I recently returned to undertake two days of electro-fishing and take a long, hard look at the reedbed. The major concern is the almost terminal decline in Bittern, against the national trend.
Leighton Moss supports one of the best densities of Eels and other fish that we have come across, yet the Bitterns are declining. Recession is indeed the problem; recession of the reed margins that is, making the fish less available to a foraging bird. The problem of reed recession in old reedbeds is well known. The formation of toxic by-products by the reed litter under anoxic conditions in high, stable water conditions reduces reed shoot density and vigour. Reed re-colonisation into anoxic sediments is known to be poor.
However, when reedbeds cease expanding (or start dying-back) due to the ageing processes described above, or grazing (by geese or deer for example), their extent can be increased by lowering water levels to expose mud and allow new plants to germinate and establish. Periodic drying out can be used to increase or decrease the proportions of open water and swamp vegetation and effectively rejuvenate the habitat.
Such a pattern of management has been used in the Oostvaarderplassen in The Netherlands. A mosaic of reed and open water habitat is maintained by a combination of fluctuating water levels and grazing by moulting Greylag Geese. This results in a periodic expansion and regression of reed. During high water levels, the geese graze back the margins of the reedbed. During low water levels, the exposed mud is colonised by reed, other swamp plants and ruderals. When water levels rise again, the seeds of the ruderals provide food for wildfowl and the new reed shoots form an expanding reedbed.
However, such management may seem drastic, particularly when short-term views are taken. But, in a recession, do you simply squeeze tighter and deliver the Bittern’s P45, or try to promote new growth?