Monday, 5 September 2011

Ouse Fen comes of age (what a tinca!)

Ouse Fen from the air and at boat level; a stunning Tinca (Tench) and a not-so-stunning stickleback

Amongst the headline figure of 104 booming Bitterns in the UK this year was the smaller milestone of the first successful nesting of Bitterns at Ouse Fen (Needingworth Quarry) in Cambridgeshire. The partnership project between Hanson and the RSPB will eventually become Englands largest reedbed as the habitat is created following gravel extraction.  A day spent reviewing progress was very instructive. The aquatic habitat in the developing reedbed compartments looked superb, with abundant aquatic weeds including the scarce Fan-leaved Water-crowfoot and Hairlike Pondweed.

A search for the now redundant Bittern nest was easy enough, thanks to excellent triangulation by the guys recording the birds activities during the long watches required to prove breeding. The typical small woven platform of reeds was constructed in what would have been about 60-70 cm of water at the time. All the feeding flights were from the nest location across to ‘Phase 1’, the oldest reed compartment. A further couple of hours of electro-fishing provided the reason for this. The relatively young area of habitat around the nest has few fish as yet, just a few sticklebacks, but the more mature compartment supports a respectable fish biomass of around 21kg/hectare, mainly Perch and Roach. This is above the 10kg/ha considered to be the threshold for successful Bittern breeding. So why did she not nest where the fish were? The simple answer is that the dry spring had reduced water levels in the reeds in ‘Phase 1’, thus reducing the security of any nest. However, the story is even more complicated. There were no booming males heard at Ouse Fen yet there was a nest. At nearby Fen Drayton, there were 2 boomers yet no nest. It is likely that the dry spring also affected the reedbeds at Fen Drayton and as a result the female moved across to the wetter Ouse Fen to find a secure nest location. Thus she found a mate, a nesting site and a food source in three different locations.

The electro-fishing also produced a few Tench; surely one of the most stunning of our freshwater fish with those golden scales. Less visually attractive, although interesting in a rather gruesome way, was the high parasite load of the Nine-spined Sticklebacks. The photo above shows one poor specimen with a bulging abdomen containing a Schistocephalus tapeworm (S. pungittii is the usual species in this stickleback). If that wasn’t enough it also has myxosporidium cysts (a protozoan parasite) to contend with.

Family parties of Bearded Tits (one of three) and Marsh Harriers were additional evidence that this new site is coming of age. A single Garganey, a few Little Egrets and several Hobby all helped round off a pleasant day in a boat.

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