Aquatic life

White-clawed crayfish Photo by John Hobson

White-clawed crayfish Photo by John Hobson

The Wharfedale Naturalists Society is fortunate to have a large number of freshwater habitats within their area.

These vary from the three rivers, the Wharfe itself, the Skirfare in Littondale and the Washburn, to still water habitats. These include the four large reservoirs in the Washburn Valley, gravel pits in the lower parts of the valley and smaller ponds and lakes. Peaty acidic moorland pools have their own specialist communities and garden ponds also provide much needed replacements for the declining numbers of ponds elsewhere.

The Skirfare and the upper reaches of the Wharfe are very similar in nature, where due to the speed of the current fewer species can exist. Here the river and its feeder streams have a plentiful supply of oxygen due to their numerous waterfalls and rapids, but the speed of the current prevents sediments forming on the bed of the river.

Creatures living here such as the nymphs of mayflies and stone flies are specialists at ‘hanging on’ and have streamlined bodies to enable them to do so. Caddis flies living here commonly do not live in cases but spin webs and have large hooks on their rear to enable them to hang on.

Lower down, below Bolton Abbey the current is slower and is enriched by farm manure and the treated waste from sewage works at villages upstream. Sediments are deposited enabling both aquatic and marginal plants to become established. These in turn become habitats for different species of caddis flies, crustaceans, and beetle larvae. Mayfly and midge larvae occur sometimes in vast numbers as anyone walking on the riverbank on a summers’ evening will be aware.

The Washburn has been greatly altered by the construction of the four big reservoirs in the valley. The flow varies with the amount of water being extracted and the river bed has many small pools.

Fish in our rivers include brown trout, grayling and stone loach all the way down from the headwaters, although the numbers of grayling are in decline. Below Addingham dace, chub and minnows live in the slower flow, whilst downstream at Pool pike are becoming common, predating other fish.

Angling clubs are reducing the amount of re-stocking in order to enable more robust native populations offish to re-establish themselves in the river.

The reservoirs in the Washburn are large artificial lakes. Their depth and fluctuating water levels prevent stable shorelines from becoming established and except in one or two marginal areas do not contain much life. They are stocked with brown trout rainbow trout, and blue trout, (a hybrid of rainbow trout).

One serious problem currently occurring in the Wharfe is the presence of introduced American crayfish, which are displacing our native white clawed species. As well as competing for food they carry a disease which is rapidly killing them off.

Still water bodies in the area include disused gravel pits at Ben Rhydding and the new Otley Wetland Nature Reserve. Whilst the Otley gravel pits are relatively new, and have yet to become well colonised with aquatic life (but they are quickly becoming so), the ones at Ben Rhydding have not been worked for some years now and contain a rich variety of both aquatic and marginal plants. The speciality here are large freshwater mussels.

The peaty upland parts of the area contain many pools and peat bogs. These are colonised by many micro organisms which prefer acid waters. They form the bottom of a food web attracting other creatures including insect larvae, mites and worms all providing food for upland birds such as waders.

Smaller ponds are on the decline. Many have disappeared under urban developments, and farm ponds are now rare as they are linked with the spread of diseases in cattle. When livestock no longer trample pond margins they soon become overgrown and the pond changes into a marshy area before becoming filled in altogether. Fortunately the increasing number of garden ponds and the fashion for ‘water features’ is making up for this decline. Small ponds such as these provide ideal places for amphibians such as frogs, toads and newts to live and breed in.

Many people became interested in natural history through fishing for tiddlers and tadpoles as children and ‘pond dipping’ is certainly a rewarding and fascinating wildlife activity. The lower reaches of the Wharfe are particularly rich in easily identifiable species, and with the minimum of equipment can provide an interesting natural history experience.

Members of the Society will be happy to help with locations and identification of specimens. Do you know what lives in your garden pond?