a riffle beetle

Beetle Survey Results

Aquatic Coleoptra at River Wye SSSI, Symonds Yat Rapids, Herefordshire (September 2000)

Introduction

At the request of the British Canoe Union (BCU), a survey of aquatic beetles was undertaken at the River Wye SSSI between 12th and 14th September 2000.

The BCU is proposing to purchase a 200m long section of the river known as Symonds Yat Rapids and is intending to construct permanent in-river groynes to replace the existing stone groynes and to provide access for canoeists (including the less ambulant).

The purpose of the survey was to record aquatic coleoptera present within the section of the river which the BCU is proposing to purchase and also within the river downstream from the canoe site. The quality of existing microhabitats and the potential impacts of development work on both the canoe site and the down-stream section of the river are assessed. Particular attention has been paid to the presence of riffle beetles (family ELMIDAE) within the river, several species of which are of nationally notable or Red Data Book status.

Methodology

Beetles were collected using a standard hand net of 20 apertures per 25mm in bursts varying between approximately 30 seconds and two minutes. Samples were obtained from the edges of the river, particularly in the vicinity of mossy boulders and exposed tree roots and where possible, from near the centre of the channel by kicking submerged gravel in order to disturb resting beetles. Sites were sampled for varying periods of time, the basic criterion being to sample the site until no new species were found.

Presentation of Results

A list and brief description of the surveyed sites is presented in Table 1, together with map references. The approximate positions of the sites are denoted on Map 1. A list of the total number of species in the survey is presented in Table 2, the national status of rare species being denoted where applicable. Nomenclature follows Foster, (in press). The occurrence and frequency of each species at the surveyed sites is recorded in Table 3.

As a matter of interest, national distribution maps for six of the recorded species are included in Map 2.

Discussion

The results of the survey indicate that an impoverished fauna of aquatic beetles prevails within the river, particularly in the area of the rapids. In addition to the temporary stone groynes constructed by the canoeists, the east bank of the river in the vicinity of the rapids was observed to have been modified historically by the placing of locally quarried stone in order to stabilise the bank.

Few species of the Dytiscidae occurred within the survey areas of the river and Hydrophilid beetles were notably absent with the exception of a single specimen of Helophorus flavipes.

Of particular interest however is the occurrence of six species of riffle beetles, three of which are particularly local. Macronychus quadrituberculatus and Normandia nitens are of Red Data Book classification whereas Oulimnius troglodytes is nationally notable. Although present mainly as single specimens, M. quadrituberculatus appeared well distributed along the river in gravel and around exposed tree roots.

On the continent N. nitens is normally associated with the main channel, together with Stenelmis canaliculata, the latter species not being recorded during the recent survey. In Britain, however, the predatory bug Aphelocheirus aestivalis is prevalent in the main channels of the rivers Wye and Teme. This may well account for the apparent marginal concentration of Normandia, the rarity of midstream elmid larvae and the absence of Stenelmis.

O. troglodytes was observed to be present in similar numbers to the closely related though nationally more widely distributed O. tuberculatus. In a recent survey carried out on Loch Lomond, Doughty (1993) reported that numbers of O. troglodytes increased relative to O. tuberculatus following the deposition of silt during works associated with the Loch Lomond road widening scheme. This observation is of relevance to the present survey in that subsequent silting of the River Wye following implementation of the proposed works could be construed as being beneficial to increasing the number of troglodytes when in fact the species has in any case been found to be abundant before the event.

A single specimen of Pomatinus substriatus was encountered among tree roots at Site 4. This species is also of nationally notable status.

A number of microhabitats can be identified within the surveyed area of the river. Much of the river contains large cobbles and boulders which have been imported by human activity following historic quarrying activities in the local area during days gone by. These cobbles are particularly abundant in the vicinity of the rapids and around the island, the latter having been artificially constructed from reject stone. Few beetles were found among the larger stones, the size of which rendered sampling difficult. Typical of such habitats was found to be the Hairy Whirlgig Beetle, Orectochilis villosus. Localised areas of fine gravel are evident around the downstream end and east side of the island. These were found to contain mature larvae of the riffle beetle Limnius volckmari.

Fine sand and silty margins are present on the west side of the river, some containing exposed tree roots and providing a favourable habitat for P. substriatus. Downstream from the island, the percentage of larger cobbles decreases and areas of gravel are evident on the river bed. Few species of beetles were recorded from this habitat other than Nebrioporus elegans, Limnius volckmari and the two small species of Oulimnius.

The most productive habitat was found to be amongst exposed Alder roots, particularly in areas where the current flowing across the roots is relatively fast. All six species of riffle beetle were found to occur in such habitats, occasionally with Hydraena riparia and P. substriatus.

Recommendations

At the time the survey was undertaken the method of groyne construction had not been decided upon and it is therefore difficult to predict the degree of impact that such works are likely to have upon the existing riffle beetle populations.

As the east bank of the river adjacent to the rapids has already been modified by the historical deposition of cobbles and boulders, it is considered unlikely that the installation of permanent groynes will significantly affect the beetle populations in this section of the river.

It is of course obvious that great care should be taken so as not to introduce pollutants into the river during the process of construction.

It is also important that the microhabitats which support larval development are preserved. These include the small areas of fine gravel that are present on both the east and west sides of the island and (especially) at the downstream end. Macronychus larvae are known to develop within the rotting bark on submerged logs, the presence of which should be encouraged within the river.

Above all, microhabitats formed by the exposed tree roots should be preserved and maintained as these would appear to support the majority of the adult riffle beetle population. Both M. quadrituberculatus and N. nitens were recorded only in areas remote form human disturbance and are likely to be extremely sensitive to environmental change.

Ron Carr's Signature

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