Faunal Description of an Irish Cobble Site Using Airlift Suction Sampling

Abstract

SHORT COMMUNICATION FAUNAL DESCRIPTION OF AN IRISH COBBLE SITE USING AIRLIFT SUCTION SAMPLING A. Linnane, B. Ball, B. Munday, R. Browne and J.P. Mercer INTRODUCTION One of the pnrmary goals in marine ecology is to understand themechanisms that control population dynamics in benthic habitats. One such habitat type is cobble (32-256mm particle diameter, Phi scale range = - 5 to - 8, (Wentworth 1922)), a substratum that is particularly common at the base of many subtidal reefs off the west coast of Ireland. Due to the interstitial nature of cobble, such sites have long been considered an important nursery area formany organisms (Hudon 1987; Able et al. 1988). Quantitative descriptions are sparse, however, as conventional sampling methods such as cores and grabs are unsuitable for gravel and cobble substrata. This problem was addressed with the development of the airlift suction sampler, which was first used in the 1960s (Brett 1964; Barnett and Hardy 1967) and later developed specifically for cobble areas in the United States as a means of identifying benthic recruitment habitats and nursery grounds of juvenile American lobsters Homarus americanus (Incze and Wahle 1991; Wahle and Steneck 1991). The advantages of suction dredging over manual sampling are that divers are not constrained by low visibility and replicate samples are easily achieved. Furthermore, tests indicate that sampling efficiency ranges from 70 -80% (R.Wahle, pers. comm.). As a result, suction samplers have had considerable success quantifying recruitment densities of juvenile lobsters and other benthic crustaceans within cobble substrata (Wahle and Incze 1997; Palma et al. 1998; Robinson and Tully 2000a,b). The aim of this study was to obtain both qualitative and quantitative data on the fauna of an Irish subtidal cobble site. This was achieved by standardised airlift suction sampling over a five-day period at four sites in Galway Bay. MATERIALS AND METHODS Field sampling was carried out on the west coast of Ireland in September 1997 at an area known as Aughinish Shoal in inner Galway Bay. The experimental ground was discovered through a series of reconnaissance dives from the MRV Connemara. Visual observations by two divers confirmed that an apparently homogenous cobble habitat existed. The cobble area was situated along the base of a reefwhere thewater depth ranged from 9m to 14m. The underlying substratum consisted of sand and bedrock. Benthic sampling of cobble habitat was conducted using a diver-operated airlift suction sampler (Incze andWahle 1991; Wahle and Steneck 1991). The airlift consisted of a 170cm length of 7cm diameter black PVC tube, with a scuba cylinder supplying air, fitted 10cm above the mouth of the tube. The other end of the tube was affixed to a 1mm mesh nylon bag that could be removed, closed and replaced underwater. The suction sampler was operated by two divers and was used in conjunction with a 0.5m2 quadrat constructed from buoyant PVC piping, a30cm high sheet of lnmmmesh curtain and a heavy chain base. This design prevented the escape ofmobile decapods from the quadrat. The cobble area was sampled at four sites; ten quadrats were sampled in each of two sites, while nine quadrats were sampled in the remaining two, giving a total sample area of 19m2.On sampling, the quadrat was placed randomly on a suitable cobble patch. One diver then removed the cobble stones by hand, while the second diver operated the airlift, ensuring that themouth of the tube reached into the interstitial spaces of the substratum. The divers worked slowly until all of the interstitial spaceswere exposed and sampled. Periodically, sample bagswere returned to the surface where they were analysed immediately or preserved for future examination. Prior to identification, samples were preserved by freezing or storage in 70% alcohol. All animals were identified at least to class level. RESULTS In total, 74 families of animals were sampled across the four sites (Table 1). The molluscan gastropods were the numerically dominant group (Fig. 1)with A. Linnane (corresponding author; email: adrian.linnane@ nuigalway.ie); B. Ball, B. Munday and J.P. Mercer, Martin Ryan MarineScience Institute, National University of Ireland, Galway R. Browne, National University of Ireland, Galway, Martin Ryan Institute Laboratories, Carna, Co Galway. Received 30 April2002. Read7October2002. Published 31 July 2003. BIOLOGY...