Abstract
Several methods of assessment have been used to document variation in grazing pressure on temperate rocky shores, although often these methods are applied without consideration of local conditions or species. In this study, a comparison was made between abundance counts of inactive molluscan grazers at low tide, direct observations of grazer activity and distribution throughout day and night tidal cycles, and records of grazing marks on wax discs, for the mid-shore of Hong Kong. The abundance of grazers found during low-tide counts varied among dates, sites and species. This method, however, did not record all grazer species that day/night observations showed to migrate from the low shore with the rising tide. Low-tide counts, therefore, underestimate grazing pressure (number of active grazers per unit area) and grazer guild (number of species). Grazing marks on wax discs also recorded a greater number of species than the low-tide counts of inactive grazers, and included grazers that were seen to migrate up shore during day/night observations. Certain limpet species, however, avoided the wax and did not leave grazing marks, showing this method to also underestimate grazing pressure. All methods showed grazing pressure to be variable at spatial scales of tens of metres or less and also temporally variable between sampling dates. The sole use of either low-tide counts or wax discs is likely to underestimate grazing pressure, due to variation in shore topography and grazer foraging behaviour, especially on shores with a narrow tidal range such as in Hong Kong. To gain a more accurate assessment of total grazing pressure, it is suggested that recording of grazing marks on wax discs should be used in conjunction with direct day/night observations.
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Acknowledgements
The authors wish to thank R. Thompson for a draft of his paper and for showing N.H. his set-up at the Dove Marine Laboratory and to B. Darvell for providing us with various dental waxes. G.R. Blackmore and Z.Y. Li assisted during 26-h watches. Many thanks to S.J. Hawkins and the H.K.U. rocky shore ecology group for critical comments on an early form of the text and to an anonymous reviewer, whose suggestions helped to improve the manuscript. This research was carried out in partial fulfilment of the requirements for a Ph.D. submitted by N.H., who was supported by a part-time demonstratorship at The University of Hong Kong. Permission to work in the Cape d'Aguilar Marine Reserve was granted by the Agriculture, Fisheries and Conservation Department, The Government of Hong Kong SAR.
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Communicated by T. Ikeda, Hakodate
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Hutchinson, N., Williams, G.A. An assessment of variation in molluscan grazing pressure on Hong Kong rocky shores. Marine Biology 142, 495–507 (2003). https://doi.org/10.1007/s00227-002-0985-4
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DOI: https://doi.org/10.1007/s00227-002-0985-4