Comparison of two methods of sub-sea sampling of the Guernsey ormer,Haliotis tuberculata L.

Abstract The ormer, Haliotis tuberculata L., is the only abalone species found on the coasts of western Europe, its northern most limit of distribution being the Channel Islands. Previous Guernsey ormer population studies were based upon a form of catch-per-unit-effort sampling where the unit of effort was one diver, searching the substrate for 5 min continuously. An ormer population survey was carried out in the waters around Guernsey, using this former method concurrent with an alternative system of sampling by area using 10 m² quadrats at randomly selected locations. The published results of a series of earlier surveys include a measure of ormer density m^-2 which, along with knowledge of the extent of suitable substrate, allows tentatively for absolute abundance to be estimated. This study demonstrates that an estimate obtained in this way may be unreliable. Statistical comparison of the population size structures derived from the two different sampling methods also showed considerable inconsistency. The implications of this for management of the Guernsey ormer stock are considered.