The principal method of fishing for king scallops, Pecten maximus, in Orkney waters is hand collection by divers. Divers typically target areas at a relatively small spatial scale (tens of m 2) and catchability of scallops at this scale is considered to be very high. Divers nevertheless observe that exploited areas are re-populated by fishable scallops over very short time periods, often over periods of weeks, meaning that sites can repeatedly be targeted within the same fishing season. Given high visibility of individual scallops coupled with a high efficiency of removal by divers, this re-population seems most likely to be accounted for by immigration. This inference is, however, difficult to reconcile with the usual assumption that the mobility of scallops is very limited. Moreover, suitable sources of immigration are sometimes separated from fished sites by areas of apparently unsuitable habitat. The extent to which scallops are mobile between areas and can re-colonise areas of local depletion could be a crucial mediating factor controlling the availability of scallop stocks to exploitation and potentially of great importance in defining a sustainable fishery for scallops in Orkney waters.
The objective of the project was to determine spatial turnover rates of scallops at two spatial scales: (i) at the scale of individual fishing patches (tens to hundreds of metres), determining immigration rates following fishing; and (ii) at the local fishery scale (1-10 km and greater), determining potential sources of immigration to fishing patches from more distant areas. This report addresses the first of these spatial scales, using depletion fishing experiments coupled with tagging to estimate size-specific population size and site-fidelity to small experimental areas.