Can patch size and patch distance improve the recolonization of mussel-seed beds exploited for aquaculture?

Industrial-scale exploitation of mussel seed from the rocky intertidal of north-western Iberia is known to be detrimental for many organisms that live on or among the competitively dominant Mediterranean mussel (Mytilus galloprovincialis). Searching for practices that may ameliorate the damage caused by mussel-seed harvesting, the influence of two properties of the exploitation (patch size and patch distance) on the recolonization of the intertidal assemblage was assessed using a split-plot design, replicating the techniques used by professional collectors. Patch recolonization was monitored by monthly records of the cover of sessile and sedentary organisms within and outside clearings throughout the official closed season (7 months). Patch distance and patch size had a moderate influence on recolonization. Regardless of size and/or distance, richness and diversity stayed below control values until the end of the study, whereas total coverage was significantly smaller in the smallest patches than in control plots, but it recovered to control values in medium and large ones. Patch separation had a mostly non-significant effect on assemblage attributes, except for a slightly greater richness and diversity in patches cleared close together than when they were apart. Larger, close patches were more readily recolonized than smaller, isolated ones, possibly because the neighbouring herbivores had a negative impact during the early recolonization of the open space during the season when the shore is closed to commercial seed mussel harvesting. Nonetheless, the recolonizing assemblage consistently displayed a distinctive species composition until the end of the study, in part because clearings favoured the arrival of early colonizers, such as Ulva spp., Chthamalus sp., or Porphyra umbilicalis. The consistently distinctive species composition of the recolonizing assemblage suggests that time-a too short closed season-rather than patch size and/or distance is the major obstacle to a more complete recovery of mussel-seed exploited beds.