| Summary: | Rocky intertidal habitats are diverse and productive ecosystems which deliver various ecosystem services including nursery grounds for a plethora of species, some of which are commercially important. In Iceland, the rocky intertidal zone of sheltered areas such as Breiðafjörður is usually dominated by virtually monospecific stands of canopy-forming Ascophyllum nodosum (L.) Le Jolis, providing a habitat for other organisms under or amongst the canopy. The community of mobile predators in this area was characterized, with a special focus on fish and crabs. The extent of the effects of Ascophyllum harvesting on this community was also examined. Additionally, the distributions and hence expansion polewards of the native Carcinus maenas (Linnaeus, 1758) and of the non-native, invasive Cancer irroratus Say, 1817 within Breiðafjörður were tracked, with any effects on the population of the previously sole native brachyuran crab of the Icelandic Westfjords, the spider crab Hyas araneus (Linnaeus, 1758) investigated. The effects of harvesting in Breiðafjörður were not significant, except on young Gadus morhua Linnaeus, 1758, which were significantly bigger in the control areas. Nevertheless, this has to be interpreted carefully as total n caught was only 29. The occurrence and hence the potential impacts of C. irroratus is so far negligible in the intertidal zone, yet C. maenas was shown to displace H. araneus at one of our sampling sites, especially in the mid-shore. While harvesting did not have major detectable effects on the fish and crabs tested, a precautionary approach is recommended as the area is used as a nursery by commercially important gadoids.
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