Hierarchical analysis of spatial distribution patterns of patellid limpets in the Canary Islands

We studied the distribution, abundance and size structure of three limpet species (Patella rustica, Patella candei crenata and Patella aspera) throughout the rocky interridal zone of the Canary Archipelago. We used a stratified and hierarchical sampling strategy that involved the study of three intertidal zones (high, intermediate and low intertidal) across eight islands with three random locations per island and three randomly selected sites within locations. We did not observe any specimens of P. candei candei. For the 2 entire archipelago, the least abundant species was P. candei crenata (0.02 +/- 0.05 individuals/0.25 m(2), average +/- SD), the species with significantly greatest sizes (26.8 +/- 11.08 mm, average +/- SD) and of highest commercial interest. Patella aspera (0.08 +/- 0.37 individuals/0.25 m(2)) and P. rustica (0.069 +/- 0.16 individuals/0.25 m(2)) showed higher abundance, but smaller sizes (21.0 +/- 9.21 and 18.1 +/- 7.94 mm, respectively). The three zones influence how these species are distributed and these distribution patterns are homogenous among islands. Patella rustica was observed to be more abundant in the intermediate zone and P. aspera in the lower zone, while P. candei crenata showed similar abundance in the low and intermediate zones. This vertical variability is masked by a horizontal variability on small and intermediate scales (10 s to 1000 s of meters), but not among islands (10 s to 100 s of km). This fact can be attributed to the low abundance of the limpet species in the locations studied, as a result of overexploitation of this resource in the Canary Islands. Consequently, there is a need for management and control measures.