Human harvesting of Mytilus galloprovincialis Lamarck, 1819, on the central coast of Portugal

The Mediterranean mussel, Mytilus galloprovincialis Lamarck, 1819, has traditionally been removed from the shore by humans in coastal areas to supplement diet, for commerce or for bait. On exposed rocky shores of the central coast of Portugal, humans are an important intertidal predator, especially of mussels and of the pedunculate barnacle Pollicipes pollicipes (Gmelin, 1789). Four rocky shore sites near Cabo Raso with different levels of accessibility were compared in terms of harvesting pressure, substratum cover, density and size structure of M. galloprovincialis. Sampling surveys were conducted from March to September 2002. For substratum cover determinations a 50 cm x 50 cm square was used, while density and size structure were estimated based on scrapings performed on a 10 cm x 10 cm area. A log-linear model was used to evaluate human harvesting according to several factors. Significant differences in the harvesting intensity were related to accessibility, type of day (weekday, weekend or holiday), period of the day, weather and tidal amplitude. The differences relative to zone were not significant. The least accessible site (0.30 person day-1) showed the highest values of substratum cover, and a negative correlation was found between number of harvesters and substratum cover by mussels. Size distribution also varied considerably according to site. The major difference that was noticed was that less accessible sites showed a higher number of large individuals than more accessible areas. Density in the most accessible location showed a clear decline, above all after summer holidays. Over the study period, in the more accessible sites biomass decreased while in the other locations it increased. Long-term studies are necessary to determine the real magnitude and effects of human disturbances in intertidal communities.