Natural and human disturbance in a rookery of the California sea lion (Zalophus californianus californianus) in the Gulf of California, Mexico

Los Islotes is the southernmost breeding site of the California sea lion (Zalophus californianus californianus) in the Northern Hemisphere and represents one of the principal tourist attractions for the city of La Paz, Mexico. The tourism has been growing without control and could be the cause of perturbation in the reproductive and haul-out patterns of the sea lion. The aim of the present study was to determine the effect of tourism activities on the behavior of sea lions, in order to help design regulations to assure better use and conservation of the site. The nature and intensity of tourism activity and sea lion behavior were determined based upon monthly sampling over a one-year period (May 2000 - April 2001). Tourist activity was higher during autumn and winter, with visits by 'panga' boats being the most numerous. A total of 112 disturbances were recorded, the majority of which being non-anthropogenic causes. A quarter of disturbances were triggered by human activity within 20m of the rookery and with harassment reaction (animals going into the water) in 32% of the anthropogenic disturbances. Principal component analysis (PCA) described the conditions under which disturbance was generated (74% of the variation explained). Using three factors (47% of the total variation) the major contributing variables were month, tidal level, relative humidity, Beaufort number, total number of boats and number of powerboats and sailboats. In Los Islotes, the perturbation occurs mostly in autumn and winter coinciding with the highest frequency of tourism, large number of adult and subadult males and unfavorable environmental conditions for haul-out, such as high tide level and strong winds. The lowest number of disturbances occurred in summer, during the breeding season of the sea lion. Most of these, however, were caused by human activity