Shoaling fish and red crab behaviour related to tidal variations in Bahيa Magdalena, México

Hydroacoustics was used to study the influence of tides in the main pelagic fauna composition in Bahía Magdalena in the south of Baja California, Mexico (24°50′N, 112°50′W). The target species were the Pacific sardine (Sardinops caeruleus) and the red crab (Pleuroncodes planipes). Measurements of fluorescence, temperature, and salinity were made along a transect 18 km long covering three zones inside the bay, around the mouth of the bay (the main entrance), and the continental shelf. The mouth of the bay was the region of highest fluorescence and lowest temperature, this latter caused by upwelling pulses. Results show two locations where most of the fish were observed, inside the bay and at the main entrance. The preferred locations for the red crab were outside the bay and also the main entrance. The fish and the red crab aggregations were observed in the area of the main entrance particularly during low tide at night. Beside fish density distribution, there were two distinct fish shoaling behaviors along the transect: inside the bay, fish where located in large and compact schools, and in the region of the main entrance where they were scattered in large and small shoals. Results are discussed in terms of the adaptative behaviour of fish shoaling, food availability and seasonal abundance of small pelagics in Bahía Magdalena influenced by tides.