Immigration and emigration of a pelagic fish assemblage to seamounts in the Gulf of California related to water mass movements using satellite imagery

The appearance of an assemblage of 11 planktivorous and predatory fishes at El Bajo Gorda seamount within the Gulf of California in May 1981 coincided with an increase in surface water temperature and chlorophyll concentration. The assemblage may have either concentrated at the seamount from the surrounding pelagic environment or immigrated from south of the Gulf of California. At this time the fall to early spring current flow southeastward toward the Gulf of Tehuantepec reverses to the spring to summer flow northeastward along the mainland into the Gulf to California. One assemblage member, the scalloped hammerhead shark Sphyrna lewini, was monitored as an indicator of assemblage movements in response to water mass changes. The occurrences of 18 individuals tagged with individually recognizable transmitters were recorded over a 10 d period in July and August 1981 by 2 microprocessor monitors deployed at El Bajo Espiritu Santo seamount. Shark returns diminished as a mass of upwelled cold water, originating in the Bay of La Paz, moved over that seamount. The return rate then increased on a single day coincident with the incursion of a warm water mass before the rate decreased as this water mass cooled.