Unusual Mortality of Krill (Crustacea: Euphausiacea) in Bahı´a de La Paz, Gulf of California.

Abstract

Surface aggregations and beach strandings of a species of krill, Nematoscelis difficilis Hansen, were observed in June 2003 at locations along the shore of Bahı´a de La Paz in the Gulf of California. For 10 days before the krill die-off, a steady wind blew from the south at speeds between 4 and 5 m/sec. For that period, satellite images showed water temperatures between 18 and 22 _C along this coast, which is low compared with typical seasonal water temperatures of 26 to 28 _C for June. Phytoplankton biomass, determined by pigment concentration and cell counts, was the highest in the area in June. The diatom Chaetoceros debilis represented more than 96% of the phytoplankton community. Nutrients were in relatively higher concentrations. These data suggest that upwelling conditions occurred and the diatom bloom was in its final phase. Based on this limited data set, we present a hypothetical scenario describing the sea-surface aggregations and beach strandings of N. difficilis.