Interannual variability in krill off Baja California in the period 1997–2005

We document interannual changes in the species composition of euphausiids inhabiting the waters off Baja California during the period of 1997–2005. Thirty-two euphausiid species were identified from seventeen cruises, with the dominant species being Euphausia eximia, Euphausia gibboides, Nyctiphanes simplex, Thysanoessa gregaria, Nematoscelis difficilis, and Stylocheiron affine (present in 74–92% of the samples). Species composition had the strongest difference between coastal (Vizcaino Bay and the Ulloa Gulf) and oceanic waters. The coastal shelf was dominated almost exclusively by N. simplex while the oceanic regions showed a high diversity. In oceanic waters the most abundant species was N. difficilis off both the northern (32–30°N) and the central (30–24°N) Baja California peninsula. The highest krill biomass, however, was recorded in the central region in January 1998 during an El Niño and consisted principally of abundant larvae of E. eximia. La Niña was characterized by an increase of the ‘transition zone’ species (E. gibboides, N. difficilis, T. gregaria) and the subarctic Euphausia pacifica. This last even entered in Vizcaino Bay but was not apparent in the central region compared to the north. High abundance of E. pacifica was also observed in July 2002 and was linked to subarctic water intrusion and July 2005. Some swarms of Thysanoessa spinifera were observed during April and July of 2005. The presence of subarctic species E. pacifica and T. spinifera in Baja California waters contrasted with their strong decrease in northern sectors of the California Current System during 2005. This biological information, joined with high upwelling activity (compared to delayed upwelling off Oregon and northern California), reinforces the importance of regional studies in eastern boundary currents.

► During 1997–1998 El Niño Euphausia eximia was the best indicator off Baja California.
► Nyctiphanes simplex dominated the coastal shelf through the ENSO cycle.
► Abundance of Euphausia pacifica and Thysanoessa spinifera increased in July 2002 and 2005.