Distribution patterns, abundance, and population dynamics of the euphausiids Nyctiphanes simplex and Euphasia eximia off the west coast of Baja California, México

Abstract

Distribution patterns and population structure of the euphausiids Nyctiphanes simplex and Euphausia eximia were examined In the Callfornia Current off the west coast of Baja California, Mexico with emphasis on the southern part near Bahia Magdalena, in relation to temperature zooplancton biomass, distance from the coast as a functlon of depth, and interannual variability. The data were collected in 6 surveys from May 1986 to October 1987. High densities of N. simplex were found in the shallow coastal waters (<300 m) For E. eximia the maximum abundance occurred offshore at stations with depths of 200 to 3000 m. These distribution patterns (offshore-inshore) remained relatively constant throughout the surveys, but the latitudinal distributions showed important variations in relation to the current patterns of the region and seasonal differences in the upwelling events, particularly near Bahia Magdalena (24 to 25° N). Both species displayed continuous breeding, with 25 to 77% of the adult female N simplex in different fertilized stages. A slightly lower proportion of E. eximia females were reproductively active (37 to 48%). The breeding areas, indicated by early stages (calyptopes) and adults in the reproductive phase, showed that N. simplex is dependent on the upwelling system along Baja Callforia. High densities are maintained during spring and summer, then decrease during autumn as the physical environment changes. The population dynamics of E. eximia indicated breeding areas offshore with a continuous recruitment. Breeding areas were found in regions of relatively low biological production. Maximum abundance occurred during autumn.