Annual life cycle of the copepod Eucalanus inermis at a coastal upwelling site off Mejillones (23�S), northern Chile

Abstract

Eucalanus inermis is an abundant species in the eastern tropical and subtropical South Pacific, including the oceanic and coastal waters off Chile and Peru. Its annual life cycle was studied through a time-series sampling (weekly intervals) during 2002, at a fixed coastal station at an upwelling site (Mejillones Bay, 23°S) off northern Chile. The more-or-less continuous occurrence and abundance of naupliar and copepodid stages indicated that the species reproduces during most of the year, with two peaks: one during the austral spring and the other during the summer. Thereafter, an abrupt decline in the population was observed during winter. The abundances of E. inermis copepodids and nauplii were positively correlated with sea surface temperature, suggesting temperature-dependent development and growth during the spring–summer period. Three cohorts could be distinguished during one annual cycle, with generation times >30 days. The estimate of mean weight-specific daily growth (0.12 day−1) is lower than that of other species in the area, but it is consistent with a slower development rate. The distribution of this copepod is associated with the equatorial subsurface waters characterized by low-oxygen content ( 200 m depth), probably in a lethargic mode, within the oxygen minimum zone in the adjacent oceanic area.