High resolution vertical distribution of the copepod Calanus chilensis in relation to the shallow oxygen minimum zone off northern Peru using LOKI, a new plankton imaging system

•The vertical distribution of Calanus chilensis was measured off northern Peru.•An optical zooplankton sensor allowed high resolution measurements.•The species inhabits Cold Coastal Water within a narrow band close to the coast.•Copepodids V and both adult stages preferred hypoxic water layers.•Tolerance to hypoxia makes a large part of the population inaccessible to fish predators.

The vertical distribution of copepodite stage V and adult Calanus chilensis was studied on two transects across the Humboldt Current System off northern Peru using the LOKI system. LOKI is an optical plankton recorder, which simultaneously collects images of zooplankton and environmental data such as temperature, salinity, oxygen, and fluorescence. Image quality allowed determination of CV, females and males and identification of C. chilensis from 3 co-occurring Calanid copepods. C. chilensis was inhabiting the upper 250 m. Highest abundances with a maximum of ca. 44.000 Ind. m−2 were observed in a narrow band within Cold Coastal Water at stations closest to the coast, coinciding with the Poleward Undercurrent. This raises questions for the life cycle closure within the Humboldt Current system. In contrast to observations in the southern part of the Humboldt Current System, the three stages studied were most abundant in hypoxic waters at oxygen concentrations between 5 and 50 µM. Thus C. chilensis seems to be the only species of the family Calanidae where not only a resting stage can tolerate hypoxia, but also both adult stages. This impacts availability to predators, as despite a locally high biomass only part of the population is available to anchovy and other important fish species which are restricted to waters with higher oxygen concentrations.