Analysis of distribution and migration of mesopelagic organisms based on 38 Khz backscatter around Cabo Verde

Abstract

The Cabo Verde archipelago is located in the Eastern Tropical North Atlantic between oligotrophic open ocean waters and the productive coastal upwelling of the Canary Current System. The Cape Verde region features were relatively low primary productivity although models showed the highest biomass of mesopelagic micronekton in the tropical and subtropical North Atlantic. of mesopelagic micronekton in the tropical and subtropical North Atlantic. However, these occurrences are extremely patchy and related to eddies, filaments and island- or seamount-induced upwelling. How this impacts the distribution of higher trophic levels is currently not well understood. In this study, distribution and diel vertical migration of mesopelagic organisms were analysed in regard to features that potentially enhance their biomass using two saildrones equipped with an ADCP and a Simrad EK80 echosounder operated with a dual-frequency combi-transducers (38 Khz and 200 Khz), respectively, during the ALT2MED mission in October - December 2019. The general vertical migration pattern was confirmed and was related to surface irradiation Organisms move from shallow waters to deep waters during the day migrate into shallower water in the nighttime. The vertical distribution and migration showed that the daytime depth is dynamic. Mesopelagic organisms change and adapt their position during the daytime as a response to the intensity of light they could detect. Acoustic backscatter at 38kHz was enhanced both in the eddy and close to the island of Sal, but not near Senghor Seamount. The eddy impacted the distribution and the abundance of mesopelagic organisms around Cabo Verde by creating conditions that increased productivity, which was also visible in chlorophyll-a. In the eddy, the nighttime distribution was also confined to a shallower layer compared to outside of the eddy. This study could be of benefit to other islands and areas that have similar environmental conditions. It could be used also for socio economic purposes through developing fisheries activity