Multifrequency study of the epipelagic food web in Alboran Sea

Abstract

In order to characterize the epipelagic ecosystem from a trophic point of view, a multidisciplinary acoustic survey was carried out in July 2013 in Alborán Sea. A calibrated EK60 scientific echosounder (Simrad) equipped with 5 frequencies (18, 38, 70, 120 and 200 kHz) was used. Fish echotraces were groundtruthed by mean of a pelagic trawl (20mm cod end; 20m vertical opening). The main daytime epipelagic acoustic plankton layer was sampled using plankton nets with 4 different mesh sizes (250, 333, 500 and 2000 µm); the great progress was monitoring in real time the plankton net track. Finally fluorescence was recorded using a CTD (Seabird 19+). Our results show that primary producer (fluorescence data) can´t be acoustically clearly detected; small primary consumers (Copepoda, Cladocera) are detected at 70 kHz frequency near coast (30-45m deep). Going offshore, frequency response change due to the presence of secondary zooplankton consumers like Siphonophora, Chaetognatha, fish larvae and Apendicularia, which are the main targets detected at 18 and 38 kHz frequencies. Tertiary consumers (fish) are detected properly at 18 and 38 kHz.