Spatiotemporal dynamics in the acoustic backscatter of plankton and lesser sandeel (Ammodytes marinus) in the North Sea measured using a Saildrone

Abstract

With accelerating global warming and human activities, the North Sea is one of the marine ecosystems undergoing rapid change. The need for spatially-temporally extendable survey platforms for assisting well-established vessel-based surveys are increasing. In this thesis short term variation in spatial structure of plankton and lesser sandeel (Ammodytes marinus) were investigated in the North Sea by using unmanned surface vehicle (USVs) Saildrones equipped with dual frequency (38, 200 kHz) echosounder. The data was collected in two areas, a part of the standard Aberdeen-Hanstholm transect and English Klondyke, an important sandeel fishing ground. These areas were repeatedly covered by two Saildrones in May-June 2019. Repeated surveys witnessed high plankton density in the western part of the Aberdeen-Hanstholm transect constantly during the survey period. Salinity seemed to be one possible factor explaining the heterogeneity of plankton density in both vertical and horizontal structure. Sandeel appeared diurnally at various depths from 2 m to near the sea bottom. There was only a weak tendency that the schools were distributed deeper around midday. However, their diverse vertical distribution indicated underlying drivers of their behaviour other than light. Despite the existing uncertainty of species identification due to lack of ground-truthing and limited frequency availability, this saildrone survey conveyed little but purposeful information of the dynamics in spatial utilization of plankton and sandeel over a short period of time.