Resolving the variability in habitat use by juvenile small pelagic fish in a major tidal system by continuous echosounder measurements

Shallow coastal areas are important nurseries for larvae and juveniles of many fish species. However, empirical data on small pelagic fish (SPF) in such regions are lacking, and temporal variability in abundance and habitat use by SPF are unknown. Given the critical role of SPF as a trophic link, their commercial value and their sensitivity to climate variability, there is a need for a quantitative and high-resolution monitoring method. We deployed a bottom-mounted echosounder, combined with a water current profiler, to examine the temporal variation in density, vertical distribution and movement behaviour of SPF in the Marsdiep, a dynamic tidal inlet between the North Sea and the Wadden Sea. The acoustic setup provided year-round records of fish density distribution in the water column every 90 min. Monthly fish samples were collected to help interpret acoustic observations. Our analyses indicated that the Marsdiep is a migration gateway for SPF from the North Sea into the Wadden Sea, particularly for juvenile herring and sprat. We observed clear seasonality with high fish densities from mid-spring to early autumn. Fish typically used the upper half of the water column. Swimming behaviour was primarily driven by currents. Yet, during outgoing tide, SPF resisted the outflowing current, suggesting efforts to remain in the Wadden Sea, supporting the hypothesis that the Wadden Sea serves as a preferred nursery area for SPF. Our high frequency, multi-seasonal and vertically resolved observations provide unique insight into the use of the Marsdiep by SPF. Furthermore, potential applications of autonomous echosounder moorings are discussed