Abstract/Summary

This thesis provides new understanding on the trophic role of Small Pelagic Fish (SPF) within the Celtic Sea ecosystem. SPF facilitate energy transfer, and changes in their community assemblages could impact food web dynamics. Anchovy, sardine, herring, mackerel, horse mackerel, and sprat spend some or all of their life within the Celtic Sea ecosystem and are of commercial and ecological importance. SPF are predominantly planktivorous, transferring energy from lower (e.g., plankton) to higher trophic levels (e.g., seabirds). The Celtic Sea is relatively understudied despite having undergone environmental changes within the last decades (e.g., recent increase in anchovy, and a decline in herring). The trophic interactions of SPF with planktonic communities and the surrounding environment of the Celtic Sea were investigated through i) diet analysis of SPF (stomach sampling campaign); ii) validation, and analysis of temporal/spatial variability of satellite retrieved phytoplankton chlorophyll-a; and iii) ecosystem modelling (Ecopath with Ecosim, EwE) to investigate the effect of SPF dynamics on the wider Celtic Sea food web. Stomach analyses indicated seasonal variation in SPF diet, although, calanoid copepods were consumed year-round. As these species are predominantly planktivorous, and chlorophyll-a is a proxy for phytoplankton biomass (a key input into ecosystem models), validation of satellite retrieved chlorophyll-a was undertaken for three algorithms, Sentinel-3A OLCI-OC4ME, Sentinel-3A OLCI-NN, and MODIS-Aqua-OCI+OC3, for the region. Satellite outputs were then considered for use in the Celtic Sea ecosystem model (EwE). Diet of SPF from this study and updated biomass for 2013- 2021 were used as an input. The EwE model of the Celtic Sea demonstrated potential to address management related questions regarding SPF. Through the improvement and evaluation of ecosystem model inputs (diet studies and satellite validation), and implementation into the current EwE model, this research has highlighted SPF are key in pelagic energy transfer in the Celtic Sea.