Quantifying the fish contribution to on-shelf carbon sink: a case study of the Irish Sea

The marine biological carbon pump plays a crucial role in the sequestration of atmospheric carbon and, therefore, is paramount to global climate regulation. Most existing research on biological carbon sinks has focused on understanding the role of oceanic (off-shelf) species and processes where the carbon can quickly be removed from contact with the atmosphere.  We know little about how species living on continental shelves contribute to and influence carbon sequestration due to complex biological and physical transport process dynamics. This knowledge gap is becoming an issue as decision-makers seek to consider the impacts of anthropogenic pressures (e.g., fishing) on the flow and storage of carbon across shelf ecosystems. Most commercial fishing takes place in shelf seas. Fishing could impact carbon sinks and sequestration by altering population biomasses, ecosystem dynamics, and trophic interactions. Here, we explore the potential contribution of a selected fish population in the Irish Sea to carbon sinks and the impacts of fishing on that. The Irish Sea ecosystem, situated between Ireland and the UK, is a shelf ecosystem encompassing important commercial species populations, including herring, the species of interest in this study. An Ecopath with Ecosim (EwE) model of the Irish Sea has been developed to reconstruct the region's food web. We combine the Irish Sea EwE ecosystem model outputs of biomass with faecal egestion and attenuation rates under alternate fishing scenarios to provide a novel quantitative assessment of the annual flux of carbon that sinks to the continental shelf seafloor to then, in part, becomes buried in the sediment. Under the baseline fishing scenario, the Irish Sea Atlantic herring faecal pellet flux contribution was 84% higher than the carcass flux deposited on the seafloor. This carbon flux substantially increased for the non-fishing scenario with bigger changes in carcass carbon flux. Pelagic fish contribute significantly to the carbon flux, particularly with faecal pellets. The overall carbon deposition from the fish community changed little between scenarios with and without fishing on Atlantic herring due to food-web balancing. Our results provide an early insight into the relationship between commercial species, fishing, and biological carbon sink for shelf ecosystems.