Trends in Lusitanian and boreal marine fish assemblages in the Celtic Seas ecoregion in the context of fisheries management and environmental change

Ocean warming is expected to drive shifts in fish species distributions, with Lusitanian (warm water) species expanding northward and Boreal (cold water) species retreating in the Northeast Atlantic. This study analysed data from International Bottom Trawl surveys to investigate community-level trends in marine fish populations in the Celtic Seas Ecoregion around Ireland. Using a simple community analysis approach, we investigated if previously reported shifts in the distribution of Lusitanian and Boreal species persisted after a period of temporary cooling and the introduction of fisheries management measures in the region. We then re-estimated trends using a random walk model to account for spatial variability and used dynamic factor analysis to identify common trends across species. No clear community-wide shifts in distributions of Lusitanian and Boreal species were detected, instead, abundance changes were localised and species specific. Significant increases in Lusitanian species were observed in the southeast (ICES area 7g) where both sea surface and near-bottom temperatures have shown some evidence of warming. Several commercially exploited species including European anchovy (Engraulis encrasicolus), hake (Merluccius merluccius), and elasmobranchs such as thornback ray (Raja clavata) showed strong increasing trends. Post-2007 cooling in water temperatures followed by renewed warming, together with management measures may have moderated large-scale biogeographic shifts while facilitating recovery of certain stocks. Overall, changes in fish community composition around Ireland appear to reflect complex interactions between environmental variability and fisheries management.