Long-term dynamics, population structure and connectivity of the helmet jellyfish <i>Periphylla periphylla</i> in a Norwegian fjord and adjacent waters

Mass occurrences of <i>Periphylla periphylla</i> in Norwegian fjords cause major concerns related to potential regime shifts that could affect ecosystem stability. 15 years of trawl data (2006-2015), complemented with comprehensive sampling in different areas and seasons (2018-2021) allowed new insights on the dynamics, structure and connectivity of <i>P. periphylla</i> populations within and beyond Trondheimsfjorden. Despite assumed population bursts, no clear trend on <i>P. periphylla</i> population size in Trondheimsfjorden were identified. Sampling frequency and population size suggest a local reproduction of <i>P. periphylla</i>, especially in the inner part of the fjord where young-of-the-year (YOY) individuals occur. Size variations occurred in relation to sampling month, thus pointing at seasonal patterns in growth and reproduction. No distinct population structure of <i>P. periphylla</i> populations within Trondheimsfjorden and over larger spatial scales (> 100 km) along the Norwegian coast was observed. Such poor geographic population structure provides evidence for a strong dispersal of <i>P. periphylla,</i> potentially triggered by frequent deep-water renewals of the fjords' basins that enable a high gene flow. Data on <i>P. periphylla</i> long-term dynamics, population structure and connectivity provide valuable information for ecosystem state assessments and enable the advancement of ecosystem management approaches, thus accounting for both stakeholder and ecosystem demands.