Connectivity of the Helmet jellyfish Periphylla periphylla i. Long-term dynamics, population structure and connectivity of the Helmet jellyfish Periphylla periphylla in a Norwegian fjord and adjacent waters

Mass occurrences of the Helmet jellyfish Periphylla periphylla in Norwegian fjords have caused major concerns due to their role as fish competitor and the consequences they have to regime shifts possibly affecting ecosystem stability and health. 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 P. periphylla populations within and beyond Trondheimsfjorden. Despite assumed population bursts, no evidence for substantial increases or decreases of P. periphylla population size in Trondheimsfjorden were identified. Sampling frequency and population size distribution point at a local reproduction of this coronate jellyfish species, especially in the inner part of the fjord where the smallest, young-of-the year (YOY) individuals occurred. Size variation of specimens occurred in relation to sampling month thus providing indication for differences in reproduction and growth patterns in relation to season. No distinct population structure of P. periphylla 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 P. periphylla, potentially triggered by frequent deep-water renewals of the fjords’ basins that enable a high gene flow. These new insights on P. periphylla 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.