In situ growth dynamics of the invasive ascidian Didemnum vexillum in Norway: Insights from a two-year monitoring study.

The spread of the invasive ascidian Didemnum vexillum represents a major ecological and economic threat in many parts of the world. Recorded for the first time in Norway in 2020, this species is now established in several localities, often characterized by busy port activities. The new detection and expansion of D. vexillum raise significant concerns in Norway, especially due to the potential extension of its distribution range into higher latitudes. This study presents the results of an in situ monitoring of the growth of eleven D. vexillum colonies in Stavanger between December 2020 and September 2022. Colony growth was calculated from underwater photographs taken approximately once a month. Overall, our findings indicate significant growth of most colonies after 624 days, increasing in size from 78 cm to 1008 cm. Seasonal variations in seawater temperature are a major factor affecting the size and growth of the colonies. Most of the growth occurred during the summer months, when seawater temperature was between 13 °C and 18 °C. A rapid decline in colony growth rates was observed in November, as temperatures dropped below 13 °C, followed by colony regression below ~8 °C. These results therefore confirm the general understanding of D. vexillum as rapidly spreading species, characterized by a high colony growth rate. Its ongoing spread toward northern Norway is likely to have negative impacts on key coastal ecosystems (e.g., kelp forests, rhodolith beds, seagrass meadows), as well as on several industries critical to Norway's economy (e.g., oil industry, aquaculture, fisheries). Based on the information provided here, it is essential to continue implementing measures to prevent further spread to the north and to develop localized mitigation strategies.