Dispersal of invasive species larvae within the Orust-Tjörn fjord system

The Orust-Tjörn fjord system is located on the west coast of Sweden and consists of several fjords connected by shallow and narrow straits. It is home to nature reserves, harbors, leisure areas, and aquaculture farms, and biodiversity is threatened by invasive species, for example brought in through shipping. Therefore, it is important to understand how larvae of invasive species are dispersed by the currents within the fjords system in order to find efficient methods for management of existing and future harmful invasive species. 

A connectivity study is performed in order to identify dispersion patterns, assuming that larvae are passively transported by surface currents. For the years 2016 and 2022, the dispersion of larvae is simulated using the open source Opendrift software (Dagestad et al., 2018). The model is forced by velocity fields modeled with a high resolution regional Nemo3.6 ocean model with lateral resolution of ~50m. A large number of particles (~700,000) are seeded with four-day intervals, covering the whole fjord system and areas of open waters near the entrances of the fjord system. For each seeding, the dispersion model runs for 3 weeks where larvae that reach a shore are transported away again when currents change (pelagic phase). This is followed by a one-week period where larvae settled as soon as they reach a shore (settling phase). On the basis of this ensemble, we perform a connectivity analysis indicating the probabilities of larvae, released at one location, settling in other locations within the fjord system.

Results show that larvae seeded inside the Orust-Tjörn fjord system mostly remain there with some even remaining in the same local fjord basin during the four-week period. Connectivity matrices also show that some larvae travel far. Larvae seeded outside the Orust-Tjörn fjord system are likely to leave the model domain as they are transported within the generally northward flowing swift Baltic current.