Numerical Modelling of Jellyfish Movements in an Irish Fjord

Injury and loss of marine-farmed fish due to jellyfish stings are a global problem. Many such events have been reported all over the world with significant economic losses. The present research aims to use a hydrodynamic coastal model to simulate the transport and fate of jellyfish so as to investigate means of mitigation/prevention of adverse interactions of jellyfish with fish farms. Due to recent fish fatalities by jelly swarms, Killary Harbour, a fjord on the Irish west coast, was chosen as the case study site for the research. A 3D baroclinic model of Killary has been developed using the Environmental Fluid Dynamics Code (EFDC). The jellyfish were modelled as passive drifters using EFDC’s particle transport module and the outputs were compared with recorded activities of tagged jellies within the fjord. The tagged jelly movements were detected by 8 GPS receivers placed at various locations along the banks of the fjord. Particle transport model results were analysed to determine the number of particles within each detector’s range during the times of observation of real jellies and comparisons made. The modelled particles agreed relatively well with the positions of the tagged jellies in the short-term periods after the tagged jellies were released. Longer-term, the results were mixed. Although the analyses revealed simultaneous presence of jellies and modelled particles even at times separated by long intervals and long distances (e.g., jellies having travelled up to 5km from their release point 125hrs after their release), the numbers of detected modelled particles were small. Results show that there was a significant difference in the long-term transport of particles released at mid-depth in the water column compared to those released at the surface. Wind-induced surface currents result in many surface particles being transported west of the release point towards the sea, whereas the sub-surface particles were all transported eastward of the release point. Ultimately, the simulations have shown that the tidal and wind-driven currents can indeed cause particles to be transported in a similar manner to the tagged jellyfish; this finding corroborates the literature. However, the results also suggest that vertical motility of the jellies played a role in their transport.