Coastal areas are submitted to lots of anthropic pressure, which is raising with the increase of demand for aquaculture and renewable energy. Coastal habitats are fragile and can be preserve by relocating aquaculture offshore with offshore renewable energy (ORE), which could help to: 1) manage space availability efficiently; and 2) decrease the risk of contamination due to the proximity between aquaculture sites and source of pollution (e.g., estuaries). Blue mussels industry (Mytilus edulis L.) in North Wales represents one third of the UK production and could be impacted by the development of other industries. Here, we used a Lagrangian particle tracking model (PTM) coupled to hydrodynamic model to study the best area to catch mussel larvae during their pelagic larvae duration. Furthermore, we showed the connectivity between the main aquaculture and ORE sites to evaluate the feasibility of Multi-use platform at sea (MUPS).
Telemac-2D (V8p2r0) model representing surface and mid-water depth currents was developed during the month of April (i.e., time of main spawning event) for 10 years (from 2012 to 2022). The domain covered an area of 165,000 km2, which correspond to the whole Irish Sea as previous studies show that larvae can potentially travel up to 300 km. To resolve the complex coastal current, the grid resolution varied from 50 m on the coast to 5,000 m at the offshore limit of the domain. A Lagrangian PTM was developed to predict mussel larvae dispersal from 6 sites for a pelagic larvae duration (PLD) of 45 days. Simulations were performed for two larvae behaviour (i.e. dispersal at the surface and at mid-water depth), then
Connectivity showed the same results every year when larvae travel at mid-water depth as expected. However, connectivity is site and PLD dependant (p-value < 0.0001). Connectivity varied inter-annually when larvae travelled at the surface and especially during windy contrasted years (i.e., westerly wind in 2014 and easterly wind in 2018; p-value < 0.0001), but release site and PLD remain significantly important. One ORE site is the most connected with aquaculture sites, however this connectivity varied through mussel PLD. During April 2014, aquaculture sites are connected with ORE sites located in the eastern Irish Sea (i.e. North Wales and North of England) whereas during 2018 they are connected with ORE located in the western Irish Sea (i.e. Dublin and North of Ireland).
The simulated larvae behaviours results showed that when larvae travel at the surface, they encountered stronger currents (e.g. wind driven currents), which increased their dispersal. Results observed when mussel larvae travel at the surface are correlate with observations made by mussel farmers during the year 2014 and 2018. The results highlight the importance of vertical position of larvae in the water column to study potential multi-use platforms at sea. However, the potential ORE site called leasing site 1 is shown to be the best area to catch mussel larvae in the Irish Sea. This study shows the importance of considering larvae dispersal for the development of MUPS, especially in the Irish Sea, where ORE will occupy approximately 14% (6,564 km2) of the space in a close future.