QUANTIFYING HARVESTABLE BIOMASS AS AN INDICATOR OF MICRO-SITE SUITABILITY FOR LOW-TROPHIC AQUACULTURE IN MULTI-USE WITH OFFSHORE WIND
Introduction
In just four years' time global demand for seafood will exceed supply by 40 million metric tonnes yet aquaculture production in Europe has stagnated, largely due to competition for marine space. Maximising spatial efficiency and sustainable development across competing Blue Economy sectors is fundamental to meeting future aquaculture needs, and thus the Sustainable Development Goals of the United Nations Agenda 2030. In Europe, offshore wind farms (OWF) dominate ocean space and present an ideal candidate for 'multi-use' with aquaculture. However, the open ocean remains largely untapped as a farming resource and consequently production potential within OWFs remains unclear. To encourage investment in and future development of multi-use of OWF and low-trophic aquaculture (LTA), scenario analyses that give an indication of site suitability, production potential, and therefore economic feasibility, are critical.
Methods
Here, we used a coupled 3D hydrodynamic-biogeochemical-sediment model combined with dynamic energy budget/growth models for two key LTA species – Mytilus edulis and Saccharina latissima – to assess harvest potential within Kriegers Flak OWF in the southern Baltic Sea. We first assessed the spatial variability of 'harvest potential' of the two species within the OWF based on the food and nutrient fluxes to the area and the maximum biomass yield given no depletion of food or nutrients. We used these metrics to assign 'high', 'medium', and 'low' harvest potential areas to both species. Using these harvest potential categories we then explored optimum farming scenarios (see Figure 1 for example) to quantify maximum harvestable biomass of M. edulis and S. latissima within the OWF area.
Results
We found significantly greater biomass for M. edulis farms located in the high harvest potential areas compared to the medium or low harvest potential areas. Equally, harvestable biomass of M. edulis significantly increased with increasing distance between farm positions, indicating a strong effect of depletion on growth. On the other hand, low salinities in the southern Baltic Sea strongly inhibited the growth of S. latissima, as evidenced by the results of a sensitivity analysis, and consequently no such patterns were evident for this species. Rather, harvestable biomass for S. latissima remained very low and non-profitable across all model scenarios. Additionally, we found little spatial overlap in high harvest potential areas for M. edulis and S. latissima, suggesting the potential benefits of co-culture of the two species in this area is limited.
Discussion
This study represents a significant advancement in spatial planning for OWF-LTA multi-use. The significant difference in M. edulis biomass between model scenarios suggests farm placement is crucial to ensuring the economic viability of LTA within OWF, and highlights the need for predictive models to enhance LTA production at the planning stage of multi-use. Our results support future micro-siting decisions of LTA within the Kriegers Flak OWF and, more broadly, outline a methodological framework for assessing harvest potential of LTA species using ecosystem models. This framework can be applied globally to increase the evidence-base for the viability of multi-use of OWF and LTA Blue Economy sectors.
Figure 1. Farm locations under the nine model scenarios used to assess harvestable biomass potential of kelp in the Kriegers Flak offshore wind farm. Ten farms were randomly placed per scenario. Farm positions are indicated by coloured polygons. The x axis indicates increasing minimum distance between farms. The y axis indicates the harvest potential category of the area in which the farms were placed.
Acknowledgment
This research has been funded by the European Union (OLAMUR grant no. 101094065) as part of the Mission Ocean and Waters and by the Danish Velux Foundation project WIN@sea.