Aquaculture for all

The pros and cons of co-locating aquaculture with offshore renewables

Environment Offshore aquaculture

The possible impact of multi-purpose platforms (MPPs) – which combine marine renewable energy and aquaculture production – on surrounding marine ecosystems is crucial before large-scale construction of such structures takes off globally.

Natalia Serpetti and co-authors of a recent paper, published in Frontiers of Marine Science, propose the solution of co-locating aquaculture systems and renewable energy devices, such as offshore wind turbines (OWTs) - providing energy for farm operations as well as potential shelter. Serpetti is a member of the Working Group on Ecosystem Assessment of Western European Shelf Seas (WGEAWESS). Her paper aligns with the main objectives of the expert group, which aims to use integrated ecosystem assessments (IEAs) to better inform marine management.

The researchers assessed single and cumulative impacts of the elements representing a hypothetical MPP off the west coast of Scotland that would farm salmon and co-locate OWTs next to the farm cages. The Ecopath with Ecosim and Ecospace (EwE) modelling approach used in the study evaluated specific ecosystem responses to top-down control pathways, changes in top predator distribution (eg harbour porpoise, gadoids and seabirds) as well as bottom-up control pathways (eg increased benthic enrichment and consequent elevation of water nutrient levels).

The results showed weak responses of the food web for top-down changes (eg attraction for food by top predators to the MPP site vs. displacement of marine mammals and seabirds due to turbine noise), without significant increases or decreases in top predators' major prey species. Predator top-down controls were weakly cascading through the food web as their impacts were distributed across multiple preys, reflecting the complexity of their trophic interactions.

While top-down control pathways were only mildly affected, the results showed high sensitivity to increasing changes of bottom-up drivers that cascaded through the food web from detritus and primary producers to benthic and pelagic consumers, respectively. Bottom-up pathways have high energy transfer efficiency, where the energy mainly flows to a few predator groups, and can strongly affect food web structure and biodiversity. The primary productivity pathway also showed an amplification of the signal through the food web, with a large increase of relative biomass of small zooplankton; however, this amplification did not cascade to higher trophic levels (eg large zooplankton and herring).

Potential environmental interactions of a hypothetical MPP structure (click on image to enlarge)

Green arrows indicate potential for attraction of species (fish, mammals, birds, epifauna); yellow arrows indicate potential wider ecosystem impacts (substrate availability for invasive species, detritus enrichment, noise); red arrows indicate potential adverse/lethal impacts (dislocations and collisions of birds and bats with wind turbines; underwater noise).

This ecosystem-based modelling approach allowed the team to investigate the cumulative effects of the different MPP elements. In the cumulative impact scenario, the increasing productivity of the ecosystem, driven by bottom-up pathways, overruled the negative effects caused by the noise pressure and by predator attraction for most of the species impacted. Only harbour porpoises and seabirds did not show cumulative mitigating impacts.

“As with any model, validation is an important aspect to produce accurate predictions," said Serpetti, in a press release on the ICES website. The limited availability of validation material for this study, and uncertainties around the assumptions made regarding noise pressure responses and species habitat preferences were the main limitations of the study. In the future, a sensitivity test should be carried out to assess the model performance.

Legislating for change

Assessing the long-term environmental impacts in terms of eutrophication and noise is a priority for both the EU Water Framework Directive and the Marine Strategy Framework Directive. In the study, the cumulative impact scenario showed that the increasing productivity driven by the presence of farming can mitigate or even overrule negative effects caused by noise pressure and predator attraction. Assessing cumulative impacts will be important in the future for the Maritime Spatial Planning under the Integrated Maritime Policy. This work will also help in advancing some of the main goals of WGEAWESS, such as moving towards implementing IEAs as a tool for marine management and updating and improving ICES Ecosystem Overviews.

“Aquaculture and marine renewable energy are two expanding sectors of the Blue Economy in Europe," concluded Serpetti. “Moving toward renewables as a greener and more sustainable option in the face of climate change and due to the necessity of aquaculture production, we propose the use of MPPs to maximise the benefits of these expansions and minimise their impacts".