ECOLOGICAL AND OCEANOGRAPHIC INTERACTIONS OF AN OFFSHORE LONGLINE MUSSEL FARM

Abstract

A growing human population coupled with the need to protect marine ecosystems, requires sources of sustainable protein. Finfish aquaculture can produce a wide range of environmental impacts while shellfish aquaculture reports fewer negative effects, being classed a sustainable protein source. Mussels are ecosystem engineers and farms can mitigate the consequences of eutrophication while providing ecosystem services. The development of mussel farming has been limited by competition for coastal space, intensification of the industry causing environmental impacts due to bioaccumulation around farms, negative public perception and lack of research. Offshore aquaculture has the potential to overcome such issues but knowledge on its impacts is limited. The aim of my thesis is to investigate the ecological and oceanographic interactions of the UK’s first large-scale offshore longline mussel farm (Offshore Shellfish Ltd) to address evidence gaps on offshore aquaculture-environment interactions. My results will support policy-makers and managers foster a sustainable offshore aquaculture industry while conserving our oceans. An ecological monitoring programme of the farm began in 2013, prior to any development. From 2014 (deployment of first spat ropes) to 2017, spat ropes were surveyed annually across two different locations (Site 1 & 2). Since 2014, the farm has continued to grow focusing on Site 2, the most developed. My PhD covers three sampling campaigns (2018, 2019 and 2020) focusing effort in Site 2. When possible, data from 2013-2017 was used to build upon the ecological assessment. My main findings show that offshore mussel farms have minimal impact on local current velocities, attenuating within farm currents and producing below-farm acceleration of the flow, with potential ecological implications. Contrary to inshore farms, my study demonstrates that the farm does not deplete the area of zooplankton. The farm is acting as a fish aggregation device (FAD), increasing diversity and abundance of pelagic fish, including those commercially valuable (Dicentrarchus labrus, Chelon labrosus) with the potential to spillover into adjacent fishing grounds. My project provides the first account of biogenic reef creation by an offshore mussel farm located in degraded seabed after years of mobile fishing gear use. The complex habitat formed provides hard substrate, refuge, shelter from predation, trapping sediment and altering water flows. Ultimately controlling benthic–pelagic exchange and playing an essential role in the benthic ecosystem around the farm. Sessile, sedentary, and mobile species diversity and abundance increased within the farm compared to control sites. Certain commercial species were exclusively or almost exclusively recorded beneath the mussel headlines (Maja squinado, Pleuronectes platessa, Raja clavata, Trisopterus luscus, Trisopterus minutus or Hommarus gammarus). The offshore farm did not significantly increase organic matter content while it increased diversity and abundance of infauna organisms. My results suggest the transformation of ecosystem functions, with an increase in functional diversity and richness. Together, these improve the resilience and resistance of the seabed to pressures while the set of traits I identified can be used to assess impacts of aquaculture. I conclude that following the exclusion of mobile fishing gear and the introduction of an ecosystem engineer to the water column, the farm is having a positive impact. The farm is enhancing seabed recovery, ultimately showing the potential of this type of development to act as a de facto marine protected area (MPA). These results will have vast implications for policy and future management of the marine environment in the UK and worldwide.