Shell-shocked: how substrate vibration from offshore wind farms affects the common hermit crab (Pagurus bernhardus)

Climate change is a global challenge, where the burning of fossil fuels from human activities has led to higher temperatures and more frequent storms. As a result, the UK has a target of achieving net zero greenhouse gas emissions by 2050. This has led to the reduction of fossil fuel use and therefore increasing the demand for efficient renewable energy sources, such as offshore wind farms. This expansion of offshore wind, though beneficial for achieving this target, has led to several environmental concerns in relation to their presence in the marine environment. Underwater noise is one of these environmental concerns, with studies showing significant negative impact on marine mammals and fish. Marine invertebrates, on the other hand, have been considerably under- and inappropriately studied in regard to their perception of sound. Marine invertebrates do not detect sound in the same way that marine mammals and fish with accessory hearing structures do and instead are sensitive to vibration that travels in the water or the substrate. Turbines are typically fixed into the seabed, and installation requires impulsive pile driving activities. Therefore, given the commercial and ecological importance of many marine invertebrates, understanding the impacts of vibration generated from construction and operation into the substrate in which they reside is crucial. This thesis aims to understand how the hermit crab, Pagurus bernhardus, responds to environmentally relevant levels of substrate-borne vibration that are generated from an offshore wind turbine. Techniques and laboratory environments were developed to replicate this stimulus and ensure that the set-up was as environmentally relevant as possible.Due to the lack of data around vibration transmission into the substrate from an operational turbine, Chapter 3 aimed to quantify this at different wind speeds and compare to vibrational sensitivities of marine invertebrates. Using a calibrated accelerometer, vibration was measured at a terrestrial 2 MW wind turbine on the turbine tower, foundation and in the soil at 10 metres distance. The results from this chapter show that substrate-vibration increased with decreasing wind speed, where highest amplitudes on the turbine tower were recorded at 0.92 ms-2, above threshold of multiple species, including the blue mussel (0.6 ms-2) and common hermit crab (0.02 ms-2). All vibration was recorded below 500 Hz; however, the strongest frequency was 114 Hz. Vibration into the soil at 10 m distance was minimal, however, there were still overlaps with particularly sensitive invertebrates, such as the brown shrimp. This chapter highlights the importance for further work to be done, especially at the larger turbines placed offshore, as many marine invertebrates colonise the structures and the sediment surrounding them.Based on the results from Chapter 3, substrate-borne vibration and frequencies were replicated in the lab using three different amplitudes at 114 Hz (0.02, 0.2 and 0.6 ms-2). In the short-term, crabs were exposed to continuous sinusoidal vibration exposure across 3 hours, where behaviour and oxygen consumption were monitored throughout, and tissue glucose was measured at the end (Chapter 4.1). Long-term exposure lasted for 10-days, measuring for behaviour, oxygen consumption, food intake and larval release throughout, with tissue glucose measured post-exposure (Chapter 4.2). Short term exposure (1 day) resulted in increased locomotion and decreased tissue glucose at 0.2 ms-2 and increased time spent hiding within the shell were documented at 0.6 ms-2. Within the long-term (10 days), exposure to 0.6 ms-2 caused a reduction in food intake, reduced parental care and subsequent larval mortality.Noise generated from pile driving has gained a lot of attention in the literature, however, replication of expected amplitudes close to source has not been tested in the lab. Chapter 5 builds upon this gap in knowledge and exposes P. bernhardus to substrate-borne vibration that would be expected within a few metres of the pile driver. After 1-day of exposure to 10 ms-2, clear reductions in individual variation across all parameters (oxygen consumption, glucose, lactate and protein) were seen, suggesting they were operating at internal capacity. After 3-days, there were no significant differences between the control and the impulsive vibration treatment. Food intake did marginally differ between treatments over time during the 3-day plus recovery experiment.The results of this thesis have shown that hermit crabs respond adversely to continuous and impulsive substrate-borne vibration through alterations of behaviour, changes in food intake, fluctuations in energy stores and larval mortality. Therefore, this thesis highlights the considerations that may potentially need to be taken during installation of turbine structures, and monitoring processes during operation to ensure that marine invertebrates are not at risk of offshore renewable energy developments.