This review provides a critical, multi-faceted assessment of the practical contribution tidal stream energy can make to the UK and British Channel Islands future energy mix. Evidence is presented that broadly supports the latest national-scale practical resource estimate, of 34 TWh/year, equivalent to 11% of the UK’s current annual electricity demand. The size of the practical resource depends in part on the economic competitiveness of projects. In the UK, 124 MW of prospective tidal stream capacity is currently eligible to bid for subsidy support (MeyGen 1C, 80 MW; PTEC, 30 MW; and Morlais, 14 MW). It is estimated that the installation of this 124 MW would serve to drive down the levelized cost of energy (LCoE), through learning, from its current level of around 240 £ / MWh to below 150 £ / MWh , based on a mid-range technology learning rate of 17%. Doing so would make tidal stream cost competitive with technologies such as combined cycle gas turbines, biomass and anaerobic digestion. Installing this 124 MW by 2031 would put tidal stream on a trajectory to install the estimated 11.5 GW needed to generate 34 TWh/year by 2050. The cyclic, predictable nature of tidal stream power shows potential to provide additional, whole-system cost benefits. These include reductions in balancing expenditure that are not considered in conventional LCoE estimates. The practical resource is also dependent on environmental constraints. To date, no collisions between animals and turbines have been detected, and only small changes in habitat have been measured. The impacts of large arrays on stratification and predator–prey interaction are projected to be an order of magnitude less than those from climate change, highlighting opportunities for risk retirement. Ongoing field measurements will be important as arrays scale up, given the uncertainty in some environmental and ecological impact models. Based on the findings presented in this review, we recommend that an updated national-scale practical resource study is undertaken that implements high-fidelity, site-specific modelling, with improved model validation from the wide range of field measurements that are now available from the major sites. Quantifying the sensitivity of the practical resource to constraints will be important to establish opportunities for constraint retirement. Quantification of whole-system benefits is necessary to fully understand the value of tidal stream in the energy system.

Minke whales comprise some of the most widely distributed species of baleen whales, some populations of which are still regularly targeted by commercial whaling. Here, we review the conservation status of common (Balaenoptera acutorostrata) and Antarctic (Balaenoptera bonaerensis) minke whale populations, against the backdrop of ongoing whaling operations and other anthropogenic threats, including climate change, entanglement in fishing gear, ship strikes, and noise pollution. Although some coastal minke whale populations have been studied in detail, others, which inhabit remote and ecologically sensitive locations, such as the Antarctic ice shelf, are among the least understood populations of marine mammals. The unresolved taxonomy of dwarf minke whales further highlights some of the existing knowledge gaps concerning these species. Due to their relatively small size and elusive behaviors, large uncertainties exist for almost all minke whale populations with respect to behavior, migratory routes and winter distributions, hindering effective conservation and management. However, recent advances in research technology, such as passive acoustic monitoring (PAM), unmanned aerial systems (UAS), multisensor recording tags, and machine learning assisted photo-identification, are increasingly being applied to study minke whales and their habitat, and are starting to open new windows into their life history and ecology. In future research, these non- and less-invasive methods should be integrated in larger-scale comparative studies aiming to better understand minke whale behavior, ecological interactions and their varying habitats to drive and support effective species conservation.

The underwater sound emitted during the operation of the Atlantis AR1500 turbine, a 1.5 MW three bladed horizontal axis tidal-stream turbine, was measured in the Pentland Firth, Scotland. Most sound was concentrated in the lower frequencies, ranging from 50 to 1000 Hz. Within 20 m of the turbine, third-octave band sound pressure levels were elevated by up to 40 dB relative to ambient conditions. In comparison, ambient noise at these frequencies fluctuated by about 5–10 dB between different tidal states. At the maximum recording distance of 2300 m from the turbine, median sound pressure levels when the turbine was operational were still over 5 dB higher than ambient noise levels alone. A higher frequency, tonal signal was observed at 20 000 Hz. This signal component appears at a constant level whenever the turbine is operational and did not change with turbine rotation rate. It is most likely produced by the turbine's generator. This study highlights the importance of empirical measurements of turbine underwater sound. It illustrates the utility and challenges of using drifting hydrophone systems to spatially map operational turbine signal levels with reduced flow noise artefacts when recording in high flow environments.

Despite frequent records from other parts of the North Atlantic, minke whales have never been acoustically recorded in the North Sea. This study investigated the detectability of pulse trains previously associated with this species in other regions, in acoustic data from ten sites along the east coast of Scotland. Since preliminary results confirmed pulse train presence, subsequently, an automated detector was applied to these data to record the seasonal and diel presence of minke whale pulse trains. Minke whales were detected from May to November, with most detections occurring in June, July and October. No acoustic detections were made in December, January or in the month of April, whilst no data were available for February and March. This pattern of acoustic presence supports available visual data and suggested an absence of minke whales from the study area during winter. Minke whale acoustic presence showed a statistically significant diel pattern, with a detection peak during night time. This study established the acoustic detectability of minke whales in the North Sea and highlights the potential of using passive acoustic monitoring to study the seasonal presence and spatial distribution of minke whales in the North Sea and wider Northeast Atlantic.

Management interventions to reduce human–wildlife conflict can have unintended consequences for non‐target species. Acoustic deterrent devices (ADDs) are used globally by the aquaculture sector. However, the potential for these sound emissions to impact non‐target species, such as cetaceans, has not yet been quantified at population relevant spatial scales. To better understand the extent of potential impacts on cetaceans, such as harbour porpoises, we used acoustic modelling to investigate levels of ADD noise throughout the west coast of Scotland and across a Special Area of Conservation (SAC) for this species. Using an energy‐flux acoustic propagation model and data on aquaculture sites known to be using ADDs, we predicted the spatial extent of ADD noise on the Scottish west coast from 1 February 2017 to 31 January 2018. Noise maps were produced to determine the risk of auditory impairment for harbour porpoises under a range of scenarios which assumed single or multiple ADDs and simultaneous use across all sites. The acoustic propagation model performed well when tested against field measurements up to 5 km, with 98% of sound exposure level (SEL) predictions within ±10% of the measurements. Predictions of SELs over a 24‐hr period suggested extensive temporary hearing loss zones (median radius: ~28 km) for harbour porpoises around aquaculture sites. Assuming a single device at each site, 23% of the harbour porpoise SAC was predicted to be exposed to ADD noise sufficient to induce a temporary threshold shift, and under the worst‐case scenario (multiple, continuously running devices per site with an aggregate duty cycle of 100%), levels exceeding permanent threshold shift could reach 0.9% of the SAC. Policy implications. This study highlights the potential for ‘collateral damage’ from interventions such as acoustic deterrent devices (ADDs) which are intended to reduce human–wildlife conflicts with pinnipeds but may affect the long‐term health and habitat use of non‐target species. This is especially true for harbour porpoises which are protected under the EU and UK Habitats Regulations. The aquaculture industry, policymakers and regulators in countries where ADDs are used should consider these findings when attempting to mitigate pinniped depredation.

Humpback whales (Megaptera novaeangliae) produce song and non-song vocalisations, which allows their presence to be detected through passive acoustic monitoring. To determine the seasonal and diel acoustic presence and acoustic behaviour of humpback whales at the migratory stopover site off Bermuda, three hydrophones were deployed between March 2018 and April 2019 on Challenger Bank and the Bermuda platform. Song was the predominant vocalisation type encountered, with 65% of song recordings containing whale chorus and a clear seasonal trend of humpback whale occurrence in the spring and winter months from late December to mid-May. A strong diel pattern in singing activity was detected. Singing activity significantly increased at night relative to the daytime (p<0.01), whilst twilight periods were characterised by intermediate levels of singing. The song structure encountered in spring 2018 consisted of 18 units, 6 themes and 5 transitional phrases. The high occurrence of whale chorus and the strong seasonal and diel patterns of male humpback whale singing activity highlights the importance of Bermuda not just on their northward migration during spring, as described historically, but also on their southward migration during winter. Bermuda therefore constitutes a two-way migratory stopover site for humpback whales. The present study also provides Bermuda’s planning authorities with better constraints on the duration and intensity of anthropogenic activities in these waters.