No abstract

Tidal-stream habitats present periodically fast-flowing, turbulent conditions. Evidence suggests that these conditions benefit top predators such as harbour porpoises Phocoena phocoena, presumably allowing them to optimise exploitation of prey resources. However, clear demonstration of this relationship is complicated by the fact that strong tidal flows often occur near-simultaneously across a wide area. The Great Race of the Gulf of Corryvreckan (western Scotland, UK) is a jetting tidal system where high-energy conditions persist across a broad range of tidal phases in a localised, moving patch of water. Porpoises can therefore actively enter or avoid this habitat, facilitating study of their usage of adjacent high-and low-energy environments. The distribution of harbour porpoises was studied using passive acoustic porpoise detectors (C-PODs) deployed on static moorings (~35 d) and on Lagrangian drifters moving freely with the current (up to ~48 h). This dual approach provided complementary perspectives on porpoise presence. C-PODs moored in the path of the Great Race registered a significant increase in detections during the passing of the energetic tidal jet. Encounter durations recorded by drifting C-PODs were longer than those recorded by moored C-PODs, suggesting that porpoises tended to move downstream with the flow rather than remaining stationary relative to the seabed or moving upstream. The energetic, turbulent conditions of the Great Race are clearly attractive to porpoises, and they track its movement with time; however, their structured movements in response to the evolving tidal situation cannot simply be represented as a direct relationship between current speed and porpoise presence.

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.

1. Bottlenose dolphin photo-identification data were compiled from western Scotland to identify individuals and ultimately investigate population size, demographic parameters, spatio-temporal distribution, and movement patterns.2. Opportunistic citizen science photographs revealed what appeared to be an adult bottlenose  Risso's dolphin hybrid along with an apparent second-generation hybrid or back-cross calf. Both had atypically short rostra and the dorsal fin of the adult was noticeably taller than is normal for bottlenose dolphins.3. Based on these characteristics, this case may represent a congenital rostral abnormality or the first intergeneric calf reported for this species combination, either in captivity or in the wild.4. The previously reported presence of several putative hybrids and mixed-species sightings in the area, in combination with the tall dorsal fin, provide support for the second possibility, i.e. intergeneric hybrids. 5. Although rare, hybridization may have disproportionate conservation consequences, with population-level impacts in very small coastal populations of long-lived, slow-breeding animals.