The harbour porpoise (Phocoena phocoena) is the most widely distributed cetacean in European waters. Although most commonly observed near coastal areas, harbour porpoises can also be found over much of the European continental shelf. Harbour porpoise are usually observed in small groups or as individual animals, although sightings of large numbers of animals have been recorded seasonally (Hoek 1992), in association with food concentrations or long distance movements. It has been proposed that in the seas around the UK, three main discrete populations of harbour porpoise exist; Ireland/west Britain, the North Sea and the English Channel (Gaskin 1984; Walton 1997).
The harbour porpoise, like other toothed whales (odontocetes), use sound to capture prey, communicate and to navigate. Porpoises produce relatively short echolocation clicks, with dominant narrow-band high frequency clicks within 110-150 kHz, 130 kHz peak frequency. Research has shown that porpoises use their echolocation almost continuously (clicking around ever 12.3 s). The harbour porpoise spends limited time at the surface, and is not seen generally above Beaufort Sea State 2, therefore is difficult to observe visually. This species is thus ideally suited to studies using passive acoustic monitoring (www.passiveacousticmonitoring.com).
The west coast of Scotland features complex coastlines and topography, and has one of the highest densities of harbour porpoises in Europe, with sightings recorded year-round (SCANS 2008). Porpoise detections are most common in regions close to the coast, with seabed depth and seabed slope being the most important covariates for porpoise occurrence. Acoustic detection rates of harbour porpoises are also generally twice that of visual sightings rates, thus demonstrating the importance of passive acoustic monitoring methods as a tool for modelling harbour porpoise distribution, while also identifying important regions of harbour porpoise habitat use. Identifying high density areas of harbour porpoises is key to designating Special Areas of Conservation (SAC) for the species. To date, no SACs have been designated for harbour porpoise in the UK (Booth et al. 2013).
Harbour porpoise and bottlenose dolphin interactions
One important factor affecting harbour porpoise distribution in UK waters is interspecific aggression from bottlenose dolphins (Tursiops truncatus) which has been recorded in the Moray Firth (Ross & Wilson 1996; Patterson et al. 1998) and in Cardigan Bay (Simon et al. 2010). Previous studies have revealed that more than 60% of stranded harbour porpoise in the Moray Firth had suffered multiple skeletal fractures and damaged internal organs from violent interactions with bottlenose dolphins (Ross & Wilson 1996).
T-PODs (www.t-podporpoiseclickdetector.co.uk) have been used to study bottlenose dolphin and harbour porpoise habitat use and partitioning in Cardigan Bay, Wales. Results found that both species were detected all year-round, with a peak in detections in April – October for dolphins and in October – March for porpoises. This study revealed an indication of temporal habitat partitioning between the two species and a previously unknown importance of the area to harbour porpoises in winter (Simon et al. 2010). One location was found to have simultaneous detections of both species; however this rare encounter was close to the shoreline where stranding of porpoise killed by dolphins was most common.
The North Sea is one of the most important habitats for harbour porpoises in the north east Atlantic; however this sea is also known for its busy shipping lanes, fisheries and a mecca for oil and gas exploration. Threats include incidental by-catch in fishing gear, habitat destruction and anthropogenic disturbances from shipping, oil and gas activities (seismic exploration) and renewables construction (i.e. wind-farm piling).
The harbour porpoise is protected under Annex II and IV of the EU’s Habitat Directive. It is therefore essential to investigate and understand the possible effects of anthropogenic noise/offshore construction on this species at both an individual and population level. Passive acoustic monitoring using porpoise detectors (e.g. T-POD www.t-pod.co.uk, and C-POD www.c-podclickdetector.co.uk) has proven to provide valuable information on the abundance and distribution of harbour porpoises.
With the growth of the offshore renewable industry, and the construction of numerous wind farms in the North Sea, evidence has shown that harbour porpoises can be affected both positively and negatively. Research to date has shown that underwater noise produced from construction (e.g. piling) of offshore wind farms is similar in nature to noise produced by other offshore activities including oil and gas extraction (e.g. conductor hammering). Noise from piling activities is of low frequencies and of high intensity, with peak levels exceeding 200 dB re 1µPa near the piling location, and are likely audible to marine mammals at large distances (Tougaard et al. 2008). Data from porpoise detectors have demonstrated behavioural reactions of harbour porpoises up to 15 km from piling sites, highlighted by a reduction in acoustic activity of porpoises (Carstensen et al. 2006; Tougaard et al. 2006; Tougaard et al. 2009; Thompson et al. 2010). One study found that porpoise behavioural reactions were affected negatively up to a distance of 18 km, with porpoise activity and possibly abundance being reduced throughout the entire construction period (Brandt et al. 2011). On the other hand, results from studies undertaken during the operational phase of offshore wind farms have demonstrated higher densities of harbour porpoises, and in particular within the wind farm area (Scheidat et al. 2011). Possible reasons for this increase in density could be related to an increase in prey, brought about from the artificial reef effect of the wind farm structures. Artificial reefs, in the form of oil and gas installations (rigs and platforms), have been found to be foraging locations for harbour porpoises (Todd et al. 2009).