Porpoises are marine mammals that belong to the suborder Odontoceti (porpoises, dolphins, and toothed whales; http://en.wikipedia.org), more specifically, to the taxonomic family Phocoenidae. In North America, the terms ‘dolphin’ and ‘porpoise’ are used interchangeably, but this is inaccurate because porpoises are about as closely related to dolphins as a dog is to a cat (Read 1999). There are six species or porpoise within the family Phocoenidae. Two major physiological features distinguish porpoises from dolphins: porpoises have spade-shaped teeth, whilst dolphins have conical teeth and, porpoises have blunt jaws instead of the classic ‘beaks’ present in the majority of dolphins (for other examples see http://thewesternisles.co.uk). With the exception of one Norwegian Fjord harbour porpoise (Phocoena phocoena) population and Dall’s porpoise (Phocoenoides dalli) which bow ride regularly, porpoises are shy creatures and tend to avoid interaction with humans and boats. Porpoises are amongst the smallest odontocetes in the world.
WESTERN EUROPEAN HARBOUR PORPOISE
The tiny harbour porpoise grows only to a maximum length of 1.9 m when fully grown (Shirihai & Jarrett 2006). The harbour porpoise is found in cooler coastal waters of the North Atlantic, North Pacific, and the Black Sea. The North Sea population is estimated to contain 250,000 individuals (SCANS 2008). Potential threats include: bycatch in fishing gear, underwater man-made (anthropogenic) noise, vessel collisions, toxins, climate change, shifts in prey distribution and abundance, and over fishing of their prey. For example, in Norwegian waters, General Additive Models (GAMs) were used to give an annual prediction of 6,900 harbour porpoise bycatch in anglerfish and cod fisheries (Bjørge et al. 2013).
Historically, porpoises are cited as ‘proxy indicator’ species, in that their presence/absence may give an indication as to the health of an ecosystem, but it is clear that the relationship between top marine predators and food abundance is not always so simple. Spitz et al. (2012) have found that different daily energy needs (known as ‘cost of living’) affect the type of prey a cetacean is able to feed on. Species with a high cost of living require prey that yield high energy gains whereas a species with a lower cost of living is able to feed on numerous prey species that each provides low energy gains. Harbour porpoises have a high cost of living and therefore have been found to feed on high quality prey rather than numerous low quality prey. Consequently, harbour porpoises may be absent from a healthy ecosystem purely because the fish species present do not represent a viable food source.
Harbour porpoises are often monitored as part of offshore industrial operations such as seismic and drilling exploration, military operations, and renewable construction activity. Methods are also being developed to improve stranding data for population indicators (Peltier et al. 2013).
Porpoises utilise sonar (echolocation, http://en.wikipedia.org) to navigate and locate their prey. Porpoises are predators that feed on fish, squid, and crustaceans. Harbour porpoises produce Narrowband High Frequency (NBHF) ‘clicks’ with a peak frequency of ca. 130 kHz and duration of 100 μs (Teilmann et al. 2002). As a porpoise approaches its target, the interval between clicks decreases until a peak is reached, known as ‘burst pulsing’ (Schevill et al. 1969), or buzzes (e.g. Nuuttila et al. 2013). Harbour porpoises also use their clicks for communication (Clausen et al. 2010). Some click repetition rate patterns have been linked to specific behaviour e.g. clicks produced during aggressive behaviour are highly directional whereas clicks produced when a calf is calling its mother are less directional and have a higher source level to enable the call to be heard from a greater distance.
Porpoise detectors are used to identify and log clicks produced by porpoises and other odontocetes. Porpoise detectors can be embedded into software such as PAMGuard (www.pamguarg.org) or as hardware dataloggers. PODs or ‘porpoise detectors’ (PODs; www.staticacousticmonitoringsystems.co.uk) are a generation of detectors used often in offshore oil and gas, and renewables industries, and as research tools for monitoring. PODs are classified among a broad range of underwater listening devices known as Passive Acoustic Monitoring (PAM; www.passiveacousticmonitoring.com) tools. Current favourites include the C-POD (www.c-podclickdetector.com), and its predecessor the T-POD (www.t-pod.co.uk), both produced by Chelonia Ltd. (www.chelonia.co.uk). In ideal propagation conditions a C-POD can detect porpoises up to ca. 300 m (Kyhn et al. 2008; Haelters et al. 2010). Please see www.dolphindetectors.com for details on how C-PODs work. T-PODs and C-POD technology has, as yet, to be rivalled. Other companies produce broadband datalogger tools that are able to detect porpoises, but are not designed specifically for that purpose alone.
Broadband underwater acoustic recorders generally utilise omni-directional hydrophones and are capable of detecting sounds from 1–150 kHz. They are suitable for shallow and deep water (in some cases up to 3000 m) deployment. They can be set up to log at varying intervals depending on the length of time of deployment and period between servicing, but in general, servicing intervals should not be longer than 1 month. Like C-PODs, some data loggers can record onto a removable memory card, rendering data transfer in the field fast and efficient.
Porpoise detectors are the ideal device for projects that require extended periods of PAM due to their capability of remaining active on location 24 hours a day for 4+ months, depending on battery life. For this reason porpoise detectors are used regularly for Environmental Impact Assessments (EIAs; http://en.wikipedia.org) and baseline studies prior to marine construction projects.
Data collected from porpoise detectors can determine presence of animals and habitat usage. With implementation of specialised data analysis, behaviour such as feeding activity can be ascertained.
RECENT PORPOISE RESEARCH
Porpoise detectors (T-PODs) have been used by OSC staff and collaborators to record echolocation clicks of harbour porpoises around offshore gas installations in the Dogger Bank region of the North Sea (Todd et al. 2009; www.osc.co.uk). Three T-PODs were deployed from the A6-A offshore gas platform (www.wintershall.com) and the Noble Kolskaya jack-up drilling rig (www.rigzone.com) from July 2005 to January 2006. Recorded echolocation click trains were divided into four sections of the diel (24-hr) cycle; morning, afternoon, evening, and night. From data obtained, Todd et al. (2009) were able to conclude that porpoises visited the installation more often at night than during the day. The ratio of feeding buzzes to search phase clicks (known as a ‘Feeding Buzz Ratio’ or FBR) was also higher, indicating that porpoises were feeding below and around the platforms during the night. This information has prompted a rethink into the possibility of leaving decommissioned rigs in situ as artificial reefs to provide safe feeding grounds for these small cetaceans (www.osc.co.uk).
Porpoise detectors (T-PODs) have also been used to investigate whether a re-established reef in Danish waters (http://en.wikipedia.org/) would attract harbour porpoises (Mikkelsen et al. 2013). Two T-PODs were placed at the reef site, with two more at a reference site (a well-developed stony reef 10 km away) between June and August every year from 2006 to 2012. The restoration project comprised approximately 100,000 t of boulders placed in a 45,000 m2 area. Researchers observed more porpoise detections during and after reconstruction of the reef than in the baseline data. This increase in porpoise activity at the re-established reef has probably been caused by an increase in prey, as fish and invertebrates quickly colonised newly available nooks and crannies. The likely increased concentration of larger fish ultimately attracts top marine predators, such as porpoises. Like Todd et al. (2009), detections increased at night compared to during the day, which in this instance could possibly be attributed to the diel pattern of the porpoises’ main prey, Atlantic cod. Cod move into the shallower water of the reef at sunset and return to deeper water at sunrise. The increase in porpoise detections at night indicates that: (1) more porpoises were visiting the reef; (2) porpoises spent more time at the reef; (3) porpoises changed their echolocation behaviour; or, (4) a combination of these factors. There were also fewer detections at the reference (control) site after reconstruction of the reef, with highest numbers observed during the day. The control site was a flat plateau, and therefore unlikely to attract porpoises to the same extent. Also, being a flat plateau, there was no diel migration of fish observed from deep to shallow water; it is also likely that there is a lower abundance of prey resulting in a lower amount of foraging activity by porpoises. The decrease in activity at the control site could, in part, be because porpoises are choosing to feed at the re-established reef with its higher abundance of prey compare to the control reef. This study showed that, at least for porpoises, reef reconstruction provided a positive impact to the ecosystem.
Seasonal & spatial variation
At present, impacts on coastal and estuarine ecosystems from defensive measures built to combat the effects of global warming and climate change are poorly understood. A study was conducted by Jansen et al. (2013) to try to ascertain impacts caused to the harbour porpoise population due to the construction of a storm surge barrier in the Eastern Scheldt, in south-west Netherlands (www.wikipedia.org). Researchers measured ‘residency’ by sampling muscle and bone tissue from stranded porpoises along the Dutch North Sea coast from 2006 to 2008. The authors found significantly higher stable carbon isotope values in the muscle of porpoises stranded within the Eastern Scheldt than porpoises stranded along the Dutch coast. As this distinct stable carbon was found in the muscle, but not bone tissue of the porpoises, it suggested that animals had only recently been feeding in the Eastern Scheldt, and that none of the analysed animals were born there. The high number of strandings also indicated a higher mortality rate within the Eastern Scheldt compared to the Dutch coast and, as appeared to be an annual increase in porpoises greater than calf production could produce alone, this suggested porpoises were regularly entering from the North Sea. It appeared that the storm surge barrier was preventing porpoises from leaving and undertaking their usual seasonal migration and trapping them inside the Eastern Scheldt. Being trapped and unable to perform their usual migration could cause changes to their behaviour and, in turn, their chance of survival. More research is needed in this area, as more coastal defence structures are likely to be built in the near future.
A study by Haelters et al. (2010) investigated spatial and temporal distribution of harbour porpoises in Belgian North Sea waters. Researchers used three methods to accomplish their goal: aerial line transects, PAM, and strandings data analysis. Aerial line transects were used to assess population size and spatial distribution of harbour porpoises. Five aerial surveys were conducted during 2008 and 2009 and all were able to locate harbour porpoises. The three summer surveys provided the lowest densities, whilst the two winter surveys showed a much higher density of porpoises. Summer and winter surveys also had distinct differences in porpoise distribution; in winter porpoises were present both inshore and offshore, whilst in summer all porpoises observed were further offshore. Passive acoustic monitoring was performed using C-PODs from November 2009 to May 2010. More detections were observed further offshore from October to December compared to inshore waters. Inshore waters showed low detection numbers throughout the survey. Stranding data were used to analyse historical abundance of harbour porpoises in Belgian North Sea waters. The study also reported a steady increase in the number of strandings since the late 1990s, with peak months being March, April, and May. Collated data showed a seasonal temporo-spatial distribution shift, from inshore and offshore waters at the start of the year, to further offshore in summer, and back to both inshore and offshore at the end of the year.
A baseline study to assess the environmental impacts of the planned Near Shore Wind (NSW) farm in the Netherlands in relation to harbour porpoises was conducted by Brasseur et al. (2004). The study used three methods to locate harbour porpoises in a 200 km2 area: bi-monthly visual surveys, towed hydrophones, and porpoise detectors (T-PODs). Researchers found that there was seasonal variation in detection rates of harbour porpoises, with the most detections occurring in winter (December-March), and least detections recorded in summer (May-August). This was also observed in visual surveys. Average encounter time was also shorter during summer months compared to winter months. Spatial variation was observed, with slightly more detections found in the northern area of the survey location.
Reactions to wind farm piling and construction
Tougaard et al. (2006) monitored harbour porpoises at Horns Reef Offshore Wind Farm in the Danish part of the North Sea from 1999 to 2005 to assess abundance before, during and after wind farm construction. Horns Reef and surrounding areas are known to be important habitats for harbour porpoises with a much higher mean density outside winter months than the North Sea and German Bight (http://en.wikipedia.org/wiki/German_Bight). Boat surveys and twenty porpoise detectors (T-PODs) were used to monitor porpoises and both showed that construction had a negative effect on porpoises, with lower numbers recorded at the wind farm area during construction than the reference site. After normal operation of the wind farm had begun, no significant difference in abundance of porpoises was observed between the wind farm and the reference site. As mentioned above, T-POD data also showed greater concentrations of porpoises during the summer than winter. Data from T-PODs showed a clear negative effect to porpoises from pile driving operations. Porpoises left the entire Horns Reef area when pile driving began and did not return for 6–8 hours. After 6–8 hours porpoise activity returned to normal levels observed during construction phase.
Carstensen et al. (2006) used T-PODs, installed at three locations within the ‘wind farm impact area’, and another three in a reference area 10 km east of the wind farm (http://en.wikipedia.org) to investigate effects of wind farm construction on harbour porpoise. T-PODs were serviced and data downloaded roughly every 60 days from November 2001 to June 2002 (baseline period), and July 2002 to November 2003 (construction period). Results showed that duration of waiting times increased significantly during construction when compared to baseline, and this increase was larger in the impact area than the reference area. The authors also found that waiting time duration was longer when ramming or undergoing vibration activity, and that the number of detections increased with increased distance from the ramming/vibration activity.
Scheidat et al. (2011) used T-PODs to monitor porpoise activity before and after the construction of a wind farm (http://en.wikipedia.org) in the Dutch sector of the North Sea, to try to understand whether porpoise abundance is affected by the presence of a wind farm. Eight T-PODs were positioned in the proposed, and later operational wind farm, and in two reference areas. The baseline study took place from June 2003 to June 2004, and the operational study took place two years after construction was completed, from April 2007 to April 2009. All sites showed an increase in acoustic activity after the wind farms became operational, compared to baseline, suggesting an increase in porpoise presence/activity. It is not confirmed what caused this increase in habitat use in the wind farm area, but is thought to be a combination of a number of factors, including: the introduction of an artificial reef, increasing the amount of food available, and a decrease in disturbance. The introduction of a solid structure into the water column can act as an artificial reef, providing a substrate for sessile (i.e. immobile) organisms to colonise, which in turn attracts more fish that feed on these organisms, and therefore more food for porpoises. A decrease in disturbance is observed, as no vessels are allowed within the 500 m exclusion zone around the wind farm. This exclusion includes fishing vessels, increasing the abundance of prey for the porpoises, and also reducing the chance of porpoises being caught in nets. A seasonal pattern was also observed in both study periods, with more acoustic detections occurring during winter months (December to March). This correlates with a previous study of the Dutch coast by Camphuysen (2004), which observed most harbour porpoises in the area between February and April. This, however, varies drastically from the pattern seen further North, at Horns Reef (http://en.wikipedia.org), described by Tougaard et al. (2006), which found greater concentrations during the summer months (detailed above).
Recent research undertaken by Dähne et al. (2013) to help understand the effects of pile driving on harbour porpoises utilised porpoise detectors (C-PODs) to investigate habitat use and behaviour. C-PODs were deployed at 12 sites between 1 and 50 km from the centre of a wind farm in Germany (https://en.wikipedia.org/) from 2008 to 2011. C-PODs were programmed to record the length of time, greater than ten minutes, without detections, referred to as the waiting time, following the commencement of pile driving. Researchers found that during pile driving, C-POD locations closest to the operation had the longest waiting time, and this decreased, as distance from the piling increased. Also, as duration of pile driving increased, waiting time at all sites increased. During piling, higher detection rates were recorded at locations 25 and 50 km from the pile driving location, suggesting that porpoises from the piling area were driven to these locations. This study also found seasonal variation in all years at all sites, with highest detection rates from September to January, and lowest between April and August; however, as there was regular pile driving taking place it is debatable as to how much can be inferred regarding this seasonal information.
The effects seal scarers (also termed Acoustic Deterrent Devices, ADDs or less favourabley, Acoustic Harassment Devices, AHDs) have on harbour porpoises was investigated by Brandt et al. (2013). The study occurred in the German sector of the North Sea and utilised C-PODs to identify the deterrence effect of a seal scarer on harbour porpoises. To achieve this, sixteen C-PODs were positioned at varying distances up to 7.5 km from the seal scarer and data recorded from mid-July to the end of November 2009. Each trial used a seal scarer active for four hours with no other noise source in the water, including the boat’s engine and sonar being switched off. Ten trials occurred and each was separated by at least four days. To prevent bias, C-PODs were regularly rotated between different positions. Two aerial surveys were also undertaken on 10th August 2009, one before the seal scarer was switched on and the other whilst it was active. A significant decrease in porpoise activity was identified whilst the seal scarer was active compared to the time before it was switched on at most C-POD positions. The C-POD locations that did not produce significant decreases in porpoise activity still exhibited decreased porpoise activity. These locations could not be classed as significant because there was a low sample size both in the baseline and during seal scarer operation. During seal scarer activity the porpoise detector closest to the scarer only detected porpoises for two minutes in a 25 hour total period. This represents a decrease from an average PPM of 2.62% before seal scarer activation to 0.12% when it was switched on. The decrease in porpoise detections recorded by C-PODs whilst the seal scarer was active could be due to the porpoises leaving the area, a decrease in echolocation, an increase in directionality of the porpoises’ swimming, or a combination of these behavioural changes and the porpoises leaving the area. Seal scarers do not appear to cause any long lasting effects, as porpoises were detected regularly 6 hours after the seal scarer was deactivated. Aerial surveys showed a significant decrease in porpoise density after the seal scarer had been activated compared to the baseline date, with 2.4 harbour porpoises km-2 observed over the entire survey area prior to activation, falling to 0.3 harbour porpoises km-2 after activation. Also, of the 38 harbour porpoises observed before activation, nine were located within 7.5 km of the seal scarer, whereas only one of the four harbour porpoises sighted after activation was within 7.5 km of the seal scarer. The results of this investigation showed that effects of the seal scarer on harbour porpoises can be observed at 7.5 km, which is much further than previously thought. This research highlights the need for careful planning before using seal scarers, especially in regards to fisheries, to ensure that porpoises do not become trapped in close proximity to them.
Research involving porpoise detectors shows that they do not make visual surveys redundant, instead when deployed in conjunction with visual surveys, data produced are substantially better than using either method alone. For the most accurate outcome of any research, it is suggested that porpoise detectors be used in conjunction with visual surveys wherever feasible.
For recent research on harbour porpoises in the Bay of Fundy see www.porpoisedetectors.com.