The order Cetacea is a specialised group of marine mammals referred to commonly, as whales, dolphins, and porpoises (https://en.wikipedia.org). Cetaceans have evolved to inhabit all major oceanic regions, some lakes and rivers, and play a vital role in maintaining a stable and balanced ecosystem.
As offshore industry continues to expand, so does interest in research and conservation of cetaceans around maritime operations (www.marinemammalseismic.com). Offshore industry is subjected to increasing pressure by Governments worldwide, as part of increasing concern over general ‘health’ of the oceans. New guidelines and legislation, such as those set out by the UK’s Joint Nature Conservation Committee (JNCC, http://jncc.defra.gov.uk), Europe’s Marine Strategy Framework Directive (MSFD, http://ec.europa.eu), New Zealand’s Department Of Conservation (DOC, www.doc.govt.nz), and the USA’s Bureau of Safety & Environmental Enforcement (BSEE, www.bsee.gov) and Bureau of Ocean Energy Management (BOEM, www.boem.gov) have obliged Governments to impose policies designed to minimise potential damage to the marine environment. Marine Mammal Observers (MMOs, www.marinemammalobserver.com), Protected Species Observers (PSOs, www.protectedspeciesobservation.co.uk), and Passive Acoustic Monitoring (PAM) operators (www.pamoperator.co.uk), in conjunction with the latest PAM systems (e.g. www.towedarray.co.uk, www.t-pod.co.uk, and www.c-podclickdetector.co.uk), are playing a vital role within non-renewable energy exploration, renewable projects, international research operations, and military exercises.
With regards to potential impacts of anthropogenic (man-made) noise on marine mammals, there have been new protocols that stipulate Environmental Impact Assessments (EIAs) must be carried out before, during, and after offshore industrial activities (for further details see www.ukooaenvironmentallegislation.co.uk). Many EIAs require cetacean monitoring to gauge distribution and abundance, as well as seasonal variation in cetacean populations around offshore operations.
Visual observations are an invaluable tool when monitoring cetaceans. Visual data can be extrapolated to produce population abundance and density estimates at any scale, in a wide range of locations around the world; this is an essential component of EIAs. Regular visual monitoring throughout seasons and over many years can offer an affordable and effective long-term solution to monitoring and management of cetacean populations.
Observations can be made by MMOs from boats or more accurately from cliffs using theodolites. For example, during construction of the Beatrice offshore wind farm in north-east Scotland, visual observations were used to obtain data on effects of pile driving on harbour porpoise (Phocoena phocoena) and bottlenose dolphin (Tursiops truncatus) populations (see www.sciencedirect.com by Thompson et al. (2010).
When appropriate, visual observations can also be obtained by MMOs from aeroplanes or helicopters. This method is very useful when surveying large areas, and due to the speed at which observations can be carried out, the potential for bias caused by animal movement is reduced. This method was used during research investigating the abundance of harbour porpoises on the Dutch continental shelf between July 2010 and March 2011 (see http://documents.plant.wur.nl by Geelhoed et al. (2013)).
Many cetacean monitoring methods are based on the principles of distance sampling, with line or point transects (Buckland et al. 1993). Distance sampling involves the use of line transects, by which a vessel (or aeroplane, for example) travels along a pre-determined route, while MMOs record data. When confined to shore based sampling, theodolites are used in conjunction with point transects. Data are then extrapolated to produce an estimated abundance and density of cetaceans within the area.
Distance sampling can produce information at a basic scale, but more intricate details can be obtained through Density Surface Modelling (DSM) techniques. Geographical Information Systems (GIS) combined with mathematical models are used to produce information explaining population dynamics in relation to environmental variables (e.g. depth, substrata, and distance from the coast). Similar to distance sampling, data can be extrapolated to the entire survey area. Between 1994 and 2005 the Small Cetaceans in the European Atlantic and North Sea (SCANS) project used DSM to compile density maps of areas in which harbour porpoises were most likely to occur, in an attempt to mitigate cetacean bycatch (see http://biology.st-andrews.ac.uk by SCANS (2008)).
PASSIVE ACOUSTIC MONITORING (PAM)
Passive Acoustic Monitoring systems (www.pamsystem.co.uk) are at the forefront of cetacean monitoring. Passive Acoustic Monitoring is split into Static Acoustic Monitoring Systems (SAMS, www.staticacousticmonitoringsystems.co.uk), towed hydrophone arrays (www.towedhydrophonearrays.com), and Digital Acoustic Recording Tags (DTAGs, developed by Woods Hole Oceanographic Institution (WHOI, www.whoi.edu).
Static Acoustic Monitoring Systems (SAMS)
Static Acoustic Monitoring Systems (SAMS) are surface or subsurface moored hydrophones used to detect vocalisations of marine mammals. C-PODs (www.cpodclickdetector.co.uk), and its predecessor T-PODs (www.t-podporpoiseclickdetector.co.uk), are at the cutting edge of SAMS. C-PODs were used recently as part of the international Static Acoustic Monitoring of the Baltic Harbour Porpoise (SAMBAH) project (www.sambah.org) to gauge relative density and distribution of harbour porpoise populations throughout the North Sea (see www.c-podporpoiseclickdetector.co.uk for an up-to-date overview of the SAMBAH project). C-PODs and T-PODs are used commonly in conjunction with visual observations by MMOs (www.marinemammalobservers.co.uk). This multifactorial approach has proven highly advantageous when environmental conditions are suboptimal and/or inconspicuous animal behaviours are such that, visual observations cannot be made. The inclusion of PAM is a valuable means of limiting operational downtime.
The manufacturer of C-PODs and T-PODs, Chelonia Ltd (www.chelonia.co.uk), announced in late 2012 that they were developing a new C-POD, incorporating a new low power CPU which will enable C-PODs to remain operational in excess of one year (www.chelonia.co.uk).
Towed Hydrophone Arrays
Towed hydrophone arrays are a series of subsurface hydrophones deployed astern a vessel. Towed array systems receive and process real time acoustic data. Passive Acoustic Monitoring operators decipher signals and attempt to constrain calls to marine mammal species through careful observation of amplitude, frequency, duration and call contours etc. More recently, PAMGuard detection software (www.pamguard.org) has been used, whereby algorithms are employed to identify marine mammal species (www.dolphindetectors.com). Recent research by Yack et al. (2013) used towed hydrophone arrays in conjunction with PAMGuard software to detect previously inconspicuous beaked whales in the southern California Bight (http://sdsu-dspace.calstate.edu).
In certain circumstances, it is possible to obtain the origin of sound production to determine approximate location of cetaceans in relation to the array; this is achieved by using triangulation methods. Triangulation is the process by which the origin of sound emission can be verified through analysis of data from two or more stereo hydrophones using the Time Difference of Arrival (TDOA) of sound to each of the hydrophones. A radar plot, showing rough loci can then be produced. While not strictly necessary for EIAs, information on distance between sound source and array can be used to estimate detection range of marine mammals.
Digital Acoustic Recording Tags (DTAGs)
Digital Acoustic Recording Tags (DTAGs) are a form of satellite telemetry attached directly to the host by four medical grade silicone suction cups. The DTAGs themselves comprise a hydrophone, orientation sensors, memory unit, software, infra-red data offload system, and rechargeable battery enclosed within a plastic skeleton and sealed by urethane sheeting. Unlike any other cetacean monitoring technique, once attached, DTAGs record continuously for up to 24 hours traveling with the host down to over 1900 m (www.whoi.edu). The orientation sensors are so sensitive, DTAGs can record individual fluke strokes. When used in conjunction with other cetacean monitoring techniques, DTAGs can bridge the gap between marine mammal underwater behaviour and surfacing contact with MMOs. In recent research Tyack et al. (2006) used DTAGs to investigate the acoustic diving characteristics and foraging habits of beaked whales in the Ligurian Sea and off the coast of the Canary Islands (http://jeb.biologists.org).