1. a critical approach for collecting and analysing data
Aco 04
Passive acoustic monitoring of cetaceans:
a critical approach for collecting and analysing data
Di Clemente J. and Wahlberg M.
University of Southern Denmark, Sound and Behaviour Group, Department of Biology, Campusvej 55, 5230, Odense M, Denmark.
Why Passive Acoustic Monitoring? Marine mammals emit sounds from below 7 Hz to more than 150 kHz (Barker & Lepper, 2012). Passive Acoustic Monitoring is therefore a useful method to assess the impact of human activities on cetaceans and understand their distribution and behaviour. There are many choices of experimental design and algorithms for analysing data.
Pros & Cons: Fieldwork issues. Compared to the classical visual survey, PAM surveys can detect cetaceans at up to ten times longer ranges (Rankin et al., 2007). Moreover, acoustic surveys are not critically affected by weather conditions and can be carried during night-time (Mellinger & Barrow, 2003). It is important to carefully plan detector deployment. Currents, tides, storms, and waves as well as the background noise can effect the performance of PAM systems (Dudzinsky et al., 2010). Costs of hydrophones and related equipment - coupled with the long term need for analysis of the large data sets that are generated - needs to be carefully considered.
Fig. 1. C-pods ready for deployment (
Instrumentation Fixed detectors can record for a longer period at lower cost (Thompson and Freidl, 1982), whereas mobile detectors cover a larger area (Mellinger et al., 2007). According to Mellinger et al. (2007), hydrophones can be deployed either in isolation from one another or in large arrays (ranging from 2 to many 100 receivers) for a larger coverage area. With more than three receivers individual whales can be localized from time of arrival differences, given that the detectors are carefully synchronized. Autonomous recorders (AR) acquire and store acoustic data internally. They remain underwater via moorings and must be retrieved before data analysis. C-PODs (Fig. 1) are widely used for detection of click trains of toothed whale, and they are now more used than T-PODs (Fig. 3). They combine an omnidirectional hydrophone with an analysis software that allows filtering out undesired frequency. Real-time monitoring is often difficult with AR (Sousa-Lima et al., 2013). RATS (Real-Time Acoustic Tracking System; Fig. 2): Baumgartner et al. (2008) developed a combination of acoustic tag and four free-floating buoys in an array. The buoys can detect 36-kHz pings and their position and arrival timing are transmitted to a computer on board. Differences in arrival times of the pings from all of the buoys are used to estimate the location of the tagged animal.
Fig. 3. T-pod linked to the seafloor (
Fig. 2. A RATS buoy (
Applications
Large area survey: during oil and gas explorations, PAM is often used in conjunction with other survey methodologies (satellite tracking, aerial, visual) to assess spatial patterns of occurrence of a species (Thompson et al., 2013; Visser et al., 2010). Wind farm: PAM of cetaceans is a popular research tool to investigate whether or not offshore wind farm impacts the biology of cetaceans (Thomson et al., 2010). Determining seasonality: this can be achieved counting the n° of vocalizations in the time period analysed (Širovi´c et al., 2004), or using the amount of energy in the frequency band as indication of n° of calls (Burtenshaw et al., 2004). Dolphins produce sequence of sounds in such short time that matching the n° of vocalizations with the real n° of individuals can lead to bias. Abundance: derivation of the “probability of detection as a function of range” is a useful way to properly estimate abundance of cetaceans. This can be done essentially either using “time of arrival differences” of multiple hydrophones in an array - to assess position (Cummings et al., 1964), or estimating abundance range from received levels (Cato, 1991) or acoustic multipath propagation effects (Cato, 1998). Combining the average cue rate (animal/time) with the n° of vocalizations can lead to assess the n° of animals present in the study area (Buckland et al., 2001). However, for accurate estimates of animal density in a designed area, the position of fixed hydrophones has to be carefully chosen (Mellinger et al., 2007). In PAM projects, behavioural knowledge of the species of study is crucial to avoid bias in assessing abundance, distribution and population dynamics of cetaceans.
IMSP
Species implications: better know your species! PAM research has been extensively used in the last decades to study both toothed and baleen whales.
Frequency. The focal species determines which frequencies the detector should register. Baleen whales emit low frequency sounds, whereas toothed whales emit higher frequencies (Mellinger et al., 2007). Vocal behavior. Some species produce sounds more frequently than others (Mellinger et al., 2007) and are therefore better subjects for acoustic surveys.
Source level. Large whales - like most of the baleen and sperm whale – are well known to produce vocalizations potentially detected by a single hydrophone on arrays at several 10s or 100s of kilometres (Barlow and Taylor, 2005, (Clark, 1995).
Directionality. Mysticetes are believed to generate omnidirectional low-frequency vocalizations (Mellinger et al., 2007). In contrast, Odontocetes produce both omnidirectional whistles and directional high frequency clicks.
Key References Mellinger D.K., Stafford K.M., Moore S.E., Dziak R.P. and Matsumoto H., An overview of fixed passive acoustic observation methods for cetaceans. Oceanography, 20 (4),
Sousa-Lima R.S., Norris T.F., Oswald J.N. and Fernandes D.P., A review and inventory of fixed autonomous recorders for passive acoustic monitoring of marine mammals. Aquatic Mammals, 39 (1),