Cetacean Monitoring Systems
PODs have made it relatively easy to both obtain and analyse long records of tonal ultrasound where these tasks were previously logistically too difficult or too slow. Like other new research tools this has thrown up a range of findings, some very unexpected, and many more are likely in the future. Most of these still have no description in scientific publications.
Sediment transport noise is hardly mentioned in current textbooks which mostly give the classic descriptions of ambient noise that come from deep water and concentrate on ultrasound created by wind, wave and rain noise. Use of C-PODs in shallow water shows how prevalent sediment transport noise is wherever the finest sediment on the sea bed is not finer than very fine sand. Sites with strong tidal or wind driven currents show dramatic noise profiles. The example below shows strong neap/spring variation and ebb/flood asymmetry – the black line in the lower panel rises with increasing noise.
For cetaceans sediment transport noise levels can be far above the echoes they need to hear, and at the same frequency, and probably cause animals to move to deeper water. This interesting aspect of acoustic ecology merits more investigation.
Construction noise: T-POD and C-POD studies have shown that driving piles for marine wind farms can drive porpoises more than 20km from the source.
ADCP: Acoustic Doppler Current Profilers commonly produce sound audible to cetaceans and C-PODs, which can monitor their duty cycles. Their technical specification may only mention much higher frequency sound outputs.
Acoustic Harassment Devices: C-POD studies have shown exclusion zones for porpoises of up to 7km from AHDs used on fish farms to keep seals away – which they generally fail to do!
Boat noise and sonars: The presence of porpoises only at night has been found in some busy coastal areas where they were formerly seen in the day. Whether this is due to low frequency boat noise or to their high frequency sonars is not known. Detection of the sonars of a slow moving vessel has shown that it was continuously detectable when within 1km range from a C-POD.
Weak unknown train sources: or WUTS first showed up in data from rias in southwest Britain, then in mangrove areas in Australia, then in tidal estuaries in New England and other locations, with particularly dramatic examples from the Gulf of Alaska. A very small crustacean is suspected!
Echo-location acoustics and Click Train structure
Automated train detection allows analysis of much larger data volumes than have been analysed previously and show that many published statements on the distribution of inter-click intervals in click trains are based on inadequate data volumes.
Species differences: A study of Amazon cetaceans has shown a striking difference between the two species in the distribution of their click rates that may be explained by their differing capabilities in using flooded forest. The species less capable of entering the forest, where entanglement is a danger, uses more long inter-click intervals that could enable it to maintain a longer-range map of the complicated waterways.
NBHF versus broadband species: The trains made by NBHF species (porpoises and a few dolphins) differ from those made by the rest of the dolphins in showing less rapid modulation of ICI intervals. This, combined with their less variable click structure makes them an easier target for the train detection.
Prey differences: T-PODs and C-PODs used in the same area over a number of years have shown that the same resident population of bottlenose dolphins may shift the dominant frequency of their echo-location clicks in the absence of any obvious change in ambient noise. This may be an adaptation to prey size.
Acoustic detection of behaviour: Train detection also allows behaviour to be inferred. Several studies have looked for, and found, correlations between visually observed surface behaviour and click train structure.
Porpoise social communication: Distinctive smoothly modulated fast click trains have been noted in porpoise data that correspond in part to the few descriptions of porpoise social communication.
NBHF click variability: The frequency of NBHF (narrowband high frequency) clicks was widely thought, on the basis of early acoustic studies, to be fixed for each individual. C-PODs give accurate measurement of the dominant frequency of these long clicks and show that the frequency rises when inter-click intervals are long i.e. when the animal needs to hear an echo from a distant target, and commonly falls during the feeding buzz.
Click rates and water depth: The most prevalent Inter-click interval in click trains of porpoises have been shown in T-POD studies to be related to water depth, and fit the possibility that animals at the surface inspect the bottom some way ahead of their surface position and match their click rate to the two-way travel time of the click.
Detection range and Landmark sequences: T-PODs and C-PODs have shown that porpoises echo-locate on targets at far greater distances than was previously known. In the example below a porpoise is approaching a buoy with a T-POD below it. The range (initially approximately 150m) and swimming speeds can be calculated.
Click rate range: The longest inter-click intervals are longer in trains from dolphins than NBHF species. NBHF species produce trains with click rates up to about 1250 clicks/sec (surface echoes can easily be identified in POD data).
Cetacean feeding opportunities
Habitat modification: A detailed study of a new wind farm showed nocturnal porpoise feeding activity over the rock armour associated with the presence of fish in the water column.
Oil rig in a storm: Bursts of porpoise feeding activity have also been shown around marine oil rigs following severe storms, perhaps due to scouring around the legs of the rig exposing prey for fish that move in to this new feeding opportunity, and are then hunted by porpoises.
Diel and seasonal activity
The long recording times of C-PODs allows detailed study of these patterns.
Lunar cycles: In shallow water porpoises commonly show both tidal and daily cycles of activity and may switch between the two in different phases of the lunar cycle i.e. the spring/neap tidal cycle. The auto-correlation of dolphin detections is shown below. The X-axis units are tidal cycles. At this site tidal patterns strongly determine the presence/absence cycle.
Beaked whales: In deep water very strong diel patterns can be seen in the activity of beaked whales:
Seasonal patterns and visual surveys: Seasonal patterns in some locations are so strong that it is clear that visual assessment of use of such sites would be very misleading unless it was based on multiple surveys through the whole year. The graph below shows seasonal presence of porpoises at a high density site in southwest Britain.
Data: Dr Matthew Witt Environment and Sustainability Institute, Exeter University, UK.
Bed time for dolphins: A C-POD study of bottlenose dolphins in a sea pen showed a daily period of nearly complete acoustic silence from late afternoon to midnight.
Killer dolphins: Bottlenose dolphins are known to attack and kill harbour porpoises in various locations. C-PODs have shown non-overlapping use of habitats by the two species in relation to seasonal, diel and tidal patterns.
Extremely low densities: … make all line transect methods impractical as the cost of a single sighting becomes huge. Use of PODs to assess porpoises in the German Baltic was able to show both that they were regularly present and that they still showed seasonal patterns similar to those reported before their massive decline.
Measuring very small changes: This is very difficult with line transect methods because the number of sightings is usually too low to give usefully narrow confidence intervals. A feasibility study on the use of acoustic monitoring to detect a 5% decrease or 4% increase in Vaquita Phocoena sinus in 5 years is reported in the NOAA Technical Memorandum Assessing Trends in Abundance for Vaquita using Acoustic Monitoring.
Cetacean conservation and Fishery studies
Cetacean predation on nets: Acoustic studies of possible predation by bottlenose dolphins on fish caught in gill nets in Spain showed that the dolphins were present at night when net damage was found.
Pinger efficacy: C-PODs have shown that pinger efficacy in keeping cetaceans away from the location of a pinger can be very efficiently demonstrated using a cycling pinger that is ON for 21 hours and OFF for 21 hours. Such pingers can be obtained from Fishtek Limited or via Chelonia.
Cetacean response to nets: PODs on nets have shown that entanglement is an unusual result of porpoises encountering bottom set gill nets, and not the usual result as was previously thought.
Species range: POD studies of the areas used by the Maui's dolphin or popoto (Cephalorhynchus hectori maui) have been able to show that the dolphin uses areas in which it has rarely been sighted. This fits experience with other small cetaceans that are hard to see or use specific areas only at night.