Chelonia Limited

  Cetacean Monitoring Systems

 

Choosing a system

The following information provides a brief summary of the main features of different acoustic and visual methods of monitoring small cetaceans (excluding photo-identification) and how to select a monitoring device.

Small cetacean sounds - what can be monitored

Whistles

Clicks

Louder and fairly omni-directional from the animal, so that detection is easier.

Clicks are highly directional with 3 dB beam widths of around 15 degrees.

Lower frequency so less absorbed by water.

Higher pitch, so larger data volumes if waveforms are stored.

Individuals may be identifiable.

Individuals are not identifiable at present.

Porpoises, beaked whales and some others do not whistle.

All odontocetes click.

No species whistles continually.

Many species seem to click most of the time, particularly porpoises.

Comparing monitoring methods

PODs
 

Advantages

Disadvantages

Data volume less than 1 MB/day.

 

Run for > 4 months

 

Standardised sensitivity.

 

Fully automated detection process.

Unable to track individual animals.

Very low false-positive rate.

 

Behaviour analysis - using train data.

 

Porpoise density assessment possible.

 

Can distinguish porpoises from dolphins.

Cannot distinguish between dolphin species.

Detects all odontocetes...

...except sperm whales.

Static deployment down to 2000 metres.

 

Cost-per-detection is low.

 

Hydrophones and broadband recording
 

Advantages

Disadvantages

2D or 3D tracking of vocalisations from baleen whales and whistles from dolphins using multiple hydrophones has been achieved.

Large volumes of data.

Accurate characterisation of clicks, whistles, etc, is possible.

Data volumes for continuous recording of clicks can be very large - porpoise clicks at 130 kHz need sampling rates of 500 k/s i.e. around 1 MB/s or 1 TB/10 days.

 

Detection is done through a user-designed process generally involving spectral analysis. Standardisation and testing are needed. This process is generally very labour intensive and somewhat subjective. Many projects fail to fully analyse their data.

 

Click train recognition is usually by eye.

 

Cost per detection is usually high.

Towed acoustics*
 

Advantages

Disadvantages

 

The major issue for small cetaceans is responsive movement: porpoises are typically less averse to vessels in places with heavy boat traffic, while dolphins often show attraction to vessels.

 

Cost per detection is relatively high.

* Various systems have been developed and are not described in detail here.

Visual methods
 

Advantages

Disadvantages

The best established methods.

Cost per detection is very high.

Capable of producing absolute density estimates based on well developed theory.

Serious problems in the estimation of the detection rate on the trackline, which is essential to the density estimation.

Give by far the best species identification.

Small studies often fail because too few sightings are made to support statistical analysis unless animal densities are high.

Choosing static acoustic monitors

Listed below is a brief checklist of factors to consider when making a choice between monitoring systems intended for use over periods of weeks or more.

  • Standardisation and sensitivity
  • Time required for data processing
  • False positive rate in adverse conditions e.g. sand in suspension, exposure to varied boat sonars
  • Method of maintaining consistency of data analysis
  • Train recognition methods provides behavioural information
  • Data export formats
  • Robustness
  • Attachment
  • Buoyancy
  • Depth
  • Running time
  • Battery costs
  • Ease of removal of bio-fouling

For C-PODs these are detailed in C-POD specification.