Sunday, January 5, 2014

Cetacean Survey – A whole new approach to surveying cetaceans

The Mossel Bay Cetacean Project has been running since 2010 and has previously looked at movement patterns, human impacts and the impact of environmental factors. Since May 2013 we have also attempted to incorporate the use of acoustics in the monitoring of cetaceans. Cetaceans have a reduced sense of smell and vision, therefore they needed to develop other means of communication. Sound travels much faster in water (in water sound travels 1482m.s. -1 whilst in air sound travels only 343.2m.s.-1), therefore cetaceans used this speed to their advantage and evolved highly specialized ‘sound systems’ in order to communicate, echolocate and orientate themselves.

Acoustic monitoring of cetaceans is achieved through the use of hydrophones. A hydrophone is a microphone specifically designed to record underwater sounds. There are two types of hydrophones omnidirectional and directional hydrophones. Omnidirectional hydrophones detect sounds in all directions whilst directional hydrophones are set to have a high sensitivity for a particular direction.

Hydrophones can be divided into 4 categories and use is dependent on specific aims of the project in question.
Figure 1. Simple Hydrophone (Used for monitoring small groups.)

Figure 2. Cabled Hydrophone (Spatial surveys and distributions using transect lines)

Figure 3: Sonobouys (population surveys long term trends.)

 Figure 4: CPOD (long term trends in an area)

In Mossel Bay, we focus on a branch of acoustic monitoring known as Static Acoustic Monitoring (SAM) which is defined according to Marques et al. (2012) as “the use of sounds made by animals to make inferences about their distribution and occurrence over space and time.” The advantages of using SAM are that it operates under any type of light & weather, is cost effective and can be used over extended time intervals. These differences are very distinguishable when comparing to visual land based surveys. Visual based surveys are limited to daylight hours with no rain and little wind. By incorporating acoustic techniques we can look at what they are doing at night or during bad weather.
We are currently using a SAM device referred to as a Cetaceans and POrpoise Detection device or (C-POD). The CPOD is basically a hydrophone along with a data logger that detects echolocation clicks made by odontocetes (C-POD User Guide 2013).  For this portion of the project we will be studying two species of cetaceans known to occur in Mossel Bay namely the Indo-Pacific bottlenose dolphin (Tursiops aduncus) and the Indo-Pacific humpback dolphin (Sousa chinensis). 

Both species are found throughout the bay all year round spending most of their time in groups close to shore. Very little is known about either species according to IUCN’s endangered species list, bottlenose dolphins are classified as Data Deficient and humpback dolphins have recently been classified as Endangered (Still debated among scientists).

The aim of this part of the project has three main areas of focus:
  • Detection ranges of a C-POD for T. aduncus and S. chinensis – Maximum distance clicks can still be detected and the impact of environmental and behavioural factors.
  • Investigate species specific patterns in echolocation clicks - Possible to distinguish the two species based solely on clicks. 
  •  Critical Comparison of visual and acoustic survey methods - look for trends between the two   methods. 

Combining traditional land based survey using a surveyor’s theodolite with acoustic monitoring:
Figure 5: A look at a traditional land surveyor’s theodolite

Set up theodolite at Klein Brak (– see figure in red)

Figure 5. Map of Mossel Bay showing survey locations

Figure 6: layout of C-POD once deployed.

The C-POD is deployed and retrieved on a monthly basis. Data is recorded on a SD which can then be transferred onto a laptop computer for analysis.
Land based surveys take place three times a week where observations are made every half hour for a period of 4 hours. Weather conditions are recorded as well. If a dolphin is sighted then it is tracked using the theodolite making note of their behaviour and overall dispersion.
The majority of the work on this portion of the project takes place behind a computer. Once all the visual data and acoustic data has been transferred to a computer the real analysis can begin. Acoustic data is processed by a program known as C-POD. Here it assigns a probability value to each click segment which enables us to sort out what is dolphins clicking vs. sea noise. It also allows us to import useable data into various spreadsheet for further analysis.
The data from the CPOD will allow us to determine the exact detection range of the CPOD. By knowing the maximum detection distance we will be able to place more than one CPOD in and around the bay ensuring the whole bay is covered. In addition to the detection range if we can distinguish different species from each other based on acoustics it will reduce cost as well was time spent in the field and lastly acoustic surveys will mean we can obtain data in the field 24/7.

This project is currently in its 8th month with at least 5-7 more months of fieldwork. We currently in the process of analyzing data already collected to see if any trends are starting to come through. Although there is still quite a bit more work to do, this project will provide valuable information to researchers regarding the cetaceans we find in this area.

Monica Betts
M.Sc. student 
University of Pretoria

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