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 4: CPOD (long term trends in an area)
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).
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.
Methodology:
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
Monica Betts
M.Sc. student
University of Pretoria
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