Are They
Absent or Just Undetected: Using Sound to See Secretive, Rare Marine Species?
Tracey Rogers - tracey.rogers@unsw.edu.au
Michaela. Ciaglia - m.ciaglia@unsw.edu.au
Carolyn. J. Hogg - m.ciaglia@unsw.edu.a
Evolution &
Ecology Research Centre, School of BEES, University of New South Wales
Australia
Douglas Cato
Defence Science
Technology Organisation
NSW, Australia.
Colin Southwell
Australian
Antarctic Division
If we cant see
animals because of their naturally secretive behavior,
how do we know they are really thereand in what numbersso that we can make
responsible conservation policy decisions about vulnerable habitats?
This situation is
exacerbated in Polar Regions where peculiar logistical difficulties of working
in pack ice make survey effort enormously expensive, so a further challenge is
to gather data cost-effectively.
Acoustics, seeing
animals with sound using an animals vocalizations as a proxy for field
sightings, can be a simple, cost-effective tool. Some marine animals have
stereotyped long-range signals which are ideal for cost-effective, repeatable
surveying. We have been modelling sounds-per-animal over a unit of time
as a relative population index for a given speciesa sound census. We are
working with the leopard seal, an Antarctic top predator that is difficult to
survey (Figure 1). We have been following changes in their behaviour on
disparate sides of the Antarctic continent. Weve been using their
acoustic behavior to follow changes in their spatial
behaviour as all five of the call types produced by the leopard seals within
the Davis Sea off Eastern Antarctica, are in a stereotyped fashion. Adult male
leopard seals call at a constant, stereotyped rate, and as we have an
understanding of age-related differences, the number of leopard seal calls over
a given period can be converted into the number of vocalising seals present at
that time (Figure 2).
This kind of model
requires information on the production of vocalizations including seasonal
calling patterns, daily calling patterns, and vocalizations that identify
individuals and gender. A calls usefulness, in this context, is the interplay
between the stereotypy of the calls production with its signal propagation characteristics
which determine its detection threshold distance, which is the area over which
it can be detected.
We are enormously fortunate to work within a field where our
international colleagues from diverse disciplines are pushing forward the frontiers
of possibilities. The advances in sophistication due to acoustic engineers,
physical acousticians, oceanographers and modellers, has allowed ecologists
like myself to explore cost-effective ways, using sound, to glean information
about the behavior of secretive, rare, or low‐density marine
species, which not that long ago was not possible.
Acoustics offers
enormous possibilities for marine studies, and although we used a simple system
-- recordings from remotely deployed hydrophone units (sonobuoys) -- the recent
increase in sophistication of the capabilities of acoustic devices provides an
array of systems from stationary units consisting of single or multiple
hydrophones, single hydrophones deployed on individual animals, to multiple
sensors on towed or bottom-mounted hydrophone arrays, providing data across a
range of spatial scales. Additionally, passive-acoustic surveying is a robust
data collection system. Data is largely independent of collection error and
inter-observer bias, and it can be archived providing useful information on
multiple species, not just the target species at the time of the study, which
could prove useful if we are to see long-term community composition change.
Significant
changes in physical and biological systems are occurring on all continents and
in most oceans, but the concentration of data available has been biased towards
terrestrial systems. Recent work is revealing that both the abiotic
changes, such as ocean circulation and ocean chemistry, along with biological
responses are substantially more complex within the ocean than within
terrestrial systems. Also, synergistic effects between climate and other
anthropogenic variables, particularly fishing pressure, are likely to
exacerbate climate-induced changes within the marine system, where following
change is likely to be substantially more difficult, both logistically, as well
as more expensive. Where should we
invest limited conservation/research funds to protect biodiversity? It is
an important question.
The first time I walked up a gangplank to work in Antarctica on
leopard seals was twenty years ago, and at the time the ability to work on
marine mammals within the pack ice was rather limited. Today however, with the advent of sophisticated marine
engineering, coupled with an understanding of behavioural ecology, tables have
turned.
As climatic change
affects a broad range of organisms across geographic boundaries and more often
in logistically difficult regions which are remote, marine and/or polar, using
simple (as well as complex) passive-acoustic methodologies are cost-effective
tools that are robust, archival, and target multi-species within many
systems. Information yielded by these
new approaches can include data about population density, natural history and
habitat useall foundational elements of conservation management and
policy.
Figure
1. Team working with seal in Antarctica
Photo credit: Tracey Rogers
Figure
2. Leopard seal in Antarctic waters
Photo credit: Michaela Ciaglia