Location of the study site in the Hawaiian Archipelago
Over-eager eco-tourists intent on seeing spinner dolphins up close
may inadvertently be disturbing the charismatic animals' daytime rest
periods and driving them out of safe habitats in bays along Hawaii's
coast.
Scientists at Duke and Stony Brook universities have developed a
promising new tool that may help to limit repeated human disturbances
and help reduce their negative impacts on the dolphins.
"Using the maps produced through this study we can identify the bays
where the effects of human activities on spinner dolphins should be
monitored most closely, and where immediate conservation actions are
required," said David W. Johnston, research scientist at Duke's Nicholas
School of the Environment.
The researchers' tool shows that only a small number of bays – 21
out of 99 – in a study area along the western coastlines of the main
Hawaiian islands were suitable habitats for resting dolphins. Knowing
this, "conservation efforts can be focused on specific areas of
importance," Johnston said,
"We may be able to minimize detrimental effects on dolphins by
putting restrictions or preventative measures into place in a relatively
small number of bays, rather than limiting access to dolphins along the
entire coast," said the study's lead author, Lesley H. Thorne, a
lecturer in marine science at Stony Brook University, who received her
PhD from Duke in 2010. "That benefits tourists and tourism operators as
well as the dolphins."
In the study, the researchers used the geographic coordinates and
key environmental factors – such as water depth and calmness, the size
and proportions of the bays and distances from deep-water foraging
grounds – for hundreds of spinner dolphin sightings made in the study
area between 2000 and 2010. The results appear August 27 in the online,
open-access peer-reviewed journal PLoS ONE.
Spinners are small dolphins famed for their graceful aerial
movements and balletic spins. They are found in tropical and
subtropical oceans around the world and some divide their time between
daytime rest periods in shallow, protected bays and nighttime foraging
in more exposed waters.
Distinguishing between sightings of resting and active dolphins was
key to defining critical habitats, Thorne said. While socially active
spinner dolphins are more tolerant of humans' presence, resting dolphins
will leave the safety of a bay and retreat to less suitable open waters
if humans repeatedly disturb them.
"Sleep is essential for most animals," added Johnston. "When
deprived of their necessary 'zzzz's,' they gradually show a decreased
ability to process information and remain attentive to environmental
stimuli. In technical lingo, we call this a 'vigilance decrement'."
Spinner dolphins are no exception to the rule. The researchers say
dolphins that experience human harassment every day during their rest
periods never fully recover their vigilance decrement. Their ability to
forage successfully and detect the presence of nearby predators is also
degraded, and their ability to produce sounds to communicate and
navigate may also be impaired, he said.
Scientists and conservationists have long worried that spinners'
popularity with tourists – and overlap of their resting habitats with
popular ocean recreation destinations – may be placing them at risk.
Reports of interactions have increased sharply in recent years, but few
published studies have examined the detrimental impacts these
interruptions may have on the animals, especially at the population
level.
"It would be next to impossible to survey spinner populations and
human activities in every bay that might be a resting habitat," Thorne
said. "We're talking about hundreds of bays in the Hawaiian islands
alone."
"Using predictive models, such as the maximum entropy spatial
modeling approaches we've produced, is a much more cost-effective
method," she said. "This type of modeling has only recently been
applied to the study of marine mammals, but our study suggests it may be
especially useful."
Thorne and Johnston plan to test their models by conducting similar
studies of spinner dolphin distributions and habitat use in the
Northwestern Hawaiian Islands and other sites in the Pacific islands
region. Results from those studies, they said, could confirm the new
models' usefulness.
Johnston and Thorne's co-authors on the PLoS ONE study
include Dean L. Urban, professor of landscape ecology at Duke's Nicholas
School, and Lars Bejdar, associate professor at Murdoch University,
Australia, and adjunct assistant professor at Duke.
The data on spinner dolphin sightings used to develop the models in
the study were provided by a team of eight additional co-authors from
Murdoch University, the Pacific Islands Photo-Identification Network,
the Cascadia Research Collective, the Hawai'i Marine Mammal Consortium,
the Hawai'i Association for Marine Education and Research, the Dolphin
Institute, the University of Hawai'i (UH) at Hilo, UH at Mānoa, and
Marine Mammal Research Consultants.
Reference: "Predictive modeling of spinner dolphin
(Stenella longirostris) resting habitat in the main Hawaiian Islands,"
L.H. Thorne, D.W. Johnston, D.L. Urban, J. Tyne, L. Bejder, R.W. Baird,
S. Yin, S.H. Rickards, M.H. Beakos, J.R. Mobely Jr., A.A. Pack, M.C.
Hill. PLoS ONE, August 24, 2012. DOI: 10.1371/journal.pone.0043167
Contact: Tim Lucas
tdlucas@duke.edu
919-613-8084
Duke University
