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Meekan MG, Speed CW, Planes S, McLean C, Bradshaw CJA
Australian Institute of Marine Science, Townsville
Report prepared for the Australian Government Department of the Environment, Water, Heritage and the Arts, May 2008
ISBN 9 780 6423 22470
This project aims to: (1) review current monitoring methods and highlight viable data collection and analysis techniques; (2) develop new protocols for genetic analysis to assist in understanding the population structure of whale sharks at Ningaloo Reef; (3) use photo-identification techniques as a basis for mark-recapture and demographic analyses of population and stock structure; (4) use satellite tagging techniques to document migration and diving patterns and to compare these to environmental variables such as water temperature and ocean productivity; and (5) analyse historical databases of whale shark sightings provided by ecotourism operators to determine how abundances of sharks are influenced by oceanographic phenomena and trends in population composition and abundance through time.
The project is an international collaboration among staff from AIMS, Charles Darwin University, University of Adelaide, South Australian Research and Development Institute, CSIRO, NOAA, Hubbs Seaworld, the University of Texas and the University of Perpignan (France).
The results described in this report have been obtained from field work undertaken at Ningaloo Reef, Western Australia during April-May in 2004-2007. This project is ongoing with the next scheduled field work in April-May 2008.
Development of microsatellite markers for genetic tagging proved to be a far greater technical challenge than was originally anticipated and took 24 months to complete. A full library of markers is now available and work has commenced on collating and processing genetic samples from the Ningaloo Reef and other Indian Ocean populations.
We contributed to a worldwide study of the genetics of whale sharks. There was an absence of population structure across the Indian and Pacific Oceans indicating that oceanic expanses and land barriers in Southeast Asia were not impediments to whale shark dispersal. We did, however, find population structure (AMOVA, FST=0.153, P<0.001) between the Atlantic and Indo-Pacific ocean basins.
The global pattern of shared haplotypes in whale sharks is a compelling argument for development of broad international approaches for management and conservation of whale sharks.
We assessed the use of open-access software for automated matching of photo-identification images of whale sharks. We developed an information criterion (IC) algorithm that resulted in a parsimonious ranking of potential matches of individuals in an image library.
The software provided accurate and reliable image matches.
We used the 12-year photographic identification library of whale sharks from Ningaloo Reef to construct Cormack-Jolly-Seber estimates of survival within a capture-mark-recapture framework. Of the 16 model combinations considered, 10 (63 %) indicated a decreasing population.
Assuming relatively slow vital rates, size-biased survival probabilities suggest the Ningaloo Reef population of whale sharks is declining, although more reproductive data are needed to confirm this conclusion.
We interrogated the photo-identification data bases focusing on potential threats to this species. We recorded scars on whale sharks in three Indian Ocean aggregations (Australia, Seychelles and Mozambique), and examined whether scarring (mostly attributed to boat strikes and predator attacks) influenced apparent survival rates using these photo-identification libraries.
Scarring was most prevalent in the Seychelles aggregation (67 % of individuals). Predator bites were the most frequent source of scarring (aside from minor nicks and abrasions) and 27 % of individuals had scars consistent with predator attacks. A similar proportion of sharks had blunt trauma, laceration and amputation scars, the majority of which appeared to be caused by ship strike. Predator bites were more common (44 % of individuals), and scars from ship collisions were less common at Ningaloo Reef than at the other two locations.
We found no evidence for an effect of scarring on apparent survival for the Ningaloo or Seychelles populations.
We conclude that while scarring from natural predators and smaller vessels appears to be unrelated to whale shark survival, the effects of deaths related to ship strike need to be quantified to assist in future management of this species.
Ongoing work aims to quantify the extent of interchange among three major whale shark aggregations, representing the approximate eastern- (Ningaloo) and western-most (Seychelles, Mozambique) extent of the distribution of whale sharks within the Indian Ocean.
Satellite tagging of whale sharks has aimed to determine not only the migratory pathways of whale sharks, but to also understand the mechanisms by which these long distance voyages take place.
We validated location estimates from Pop-up archival Satellite Tags (PSAT), by attaching two PSATs and one Argos satellite-linked transmitter (SAT tag) to one whale shark at Ningaloo Reef. Our findings support the use of archival light data from PSATs to reconstruct the large-scale movements of these animals.
Long distance (100-1000 km) migrations of sharks from Ningaloo were recorded by SAT tags in 2005, 2006 and 2007. Tracks from these tags show that the Ningaloo population of sharks is part of a wider Indian Ocean stock that is likely to encompass much of the south eastern Indian Ocean and the waters of South East Asia.
We used these tracks to investigate how migratory patterns of whale sharks were influenced by geostrophic surface currents. This was done by utilizing a passive diffusion model parameterised with observed whale shark starting positions and weekly maps of surface current velocity and direction (derived from altimetry). Our results indicate that whale sharks departing from Ningaloo are likely to use active locomotion in their migration, rather than surface currents to passively drift.
SAT tags also record and transmit information about diving behaviour by whale sharks. This allowed us to investigate how whale shark dive patterns during long distance migrations were linked with ocean temperatures and dissolved oxygen levels by overlaying 3-dimensional satellite tracks of tagged sharks with oceanographic data.
Whale sharks appear to selectively dive within water bodies of warm temperatures (24.01 < 30 ºC) and high levels of dissolved oxygen (4 < 5 ml l1) for the majority of dives (usually >60%). This pattern of habitat selection may relate to physiological limitations of large aquatic poikilotherns and energetic conservation mechanisms.
Ongoing work includes analysis of SAT tag tracks from tags deployed in 2006 and 2007. In two instances, SAT tags were recovered from beaches at Ningaloo after they had detached from the animal. This allowed the detailed (every 2 sec) records held in the archive of the tag to be downloaded (while attached the tags only transmit summary information to satellites). These are now being compared with oceanographic data collected by Acoustic Doppler Current Profilers and water temperature loggers deployed by AIMS at Ningaloo.
A summary of migration tracks and diving behaviour obtained from the 43 PSAT deployments on sharks from 2002-2007 is also currently in preparation. A PhD student will commence a detailed analysis of the dive records from these tags in April 2008.
Seasonal observations of whale shark abundance recorded by ecotourism operators at Ningaloo Reef from 1995-2004 provide a historical data set that can be used to investigate temporal patterns in abundance of whale sharks in relation to oceanographic phenomenon and decadal trends in population composition and size.
The SOI positively influenced whale shark abundance such that during La Nina years, more sharks were sighted, and fewer were recorded during El Nino years. This may reflect changes in the strength of oceanographic processes such as the Leeuwin Current in response to the Southern Oscillation, which may act to transport sharks to the region and/or affect their prey by driving productivity events.
Analysis of ecotourism records shows that mean shark length declined linearly by nearly 2.0 m and relative abundance measured from ecotourism sightings (corrected for variation in search effort and environmental stochasticity) has fallen by approximately 40 % over the last decade.
This population-level result confirms previous predictions of population decline based on mark-recapture estimates of survival. The majority of these changes are driven by reductions in the number of large individuals in the population.
These reductions have occurred despite the total protection of whale sharks in Australian waters. As this species is highly migratory, the rapid change in population composition over a decade (< 1 whale shark generation) supports the hypothesis of unsustainable mortality in other parts of their range (e.g., ship strike and over-fishing), rather than the alternative of long-term abiotic or biotic shifts in the environment. As such, effective conservation of whale sharks will require international protection and collaborative tagging studies to identify and monitor migratory pathways.