Rachel Graham, Ph.D.
Associate Conservational Scientist
Wildlife Conservation Society
P.O. Box 37
Punta Gorda, Belize
Dan Castellanos, BlueBelize Tours
Elasmobranch populations are in global decline and many conservation efforts to protect populations are thwarted by the lack of knowledge on their abundance, distribution and behavior. Many species of large elasmobranchs are also highly migratory, such as the whale shark, which places them at increased risk from anthropogenic impacts such as fishing and boat traffic. Studies on elasmobranch movement are gathering momentum thanks to greater availability of new technologies that permit remote monitoring of animals throughout their marine habitats.
There are currently no studies focusing on large elasmobranch movements and use of habitats along the Gulf of Mexico’s topographic features such as Flower Garden Banks National Marine Sanctuary (FGBNMS) and other prominent underwater features located near the shelf edge. The three banks that make up FGBNMS are rapidly gaining a reputation as a site for predictable marine megafauna encounters. Little is known about the populations and behavior of these charismatic megafauna in the Gulf of Mexico. Yet these species are economically important to the tourism industry worldwide, and the sanctuary is no exception.
During the winter months of February and March, the Flower Garden Banks sanctuary hosts tiger sharks (Galeocerdo cuvier), dusky sharks (Carcharhinus obscurus), sandbars (C. plumbeus) and hammerheads (Sphyrna spp.) as well as manta rays (Manta birostris) and mobulas (Mobula spp.). During the spring and summer months the banks host mantas, nurse sharks, dusky sharks and later when the water warms up more, whale sharks (Rhincodon typus)
Knowledge of how and when these animals use the banks in relation to biological and physical characteristics such as primary productivity, temperature, salinity as well as anthropogenic pressures will help sanctuary managers assess the importance and effectiveness of the banks in protecting them throughout part or all of their life cycle. The Flower Garden Banks are truly notable for encounters with the world’s largest planktivores: whale sharks, and with greater frequency, manta rays. Both species are protected in U.S. waters and, as of March 2007, they are further protected throughout Mexico. Requiem sharks (tigers, blacktips, hammerheads, etc.) do not currently benefit from the same protection as the large planktivores and are targeted by longliners operating near the sanctuary. Retrieval of longline gear on the banks’ coral caps further indicates illegal gear use within the sanctuary, however the level of fishing effort on or near the banks is currently unknown.
The Wildlife Conservation Society is currently spearheading research on the sanctuary’s populations of mantas and whale sharks in partnership with the sanctuary. In 2003 we established the collaboration with Flower Garden Banks National Marine Sanctuary to investigate potential connectivity with the Gladden Spit Marine Reserve with a focus on the highly migratory and planktivorous whale shark as our study species. Our collaboration was based on the identification of sharks in Belize versus FGBNMS using images of the sharks’ unique spot patterns. Although the IDs have not yielded a match between sharks sighted, the promise of connectivity remains, particularly in light of recent remote telemetry results showing the movement of two whale sharks from Gladden to the tip of the Yucatan Peninsula (Graham, 2003). The whale shark research in particular compliments that conducted in Mexico, Belize, Cuba and Honduras and will help to determine movements between the sanctuary and key feeding sites located near the Mississippi delta (Hoffmayer et al. in press), and in four Western Caribbean sites (Graham in press): Holbox (Mexico), Gladden Spit (Belize), Jardines de la Reina (Cuba) and Utila (Honduras).
Through the course of the field work, Rachel and sanctuary staff encountered substantially more mantas than whale sharks and subsequently expanded the study to assess manta use of the sanctuary using acoustic and satellite telemetry. The manta research represents the first study of a population and their movement behavior in the Gulf of Mexico, Caribbean or Atlantic basins. Collaborative manta research is expected to begin shortly in the northern Gulf of Mexico and Mexico’s Yucatan Peninsula.
top of page
Whale sharks - occurrence of individuals using spot patterns.
Satellite tagging of whale sharks.
Acoustic tagging of elasmobranchs.
Population study and photo-identification catalogue
Rachel is collecting images of whale sharks and mantas, taken by recreational and scientific divers, to determine the size and structure of the visiting populations of mantas and whale sharks at Flower Garden Banks National Marine Sanctuary. Individual animals are identified by their unique spot patterns. With mantas, the focus in on their undersides. With whale sharks the focus is on the region behind the gills and the first dorsal. These images are then compared to those gathered in Belize and Honduras for possible connectivity between these sites. To date, we have identified 10 whale sharks at the sanctuary and Gulf of Mexico oil platforms and at over 60 manta rays.
top of page
Marine ears: the acoustic tagging program
To assess whether mantas and whale sharks are resident or “site faithful” we are attaching coded acoustic transmitters to animals encountered at the Flower Garden Banks National Marine Sanctuary using intra-muscular darts applied with a polespear. Each tag possesses a unique code to identify each individual tagged and a battery life of approximately three years. The acoustic transmitters or “pingers” are used in conjunction with Vemco’s VR2 passive acoustic receivers. These instruments are attached to an instrument rack (i.e. modified railway wheel) at 24m depth next to water quality equipment that records temperature, conductivity, pH and dissolved oxygen. Each time an acoustically tagged animal passes within 500m of a receiver, the pinger signals to the receiver to date and time stamp the visit. If the animal stays within range of a receiver for a period of time, the duration of its stay will be recorded. However, we do not yet know where the animals are going outside of the receiver range and the intricacies of their diving behavior. To this end, we hope to expand the behavioral study in the near future with the deployment of satellite tags that will record their diving depth, ambient temperature and large-scale movements.
top of page
Results to date:
Although this research initially began in 2004 with the deployment of two receivers and tagging of two animals, the array was marred by losses and malfunctions. After Hurricane Rita and Katrina's impacts in 2005, we started over again in March 2006 with the installation of an array of three receivers on the coral caps of East, West and Stetson Banks. During the August and September Coral Spawning cruises, we met with good success thanks mostly to Dan Castellanos' superb tagging efforts.
Emma Hickerson has been working with many of the sanctuary’s visitors to retrieve images and video taken of mantas over the past three decades. Due to Emma’s fantastic videography, photography and organization skills coupled with the generosity of many underwater photographers we now possess a historical photographic identification catalogue of mantas at the sanctuary. These images have enabled us to identify at least 32 individual animals to date and we still have much of the video footage to review. Our encounters and underside shots indicate that the manta population is mixed and the majority of sighted or tagged animals are under 2.5m in disc width which suggests that most mantas encountered at the sanctuary are juveniles. This is based on the observed lack of development of claspers for males and the limited current knowledge of size at maturity for both males and females.
Our acoustic receivers have yielded data that confirms manta movements and hence connectivity between the sanctuary’s three banks. One animal tagged at Stetson moved to East Flower Garden Bank (40 miles) and then to West Flower Garden Bank (11 miles); two animals tagged at East Flower Garden Bank moved to West Flower Garden Bank, with one individual returning to East Flower Garden Bank shortly thereafter. At this stage, none of these movements can be characterized as migrations. We need much more data before we can determine if these movements form patterns held by males and females, juveniles and adults. However, two mantas showed strong site fidelity to East Flower Garden Bank and may be residents.
Next steps: We will deploy additional acoustic tags on mantas and any whale sharks encountered during the summer coral spawning cruises and implement a collaboration with Tim Clark, a colleague who will soon implement a manta study at the northern Gulf’s oil and gas platforms.
As a final note, thanks again to all those who have contributed their images or video clips to the photo identification effort. Should you know of anyone who has old underwater photos of whale sharks or mantas and can remember the date and location of the encounter, please have them contact Emma Hickerson so that we may add these images to our growing catalogue. Please note that these images will only be used for research and outreach purposes, although Rachel may ask to use selected images to raise funds for the research. In all cases, the images will be properly credited and we will ask if we may use them for these purposes.
top of page
Earlier work on manta rays at Flower Garden Banks National Marine Sanctuary was conducted by Jeffrey N Childs (2001), a graduate student from Texas A&M University.