Oct 5, 2012

Stingray Census, Grand Cayman, July 2012

by Guy Harvey

The annual census of the stingray population in Grand Cayman was conducted the week of July 10 – 13, 2012. The research work was done by personnel from the Guy Harvey Research Institute in collaboration with the Cayman Island Dept. of Environment. The difference this year was that three veterinarians from the Georgia Aquarium visited to assist with analyzing the health of the stingrays.

The situation at the Sandbar in North Sound is unique, with a large number of rays, which are wild animals that are not fenced or contained but inhabit the shallow clear water with accessibility every day of the year. The socio-economic value of the rays to the Cayman economy is enormous. Rays are slow growing long lived animals, a close relative of sharks. Each animal may generate USD 500,000 in revenue per year therefore in its life time assuming they live more than 20 years, may generate USD 10,000,000 in a lifetime.

From a historical perspective, it is worth setting out the track record of research work conducted on the population of stingrays in Grand Cayman. Research was started by the GHRI in 2002 when all the stingrays that frequent the two main sites were caught by hand and tagged with a PIT (passive integrated transponder) at the base of the tail where it meets the disk on the left side of the animal. Tag retention remained at 100%, so many animals tagged ten years ago still have their PIT today. This has been a very simple and valuable tool to track the life history and growth rates of these animals.

For the period 2002-2003, one hundred rays were sampled each month over a three day period at the Sandbar. One hundred sixty-four rays were tagged, weighed and measured at the Sandbar over two years. There was never any difficulty in catching a hundred animals. The same situation was experienced in a subsequent census conducted by GHRI in 2005 and in 2008. There was recruitment of new (untagged) rays to the Sandbar and loss of individuals due to migration, natural mortality and possibly some predation. The sex ratio of 90% females to 10% males has remained historically skewed in favor of the larger females.

From 2010, tour operators and casual observations indicated a sudden decline in the number of rays at the Sandbar. The GHRI conducted a census in January 2012 and sampled only 61 rays in the standard three day research period at the Sandbar which represents a significant (38%) decrease in number of rays compared to the last census in 2008. Now that we had some facts, the next step was to find out why? What was causing the decrease in numbers? How would this affect the tourism value of the interactive programme? What action would the Dept. of Environment and therefore the CI Government take to learn more about this potential problem?

The numbers of rays have been constant since research was started in 2002 with recruitment and mortality being well balanced. GHRI personnel ruled out predation by sharks in the January census due to lack of evidence of shark bites (near misses) and the corresponding demise of sharks in the last ten years. However, some tour operators have reported seeing rays injured by sharks from time to time but no more than normal. Fishing mortality (intentionally or by accident) is a consideration. I say this because outside of the Wildlife Interactive Zones (WIZ) this species has no protection and can be removed and consumed by residents. There is no national protection for stingrays.

The health of the rays was another consideration, which is why the GHRI enlisted the support of the Georgia Aquarium veterinary staff. The research work was now becoming much more technical. Dr. Tonya Clauss (Director Animal Health, Georgia Aquarium), Dr. Lisa Hoopes (Nutritionist, Georgia Aquarium) and Nicole Boucha (Senior Veterinary Technician, Georgia Aquarium) all arrived here loaded with equipment to take blood and store these precious samples in liquid nitrogen until analysis could be achieved back in Georgia.

Over three days, the team sampled 57 rays (only 5 males) at the Sandbar (down from 61 in January) with assistance from DoE staff and several volunteers. The team spent a day at the original Stingray City and sampled 11 rays (2 males) and caught 3 rays (1 male) at Rum Point bringing the total to 71 rays sampled. The low number of males generally is cause for concern.

Each ray was caught by hand and transferred to the pool in the work boat where they were measured, tagged and then blood was taken from the underside of the base of the tail. Some of this blood was immediately centrifuged to make counts of white blood cells. The rest was frozen in liquid nitrogen for shipment back to the lab in the Georgia Aquarium.

From the blood samples the vets will be able to determine if the (monotonous) diet of squid fed the rays by the majority of tour operators is affecting the animal’s health. The processing of samples and data will take several weeks. At the end of this process we will have more knowledge about these valuable creatures and how better to manage their supplementary diet and well being.

Overall, a long term plan of monitoring the numbers of rays and their health is required. Everyone in the Cayman Islands benefits from the presence of this unique marine interactive site. Every advertising campaign or tourism related article featuring the Cayman Islands has these iconic animals up front and prominently displayed. It is time the CI Government returned the favor by supporting ongoing research of the stingrays’ population status and wellbeing by releasing funds in the Environmental Protection Fund collected for this purpose.

More updates to come.

Fish responsibly, dive safely.

Guy Harvey

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