[dropcap]T[/dropcap]hose of us living on the Gulf Coast know our lives are distinctly intertwined with the great body of water to the south. Most of our recreation, food and the way we make a living are connected to the Gulf. However, we don’t always have time stop and consider the vastness of this incredible place we call home. This large marine ecosystem provides a home to creatures many of us have never seen in person, for example, whale sharks. Whale sharks— those spotted gentle giants that feed on fish eggs and plankton—are one of the Gulf’s most unique creatures. Whale sharks are the largest fish in the world, and although the Gulf is home to a number of them, little is known about their behavior in the Gulf. Ocean Conservancy interviewed Dr. Eric Hoffmayer, a preeminent whale shark expert and research fishery biologist with the National Marine Fisheries Service, to find out more about this fascinating fish and what research is needed to ensure their protection.
Ocean Conservancy: How much is known generally about the whale sharks found in the Gulf of Mexico? What is the size of the population?
Dr. Hoffmayer: Ironically, even though whale sharks are the largest fish in the ocean, we still know so little about them, specifically here in the Gulf of Mexico. We know from our research efforts, as well as from research efforts of our colleagues in the southern Gulf, that whale sharks are relatively common in the Gulf. Unfortunately, due to their highly migratory nature and preference for offshore habitats, we still do not have a good population estimate for this region. However, colleagues working in the southern Gulf have estimated that between 500 and 900 individuals occur off the Yucatan Peninsula. In the northern Gulf, whale sharks occur along the continental shelf edge from Brownsville, Texas, to the Florida Keys and commonly occur off the mouth of the Mississippi River.
OC: Tell us about the relationship between tuna and whale sharks.
Dr. H.: Whale sharks associate with other tuna species (e.g., yellowfin, blackfin and skipjack) in the fall. Whale sharks work together with the tuna to feed on sardines. Tuna will concentrate the sardines at the surface and whale sharks will position themselves vertically in the water column to feed on the sardines. It is amazing to see a whale shark feeding vertically in the water column. We don’t know how often whale sharks associate with tunas during this time, but there have been days where every large school of tuna that we came across had at least one whale shark in the middle feeding.
OC: What do we know about whale shark movements through the Gulf?
Dr. H.: We know from various tagging efforts that whale sharks move throughout the waters of the Gulf of Mexico. During summer, they tend to aggregate off the mouth of the Mississippi River in the northern Gulf and the Yucatan Peninsula in the southern Gulf. They radiate throughout the Gulf during the fall; however, the biggest mystery is where the sharks reside during winter and spring. Our satellite tag data is relatively limited during this time, due to the short retention times of the tags (less than six months). As technology improves, we are hoping to be able to track individuals for longer periods (one to two years), and we should be able to gain a better understanding of their seasonal movement patterns in this region.
OC: How many Gulf sightings of whale sharks were logged in 2013?
Dr. H.: There were 35 whale shark sightings reported to us from the northern Gulf during 2013, the majority of which occurred during May and June. We have been averaging about 80 sightings a year since 2008, but our numbers were down in 2012-13.
OC: How many whale sharks did you and your colleagues tag in the Gulf during 2013?
Dr. H.: In collaboration with our partners at the Louisiana Department of Wildlife and Fisheries we were able to deploy satellite tags on 10 whale sharks this year. Five sharks were tagged with pop- up satellite tags that provide daily position estimates along with detailed temperature and depth data, and five were tagged with towable satellite position tags (SPOT) that only provide detailed location data. The SPOTs only send data to the satellites when the sharks are at or near the surface. Our data has shown that whale sharks in the northern Gulf spend roughly 80 percent of their time in surface waters, usually coming to the surface 10 to 12 times a day.
While the pop-up satellite tags tend to remain attached to the sharks anywhere from one to six months, the satellite position tags will stay on the shark from about two weeks to three months. We are currently in the second year of a three-year project. Our goal istotagandtrackthemovementsof20to30 whale sharks to gain a better understanding of their movement and habitat-use patterns in the northern Gulf.
OC: Why is whale shark tagging important?
Dr. H.: Our tagging work, which consists of the use of photo identification and satellite tags, will help us answer basic questions about movement and habitat-use patterns as well as help estimate population size for whale sharks in the northern Gulf. For example, we are trying to get a basic understanding of what the population is doing. Is it a migratory population? A resident population? A mixture of both?
Ultimately, if we tag enough sharks, we can start identifying their essential habitat requirements in the Gulf. Then we can compare habitat use based on satellite tag and sightings data to get a better understanding of their movements and distribution. Ultimately, we want to know their long-term migratory patterns.
OC: The Northern Gulf of Mexico Whale Shark Research Program encourages public involvement in this ongoing research effort, and there’s a website for reporting sightings, correct?
Dr. H.: Yes, we are interested in sightings and photographs. As the website will also tell you, we are very interested in photographs of whale sharks, especially underwater photographs from the left side. We can identify individual animals using the unique spot patterns on their left sides. People can report sightings here: http://www.usm.edu/ gcrl/whaleshark/whaleshark_survey.php.