(This blog is part of a series of interviews with scientists who are championing marine research in the Gulf of Mexico.)
A preeminent whale shark expert and ecophysiologist, Dr. Eric R. Hoffmayer is a research fishery biologist with the National Marine Fisheries Service, Southeast Fisheries Science Center, Mississippi Laboratories. His interest in coastal shark species ranges from their reproduction and life history to their specific abundance, distribution and feeding ecology in nursery grounds. He has pursued a particular interest in the Gulf of Mexico’s whale sharks, the largest fish in the ocean, compiling information on their basic biology, habitat use and movement patterns.
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 off 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 (LDWF) 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 is to tag and track the movements of 20 to 30 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: Tell us a bit more about the questions that still need to be answered about whale sharks.
Dr. H.: There are always more questions to be answered. We still need to answer some very basic questions. For example, do whale sharks tend to stay together in aggregations or are they solitary most of the time and only sporadically form these large aggregations? In the southern Gulf, large aggregations of whale sharks occur consistently in relatively localized areas off the Yucatan Peninsula. We are just starting to understand the aggregation behavior in the northern Gulf.
We are also eager to learn more about where whale sharks go in the winter and spring. There is some indication whale sharks from both the northern and southern Gulf regions move to an area in the southwestern Gulf to overwinter. This could represent another seasonal aggregation site for whale sharks in the Gulf. Additional tagging data collected during winter and spring will help answer this question.
Then there are questions about age-based movements. Are the smaller sharks staying local and the larger animals moving? Or vice versa? It appears from our work that the large aggregations we encounter predominantly consist of smaller sharks. Are the larger sharks more nomadic?
We’re also lacking information on multiple-year movement patterns that will help us answer some of these questions. I hope that as technology advances, we will be able to address some of the longer-term movement questions.
OC: What do we know about whale shark aggregation in the northern Gulf?
Dr. H.: Whale sharks tend to aggregate in large numbers in the northern Gulf during summer at some of the topographic features along the continental shelf edge. One of those features that we have been monitoring the last few years is Ewing Bank. We would not have known about these aggregation sites without the help of the various “citizen scientists” who contributed to our Northern Gulf of Mexico Whale Shark Sightings Survey. The numerous reports of aggregations from the public allowed us to direct our fieldwork to places such as Ewing Bank to maximize our research effort.
OC: Why do aggregations occur in certain areas, such as Ewing Bank, west of the mouth of the Mississippi River?
Dr. H.: Most of the aggregations reported to us occurred around Ewing Bank near the continental shelf edge. Other banks may also be utilized by whale sharks, but we need more sightings and tag data to direct us to them. When we investigated the Ewing Bank area we found that sharks were feeding on fish eggs.
Thus far, we have had four aggregation encounters where whale sharks were feeding on little tunny (Euthynnus alletteratus) eggs. All four aggregations documented in the northern Gulf occurred in June, all around the time of a full moon. The little tunny spawn during full moons, so we are conducting surveys around full moons to see if sharks are aggregating and eating these fish eggs. As a side note, four of the five SPOT-tagged sharks from 2013 spent eight consecutive days near Ewing Bank after we tagged them, presumably feeding on little tunny spawn. We need to gain a better understanding of the dynamics of these aggregation events.
OC: The Northern Gulf of Mexico Whale Shark Research Program originated at the University of Southern Mississippi Gulf Coast Research Laboratory, but the program has been expanded in recent years?
Dr. H.: Yes, our Whale Shark Research Program has several partners, including the University of Southern Mississippi’s Gulf Coast Research Laboratory, NOAA’s National Marine Fisheries Service, LDWF and On Wings of Care. In fact, LDWF has been funding our most recent tagging efforts.
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.
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- Stay tuned for more from this series…