January 24, 2011
University of Southern Mississippi ecophysiologist Eric Hoffmayer received a National Geographic Society/Waitt grant to tag and track whale sharks in the northern Gulf of Mexico. Last June while diving with Sylvia Earle and a filmmaking team led by Bob Nixon, Hoffmayer witnessed roughly 100 of the sharks–the largest gathering ever recorded of this, the world’s largest fish species–at Ewing Bank off the Louisiana coast. Several days later, three of the filter-feeding sharks were filmed at the surface near the Deepwater Horizon blowout site, skimming waters coated with spilled oil.
Aboard the research vessel Brooks McCall with the Mission Blue team, Hoffmayer relates what’s known about the Gulf’s whale sharks and some of the many mysteries surrounding their migrations.
By Eric Hoffmayer
Whale sharks (Rhincodon typus) are probably more susceptible to oil than most sharks or other fish because of their surface filter-feeding behavior. It’s not the toxicity of the oil, per se; it’s the physical properties of the oil coating their gills. Once these are coated, whale sharks are going to sink to the bottom. They’re negatively buoyant. We’ll never see the ones that die, which makes it hard to know their fate.
What brought attention to this was our encounter back in June followed, five days later, by NOAA doing overflights and getting pictures of three animals within four miles of the wellhead site, swimming in thick oil. Those animals most likely died.
One question is how many in the population we saw have been affected? And because we know there’s a lot of connectivity, how far-reaching are the consequences of the spilled oil for these sharks? There’s lots of ecotourism going on in other parts of the Gulf and the Caribbean. A lot of people are concerned about whether the animals they’ve seen year after year are going to show back up again in 2011.
We’ve documented whale sharks swimming down to depths of 6,300 feet. If there’s submerged oil somewhere, they have a high likelihood of encountering it. As long as there’s oil in the water there’s still a threat.
Why is Ewing Bank so important? We first encountered whale sharks back in 2002. We were studying Sargassum (a seaweed genus) in the Gulf, and we came across a school of yellowfin tuna. In the middle of this school there were two whale sharks. We knew they were in the Gulf, but no one had documented their presence.
So I started researching, and I found out that’s a very common association throughout the world: Whale sharks and tuna seem to hang together, probably because of a common food source.
One thing we wanted to do was talk to the offshore fishermen, the guys who go several hundred miles out. We wondered if they ever saw whale sharks. We were just amazed at the response we got.
First off, it was “We see them all the time!” This was a classic example of the disconnect between fishermen and scientists: They knew it, but didn’t know to tell us, and we didn’t know to ask. Not only that, but we had four different people tell me they saw groups larger than 20 animals, with perhaps as many as 100 animals at a time. In 2002, that was unheard of anywhere in the world, even in Australia, the Caribbean, other places.
In 2009, I was contacted by a commercial snapper fisherman. He had encountered 44 whale sharks the year before, and he wanted to help out any way he could. We started talking and realized we didn’t have a lot of funding, so he said “Hey, would you come out on my boat? I want to help and bring awareness to this.”
We went out to Ewing Bank, and we encountered on, I think, day three, we don’t know how many whale sharks–at least 30, I would say. We had several more sightings in 2009. We’ve had 19 now in the past three years. The smallest group at Ewing Bank was five.
There’s something unique to that area, and we think it’s the spawning. Every large encounter we’ve had in the northern Gulf, little tunny has been spawning, and the whale sharks feed on the little tunny spawn.
Bob Nixon called me and said Sylvia wanted to come out and take a look. So we went to Ewing Bank. I didn’t think we were going to have the success we had. I was hoping we would see a couple animals, maybe five or ten.
The little tunny started circling below our boat. They were shining, and I was thinking “I don’t know what little tunny spawning behavior is like, but I’m hoping this is it.” Sure enough, I awoke at about five in the morning to screams of “Sharks all around us!” And it just got better as the day went on.
We think what is going on is that these fish are spawning maybe 50 or 60 feet down, maybe as much as 200 feet down, and their eggs are slowly rising to the surface. The whale sharks are there at depth and follow the eggs to the top. The eggs would concentrate once they reach the surface.
The big question is this: How did 100 whale sharks find this event, a 12-hour event? How did they know to show up? They weren’t there the day before, because we were there looking and didn’t see them. They were there at the perfect time to be there, and the event’s over and done in 12 hours. Do these whale sharks stay together, do they disperse, do they come back? What do they do in the winter time? Where are they going, what are they doing now?
Those are some of the questions I’m hoping to answer in the future by tagging and tracking multiple animals at the same spot.
Speaking as a scientist, this oil spill in the Gulf of Mexico sort of caught us off guard. We don’t know a lot about many of these animals. Whether it’s whale sharks, tiger sharks, makos, whatever, we don’t know what their habitat use is in the region. We don’t have the baseline data. Without understanding how they use this environment, we don’t know how the spill will affect them.
Support for the Mission Blue Gulf of Mexico expedition is provided by the National Geographic Society, Google Inc., the Waitt Institute, and Hope Spots LLC. Follow along in context by clicking on the ship icon near Pensacola, Florida using Google Earth.
Read all Mission Blue expedition coverage here.
Photos of Sylvia Earle diving with a whale shark by Bryce Groark; photo of Eric Hoffmayer reviewing video footage with Sylvia Earle by Ford Cochran