The lemon shark's yellow coloring serves as a camouflage when swimming over the sandy seafloor in its coastal habitat. (Creative Commons)
Catching lemon sharks for research to maintain healthy ecosystem
They told us that sharks are like spiders: they’re more afraid of you than you are of them.
This presentation was actually keeping me awake in the sweltering heat of the Cape Eleuthera summer. On the screen swam large, finned figures, footage from ‘The Medusa’ – a specialized deep-water, baited video camera capable of reaching depths of up to 6,600 ft. It seemed a whole different world from the one I knew above the surface of the water. The world where I sat in class, in my Island School polo shirt and khaki shorts, usually fighting the oncoming sleep induced by the fatigue from our morning workouts in combination with the heat.
But today was different. Today we were going out in the field with a team of researchers from the Bahamas’ Cape Eleuthera Institute to catch baby lemon sharks for research. This was an overview of their topic of study.
Sharks are among some of the most threatened fishes in the world’s oceans, they told us, as we watched a huge Bluntnose Sixgill shark, Hexanchus griseus, swim directly into the camera. I raised my hand and asked them about scale… exactly how big were these sharks?
The Bluntnose Sixgill can grow up to 26 feet in length, and is completely harmless to humans unless provoked. However, it is listed as Near Threatened by the International Union for the Conservation of Nature. While the Bluntnose Sixgill is hunted for both sport and food, severely depleting its population, it is also commonly caught as bycatch, the result of unsustainable commercial fishing practices. According to the final bycatch report, published by Oceana.org, some estimates put bycatch as accounting for 40 percent of the world’s catch. That means that for every six fish caught and eaten at a restaurant, four animals are caught by accident and harmed or killed unnecessarily. The result is 63 billion pounds of wasted animal matter each year, according to the report.
And it’s not just wasted matter, it’s a negative space in the ecosystems from which these animals come. Sharks are top-of-the-chain predators; the Cape Eleuthera Institute says they are crucial to their ecosystems, providing the “regulation of the distribution and abundance of species at lower trophic levels.” Without them, the populations of these lower trophic level species would grow unchecked, resulting in the near extinction of the species in the trophic level directly underneath these, throwing off the entire ecosystem. While the overfishing of lower level fish species is already causing major ecosystemic disruption, the elimination of shark species would be catastrophic.
Already, as a group, sharks face possibly the largest global population declines in modern history, which is why the Institute focuses a large portion of its research on shark conservation.
We boarded the research vessel from the main dock, and cut through the clear, blue water towards the flats, a low level ecosystem characterized by mangrove trees, hosting many juvenile fish species including our juvenile lemon sharks. We anchored a ways off shore, to account for the outgoing tide, and started wading towards the stretch of beach on the right side of the entrance, where, on the left, an island created by the low tide bordered the stream of water steadily entering the greater ocean. We walked against this current, holding coolers, nets, and equipment overhead.
As the tide goes out of the flats, the water level gets very shallow and hot. So hot that all of the wildlife must move temporarily out of the flats and into the deeper water just outside of their cozy mangrove-protected home. We trekked against the current to the other entrance, where our job was to run through the flats, stomping and churning, chasing the lemon sharks into our trap at the other end. About 10 of us ran through the water, thigh deep most of the way, sometimes deeper, spanning the width of the outgoing water. We shouted encouragement to each other, needing to keep a steady pace so that we maintained an unbreakable line. It was a work out.
After about 30 minutes of this, we reached the stretch of water where our counterparts had already set up a net between island and shore. As soon as we entered this stretch, they hastily threw another net into the water behind us, closing off the section of water from escape. We were now stuck in the trap with the sharks.
There appeared to be about six juvenile lemon sharks in hot water with us – now we had to catch them. They gave us hand held nets, while the others attempted to chase them towards the ready mesh cloth. This was a difficult but time-sensitive task, as we had to catch and store each shark in a cooler before the tide started back in. After our unsuccessful attempts with the nets, the professionals took over, and had logged five out of the six sharks. We could feel the water changing direction as the tide turned, marking the rapid close of our expedition. We started back to the boat with the five sharks in coolers over head, while two research assistants made a last ditch effort to catch the sixth. Eventually, they were successful.
We arrived back to the Institute’s dock just in time for dinner, but saw the lemon sharks to their temporary homes first. The Institute’s researchers would be measuring the energy cost of their capture, and from there, calculating their daily energy budget in order to further assess their crucial ecological role.
Understanding the role that sharks play in their fragile, local marine ecosystems will help researchers understand the impact that their population declines will cause. This research-based evidence will help scientists influence policy and find solutions to prevent their decline and the concurrent demise of their ecosystems.
The next week we went back to visit the lemon sharks in their tanks. Four of them had already been tagged and released back into the flats, they kept one for study, and another one had died after capture, the energy expenditure being too great. In other words, we had scared it to death.