Galapagos sharks, silky sharks, and pilot whales
A Galapagos shark investigates the camera (image courtesy Alp Gokgoz).
Over the past few months we've done a lot of field work in 500m+ water. While it tends to add a lot of logistical challenges to our work and induces seasickness in a number of our visiting students, it also provides us with some exciting moments that make the daily routine worthwhile.
Among them, in between hauling and coiling 1 km+ lines into 55-gallon drums, is the occasional chance encounter with pelagics that would normally go undetected past the 'wall' that we regularly dive and snorkel, but seldom venture beyond. The past few weeks have provided those opportunities pretty regularly as we have begun sampling gulper sharks at the deeper end of our fishing range.
Two of the predators we've encountered are the silky shark (Carcharhinus falciformis) and the Galapagos shark (Carcharhinus galapagensis), both globally distributed in warm and temperate waters and historically abundant in numerous locations worldwide. They seem to be drawn to the low frequency hum of the boat as we go in and out of gear in a relatively barren seascape.
Here, few data are available on their populations and no work has been done on those inhabiting Exuma Sound. My only previous experience with either of them was during a research expedition to an area called 'The Bridge', an undersea spit of land connecting the southern tip of Eleuthera to Little San Salvador (aka Half Moon Caye), seen below.
There, sharks are highly abundant and dwarf the common individuals we find around Cape Eleuthera. Researchers from here at CEI caught and tagged numerous tiger sharks around the 4-meter mark a few years back, and also encountered a few silky sharks and one Galapagos shark during bi-annual surveys along The Bridge. Towards the northern end of Exuma Sound, however, our knowledge is limited to now 2 silky sharks that we've caught to date, and the one Galapagos shark that just last week investigated our discarded bait.
Given their nature as inquisitive predators in blue water, both sharks have a tendency to check out unknown objects and have a certain persistence that lends a hand to our fear of big ocean predators. The Galapagos shark did not disappoint as it came right up to us time and time again.The silky shark from two weeks ago, in contrast, was the perfect shark for visiting students from Queens University (Ontario, Canada) who we convinced to hop in after some hesitation on the boat. The small shark was curious but did not approach so closely, and the afternoon was a self proclaimed once-in-a-lifetime experience for our guests... one which we were happy to be a part of.
A silky shark investigates the boat while we haul the cage (photo courtesy Kat Magnenat).
Most recently, we left the dock on a Tuesday afternoon to haul our longline and hopefully catch some gulper sharks to increase our sample size and break our current bad luck streak with the Island School students. A visiting program was with us as well, typically a source of good luck, so we figured our chances were pretty decent to catch something. We're very superstitious. On this occasion, one of the two boats spotted a pod of 15-20 pilot whales (Globicephala sp.) resting at the surface. We observed the whales, which are actually one of the largest members of the dolphin family (Delphinidae), for around 30 minutes before heading over to the line and pulling up four Cuban dogfish amid a boil of baitfish getting chased by tunas from below and seabirds from above. A few members of the group hopped in to check it out and reported a few large requiem sharks around as well... Overall it was a fantastic afternoon for our team as well as for the fifteen eighth graders who were supporting our research that day.
A Cuban dogfish swims back into the deep ocean. As our sample size is now quite large, we are releasing most individuals of this species while in search of gulper sharks (photo courtesy Chris Ward).
Check out the video below highlighting some of our recent interactions and lastly a big thank you to the PADI Foundation and Sigma Xi Scientific Research Society for their recent, and generous, financial support.