As we set out to the immense Saba Bank to do regular dives last Thursday, our captain spotted two small sharks at the surface. One glance at the surface and I realized these sharks were a bit more special than the regular residents, Caribbean reef sharks and nurse sharks. In fact, I soon started screaming “Silkies!”, as the sharks that were swimming around the boat were silky sharks (Carcharhinus falciformis; I did not scream that scientific name though..). Next we quickly readied our gear and hopped in the water in the middle of the Saba Bank.
The last couple of months I have spent my time on the small Caribbean island of Saba, the place where I focused on shark movement ecology over two years ago. This immense area is the largest submerged atoll in the Atlantic Ocean, one of the most biodiverse areas in the Caribbean Sea and the largest protected area within the Dutch territories.
We set out that morning to dive at known sites of the Saba Bank to collect data loggers and inspect a spawning aggregation site of red hinds (Epinephelus guttatus). At approximately 1.5 to 2 hours from Saba our captain spotted the two sharks, which were most likely attracted by the sound of the boat. As we jumped in two sharks became four, four became seven and the last couple of minutes in the water I counted up to 10 juvenile silky sharks as I swam back to the boat (I did not want to leave..).
Silky or reef shark, huh?!
Silky sharks have, unknowingly, been documented before by scientists working on the Saba Bank, but were misidentified as juvenile Caribbean reef sharks (Carcharhinus perezi). So what makes it a silky shark I hear you ask? Well, there are three main distinctive features of this species that can help you distinguish them from any other shark: (1) the first dorsal fin is rounded, (2) the first dorsal fin originates behind the end (free tips) of the pectoral fins, and (3) the second dorsal fin is small, with a free tip that is almost twice as long as the height of the fin. Personally, I quickly recognize silky sharks by the strong edge along both sides of the head in combination with feature 1 and 2.
Contrastingly, the first dorsal fin of a Caribbean reef shark is more triangular and located more anterior on the body compared to the silky shark. The body of silky sharks is long and slender, compared to the more robust body of Caribbean reef sharks. Also, the free-rear tips of both fins of reef sharks are short.
So why is this important?
Besides that documenting a new shark species for your personal species-list (nerd-alert) is pretty cool, I genuinely believe observations like these are important for a couple of reasons. First of all, to effectively conserve natural areas, you have to get a grasp on their biodiversity. What species occur within the area, and how do they use the area? Up until now, studies (one, two and three) focussed on shark (and fish) diversity of the Saba Bank have missed this species, making observations like these very welcome additions to our knowledge of these coral reef systems and local shark populations.
Although silky sharks are common in the pelagic zone of tropical seas, they’re also listed as ‘vulnerable‘ on the IUCN Red List of Threatened Species. Being a pelagic sharks feeding on schooling fish like tuna, causes these sharks to be a major proportion of the bycatch of pelagic longline and purse seine fisheries. Being caught mostly on the high seas, this species also makes up a large proportion of the international fin trade. On the international Red List, this species has gone from “near threatened” in 2016 to “vulnerable” in 2017, and populations continue to decrease according to the assessment of IUCN. This previously undiscovered population within a shark sanctuary (the Saba Bank is part of the Yarari whale and shark sanctuary) can act as a refuge for this species within the wider region. More importantly, the individuals we observed were all small (± 120 cm total length) and within a small length range, indicating the Saba Bank homes a specific life stage of the population.
Little background story on those guys (and gals)
This species of shark got its name from the silk-looking skin (no joke), and has a circumglobal distribution in tropical waters. Within its geographical range the silky shark mainly occurs in pelagic habitats, although younger individuals prefer shallower reefs (which agrees with our observation). The size at birth is around 75 to 80 cm and the maximum size of this species is 3.30 meters. Males reach sexual maturity at a total length of 215 to 230 cm (9-10 years of age), and females at a slightly bigger size of 230 to 245 cm (± 12 years). Silky sharks reproduce viviparous with a placenta, meaning these sharks feed their pups through a placenta in the uterus (contrasting to oviparous and ovoviviparous shark species), before giving birth to live young. A female gives birth to 6 to 14 pups (although this fluctuates among regions within the species’ geographical range) after a gestation period of approximately 12 months. The diet of the species mainly consists of bony fishes like tuna, mackerel, mullet and other schooling fish. In addition, the diet can also consist of cephalopods and to a lesser extent of crustaceans.
These observations were not only very important to me, but to everyone onboard that day. We spent over 45 minutes in the water with them, without ever using bait or attract them in any other way.
See also this cool video about our encounter with the silkies, shot and edited by Ayumi!
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