Silky sharks under the ship’s lights

Location: Roca Partida (70 miles west of Socorro Island), Islas Revillagigedos.

Comments: Captain Dave reports another great dive day at Roca Partida along with an interesting observation that they only noticed 2 silky sharks swimming around under the ship’s lights at night. This is highly irregular as we usually see 30 – 50 animals at night out there and sometimes a lot more!! Our theory is that the number of sharks relates directly to the phase of the moon, ie. on a new moon there is very little reflected light in the water and the overside lights on the Nautilus become highly useful for the flying fish to feed on their prey. Presumably the deck lights illuminate the “silvery” flying fish and in turn make them prey for the silky sharks and dolphins. Conversely, on a full moon, there is so much light on the water that there is no need or significant benefit to come into the ship’s lights. A full moon in the open ocean under clear skies is very bright indeed, and probably does a more than adequate job of lighting up prey!? Hmmm, we will continue testing this theory out. Dave reports that the divers saw 7 silver tip sharks between 5 and 7 feet in length circling at the north end of Roca Partida, along with at least 10 silky sharks and some good-sized Galapagos and hammerhead sharks. One interesting note is that one small silvertip shark apparently chased away 4 larger silky sharks (4 – 5 feet in length) that were hanging around a tag line hanging off the back of the Nautilus Explorer. Not sure on the dominance behaviour of silky and silvertip sharks, but that strikes me as highly irregular behaviour. Only 1 giant manta ray today, unfortunately.

–Captain Mike

Weather: High overcast, winds from the north at 15 knots, seas 3 to 4 feet, air temperature 75°F

Water: Water temperature 73°F (so it has warmed up a LOT again!), visibility 100 feet.

By Nautilus Staff

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