Current Waters: Dusky Sharks

We’re in the thick of summer this late in July. The fishing is still great. Striped bass fishing is still going strong. The new moon brought a batch of bigger fish to some areas. So we’re having fun with them. We’re still able to get some of the top-water bite at times. The water definitely cooled down a bit with those bigger tides, so that will help continue the pattern.

We’re also still having fun with our bluefish. The trips we’ve had to Great Point this past week have produced.

For fluke, we’re getting them but I certainly hope it continues to improve. There has been no consistency to black seabass.

Rumor has it we have a few bonito around – though I haven’t seen any myself yet.

Offshore, the tuna fishing has been great to the Nantucket fleet. There’s a good stretch of weather ahead of us. Anytime offshore in this weather is great. Add a bluefin tuna to that and you have a fantastic day!

On our bluefish trips, there are often sightings of silhouettes of sharks. Think of the long shadow cruising around the boat. Everyone wants to have a great white experience and/or photo. We do get a sighting of great whites sometimes, but there are a lot of other shark species cruising around Great Point as well. One of the larger ones we encounter pretty regularly is the dusky shark.

The dusky shark (Carcharhinus obscurus) is one of our larger sharks which can reach up to 12 feet or so at maturity. This large shark species has a broad, rounded snout, and triangular saw-edged upper teeth. They have a small flap of skin on the outer side of each nostril. Their curved pectoral fins are moderate in size, and they have an interdorsal ridge on their backs. The dusky is blue-gray to dark-gray on the dorsal side, white on the ventral side. They are sometimes difficult to distinguish from large sandbar sharks. Sandbar sharks have higher first dorsal fins originating farther forward. Sandbar sharks also don’t get as big as duskies.

Dusky sharks are primarily found in neotropical waters. We’re basically at the northern range limit of the species here on Nantucket. The northwest Atlantic population of dusky sharks occurs from southern Massachusetts to Florida, the Caribbean Sea, and the Gulf of Mexico. The range of the dusky shark extends worldwide, though somewhat disjointed. They’re also found in the eastern Atlantic, in the Indo-West Pacific, Red Sea, Mozambique, South Africa, Japan, China, Vietnam and Australia. This is a highly migratory species generally found in coastal and offshore waters at depths between 200-400m. They undertake lengthy migrations up and down the coast in summer and fall.

They can be found at great depths, but this time of year, duskies migrate to coastlines where they overlap with other shark species usually found inshore. Like many migratory species, they are here during the warmer months and move back towards the equator as waters cool.

As generalist apex predators, duskies feed on bony fish, other sharks, skates, rays, cephalopods, crustaceans, and occasionally on mammalian carrion, and even garbage. In the northwestern Atlantic, around 60% of the dusky shark's diet consists of bony fishes, from over ten families with bluefish and summer flounder being especially important. A good indicator of why they hang out at Great Point. Cartilaginous fishes, mainly skates and their egg cases, are the second-most important dietary component, while the lady crab is also a relatively significant food source.

Dusky sharks are a protected fish on the prohibited harvest list. If caught, they should be released immediately and not removed from the water.

Duskies are not known to be aggressive (unless provoked). However, their behavior makes them vulnerable. They are among the slowest-growing, latest-maturing of large sharks making them more vulnerable to overfishing than some other sharks. Their very low reproductive rate also contributes. The International Union for Conservation of Nature has assessed this species as Near Threatened worldwide and Vulnerable in the northwestern Atlantic and the Gulf of Mexico. The American Fisheries Society has also assessed North American dusky shark populations as Vulnerable.

Full-grown dusky sharks have no significant natural predators with overfishing being the primary threat to the species. Major predators of young sharks include the great white shark , the bull shark , and the tiger shark.

Research by PhD student Caroline Collatos on sandbar sharks which we highlighted last summer, will also help us learn more about the dusky sharks around Nantucket shores. Stay tuned as we learn more!