A Sixgill Shark on a Longline: What One Observer's Catch Taught Scientists in Hawaiʻi

It was a calm late-spring day northeast of the Big Island of Hawaiʻi when the crew of a longliner felt something heavy pull on the line. As they hauled it in, an animal surfaced that nobody aboard could name.

"The first thought for me was, 'We got a whale!'" said the fisheries observer on board, there for NOAA Fisheries' Pacific Islands Regional Office to record the catch and the crew's work. "But it looked 'one-off.' The tail was wrong."

Keeping the catch

Some of the crew wanted to cut the line. The observer asked them to bring the animal aboard instead, and counted its gill slits: six on each side, where most sharks have five. "I knew it was a sixgill," the observer said. "I don't know much about it but I can count six gills."

The shark on deck: six gill slits on each side, one more than most sharks have. NOAA Fisheries.

The captain saw no money in a live, inedible shark and argued for throwing it back. The observer argued for its value to science, answering each objection until the crew agreed. With the deck boss's help, the nearly 9-foot shark went below on ice under a tarp. In Honolulu, the head of the Pacific Islands Fisheries Science Center's life history program and a shark expert from the Waikīkī Aquarium met the boat. It was the first chance either had had to dissect one.

Which sixgill?

Only six living species of shark have more than five gill slits: four cow sharks (the bluntnose sixgill, bigeye sixgill, bluntnose sevengill and sharpnose sevengill) and two frilled sharks. The scientists hoped for a bigeye sixgill, known around Japan and Taiwan but never recorded near Hawaiʻi. On the dock it looked more like a bluntnose sixgill, a far commoner species and no stranger to Hawaiian waters.

A bluntnose sixgill shark in Pacific Islands waters. NOAA Ship Okeanos Explorer.

The catch still mattered. A pelagic longline fishes shallow water, and the observer put this shark at 200 to 300 feet, where sixgills in Hawaiʻi are usually found at 900 to 3,000 feet. That fits a 2015 study from the University of Hawaiʻi, published in Deep Sea Research Part II, which suggested the sharks rise through the water column at night to follow their prey.

Measuring and sequencing

The shark went into a deep freezer at the science center on Ford Island, Oʻahu. Months later the team thawed it and took 60 measurements, which matched the bluntnose sixgill described in the literature. It was 8.86 feet long, weighed 200 pounds, looked a little thin, and was too small to be mature: males usually mature at about 10 feet and females at 12, and some adults are estimated at 18 feet or more.

Scientists at the Pacific Islands Fisheries Science Center take the shark's measurements. NOAA Fisheries.

Bluntnose sixgills live in temperate and tropical coastal waters of the Pacific, Atlantic and Indian oceans, usually deep but sometimes surprisingly shallow, as in Puget Sound. They are thought to be one global species. To test that, a tissue sample went to a biology professor at the College of Charleston, who compared its mitochondrial DNA with sixgills from elsewhere. It was virtually identical to one from a fjord in Chile and only 1 to 3 percent different from sixgills in Atlantic and Indonesian waters. "At this point, all specimens sampled reveal one worldwide population," the aquarium's expert said, which suggests the sharks travel and breed over large areas of ocean.

The necropsy

About six months after the catch, the two scientists performed a necropsy, an animal autopsy, on the lab floor. Samples of white muscle can show the shark's diet and nutrition.

White muscle tissue, sampled to study the shark's diet and nutrition. NOAA Fisheries.

A huge liver, which keeps the shark buoyant, filled the length of its body cavity; samples of it can be tested for pesticides, heavy metals and other contaminants. Because the observer had iced the shark so quickly, it had hardly begun to decompose. Its ovaries were barely developed: "Even though it looked like a large shark, it was very much an immature female," the life history scientist said.

The stomach was empty, apart from ice crystals. The shark may not have eaten recently, or may have emptied its stomach after being caught, as some fish do. "People tend to think the animals are like eating machines, but they are not always feeding like that," the aquarium's expert said. The spiral valves of the intestine held only mucus. The team also removed some of the six rows of comb-like teeth, with difficulty, and the palm-sized eyes, whose lenses can be radiocarbon dated to find the shark's age. A sixgill can roll its eyes backward to protect them while it bites.

The bluntnose sixgill's comb-like teeth. NOAA Fisheries.

Why observers matter

NOAA Fisheries says it does not take such samples lightly. Bluntnose sixgills are not rare in Hawaiʻi, but a specimen for biological and genetic study is, so the team took as many samples as it could and wasted nothing. One animal can add a great deal to what is known of a species' distribution, age, growth, reproduction, diet and population structure, the basis for managing it sustainably.

It also shows the value of trained eyes on fishing boats. "For every time we get a specimen like that, there must be 10 other things caught that are unusual that fly under the radar because there are no observers onboard," the life history scientist said. The observer saw it more simply: "This is the kind of stuff I live for. Every day is Christmas out here."

Sources

• NOAA Fisheries, "Revealing the Unknowns of an Unusual Catch"; rewritten in hubnx's own words. Photographs: NOAA Fisheries and NOAA Ship Okeanos Explorer.