Tagging sharks a chilling task
|||Catching powerful sharks takes big hooks, lots of line|
By Jan TenBruggencate
Advertiser Science Writer
By Jan TenBruggencate
FRENCH FRIGATE SHOALS — The float ripped through the water, occasionally submerging, dragging a wake, headed directly for our boat.
"It's swimming the float right at us! This is 'Jaws' stuff right here!" biologist Matt Craig shouted.
And just like that, the float stopped moving. We watched as it just sat there, resting on the dark sea's surface.
Welcome to the dangerous world of shark tagging where, inevitably, the job offers jolt after jolt of excitement and anticipation.
Among the goals the researchers on this 25-day trip to the Northwestern Hawaiian Islands have is to learn more about shark behavior and travel patterns.
Tagging the sharks is the one task that reveals the cold ferocity of the natural world.
A couple of minutes after the float stopped moving we hauled up — in one gory gorgonian bundle — a complex tangle of twisted line with the head of one Galapagos shark, the gutted carcass of another and the top half of a third.
The "Jaws" reenactment we had watched outside the reef at French Frigate Shoals may have been a tiger shark feeding on hooked sharks, dragging line and floats around as it tore the trapped animals apart.
Sharks feeding on other sharks is not just a function of the victims being hooked. Biologist Randy Kosaki said new research is showing that tiger sharks feed higher on the food chain than anything else on the reefs — meaning that in addition to other things, they regularly feed on other predators, including other sharks.
Tagging is an amazing process to take part in.
At one point, I held a 10-foot tiger shark by the tail, feeling its fluid but powerful muscles and tendons flex under the striped sandpaper skin.
Another time, I held a 6-foot Galapagos shark by its pectoral fins as its eye watched me intently.
In an instant, the shark twisted and snapped.
"Keep your arm in tight. They can come right around and reach it," said Carl Meyer from the Hawaii Institute of Marine Biology. Meyer's team is charged with putting satellite, sonic and visual tags on the biggest predators around the French Frigate Shoals. The reefs and sandbars lie roughly 400 miles beyond Kaua'i in the Northwestern Hawaiian Islands.
"The Northwestern Hawaiian Islands is in the process of becoming the world's largest marine sanctuary," Meyer said. "You've got to know if the animals you are protecting are largely contained within the sanctuary."
Meyer has been surgically inserting tags in sharks in the main Hawaiian Islands for several years under the auspices of HIMB researcher Kim Holland's lab.
Only in the past few years has he added the northwestern islands, already providing insight into shark behavior.
Tiger sharks, for example, were once believed to be very territorial, but tagging research has showed they clearly can move great distances.
One of Meyer's tigers tagged at French Frigate Shoals showed up at Kona, 700 miles away. Whether the movement is common among the species requires more research.
Meyer's tagging methods are quite invasive, including surgery to insert in their abdomens an acoustic transmitter.
The hardy animals survive the surgery and related tagging procedures quite well. Meyer said the damage they do to each other is far greater than a little slice in the belly.
He has heard from nearly every animal he has tagged with electronic gear, and they continue to move around, indicating a very high survival rate.
During the past week, Meyer did his first installations of a new kind of satellite transmitter. The installation sounds brutal, but the sharks barely flinch. He drills four quarter-inch holes in the cartilage of the dorsal fin, slips the transmitter with its trailing antenna so plastic bolts pass through the holes.
Then, with plastic washers and stainless nuts, he screws the unit down. This tag reports the shark's position to an ARGOS satellite whenever the dorsal fin breaks the surface of the water.
For the sonic tags, sharks are flipped upside down, and Meyer pounds a sharp knife through the tough skin, checks the hole with a gloved finger, and slips in a lipstick-sized acoustic transmitter.
The incision is about an inch long, and he closes it with a single suture. These tags emit a coded sound that is picked up by an array of seafloor receivers positioned up and down the Hawaiian archipelago.
On a few sharks, Meyer also installed pop-up tags that are attached by sliding a small stainless steel anchor into the animal's back muscle.
This tag tracks depth and other data, and after a set period, releases itself and floats to the surface, where it reports the shark's behavior to a satellite.
The tagging is an attempt to learn more about the movement of the marine animals at the top of the food chain throughout the Hawaiian chain.
"We're looking at broad questions about how top predators move — do they stay in one area, between island, between the Northwestern Hawaiian Islands and the main islands, or between Hawai'i and the wider Pacific — and whether there are patterns in their movement," Meyer said.
Work with ulua already has shown that they have day-night movement patterns, changing habitat depending on the time of day. The most recent tag data suggests they also have a lunar cycle — meeting in huge groups at specific sites during certain phases of the moon.
"Other places in the Pacific show aggregations of ulua for spawning at certain phases of the moon, and local fishermen in those communities knew about it," Meyer said.
On big sharks, there is only preliminary data.
At French Frigate Shoals, Meyer has only caught female sharks. At a couple of more northerly atolls, it's all males.
He said he doesn't have enough information yet to determine whether the pattern will hold, but wonders what it means.
Reach Jan TenBruggencate at firstname.lastname@example.org.