Few adult smooth hammerhead sharks have ever been tagged and tracked by scientists. Photo: Scott Tindale
Few adult smooth hammerhead sharks have ever been tagged and tracked by scientists. Photo: Scott Tindale
Scientists who sought to solve mysteries about hammerhead sharks were only left with another when one of the first adults ever tagged ended up eluding them.
Last week, eight months after the smooth hammerhead shark was caught and tagged in the Hauraki Gulf, the electronic pop-up tag started transmitting data back via satellite.
But they soon discovered the 3.2m shark, named Sophie, had long since shed the device.
Sophie was hooked at Simpsons Rock, near the Mokohinau Islands, by long-time research collaborators and Auckland fishers Scott and Sue Tindale.
Finding an adult smooth hammerhead shark had been a coup; while previous studies had successfully tracked juveniles, only one other adult in the species had ever been tagged with an electronic tag, and was thought to have died soon after.
"They're hard to find, they're hard to tag and they're quite susceptible to stress and handling," National Institute of Water and Atmosphere (Niwa) marine scientist Dr Malcolm Francis said.
"Elsewhere in the world, it's been found that hammerheads die a lot more easily after capture than a lot of other sharks do."
The scientists had been optimistic about Sophie, which proved a challenge to tag and appeared to be healthy and lively when released.
For Francis - whose Government-funded research has led to stunning new insights into the behaviour and range of hammerhead, mako and great white sharks - the tag presented the first real opportunity to learn where adult smooth hammerhead sharks travel over a year, and why.
Scientists are still trying to understand the migratory behaviour of smooth hammerhead sharks. Photo: Scott Tindale
"Although we know juveniles hang around New Zealand coastal waters for the whole year, until they're up to the age of three maybe, we still don't know what the adults do - they seem to disappear."
One fascinating clue was offered by an adult that was tagged in 2011 by a game fisher with a plastic tag near Cuvier Island, off Coromandel, and was later recaptured in Tonga.
"The suggestion was that adults migrate between New Zealand and tropical waters each year - they come here for summer, give birth to their young, mate, then leave again probably in autumn and head up to the tropics where it's warmer."
The popup tag attached to Sophie recorded data on temperature, depth and light levels, which the researchers could later use to plot its course around the ocean.
But only when the tag popped back up did he see it had hardly left the area where it was attached, having somehow come off Sophie just seven weeks after her capture.
"It had actually been sitting on the seabed, in about 120m of water," he said.
"It's got what we call an umbrella anchor at one end that we put under the skin of the shark to hold it in place - that easily could've snagged on a fishing line, or a net, or who knows what."
Nevertheless, the little data that was collected, now being analysed by Francis, was yielding some fresh information.
"We can see that it spends most of its time in the top 10m, and rarely going deeper than 50m of water - slightly more during the day than at night - so we think it's probably hunting for fish on the surface during the day," he said.
"At night, it's in slightly deeper water, either resting or hunting fish ... we're still not quite sure about that."
Auckland fisherman Scott Tindale attempts to haul up a captured smooth hammerhead shark in the Hauraki Gulf. Photo: Scott Tindale
The maximum depth recorded so far - around 180m - wasn't particularly deep, especially when compared with great whites, which are known to dive to more than 1200m below the surface.
Despite the setback, he was keen to have another attempt.
"It's more a case of the difficulty of finding and catching these sharks ... if we could do that, we could get more tags out.
"Then we could answer questions about how these fascinating animals spend their lives."
Smooth hammerhead sharks
• A coastal pelagic species that can also be found in deeper ocean waters, but is usually observed over continental and insular shelves and inshore waters.
• Found around New Zealand and worldwide in temperate waters and are thought to be the hammerhead species most tolerant of temperate waters.
• Can weigh up to 400kg, measure up to 3.5m and live around 20 years.
• Distinguishable by its expanded head, in the shape of a hammer, smooth back, tall and curved first dorsal fin with rounded tip, fusiform body with dark olive to brown/grey colouring on dorsal side and white underside.
Source: National Oceanic and Atmospheric Administration
