Scientists have evidence great white sharks dive to far greater depths than previously realised
A great white shark tagged with a scientific data recording tag in New Zealand waters has been recorded to dive to almost 4000 feet below the ocean surface.
Data revealed by the scientists shows that the 4.8m shark, dubbed "Shack" probed to 1,200 meters, or 3,937 feet, as it crossed deep open ocean towards its summer ranges.
The scientist in charge of the tagging study, said this is "the world's deepest shark dive record" and said it extends the predators' known vertical range by about 600 feet--which is substantial given that great whites, until fairly recently, were regarded as coastal predators.
“A big shark called ‘Shack’, the biggest great white we have tagged, at 4.8 metres, has set the world’s deepest great white shark dive record,” says NIWA Principal Scientist, Malcolm Francis.
“And he made several other very deep dive records between 1000 and 1200 metres while crossing the ocean. Prior to this, we had recorded several at around 1000 metres, so it’s quite a substantial extension.”
The shark passed through the Mesopelagic Zone (600-3,300 feet), also referred to as the "Twilight Zone," and continued well into the Bathypelagic Zone (3,300-13,000 feet), or the "Midnight Zone."
This is the realm of alien-like sea jellies and squids. It's also home to monster-like, needle-toothed predatory fishes and eels that utilize bioluminescence for light.
What was the great white doing in the company of viperfish, hatchetfish, dragonfish, sabertooth fish, fangtooth fish and gulper eels?
A Southern California-based researcher, who has tracked great whites to about 3,000 feet, believes they're searching for food at deep-water haunts. The chief food source would be various squid species, including the fabled giant squid, whose epic battles with sperm whales are legendary.
His study has tracked white sharks from Guadalupe Island off Mexico to a vast, mid-Pacific area between Baja California and Hawaii. A similar tagging effort at the Farallon Islands west of San Francisco has followed white sharks to the same spring-and-early-summer habitat.
Great white sharks are found in waters all around New Zealand. National Institute of Water and Atmospheric Research (NIWA), and Department of Conservation (DOC) scientists are investigating the long distance movements of great white sharks inside and outside New Zealand’s territorial waters to improve our understanding of their species’s migratory patterns.
It will assist with designing management measures to reduce shark bycatch in fisheries.
While great white sharks have been protected in New Zealand waters since 2007, little is known about their habitat requirements, and their interactions with great white sharks elsewhere.
New Zealand scientists tagged 25 great white sharks with electronic “pop-up” tags at Stewart and Chatham Islands, over a five year period.
The high-tech tag records location, depth and temperature, and releases itself after a pre-determined time, usually 6–9 months, to transmit its data via satellite.
Of the 25 tagged sharks, four tags came off within the first two weeks. Movement tracks could be determined for nineteen sharks, and only one of those stayed near the tagging site the whole time; ‘Kara’, a big female shark, stayed near Stewart Island to Fiordland. The other 18 headed towards warmer tropical waters.
“Before we started this work, five years ago, it was thought that great white sharks were cold water animals. But it seems the great white sharks are taking tropical winter holidays, departing New Zealand between April and September, for somewhere warmer.
The maximum distance migrated was 3300 km,” says Francis. One shark returned to its Chatham Islands tagging site after spending 6 months at Norfolk Island.
"Our sharks don’t cross the equator; so far our tagged animals have only gone as far north as 17 degrees south, north of New Caledonia,” says Francis.
They take long migratory trips making deep dives as they go. They can travel 150 km a day and it takes them just three weeks to get to Australia.
Great white sharks experience a huge range of water temperatures between 3 and 27 degrees Celsius.
Tagging a shark is not an easy process.
"We have to attract them to the boat, with a berley of tuna oil and minced tuna. Then we use a long pole that has a needle tip on it. The tag has a monofilament nylon leader with a barbed plastic anchor on it. The anchor slides over the needle tip, which is injected under the skin of the shark with the pole,” says Department of Conservation scientist Clinton Duffy.
"When the shark is close enough and at (hopefully) the right angle, we use the pole to stab the anchor into the muscle below the dorsal fin as it swims by," says Duffy. “Lots of patience is needed because usually the shark is moving around, its back is exposed only for a short amount of time and the dorsal fin is out of reach.”
After nearly a year, the tag releases itself from the tether, floats to the ocean surface and starts sending the data via satellite, to France. “If the tag itself is recovered after washing ashore, we are able to extract much more high resolution data from it. So far, we have recovered 6 of our tags from the Pacific Ocean. These “pop-up” tags cost $5000 each, and they have a return to sender message on them.
“The tag leaders have a small guillotineon them, which can cut through the monofilament nylon, and release the tag if it goes too deep,” Says Duffy.
There seem to be two distinct populations of great white sharks, so far there has been no direct mixing between the Stewart and Chatham Islands sharks, though there is some overlap of these sharks in Australia.
Australian tagged sharks have turned up in New Zealand waters which suggests that the southwest Pacific may comprise a single population.
New Zealand sharks have migrated to eastern Australia (New South Wales and Queensland), the Coral Sea, New Caledonia, Vanuatu, Norfolk and Tonga.
"Most of the Stewart Island sharks head to the northwest of New Zealand, whereas most of the sharks tagged around the Chatham Islands head north of New Zealand. They tend to stay separate even up in the tropics,” says Francis. “We need more biopsy DNA samples to see whether they are separate populations or are interbreeding.”
DoC and NIWA scientists have just completed another two week trip to Stewart Island to tag sharks.
The expedition was extremely successful with about 30 different sharks being recorded using photo-identification techniques, six sharks being tagged with popup tags, and one with a dorsal fin tag. The dorsal fin tag sends messages to a satellite every time the dorsal fin breaks the surface, so it provides real-time information on the shark's location.
"Positions determined this way are much more accurate than those obtained from pop-up tags, so they allow us to get finer-scale information on the shark's habitat and behaviour. The focus of our research is now moving from the large-scale picture of movements around the South-West Pacific to the smaller-scale habitat use in our own backyard," said Dr Francis.
In future we will be deploying more dorsal fin tags and also acoustic tags which send out sound pulses which can be picked up by listening stations up to half a kilometre away. These tags will hopefully reveal whether sharks repeatedly return to the same locations, how long they spend in one spot, and what their favourite spots are.
Most of the sharks tagged so far in the programme have been sub adults of both sexes, or mature males. Clinton Duffy is also trying to tag juvenile great white sharks in places like the Manukau and Kaipara harbours, to determine whether they are as mobile as the larger sharks.
This research is funded mainly by the Department of Conservation, the Foundation for Research, Science and Technology, and NIWA.
For more information, contact:
Dr Malcolm Francis
NIWA Principal Scientist,
Tel: 021 072 6100
Department of Conservation scientist
Tel: 027 262 0383
Source NIWAS National Institute for water and atmospheric research, http://www.niwa.co.nz/
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