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Advances in tagging technology help scientists monitor and protect fish populations

Riding piggyback on advances in computing, electronic tags placed on marine animals are increasing our knowledge of the oceans and its many denizens at a dizzying speed.

A new generation of electronic tags, using ever-smaller, off-the-shelf microchips, have allowed marine biologists to learn what fish do under water—not only how deep they go and how often, but also how far they travel and how fast.

“We’ve seen an explosion of knowledge in the past 10 years,” says Ellen Pikitch, the director of the Pew Institute for Ocean Science in Miami.

“Tags have given us insights into movement patterns in animals that were almost impossible to observe,” adds Kim Holland, who heads the Hawai‘i Institute of Marine Biology Shark Research Group in Kane‘ohe Bay.

Some tags, which ping underwater, allow an observer at a console to follow a fish for several days, knowing when it dives and when it rises. Others accumulate data for years—but must be recovered. The latest type allows scientists to receive the data via e-mail once the tag has transmitted it to a satellite.

The information not only has brought startling new insights to marine biologists, but is opening up new avenues for achieving a goal that unites commercial fishermen, conservationists and scientists: how to reduce by-catch, the living creatures unintentionally killed by fishing gear and usually discarded.

The technology is poised to take another giant leap forward by the end of this decade by allowing tags to download each other’s data, then transmit them to a satellite or an underwater listening station, turning a torrent of new information on the ocean depths into a flood.

reducing by-catch

The first tags date from the 1950s and were called streamer or spaghetti tags. They were implanted on the outside of fish and simply recorded where and when the tag was placed. When the fish was caught, it was therefore possible to infer how far the fish had traveled. They are still widely used today to estimate fish stocks.

The first generation of electronic tags was acoustic. By pinging at regular intervals, they allowed scientists listening to hydrophones to follow the fish for a few days and to know how deep they dived (the deeper the dive, the longer the intervals between beeps) and how far they swam. These tags disclosed that many fishes dived deeper and longer than anyone had expected, and that they inhabited different layers of the ocean.

Holland used pressure-sensitive acoustic tags to determine that juvenile tuna that congregate around buoys placed by fishermen to attract fish occupy different depths according to their size. The larger ones live deeper than the smaller ones.

“Juvenile by-catch is the biggest problem faced by the tuna fishery,” he says. “More than half of the tuna caught today are caught in purse seine nets around these buoys. If we can prove to fishermen that it’s in their interest to design a net that will leave the younger fish they don’t want, then someone might design a net to do just that.”

Acoustic tags saved Hawaiian tiger sharks, which are responsible for virtually all shark attacks in Hawai‘i, more than a decade ago, adds Holland. In the 1990s, tags proved that these sharks are constantly on the move, traveling from one end of the archipelago to the other. “Until then, whenever there was a shark attack, entire flotillas would go out and fish as many sharks as they could, because they thought they could catch the attacker,” he says. “Once we showed them that the likelihood of catching that particular fish in the same place the next day was about zero, the practice stopped.”

Meanwhile, another set of radio tags transmitting to the Argos satellite system were placed on turtles, seals and whales. “Tags are the most important thing that’s ever happened in whale biology because what we knew of them was only from their appearance at the surface,” says Bruce Mate, director of the marine mammal program at Oregon State University and a leading whale biologist.

The information proved useful far beyond the biology class. “Most whale populations are kept in check from recovery by human activities, and tags are showing ways to minimize that,” he says.

Case in point: the North Atlantic population of right whales is only 350 strong and has been for three decades. Tags showed that many were getting killed or injured by ships outside Provincetown because the shipping lane passed over a deep trench that the whales feed in.

“Our data was so compelling that…years ago the shipping industry quite happily moved the shipping lane four miles to the east and the mortality went down dramatically,” he says. “Tags help us come up with reasonable solutions so whales and people can share the ocean amicably.”

Then came archival tags, surgically implanted inside the fish. A thin light sensor sticks out and gathers depth, temperature and light, sometimes for years. The light component allows daily calculation of approximate latitude and longitude. The part that sticks out also gives information on the reward, usually $500, that the fisherman who finds the tag is offered. One restriction is that to have any chance of retrieval, they must be placed on species that are likely to be caught by fishermen.

These tags fleshed out the primitive fish-travel notions yielded by the simple spaghetti tags and the short-term diving behavior provided by the acoustic tags. They revealed that in the seemingly featureless ocean, there are areas, dubbed “hot spots,” where multiple species gather, usually to feed.

The tags also strengthened beliefs that species are attracted to so-called “fronts,” border areas of waters of slightly differing temperatures. And they documented the success of the buoys known as Fish Aggregating Devices (FADs) placed in the open ocean by fishermen which, for no clear reason, attract great numbers of oceangoing fish.

The tags also yielded a startling discovery about blue-fin tuna. While it was known that tuna, once landed on a deck, had a warmer inner temperature than the water’s, it was not until internal tags recorded the fish’s temperature that they were revealed to be, like birds and mammals, warm-blooded creatures, which allows them to hunt in the very cold depths.

These tags were used with great success by Barbara Block of Stanford University. The tags showed that the adult specimens of Western Atlantic blue-fin tuna, one of the world’s most sought-after fish, spent considerable time in the eastern half. According to Gerry Scott, head of the science committee of the Madrid-based International Commission for the Conservation of Atlantic Tuna (ICCAT), which regulates the fishing of Atlantic tuna and other fishes, the eastern tuna population, which spawns (and mostly lives in) the Mediterranean, is seven to 10 times bigger than the western one, while the estimated catch, at 50,000 tons, is 25 times the Western catch of 2,000 tons.

Block points out that even though Western catches were halved 20 years ago, “There is still no sign of a recovery.” The data “suggests that the Eastern fishers are impacting Western recoveries, so they need to lower the quotas in the east,” she adds.

“The Europeans haven’t tagged nearly as many tuna, so we don’t know how many Eastern fish are being caught in the West,” Scott said. “But the European Union has started a tagging program and eventually tags will be able to show how many fish go to the other side.”

Jeff Polovina, a scientist at the National Oceanic and Atmospheric Administration (NOAA) in Hawai‘i who has tagged many loggerhead turtles, learned that they congregate in a so-called “hot spot” off Japan where long-line fishing has been going on for years. “They didn’t report their turtle by-catch, but now that we have a reason to believe they catch a lot of turtles, they can be approached and asked to set their lines deeper and use bigger hooks.”

The next generation of tags came in the late 1990s, developed by two main U.S. companies, Wildlife Computers and Microwave Telemetry.

While the archival tags opened up broad avenues of knowledge, they were restricted to species, like tuna, that were likely to be caught by fishermen who would turn in the tags for the reward.

A new generation called pop-up tags, which look like the wireless microphones pop stars use on stage, accumulate data for up to a year and at a programmed moment, detach themselves from the animal and float to the surface, where they download the data to passing satellites. They opened a new window into the habits of non-commercial species, since the data could be retrieved without catching the animal.

On great white sharks, they proved that these sharks were not coastal, as previously believed, but traveled long distances—from Australia to South Africa or across the Atlantic, for instance. “This was crucial in getting a ban on international trade in them,” said Pikitch of the Pew Institute.

Another surprise came from swordfish, which are harpooned as they bask on the surface. Pop-up tags showed that in fact, they “rarely see the light of day,” says Heidi Dewar, a researcher at the Inter American Tropical Tuna commission in La Jolla, Calif. “They spend most of the day feeding between 300 and 800 meters in very cold water and only come up at night, and they rarely bask during the day. They’re the only pelagic predator we know of that can stay that far down and forage without having to swim up to the surface to warm up.”

Ransom Myers, who runs a fish population dynamics lab at Dalhousie University in Halifax, Nova Scotia, says tags have proven invaluable. “To count fish, you have to know where they go,” he says. “For instance, now we know that great white sharks are wide-ranging. So from limited data that’s been taken in Canada, North Carolina and Virginia that show populations there are declining, we can now infer that the number of whites off Europe and Africa must be down too, even though we have very few data from there.”

But Keith Bigelow, a Hawai‘i-based fisheries scientist who does stock assessments of Pacific tunas and marlins for NOAA, disagrees. He says that while tags give you better understanding of population distribution and migration, they don’t help understand population dynamics. “To count fish, you need to know natural and fishing mortality, birth rate and age structure,” he says. “And electronic tags don’t give you any of that. We use simple dart [or spaghetti] tags, we tag 100,000 fish at a time every decade or so and the 20 percent we get back are sufficient to make assessments.”

the future

Kim Holland of the Coconut Island shark lab sees future tagging moving in two directions.

“We now know where fish go and when they go, but not why they go and what do they do there,” he says. “To know that, we need internal sensors that will tell us more about their feeding behavior and perhaps their blood chemistry. That would help us understand why they suddenly take deep dives or dash across an ocean.”

Roger Hill, the president of Wildlife Computers, a major manufacturer of archival, radio and pop-up tags, says there’s going to be a continuous improvement in the quality of the data and a trend toward further miniaturization with the goal of tagging smaller fish. The main barrier, he says, are the batteries. “There have only been minor improvements in lithium battery technology in over 10 years,” he says.

Phil Ekstrom, who designs archival tags for Lotek, a Canadian acoustic-tag maker, says he must wait for a mass market to generate the parts he needs. “Small tags with long lives need a tiny fuel cell running on dissolved oxygen and glucose. The big market driving that research is pacemakers,” he says.

The other direction electronic tags are heading in, says Holland, is getting tags to download their data without getting separated from the fish.

“Tagged tunas and whales and turtles passing within a kilometer of each other could exchange data and one could transmit everybody else’s data to a satellite or a research buoy,” he says. That would not only increase the data from each animal, but it would reveal which animals are present at the same time at “hot spots,” fronts, FAD buoys or near coastlines. “We still have no idea how marine animals interact,” he says. “For instance, we know schools of big tuna often swim under pods of dolphin, but we’re not sure why.”

A key component of that future is unfolding at the Canadian Foundation for Technological Innovation, where officials are expected to decide by the end of the year whether to fund a proposal to spend $32 million dollars to buy from the leading manufacturer of underwater receivers (who happens to be Canadian) some 4,000 receivers that would be placed in strategic spots and in “curtains,” one kilometer apart, around the world.

This new generation of receivers would be able to transmit their data to a boat passing overhead instead of having to be retrieved. It allows them to be placed much deeper than existing ones, up to 400 meters. At first, the receivers would only download basic information on the passing fish, such as its identity and its depth, says Ron O’Dor of Dalhousie University in Halifax.

But he says they could be modified to receive much heftier loads of data from passing archival tags by using underwater broadband transmission.

“Getting marine data through satellites is very expensive,” says O’Dor, the head of the Ocean Tracking Network, which is creating a global network of receivers. “The demand for cheaper data is growing and I think that will fund the research.”

He agrees with Holland that having tag-to-tag and tag-to-receiver transmission could quite likely be done in a decade, but what’s in doubt is just how detailed that data will be.

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