And, experts say, the breakthrough opens the way to reveal some of Mother Nature's most closely guarded secrets.

Over the last decade, researchers have used tags to follow larger ocean dwellers such as sharks, sturgeon, tuna and sea turtles, and to follow migrations of mature salmon along marine coasts.

Now for the first time they have tagged and directly tracked small juvenile Pacific salmon, from their release in freshwater far upriver to distant ocean destinations, a major step towards understanding the full life experience and decline of this species.

"It may have been one of humankind's first ponderings: the fish that got away - where they come from, where they go, and what happens to them in between," says Jim Bolger, Executive Director of the Pacific Ocean Shelf Tracking (POST) Project, part of the international Census of Marine Life and hosted by the Vancouver Aquarium.

"Until now it has been difficult to follow small animals in vast oceans, and it has only been possible to infer their movements using very indirect methods. Thanks to new sound-emitting tags about the size of an almond, combined with an extensive coastal network of underwater detectors from Alaska to California, several mysteries of fish migration and survival may soon start to unravel."

In 2006, researchers implanted the new tags in 1,000 juvenile Chinook salmon roughly the same length and half the weight of a frankfurter hot dog - 14 centimeters long, 20-30 grams weight - and followed their journeys in the Columbia and Fraser Rivers.

Among the many studied, two tagged juveniles survived a 2,500 km trip that took more than three months - from the upper reaches of the Snake River (a tributary of the Columbia River) in Idaho, out to sea then north along the continental shelf to Alaska.

The two swam about the same distance as from London to Istanbul or Moscow, from Auckland to Melbourne, from Beijing to Hanoi, or from New York City to Austin, Texas.

The 1,000 tagged juvenile salmon were followed via an extensive network of POST acoustic receivers in the Columbia and Fraser Rivers and along the Pacific continental shelf. The system helps scientists, resource managers and the public to better understand our oceans and the animals that live there, something the Census of Marine Life is accomplishing on a global scale through POST and 16 other projects.

Some other Census projects and all research about animal migration can profit from the technology that POST has now tested.

The underwater array of detectors in the rivers and ocean capture signals when a tagged fish passes by, akin to an electronic toll booth that records a passing vehicle equipped with a transponder.

A new paper, published by the Public Library of Science in the prestigious peer-reviewed open access online journal PLoS Biology, describes one of the first important applications of this technology - comparing the survival of young salmon in dammed and un-dammed rivers. And the finding that survival is comparable is both surprising and controversial.

Two species of juvenile salmon migrating through the Columbia River's eight dams survived the freshwater and early marine portions of their journey to the ocean as well as those in the un-dammed Fraser River, challenging widely-held notions about factors affecting salmon abundance.

In fact, more survived in the Columbia once distance or travel time was taken into account - and survival was greater during migration within the hydropower system than below the dammed section.

Evidence does not yet suffice to tell whether the Fraser has a problem that cuts salmon survival to that of a heavily dammed river, or whether factors other than dams play a larger, unsuspected role in salmon survival.

However, "wherever future research leads on those questions, the electronic and acoustic technology has demonstrated itself as a useful tool for obtaining unique scientific data of importance in a number of public policy arenas," says lead author David W. Welch of Kintama Research, Nanaimo, British Columbia. The full authors list is appended.

Says Victor Gallardo of Chile, the Census of Marine Life's Vice Chair: "These results from North America have global implications and will be of interest in Chile, Russia, Japan, India, Ireland - indeed every nation where fish migrate between fresh and salt water."

PIT tags
The technologies used for the POST study are an alternative to Passive Integrated Transponder (PIT) tag detection systems installed at Columbia River dams starting in the 1990s, a system that provided the first accurate measurements of salmon survival in the hydropower system.

Though reliable and smaller, PIT tags are very short-range, comparable in function to the instant pay-and-go keychain devices now offered by some oil companies to speed customers through gas stations. The PIT system requires fish to be funnelled through structures at the dams to bring their tags close enough to be read by the detectors.

Since it's impossible to install such corralling structures in free-flowing rivers, however, comparing survival in the dammed Columbia River to un-dammed rivers has been extremely difficult.

The more versatile POST tags and listening arrays can be used in any river or ocean. And POST tags generate in-river survival data equivalent to that obtained from the PIT system, and extend the possible results beyond rivers out into the ocean.

Future applications of POST's tags and arrays
Tags will continue to miniaturize, allowing researchers to study ever smaller individuals and species such as herring. And next generation receivers will allow the deployment of lines in waters deeper than the current 200 meter limit, allowing scientists to follow deeper dwellers, like halibut and black cod.

POST's array of receivers will expand and increase in resolution, allowing scientists to look to the far reaches of the continental shelf for answers to questions of regional importance.

The evolving database management system will soon enable scientists to examine relationships between animal movements and behaviour and environmental variables in the ocean, which will shed light over time on the effects of climate change on marine species, populations and ecosystems.

"As network and data capabilities grow, new knowledge will become useful in testing the strengths and weaknesses of previously held beliefs," says Mr. Bolger. "New discoveries made possible by POST will eventually lead to greater conservation and management of biologically and commercially important animals."

Building on past success
Earlier, POST's ocean arrays revealed surprisingly extensive cross-border movements of both green and white sturgeon along the coast, offering insights into gaps in protection strategies.

For example, white sturgeon are listed as endangered in British Columbia but are fished on the Columbia River, while green sturgeon are protected through habitat conservation efforts in California's Sacramento River but may be threatened by trawling off northwest Vancouver Island where we now know they spend a great deal of time.

Since sturgeon meander between estuaries, a continuous POST-type system is the only way to follow these fish from, for example, the Klamath estuary to the Columbia estuary to the Fraser estuary.

Data collected on the POST array are also being used by the US National Marine Fisheries Service to develop a court-ordered critical habitat designation to protect green sturgeon.

In 2006, researchers tagged returning adult sockeye salmon (www.eol.org/taxa/17154705) in marine waters 215 km from their destination river mouth to try to learn why up to 90% of these late-run fish changed their behaviour - leaving the ocean to migrate upriver as much as one and a half months earlier than normal; a change that has caused an estimated 4 million fish to perish over the last decade.

POST is also developing data clearinghouse and mapping and visualization services for independent researchers working with acoustic tags.

For example, in Puget Sound, Washington, several university, federal, state and tribe researchers are investigating Chinook and coho salmon and other species using their own acoustic tags and detectors. Many of them provide their data to POST and, in return, receive detections of fish passing by POST receivers.

A similar arrangement is in place with some members of a consortium of organizations using their own array in the San Francisco Bay estuary to investigate reasons behind a stock collapse that led to extremely costly closures of California's entire coast-wide salmon fishery in 2008.

POST background
POST began with a simple question: What happens to salmon when they leave the rivers and enter the ocean? Before 2000, scientists knew little about salmon migration at sea. But acoustic telemetry was being developed rapidly and the Atlantic Salmon Federation recognized its potential as a tool to address this lack of knowledge.

In 2001 and 2002, the Census of Marine Life funded a small pilot project, originally named the Pacific Ocean Salmon Tracking Project, to demonstrate the feasibility of a permanent, continental-scale telemetry system on the west coast of North America. This allowed researchers to examine marine movements and early ocean survival of both hatchery-raised and wild salmon.

Preparations were made to scale up the demonstration in 2003. In 2004-2005, CoML and the Gordon and Betty Moore Foundation funded a large-scale study in the Salish Sea region, the large, dilute, estuarial inland sea that includes what is now called Puget Sound, the Strait of Georgia, and other water, including the Strait of Juan de Fuca, which connects the Georgia-Puget Basin to the Pacific.

The project was renamed the Pacific Ocean Shelf Tracking Project to acknowledge the technology's ability to track many marine species.

POST's results have begun answering questions about salmon migration. More importantly, the large-scale demonstration justified installing permanent POST listening lines in 2006, originally spanning more than 1,500km of the Pacific Coast from Southeast Alaska to Northern Oregon.

In 2007, POST maintained the permanent array and download data from all receivers, providing researchers with valuable data on movement and survival.

This year and going forward POST is maintaining and expanding its permanent system, with new lines that will fulfill the vision of an array from the Baja Peninsula to the Bering Sea, providing unprecedented opportunity to discover more about the lives of marine species.

POST's volunteer Management Board, comprised of members from Canadian and American agencies and groups concerned with maintaining the welfare of our oceans and their inhabitants, are constantly seeking novel opportunities to apply the technology and infrastructure.

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