Ocean tracking puts batteries in fish

ALLEN MARQUETTE

November 26, 2008 at 9:58AM AKST

For many years, fisherman and researchers alike have been trying to figure out where different species of fish like herring go once they spawn or where juvenile salmon go after leaving their freshwater habitat to journey out into the waters of Prince William Sound and the Gulf of Alaska.

Over the years, various tracking systems, tagging methods and models have been developed worldwide to monitor commercial fish stocks to better understand and predict where fish go, what might happen to them while in the ocean and help predict when they will return.

Since there are so many variables affecting fish populations like predation, possible commercial and sport fishing pressure, disease and changes in their habitat, predicting when and how many herring, salmon or other commercial fish species might return or where they go is a difficult challenge for biologists and oceanographers.

Scientists use various methods for tagging and tracking fish and their movements,
The simplest method includes numbered tags that have to be recovered later by researchers or fisherman. However, these tags only tell you where the fish was tagged and where it was caught again. More sophisticated methods for tracking fish include implanting a battery powered acoustic transmitting device inside the fishes’ body cavity, which transmits a signal at a specific frequency allowing telemetry equipment to receive the signal.

The information received from these devices can help biologists or researchers determine how many fish pass through a given area, if there might be predation or fishing pressure affecting the fishes’ return or when fish leave or return to certain areas.

POST in Cordova
Recently, a workshop presented in Cordova on the Pacific Ocean Shelf Tracking Project, or POST, was presented to various researchers and managers from organizations like the Prince William Sound Science Center, University of Alaska School of Fisheries and Ocean Sciences, Department of Fish and Game and other groups throughout Alaska.

The Pacific Ocean Shelf Tracking Project was developed by a nonprofit organization facilitating the development and coordination of a large-scale acoustic telemetry network along the entire length of the West Coast of North America. POST executive director Jim Bolger attended the workshop in Cordova and was excited about the possibilities POST offers to researchers and fisheries managers. Currently, there are a series of arrays beginning as far south as Point Reyes, Calif., and north to Prince William Sound with plans to have the complete system in operation by 2010.  

Many of the key aspects of the biology of threatened or commercially valuable species of fish is not known, making it difficult for fisheries assessments. Although the POST tracking project is available to all researchers, much of the current use is for salmon and steelhead tracking on the Columbia, Snake and Fraser rivers. One acoustic study tracked juvenile salmon up the coast from the Fraser River to Southeast Alaska.  

Making batteries last
Once researchers have invested the time and expense to tag a fish, they want to record data from the tag for as long as possible. One way battery life can be extended is to program the tag to turn off for a given number of months or even a few years’ time; the tag will then reactivate itself about the time researchers estimate the fish will return to spawn.

The POST arrays of receivers are placed 800 meters apart in lines across the continental shelf at strategic points along the coast. They can detect accoustically tagged animals to a depth of approximately 200 meters. The tagging devices that are implanted in an animal or fish send out a repetitive series of pings unique to that particular tag allowing the researcher to identify that animal as having passed by a specific receiver.  

The implanted tags vary in size according to the data they record or provide and the battery size, which determines how strong the signal is and how long the device will operate. Some of the larger tags will also transmit the water temperature and the depth that the fish is at when it passed by the receiver.

Participants attending the workshop learned how they could use POST technology to track fish specific to their individual research or tracking projects. Many participants wanted to know how to best retreive data from migrating fish with the least amount of cost.  

As one would assume, these devices can get expensive depending on what data they record or transmit and how large the battery is that is used to send the data. The larger the battery, the longer the device will operate allowing researchers to get a better idea of what their tagged fish are doing over an extended period of time.  Recently, 10 acoustic receivers were deployed in Prince William Sound by the Prince William Sound Science Center, extending the POST systems range northward.  

Allen Marquette is an educator at the Prince William sound Science Center.

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