A fish tag that knows it's been eaten is helping endangered Atlantic salmon

New tracking devices inserted into Atlantic salmon reveal that up to 48 per cent of the critically endangered fish are being eaten while leaving Nova Scotia's Stewiacke River on their ocean migration.

The insight is the result of acoustic tags that can tell when a tagged fish has been eaten.

"It certainly is high, and it's somewhat higher than some work that was done by some colleagues of mine about 10 years ago using similar tags, but without the predation detecting capability," says David Hardie, a marine biologist with the Department of Fisheries and Oceans.

Each spring for the past three years Hardie has implanted 50 salmon smolts with predation tags before they run to the ocean.

The tag is equipped with a small dab of polymeric coating that, once eroded by the stomach acids of a predator, changes the signal emitted by the tag.

Receivers moored in the Stewiacke River and Minas Basin in the inner Bay of Fundy record a different acoustic signal from a digested and undigested tag.

Eaten on the way to the sea

"In the past two years between 38 and 48 per cent of the smolts that we tagged and released here at the head of tide at Stewiacke River Park were triggered as predated before they reached the mouth of the Shubenacadie River at Maitland," said Hardie.

A previous study has estimated between seven and 27 per cent of tagged salmon smolts in the Stewiacke were being eaten while leaving the river.

That was based on the behaviour of tagged smolts that suddenly started acting like striped bass, moving up and down the river with the tides during bass spawning in the spring.

The predation tags cost $600 each are manufactured by InnovaSea Systems of Bedford, N.S.

They were developed in collaboration with the chemistry department at Dalhousie University in Halifax, said Innovasea president Mark Jollymore.

"Like any new technology it requires a lot of research and development so it takes it takes effort and time to make it into a scalable, reliable commercialized product," Jollymore said.

It's now being used on several rivers in eastern North America.

Striped bass the main predator

It's not clear whether the higher predation level in the Stewiacke River is the result of better tracking technology or the booming striped bass population.

The spring smolt run to the ocean coincides roughly with the arrival of spawning striped bass.

"Striped bass have been coexisting with salmon in the entire Bay of Fundy for a long time but striped bass are doing quite well in this river in particular, and it's a spawning site," said Hardie. "The smolts are running through the gauntlet of these striped bass."

This year, Hardie and team members inserted the tags 60 kilometres back from tidewater, and away from striped bass spawning, to try to give the smolts more time to recover from the tag insertion.

The fish are anesthetized for the surgical operation, which would be equivalent to a soda can being inserted into a person.

"Most of our smolts, over 80 per cent of them, made it here to the head of tide and we only had four or five so far confirmed that were predated in the freshwater reach. So that was somewhat encouraging."

Invasive chain pickerel have now infested one upper branch of the river.

Advanced genetic test used for first time on Stewiacke River

For the first time this year, the study is using a new genetic test that can tell whether the fish was vulnerable before it was eaten.

Dalhousie University grad student Daniela Notte took a tiny gill sample from each smolt as it was being tagged.

The Ribonucleic acid (RNA) test provides a snapshot of the physical condition of the smolt at the time and can detect a wide range of stressors from warm water temperatures to disease to trouble adapting to salt water.

Notte will analyze the results in the fall.

She wants to know if there are shared weaknesses among the eaten.

"We're looking for commonality between the smolt that have been predated versus the ones that have survived to reach the sea," she said.

"That can give us an idea of a possible river restoration or population management."

Trying to solve a mystery

One thing has not changed: Atlantic salmon remain in deep trouble in the inner Bay of Fundy rivers where they are wiped out or on the brink of extinction.

If it weren't for a federal fish hatchery, Atlantic salmon would have gone extinct years ago in the Stewiacke River.

Every year,  tens of thousands of young salmon are released into the river as part of a Live Gene Bank program.

Many survive two, sometimes three, years to become smolts. That's when their bodies adapt to saltwater.

The problem is almost all, at least 99 per cent, die at sea.

No one is sure why.

Thanks to the live gene bank, researchers like Hardie still have Atlantic salmon they can study.

"We have what inner Bay of Fundy salmon should be like, that are representative of what we've been working so hard to conserve. So anything we can do to characterize how they do, or sadly in most cases don't, survive out at sea is helpful for the broader research."

Hardie hopes findings from this study can tell them where improvements in a river system can increase the odds of salmon survival.