Like any good murder mystery, this one has a well-liked victim, many wily suspects and investigators committed to cracking the case.

They are getting closer.

The victim is wild Atlantic salmon. And one of the investigators is Jon Carr, director of research and environment at the Atlantic Salmon Federation in St. Andrews, N.B.

Carr would like nothing better than to get to the bottom of just why salmon numbers drop spectacularly once they leave rivers and head into the ocean.

“There are more than 60 different possible reasons,” he says. “There are very many possibilities.”

Since late 1980s, Atlantic salmon runs have dropped dramatically. Many historical rivers with salmon runs, like the Miramichi, have not seen salmon return in recent years. It is the same story globally.

Wild Atlantic salmon are an integral part of the ecosystem. If salmon disappear, the economic impact on recreational fishing and tourism will be immense, says Carr.

Atlantic salmon spawns a multimillion dollar recreational fishing industry in New Brunswick and Quebec. A report commissioned by the federation said that total spending in 2010 for wild salmon-related activities was estimated at $166 million.

Salmon also creates hundreds of thousands of jobs.

When researchers started studying the phenomenon of decreased salmon runs, they found something strange: while salmon populations have gone down dramatically, smolt — juvenile salmon — manage to do well in rivers but as many as 50 per cent die when they get into a bay or estuary, says Carr.

So what is happening?

Researchers hope to get to the bottom of this continuing mystery by tracking the smolts’journey from freshwater to saltwater, to “see where exactly and how and why they are dying,” says Carr.

But smolts aren’t tracked in any conventional way. Starting in 2003, scientists surgically inserted tags — sonic transmitters — inside salmon to follow their path. Carr says each tag is barely two centimetres long, with a diameter of a pencil.

Receivers are strategically placed in waterways to record the signal and track when and where the tagged salmon travel.

About 250 salmon have been tagged each year.

The tags are expensive — $400 to $500 each — but the data they provide is priceless, says Carr.

In the Miramichi River, striped bass, native to the Atlantic coastline of North America, are likely devouring salmon, says Carr. “Striped bass are the prime suspect.”

This year, Carr and a team of researchers will also track striped bass to see if they are found in the same areas as smolt. They will even look at the stomach contents of striped bass to determine what percentage of smolt may be in their diet.

Last year, the Miramichi Salmon Association said it was worried about the huge numbers of striped bass spawning in the Miramichi River estuary because smolts migrate from the Miramichi watershed to the ocean.

As salmon smolts pass through concentrated areas of striped bass, “there is a dramatic risk that almost an entire smolt run could be consumed,” it said.

Carr says the truth about striped bass will soon come out.

This year, researchers with the Atlantic Salmon Federation will also use tags to determine if cormorants are eating the fish. They will then search areas where cormorant populations are located, seeking the tags using metal detectors.

If we can find the reasons why smolt are disappearing in saltwater, “it improves our chances of success in restoring the wild Atlantic salmon,” says Carr.