HALIFAX (AP) – It was the sign of a booming industrial fishery: Atlantic bluefin tuna so plentiful that hundreds of the giant fish were crammed wall-to-wall inside European markets between the 1940s and ’60s.

But about a decade later, the fish all but disappeared from northern European waters – a mystery researchers have still not solved.

“We have some suspicions on what the cause of the decline later on in the 20th century was, but we still don’t have the full answer yet,” said Brian MacKenzie, who has co-authored a research paper on the history of the species with the late Ransom Myers, a Halifax-based marine scientist.

The study will be published this fall in the scientific journal Fisheries Research. It details the 50-year period leading up to collapse of the once-abundant stock.

The study has been released with two others that examine the migration of the Atlantic bluefin tuna in hopes of finding ways to preserve the species.

While the cause of bluefin tuna’s disappearance has yet to be studied, it’s believed overfishing played a major role, says MacKenzie, a professor of fisheries and oceanography at the Technical University of Denmark.

Bluefins were rarely captured before the First World War, but that changed with the advent of new technology, including hydraulic net lifts and harpoon rifles.

By the 1920s, European fishermen were landing impressive quantities of bluefin tuna. The first tuna cannery also opened in Denmark during that decade.

In 1949, Norway, Denmark, Sweden and Germany landed nearly 5,500 tons of the fish, which fed mostly on herring, mackerel and squid.

Sport fishing and bluefin tournaments also became popular before the species suddenly vanished beginning in the 1960s.

“The fishery collapsed nearly 40 years ago, and many people … it’s more or less gone out of their memories,” MacKenzie said in an interview from Dartmouth, Nova Scotia.

“One of our motivations for the study is to document that this species was indeed present, and to remind people that we have lost it,” said MacKenzie, adding that other fisheries in the Mediterranean and some eastern parts of the Atlantic intensified after the European collapse.

He said studying what took place before the collapse is vital to the future of the species.

“If we don’t have that memory of what we once had, then it’s going to be hard to develop management or conservation strategy to promote recovery in the area,” he said.

Barbara Block, a professor of biological sciences at Stanford University in California, said she’s hopeful results of a bluefin tuna tagging project will lead to better protection of the declining species.

The results of the migration project are contained in two studies to be published separately this fall in the scientific journals Hydrobiologia and Marine Biology.

Block said Atlantic bluefin tuna is subdivided into two types: eastern and western. North American fishermen catch western bluefin tuna.

“There’s a big debate going on as to whether the conservation that Canadians and Americans are doing to recover our west Atlantic population will ever amount to anything if we have twenty fold higher quotas in the east Atlantic (fish),” Block, who helped author both studies, said from Monterey Bay, Calif.

“The basis for this disparity of quotas is that the scientific management of Atlantic bluefin tuna is done with a model that says there is very limited mixing between the two populations.”

But evidence collected from tagged bluefin tuna off Ireland suggest both species are mixing, she said.

The second tagging study revealed popular spawning areas among bluefin tuna in the Gulf of Mexico.

Block said she hopes pinpointing where the fish breed will play a role in the conservation efforts.

“We need to think of better ways to protect the bluefin tuna, and one way is to actually use the information we found to put up a time-area closure in the Gulf to prevent other fishers from interacting with the bluefin,” said Block.

“If we could tomorrow come up with protection of our own Gulf of Mexico spawning area to prevent bycatch … we would actually help in recovering these great fish.”

AP-ES-08-06-07 0833EDT