Travelling tuna

Bluefin tuna for sale in fish auction hall, Skagen, Denmark. Circa 1946.

Generations ago, scores of Atlantic bluefin tuna arrived in the waters surrounding Northern Europe when they warmed each summer. By the early 1960s, however, the species had virtually disappeared from the region.

Three new studies co-authored by Dalhousie researchers are helping to undercover the past, present and possible future of the bluefin tuna. The first is a historical study co-written by Dr. Ransom Myers before his death this past March. The other two studies involve the work of Dr. Michael Stokesbury, Dalhousie marine biologist and one of the founding researchers of the Dalhousie-led Ocean Tracking Network.

Dr. Myers’ historical study, put together with Dr. Brian MacKenzie of the Technical Unviersity of Denmark for Fisheries Research, dusts off sales records, fishery yearbooks and other sources to describe the shift of the bluefin from relative abundance to quick decline in the first half of the 20th century. The study links the decline to an increasingly industrialized fishing industry using better technology to catch copious quantities of the large fish.

Scientists from the Tag a Giant program catch a giant bluefin tuna for tagging and release.

Today, experts believe that there are two major stocks of Atlantic bluefin tuna left in the world that forage together in the North Atlantic but travel to opposite sides of the ocean to reproduce. Dr. Stokesbury’s work provides evidence for this. He is the lead researcher on a paper for the journal Hydrobiologia reporting the remarkable migrations of bluefin tuna tagged off Western Ireland in 2003 and 2004.

“We tagged two fish hooked from the same school, at the same time, released within five minutes of one another, and recaptured them eight months later 6,000 miles apart,” he explained. One of them travelled all the way past Bermuda to waters about 300 kilometres northeast of Cuba. “It’s strong evidence that these two fish stocks forage in the Atlantic before returning to either the Gulf of Mexico or the Mediterranean to spawn.” The tracking study was undertaken by the ‘Tag a Giant’ program administered by TOPP (Tagging of Pacific Pelagics), a partner in Dalhousie’s Ocean Tracking Network.

A third study for publication in Marine Biology, of which Dr. Stokesbury is a co-author, tagged 28 bluefins in the Gulf of Mexico, examining their movement patterns and behaviour during the winter months. It’s believed that this stock has dropped 90 per cent in the past 30 years, and temperature changes due to global warming threaten to radically alter the timing and location of their spawning.

“Together these new reports help define where bluefin spawn and provide evidence for their trans-oceanic migrations,” said Dalhousie’s Dr. Ron O’Dor, project lead of the Ocean Tracking Network and senior scientist with the Census of Marine Life. “Part of the lesson here is that restoring bluefin tuna populations to health requires us to consider and manage activities one-fifth of the way around the world.”