Posted on behalf of Jessica Marshall.
The American eel (Anguilla rostrata) makes one of the world’s most elusive migrations. After spending most of their lives in rivers and estuaries from Greenland to northern South America, mature eels journey to the Sargasso Sea to spawn and die. Researchers have encountered larvae in the Sargasso Sea, but never an adult eel, and no one has ever seen the spawning underway; the location of the spawning ground and the exact path the eels take remains a mystery.
Now it looks as if the journey may be a harder one than expected, with many eels ending up as dinner along the way. The revelation emerged earlier this week at the 2012 Ocean Sciences Meeting in Salt Lake City, Utah, where researchers have been describing recent progress in elucidating the eels’ movements.
During last year’s migration season, a team led by Melanie Beguer, a postdoctoral researcher at Laval University in Quebec City, Canada, tagged and released eight mature female eels in the St. Lawrence River. The eels — which can reach 1.4 metres in size and live more than 20-years — carried devices that recorded position, depth and temperature. The devices were designed to pop off at a predetermined time and float to the surface, from which they could send their data to a satellite.
The researchers were shocked, however, when six of the eight tagged eels ended up in the stomachs of predators before making it out of the Gulf of St. Lawrence. (Another tag worked for a short period before going silent, and the researchers believe that the eel wearing the eighth tag died relatively soon after release.) “It’s a level of predation which is very unexpected,” said fish biologist and study co-author Julian Dodson, although he acknowledges that the burden of carrying the tag may increase the tagged eels’ risk of capture by a predator.
Also curious is how the researchers learned of the eels’ fate. The data recorded by the six tags revealed a sudden jump in temperature from that of the ocean to that of a warm gut — although not warm enough to be a marine mammal. This points to either bluefin tuna (Thunnus thynnus) or porbeagle shark (Lamna nasus) as the eel eaters, and the researchers are betting on the shark.
Their inkling comes from the depth profiles revealed by the tags. The team found that the eels make drastic dives down during the day and swim near the surface at night. They compared these findings with depth profiles for porbeagle shark and bluefin tuna from tagging studies by other researchers and found a good match for the porbeagle but no clear day-night cycling for the tuna. Together the findings point to a previously unknown and potentially tight link between predator and prey.
“I think they have a pretty good case that it is indeed porbeagle sharks that are attacking their eels,” says Steven Campana, head of the Canadian Shark Research Laboratory at the Bedford Institute of Oceanography in Dartmouth, Nova Scotia, who will be working with the team to study the possible link more closely. The depth profiles match well, and Campana’s team discovered a year-and-a-half ago from tagging studies of its own that gestating porbeagle females were going down to the Sargasso Sea to give birth before swimming back.
“It sort of baffled me but in thinking about this, the one characteristic of the Sargasso Sea that everyone is familiar with is that it is a spawning ground for the eels,” Campana says. “My initial reaction was to poo-poo it. But with these new results from the Gulf of St. Lawrence suggesting they are eating eels, it does start to hang together.”
The researchers haven’t given up on the grand prize of getting a tagged eel to reveal the spawning ground in the Sargasso Sea, so their strategy for the coming season is to seek eels large enough to tag in the open ocean outside the Gulf of St. Lawrence, which they hope will be beyond the reach of the sharks.