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Eel migration study reveals porbeagle shark predation in the Gulf of St. Lawrence

Date:
March 12, 2013
Source:
Dalhousie University
Summary:
Satellite tracking tags used to elucidate the migratory path of American eels from the St. Lawrence River to the Sargasso Sea prematurely detached from the specimens leading investigators to suspect significant predation during the eels' journey. Data from the tags revealed stomach temperatures and dive patterns consistent with porbeagle sharks. In addition to data collected by the satellite tags, only 4% of acoustically tagged eels were detected migrating into the Atlantic Ocean via the Cabot Strait (a major migratory pathway between Nova Scotia and Newfoundland). The study precedes the recent vote by CITES to more strictly enforce protection of five shark species including porbeagle sharks as well as recent international studies on the worldwide decline of sharks.

Map of the estuary and Gulf of St. Lawrence showing release site of eels (purple circles), sharks (red triangles) and tunas (green polygons) as well as location of first transmission of tagged eels (blue circles).
Credit: Image courtesy of Dalhousie University

A tagging study has revealed that porbeagle sharks in the Gulf of St. Lawrence could severely impact the number of migrating American eels in the region.

Canadian researchers tagged eight adult eels in the St. Lawrence River as part of the Ocean Tracking Network's investigation to uncover the eels' pathway through the Gulf to spawning grounds in the Sargasso Sea -- a journey that has mystified scientists for over a century.

Adult eels have never been seen in the Sargasso Sea. The researchers hoped to track their eels right into the Sea to precisely document spawning areas. Eight eels from the St. Lawrence River in Quebec were tagged with satellite "pop-up tags." All eight tags detached from the eels and surfaced prematurely, suggesting the eels' untimely death.

Seven tags transmitted data which included swimming depths, times, dates, and sea temperatures. Six tags recorded a sharp and sustained spike in temperature. Water temperatures in the Gulf of St. Lawrence in October range between 0 -- 10°C. Tags recorded temperatures of 22 -- 28°C -- stomach temperatures of warm-bodied pelagic fishes.

The ingested tags continued to record data on the dive patterns and depths of the animal. By comparing the known diving and depth patterns of porbeagle sharks and bluefin tuna to those recorded by the tags, investigators identified porbeagle sharks as the predators. On March 11, the Convention on International Trade in Endangered Species voted for greater protection of five shark species including porbeagle sharks.

Julian Dodson, professor at Laval University and OTN principal investigator, coauthored the study. "Both species are in trouble, and measures to conserve one may well be at odds with efforts to protect the other. What we really need now are studies to quantify just how important eels are in the diets of sharks and just what impact shark predation has on eel abundance."

In addition to pop-up tags, investigators also fitted an additional 113 eels with acoustic tags. These simpler tags broadcast only their identity and are detected by receivers moored in the ocean. Only four of the 113 eels leaving the Gulf of St. Lawrence for the Atlantic Ocean were detected on the OTN acoustic array in the Cabot Strait. The information suggests that the porbeagle sharks are taking a heavy toll on the migrating eels.

"We could hope that there will be increased pressure to protect eels in fresh water, particularly during downstream migration through power dams," says Mélanie Beguer-Pon, OTN researcher at Laval University. "We can't do anything about shark predation, but we can limit mortality in turbines."

The study was conducted by Mélanie Beguer-Pon, José Benchetrit and Julian Dodson (OTN, Laval University), with Martin Castonguay (OTN, DFO-IML), Kim Aarestrup (OTN, NIAR-Denmark), Steven Campana (DFO-BIO) and Michael Stokesbury (OTN, Acadia University).


Story Source:

The above story is based on materials provided by Dalhousie University. Note: Materials may be edited for content and length.


Journal Reference:

  1. Mélanie Béguer-Pon, José Benchetrit, Martin Castonguay, Kim Aarestrup, Steven E. Campana, Michael J. W. Stokesbury, Julian J. Dodson. Shark Predation on Migrating Adult American Eels (Anguilla rostrata) in the Gulf of St. Lawrence. PLoS ONE, 2012; 7 (10): e46830 DOI: 10.1371/journal.pone.0046830

Cite This Page:

Dalhousie University. "Eel migration study reveals porbeagle shark predation in the Gulf of St. Lawrence." ScienceDaily. ScienceDaily, 12 March 2013. <www.sciencedaily.com/releases/2013/03/130312121110.htm>.
Dalhousie University. (2013, March 12). Eel migration study reveals porbeagle shark predation in the Gulf of St. Lawrence. ScienceDaily. Retrieved September 22, 2014 from www.sciencedaily.com/releases/2013/03/130312121110.htm
Dalhousie University. "Eel migration study reveals porbeagle shark predation in the Gulf of St. Lawrence." ScienceDaily. www.sciencedaily.com/releases/2013/03/130312121110.htm (accessed September 22, 2014).

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