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Biology

Mystery Solved: Insight into the Eel Migration

Article: Béguer-Pon, M., M. Casonguay, S. Shan, J. Benchetrit, and J.J. Dodson. 2015. Direct observations of American eels migrating across the continental shelf to the Sargasso. Nature Communications 6(8705): 1-9.

Background

For many organisms, migrating over thousands of miles every year can be the difference in between surviving or not. All sorts of species, including birds, fish and mammals will migrate every year to mate, find food, or stay within a preferred temperature. Some of us even have family that will migrate south every winter for that warm sun and ocean! Some migratory species can be very particular about migrating, returning to the same exact spot every year. The complexity and importance of migrating varies tremendously over species. The American eel (Anguilla rostrata) has arguably the most fascinating migration pattern of any animal. Adult American eels (referred to as silver eels, Figure 1) lay their eggs in the Sargasso Sea of the North Atlantic Ocean. Hatched eels will leave the open ocean by migrating north to various estuaries and rivers of the United States and Canada. The eels will even go through several distinct life stages during their migration (leptocephali à glass eel à elver, Figure 1). Eels will mature and reside in coastal habitats until they are prepared to migrate back to the Sargasso Sea as adults.

Figure 1. Life stages for the American eel

Figure 1. Life stages for the American eel

Researchers have done an incredible job discovering and characterizing the intricacies of the eel’s migration from sea to coasts, understanding when they leave and how they use currents and tides to propel them toward the coasts. However, for nearly a century, no one had actually observed adult eels at sea, and spawning in the Sargasso Sea was more so inferred by the presence of larvae. American eels are a popular prey species for marine fish and birds. Additionally, they support a major commercial fishery, with exports to countries all over the world. Not knowing the adult tendencies and migration requirements has made it incredibly difficult to manage the population and commercial harvest. So the question had always remained “How do the adults get there?”

Methods and Results

To solve the mystery migration, Béguer-Pon and her colleagues equipped 38 silver eels with pop-up satellite tags to track their migration back to the Sargasso Sea. The hope was that the tags would allow the scientists to identify the adults’ migration route, understand their behavior during migration, and learn what cues the silver eels use to guide them towards the Sargasso Sea. The eels of Canada typically start their migration from Canadian rivers and the St. Lawrence Estuary (Figure 2). To reduce the chance of eels losing their tags, being eaten, or possible negative effects from carrying the transmitters, the team collected the eels from the rivers and estuaries but released them on the Scotian Shelf off Nova Scotia (Figure 2). Translocating the eels gave them a short-cut of 1,400km around Nova Scotia. The tags themselves record and transmit information about their location, including depth, temperature, and light. The scientists used these data to match the tags to specific locations, and overlaid modeled oceanographic data (including salinity and current strength/direction) to understand what other cues the eels might use to guide them toward the Sargasso Sea.

Figure 2. Pop-up satellite tag locations for eels and estimated migration paths. The route is defined by the authors as 5 distinct segments.

Figure 2. Pop-up satellite tag locations for eels and estimated migration paths. The route is defined by the authors as 5 distinct segments.

Figure 3. Eel migration in the context of depth: eels show extensive vertical migration during their horizontal migration.

Figure 3. Eel migration in the context of depth: eels show extensive vertical migration during their horizontal migration.

Of the 38 eels, two were eaten by large predators during their migration. Based on the temperature data collected, the researchers believe one was eaten by a porbeagle shark and the other by a bluefin tuna. Eight of the eels were successfully tracked to the open ocean and on the continental shelf. From days 3 to 13 of the migration (Figure 2), the eels headed southeast toward the continental shelf. While traveling, the eels would use shallow waters at night and deep waters during the day (referred to as diel vertical migration, or DVM, Figure 3). DVM can help animals avoid their predators during the day, or could be the result of them moving after their prey. This migration segment also followed an increase in temperature toward the edge of the Scotian Shelf. The eels then moved northeastward, swimming against the southwestward flowing current. This suggests that, unlike the larvae, the silver eels do not rely on the currents very much for assisting migration and swimming. Tags revealed that a few eels crossed the shelf break near the mouth of the Laurentian Channel. Finally, the most shocking part was that one eel was shown to travel straight south across the Gulf Stream to the Sargasso Sea. This eel’s migration was roughly 2,400km over 45 days (over 33 miles a day)! Additionally, it seems that this last leg of the trip (Figure 2, segment E) was independent of the temperature or salinity gradients, indicating that the eel relied on other information to make the longest portion of the migration. Could the eels have an internal, magnetic compass that orients and guides them toward the spawning grounds? Perhaps, as eels have been shown to have such tools, but more work will need to be done to validate this hypothesis.

Significance

This work represents a major breakthrough in solving one of the greatest mysteries in fisheries science, the first direct evidence of adult American eels migrating to the Sargasso Sea! This new information on this endangered species’ life cycle may be useful in determining how to manage the commercial fisheries and protect their essential habitat. Satellite tags have been used extensively in tracking animals’ movements; however, this work provides yet another example of their importance in understanding how organisms use the oceanic and environmental cues to complete their life cycles. While this is an exciting accomplishment, it is only the first piece of the puzzle in understanding the adult eel’s life in the Sargasso Sea.

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