observed changes in wild populations are due only to the effects of parasites is a challenge, according to Dr. Troy Tuckey, research scientist at the Virginia Institute of Marine Science. The parasites were first observed in
European eels in the 1980s, at a time when both the American and Euro- pean wild eel populations were already in decline. Infested American eels, however, were not discovered until the mid 1990s.
“Another consideration is that if American eels are infected with the parasite known as Anguillicola crassus and are fighting the infection internal- ly, other stressors that might not have been lethal, may now be lethal because of the existing infection,” Tuckey says. Today, infestations have been docu- mented among wild American eels all along the Atlantic coastal states and into Canada.
AQUACULTURE AND PARASITES
Both the Atlantic and Pacific Oceans contain separate species of eels and distinct populations have evolved with natural immunities against pathogens commonly found within native waters of each ocean. Yet, A. crassus is native to Asian eel populations in the Pacific Ocean, which remain healthy today because of naturally evolved immuni- ties — immunities that Atlantic eels do not posses against the parasites.
Infestations can inhibit eel swimming speeds and the buoyancy functions of eel swim bladders.
As a result, researchers believe the
spread of the Pacific parasites first occurred when Asian eels were import- ed to Germany to stock aquaculture ponds and natural lakes in the early 1980s. Uncontrolled shipments intro- duced the nematodes to European eels. The adult parasites are worm-like invertebrates visible to the naked eye. They breed and produce eggs in eel swim bladders. Large numbers of these nematodes can cause problems for eels, including the rupture of the swim bladder, internal bleeding, decrease in appetite, enlarged stom- ach, or even death.
Fortunately for aquaculture eels, the parasites can be controlled with drug treatments. Unfortunately for wild eels,
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controlling infestations is impossible. “There is evidence that A. crassus can cause mortalities in European eel farms and stocked lakes,” says Tuckey. “The problem is that those situations are not natural and some of the eel mortality that is observed could be attributed to the aquaculture practice.” Infestations can inhibit eel swim- ming speeds and the buoyancy func- tions of eel swim bladders. These are problems
some scientists believe
might decrease wild eel chances of sur- vival during long migrations back to breeding grounds, or even during the spawn itself.
“Assuming that American eels are similar to European eels, we can hypothesize that American eels infect- ed with A. crassus may have a more dif- ficult time making spawning migra- tions than non-infected eels,” Dr. Tuckey says. “If a large part of the eel population is infected, that could mean a reduction on reproductive potential, and a decrease in glass eels recruiting to North America.”
As fate would have it, American
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and European eels are believed to share overlapping breeding grounds — areas where European eels might be spread- ing parasites to American eels.
AMERICAN EEL LIFE CYCLE
American eels range from the Great Lakes and Atlantic Coast, down through the Caribbean Sea to Vene- zuela. They are catadromous fish, meaning they live in freshwater, yet breed in saltwater. American eels breed specifically
within the Sargasso Sea, a two-million- square-mile section of tropical, deep- blue Atlantic Ocean stretching between North America and Europe above the Tropic of Cancer. Although the Sargasso Sea is unbounded by shores, it is filled with floating mats of native Sargassum grass. The sea grass that entangles Atlantic fishermen’s lines after storms also provides a great place for floating eel eggs to hide after a spawn. Within a year after hatching, the lar- vae — called leptocephali — morph into 2- to 3-inch juveniles. The young eels are known as glass eels because
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