Eels on a slippery slope, warming is suspected


The stock of eels in Lake Ontario and the St. Lawrence River is crashing, and the reason may lie wrapped within the mysteries of the Sargasso Sea.

That’s where the eels spawn, but no one knows exactly where, or under what conditions.

However, scientists do know that everywhere in their range there has been a monumental drop in « recruitment » — the numbers of juvenile eels that will eventually mature.

Canada lies at the northern edge of the eel’s range, and it is here, and at the southern limit extending from Texas to South Carolina, that the declines are precipitous. They signal that the entire species is in trouble, says John Casselman, senior scientist at the Ontario’s fisheries station on the Bay of Quinte. Casselman is a leading authority on the American eel.

In the early days of European settlement, eels may have made up half of the inshore fish biomass, Casselman says. « I’m just guessing, » he adds, « but I suspect that (when commercial fishing was at its height 22 years ago) there was only about a tenth of that number of eels remaining. »

When I look at the amount of the current catch, that figure drops to less than 4 per cent of the original number.

The life cycle of eels is fascinating. A female will release 10 to 20 million eggs somewhere in the Sargasso Sea, and scientists presume that she then dies. No one has seen a female spawning, and no one has seen an adult eel leave the Sargasso.

The eggs float in the Sargasso, and then hatch into larvae the shape and size of willow leaves, but complete with a pair of eyes. The « willow leaves » drift in the Gulf Stream for half a year, and then metamorphose into pencil-sized « glass » eels, so-called because they are translucent. The glass eels swim to the Atlantic seaboard, and females head for fresh water. By then the eels will have been about two years in salt water. It will take the females another one to three years to reach the upper St. Lawrence and Lake Ontario.

When they reach 18 to 22 years of age, with a weight of one to two kilograms, the females head back to the Sargasso to spawn. Some remain for up to another 20 years.

When they reach salt water, their pectoral fins grow larger, their eyes get bigger, and they change colour from a yellowish green to a silver underside and a darker back.

Roughly half of them die in turbines at the two hydro electric dams on the St. Lawrence. But that slaughter of females doesn’t begin to explain the lack of juveniles.

Other things that could impact reproduction are: loss of 84 per cent of historic habitat; zebra mussels (eels hate light, and zebra mussels clear the water); possible overfishing everywhere in the eel’s range during the nineteen seventies and eighties; harvesting of weeds in the Sargasso Sea; the scarcity of alewives, the preferred prey of eels; the warming of Lake Ontario; pollution in the lake; and oceanic oscillation.

It’s oceanic oscillation, variations in water temperature and ocean levels associated with changes in the Gulf Stream, that intrigues Casselman the most. Juvenile eel recruitment seems to be linked to it.

« We’ve got a big bloody mystery on our hands, » says Casselman. « We don’t understand what’s required for reproduction. » Until scientists do understand, he adds, there’s no way to effectively protect the species.

Finding the funds to study eel behavior in the Sargasso strikes me as eminently wise if we want to protect the species. In addition, since eels are sensitive to temperature change, as ocean oscillation suggests, a study could also increase understanding of the impact of global warming on life in the Sargasso and the Gulf Stream.

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