It is important to realize that the American eel is capable of living an extremely long time. Twenty, thirty, forty years, one in captivity lived past the age of eighty. It is likely that most of the eels being killed at the Benton Falls dam migrated up Sebasticook before that dam was built and went into operation in 1988.
Once these female eels get into these rivers and ponds and attain a length of a couple feet or more they would have few natural predators. They could live in these watersheds for decades close to the top of the food chain in relative comfort. The same is true as they migrate downstream. There isn't much that would eat a three or four foot eel until it got to the sea. Therefore, it is reasonable to assume that the freshwater mortality rate for an adult eel migrating down through these rivers would be quite low. What impacts do these hydro dams have on the population dynamics of adult eel populations when they are slaughtering large percentages in an environment where the mortality rate should be low?
Biologists, fisheries agencies, no one knows how many of these female eels are in our watersheds along the Atlantic coast. No one knows how many adult eels must reach the Sargasso Sea to have a successful breeding season. All we know is they are declining rapidly. For all we know it could be to late.
What impact do hydro dams have on American eel populations?
No one knows for sure. It has not been studied in any detail throughout New England. Our observations and research indicate that it is significant. All of New England's major rivers from the Connecticut River north have multiple large hydro facilities on them. We do not know of any with downstream bypass specific to the needs of the American eel. It is our understanding that significant eel kills have been incidentally discovered at the Holyoke dam on the Connecticut River in the past.
What impact do dams in general have on American eel populations by impeding and preventing upstream migration?
Here again little attention has been paid to this problem here in New England. For an example we can look to our south, to the Susquehanna River where some research has been done. The information below is from Maryland Department of Natural Resources, MBSS Newsletter March 1999, Volume 6, Number 1
"The most dramatic example of the decline of
American eel abundance is dam construction on the Susquehanna River. Prior to
the completion of Conowingo and three other mainstem dams in the 1920’s, eels
were common throughout the Susquehanna basin and were popular with anglers. To
estimate the number of eels lost as a result of construction of Conowingo Dam,
we used MBSS data on American eels from the Lower Susquehanna basin and
extrapolated it to the rest of the basin above the dam. Our best conservative
guess is that there are on the order of 11 million fewer eels in the Susquehanna
basin today than in the 1920s.
Eleven million fewer eels in this river system alone! Consider that each adult female will lay between fifteen to twenty million eggs. These numbers are to large to be ignored, and they only represent one river system. If we assume similar numbers for our big New England river systems, which have multiple dams on their mainstems and tributaries the loss in eels would be staggering.
We feel this is an important part of the eel decline equation. A brief internet search will reveal multiple questions about eel decline, stream erosion and changing ocean currents are two of the most common. While these factors may very well contribute to the decline, there isn't much we can do about these influences in the short term. They should and must be studied further. However, in the mean time we must not look past these dam issues.
It's like the chicken and egg question? Are ocean currents the primary cause of the decline? Or are the eel populations so depressed because of hydro and impassable dams that they cannot recover from what otherwise would be a minor environmental change? Given the amount of obstructions to both upstream and downstream eel passage it is likely that what we considered a healthy eel population twenty years ago was in fact a greatly diminished remnant population. This remnant population is far more vulnerable to environmental changes than the healthy and thriving populations which were likely around before so many dams diced them up and blocked their passage. It is important to bear in mind that this current eel decline is a decline of an already decimated remnant population.
There is an unknown number, a threshold. When there are no longer enough adult eels left to reach the Sargasso Sea, when they cannot produce enough babies to make the long journey back to our coast, there is no place to get more eels. The only place in the world where these animals breed is the Sargasso Sea.
They are not like other fish. A herring or alewife can be lost from the rivers and ponds of one watershed and restored from another. Think of the Sargasso as a giant watershed, a pond in the ocean. The ocean between this Sargasso Pond and our coast is the river. When there are no longer enough adult eels, male or female left in this Sargasso Watershed to produce enough babies we will lose the American eel. There is no safety net, no other watershed to restore them from. The American eel has developed some of the most amazing survival mechanisms of any animal in the world both as individuals and as a species, however, its one weakness is the fact that they all must breed and give birth in the same place. When that threshold is broken what do we do, where do we get more?