Amherst, MA — Despite evidence from their recent study that populations of two river herring species are dangerously low, ecologists at the University of Massachusetts Amherst and Stony Brook University say removing dams and adding fishways can still revive alewife and blueback herring numbers in New England and help to restore a long-neglected natural link between marine and freshwater ecosystems.
Ecologists Adrian Jordaan at UMass Amherst, Michael Frisk at Stony Brook and lead author Carolyn Hall, an independent researcher, also say if numbers of these anadromous fish, which live in the ocean but spawn in fresh water, rebound it would be good news for their predators including striped bass, tuna, Atlantic salmon and cod -- all human dining favorites that are experiencing declines.
Current management of predator fish species in isolation isn't effective, the authors say. They call for a realignment of restoration goals to aid economically important fisheries and recognize the critical interdependence of ocean and freshwater ecosystems. Their work appears in a recent issue of the journal BioScience.
Jordaan says, "Our research does point to a really big deficit in these fish populations, but if we can agree on a restoration path, using fishways and some dam removal as is already happening now, we can alleviate many of the pressures on these fish. We're not going to fix it overnight, but if we continue forward with intelligent planning, if we can get river herring into the old ponds they used to spawn in, I think we could see a great deal of improvement."
Results of his study with Hall show that damming significantly altered the ecology of coastal waterways much earlier than previously suspected, by removing a huge reservoir of forage fish that the ecologists call a "key vector of marine-terrestrial nutrient exchange."
For this analysis, Hall, Jordaan and Frisk used a combination of harvest data and a fishery-independent habitat model using historical dam records to reconstruct for the first time a picture of fish populations in nine Maine rivers between 1600 and 1900. They figure cumulative lost fisheries production of 11 billion fish occurred from 1750 to 1900 due to dams, that is, fish lost to harvest. They also estimate the number of young fish missing from the system (not available to ocean predators) from a 95 percent loss of spawning habitat at 100 million in 1700, increasing to 1.4 billion fish per year by 1850.
Jordaan acknowledges, "Those are shocking new numbers for biologists to hear, but we were also surprised that the losses came so early. Our numbers show the rate of damming and its impact was dramatic. We didn't expect to see it happening at this magnitude by 1850."
Among other techniques, the researchers identified the location of catches to show how the range shrank throughout Maine. As dams went into major rivers, the catch moved into smaller and smaller watersheds. Jordaan says, "If these nine rivers had been left completely unobstructed, they would have produced the numbers of fish caught in the entire United States in the 1950s. Our study puts it all in real terms. Within the overall perspective of the eastern United States, the loss is overwhelming."
Of course, damming the major rivers of New England fueled the industrial revolution in the United States, making it a world power, "but it came at a cost, and we're paying it now," he adds.
When fish populations are at low abundance as they are now, fish are less able to meet environmental challenges such as climate change and pollution. The current low production capacity of these fish is "coming back to haunt us," he adds. "We harvest too many alewife and blueback herring. That's where we are now, and New England is viewed as having the healthiest populations of these fish. But we have only a small fraction of habitat left, and we see climate change beginning to have an effect."
The authors say another aspect of the study that should not be lost on fishery managers, ecologists and waterway planners is that forage fish link phytoplankton in the ocean to our dinner tables, and their role in the ecosystem has been gravely undervalued.
"We feel these species represent a lost connection between marine and freshwater systems that is more important than the numbers," explains Jordaan. "The modern science of ecology grew up after all these changes had already occurred. So we've failed to appreciate how interconnected the marine and freshwater systems were in pre-colonial times. We can't appreciate its significance for a whole range of factors."
Their conclusions serve as a cautionary tale, the authors point out, for planners in China, Laos and Cambodia, where engineers are on the verge of constructing 200 new dams, 11 of them main stem, on the Mekong River. It is the second most biodiverse freshwater system and host to the world's most productive inland fisheries in the world.
"Losing the connections and ecosystem services provided by anadromous species will have lasting, ecosystem-wide impacts that may not be compensated through fish passage technology," they say.
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