From their modeling study of fishes and ecosystems in the Gulf of Maine earlier this year, researchers led by graduate student Beatriz dos Santos Dias and her advisor Adrian Jordaan, environmental conservation and director of the Gloucester Marine Station, report that modeling demonstrates that improving river-ocean connectivity by removing dams not only makes more food available to larger species, but would enhance overall ecosystem functioning.
They believe this is the first study to use historical, landscape-based estimates of anadromous fish species – those that return from the ocean to freshwater rivers to spawn – in the Gulf of Maine to model ecosystem responses. Increasing the numbers of forage fish such as river herring and Atlantic herring could promote energy flow in the gulf and benefit many other species, such as Atlantic cod, flounder and wildlife including marine mammals and seabirds, they note. Details are in a recent issue of PLOS ONE.
“Our study highlights the benefits of increased connectivity between freshwater and ocean ecosystems,” say the authors, who in addition to Dias and Jordaan include Michael Frisk of Stony Brook University. “We demonstrate the significant role anadromous forage fish could play in improving specific fisheries and overall ecosystem functioning, mainly through the diversification of species capable of transferring primary production to upper trophic levels, adding to the benefits associated with their restoration.” Trophic levels refer to where species are on the food chain, with primary producers – plants and algae – eaten by the next level, and so on.
Dias and colleagues point out that anadromous fish, those that return to freshwater rivers to spawn, such as alewife, blueback herring and American shad, Atlantic menhaden, Atlantic herring, butterfish and mackerel, are forage fish that represent a crucial link in the food chain because they eat smaller, plankton-eating organisms when young and larger invertebrates and small fish as adults. In turn they are eaten by many species higher on the chain such as bony fish, sharks, marine mammals and sea birds, adding “substantially” to ecosystem functioning.
Anadromous species require many different habitats throughout the ecosystem, and were “historically very abundant,” according to Jordaan. “Unfortunately populations have been decreased due to obstructions, mainly dams built during the period of European colonization, that prevented upstream migration.” Their recovery reconnects a lost component of the food-web and adds to the prey base for many species, he adds.
For this analysis, the researchers focused on alewife, “the flagship species” in the anadromous group, one that is “vulnerable to changes in river connectivity” and one that has had the “most responsive increases” to dam removal. They estimated potential alewife production in the Androscoggin, Kennebec and Penobscot River watersheds in Maine based on the spawning habitat potentially available. As Jordaan explains, “This work continues to build upon research from our laboratory that seeks to improve the understanding of alewife and blueback herring populations and the ecosystems they are part of.”
They used two ecological model frameworks, including one that assumed full river connectivity in the three watersheds “to assess how significantly increasing forage might impact predators in the Northeast U.S. large marine ecosystem.” For comparison, one model reflects ecosystem conditions in the year 2000, the other includes estimated alewife production on the three watersheds before 1600 and dam construction that accompanied colonization by Europeans.
The researchers conclude that “restored watersheds with incentivized dam removal and fish passage policies will raise the capacity of resilience of anadromous forage fish populations. Applying these measures, we can once again provide the benefits of the successful anadromous life history strategy” that were diminished by human changes to the environment. They urge that dam removal be evaluated case-by-case, weighing the trade-offs while quantifying and testing the idea that river restoration can increase biomass flow in marine food webs.