The Merrymeeting Bay ecosystem in southern Maine—a dynamic confluence of six rivers (the Androscoggin and Kennebec being the major two), and one of only four freshwater, tidal bodies of water in the world—once supported a healthy and intricate web of biological interactions. As early as the 1600s, colonial settlement and dam construction began to tamper with the bay’s species populations and chemical compositions; now in the 21st century, researchers are recognizing its extreme ecological complexity as they strive to undo the centuries of overfishing, pollution, anoxic waters, increased turbidity, introduced species, and obstructions to anadromous fish runs that human involvement has caused. I was one of six Bowdoin students funded by the Sustainable Solutions Partners to study the ecological recovery of the bay this summer, 2010. We examined several trophic levels of the ecosystem including the submerged aquatic vegetation, macroinvertebrates, muscles, anadromous fish, and larger game fish in order to understand the holistic nature of the bay’s recovery.
One portion of the project was to monitor the migrations of anadromous fish: species that live primarily at sea and travel to freshwater rivers, streams, and ponds to spawn. In particular we targeted river herring: shad, alewives, and blueback herring, under the genus Alosa. These alosids act as important alternative food sources for birds and larger salt-water fish, distract predators from species such as small-mouth bass, and can be harvested for food and bait, proving economically viable for humans. Over the past four centuries, pollution and dam construction decimated the alosid populations to the point that Hall (2009) estimates a state-wide production loss of 6.5 billion alewives from 1600 to 1900 due to colonial dam construction alone. However, relatively recent dam removal compounded with the Clean Water Act of 1972 has led to a slow revival of alosid populations—a rebound that we sought to monitor to further encourage stock recruitment.
Two 5-hoop, cylindrical, fyke nets were set up at Abby Point on the northern end of Merrymeeting Bay—one in vegetation and one set up in a non-vegetated area, to determine whether vegetation harbored better habitat for alosid populations. Little difference in catch quantity and species (mostly channel catfish, American eel, white suckers, and white and yellow perch) were seen between the two nets—an observation likely due to the close proximity of the non-vegetated net to vegetation, especially as the tape grass and pondweed patches grew denser over the course of the summer. Although after hatching, the juvenile alosids make their way to the ocean from mid-July to Novemeber, few alosids were caught in the fyke nets, so five minnow traps (small metal cages), and a beach seine (a 50-foot net with a weighted line on the bottom and a buoyed line on the top) were also used. The beach seine—which is dragged along the bottom towards the shore—proved most successful at catching alosids, which were measured and returned to the bay. Over the course of the five weeks that data was taken, no statistical difference was seen between the average alewife size (One-way ANOVA, p = 0.147). The small range in average length—which was consistently between 5.25 and 6.25 cm—indicates an invariable migration time from the spawning site to Abby Point.
Monitoring the number of juvenile alosids migrating each summer will give us a sense of the effectiveness of stock recruitment efforts, including fish ladders, dam removals, and re-introduction of species. Recovery of the bay is an interdisciplinary project—one that involves ecological, political, and economic analysis and support. Bringing together the interests of humans and wildlife in a way that bolsters the local economy and strengthens the health of the ecosystem at all trophic levels constitutes the eventual goal. Although the complexity of biological interactions may never be returned to pre-human status, future years of research hold the hope of creating a sustainable ecosystem for all life forms involved.
Faculty Mentor: John Lichter and Phil Camil
Funded by Sustainable Solutions Partners
Reference:
Hall CJ. 2009. Damming of Maine watersheds and the consequences for coastal ecosystems with a focus on the anadromous river herring (Alosa pseudoharengus and Alosa aestivalis): A four century analysis [thesis]. [Stony Brook (NY)]: Stony Brook University.
