Modeling Population Dynamics and Trophic Interactions of River Herring in Merrymeeting Bay

Jane Carpenter ’13 (Biology and Environmental Studies). “Modeling Population Dynamics and Trophic Interactions of River Herring in Merrymeeting Bay.” Bowdoin College. John Lichter, advisor.

Project Summary:
River herring, a collective term for the Clupeid fish species alewife (Alosa pseudoharengus) and blueback herring (Alosa aestivalis), have experienced great population declines over the past few centuries. River herring are anadromous, spending most of their lives in the ocean but spawning in freshwater. Dam construction and pollution of Maine’s rivers have blocked the fish from much of their spawning habitat, thereby reducing their population sizes to a tiny fraction of historical levels. River herring represent an essential component of the Kennebec estuary, as a primary prey item for other fish species and a vector of nutrient exchange. Recently, it has been suggested that river herring may have also historically served as an important food source for Atlantic cod, a major commercial groundfish species, and that the decline of river herring forage stocks contributed significantly to the collapse of the Atlantic cod fishery. Evidence supporting this hypothesis include the occurrence of the collapse following the alewife decline despite an abundance of other prey species for cod, as well as the correlation between Atlantic cod movements and the arrival and departure of alewives.

My component of this project is aimed at determining the potential size of the river herring population in the Kennebec estuary under historical conditions and different restoration scenarios. Life-stage population models were constructed to represent separately the Atlantic cod and river herring populations, and matrices were developed to account for the interactions between the different life stages of each species. I will continue to refine these models throughout the academic year, incorporating juvenile productivity and mortality data, and expand on the spatial component by creating maps demonstrating potential population sizes if different historical spawning areas were to be made accessible.

This summer, our group also focused on assessing the current ecological conditions in the Kennebec estuary. In order to accomplish this, we evaluated the species composition of the fish communities on the Kennebec and Eastern Rivers by beach seining weekly, and determined the water conditions during each sampling occasion. Length and weight measurements of river herring caught and released at these sites will be used to quantify juvenile productivity. We also mapped submerged aquatic vegetation beds in Merrymeeting Bay using ArcMap in order to compare the extent of vegetation in previous years with current conditions.

Our efforts this summer have been essential for shedding light on the current ecological status of the Kennebec estuary. I hope to prove through further research that past river herring population levels could have supported Atlantic cod in the Gulf of Maine, and thus that efforts to restore historical spawning habitat would bring tremendous economic benefits to Maine by aiding the recovery of the Atlantic cod fishery.

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