An Assessment of alewife (Alosa pseudoharengus) and blueback herring (Alosa aestivalis) spawning migration and performance of passage structures at Moosehorn National Wildlife Refuge
The Saint Croix River serves as an international boundary between Maine, USA, and New Brunswick, Canada. Sea-run alewife, (Alosa pseudoharengus) and blueback herring (A. aestivalis) – known collectively as river herring – are indigenous to the St. Croix River and have vital roles in the function of the watershed’s freshwater and terrestrial ecosystems and of nearshore marine ecosystems. River herring return to their natal river to spawn and require access to inland waters to reproduce. Inadequate fish passage at dams and other man-made infrastructure have historically blocked access to spawning habitat. By 1981 these issues were addressed and the river herring population on the St. Croix rapidly increased from less than 200,000 to 2.6 million. In 1995, the Maine Legislature blocked river herring passage at the St. Croix’s second and third dams (Woodland and Grand Falls) in response to concern that the rebounding alewife population was causing a significant decline in the local smallmouth bass fishery and the rural economy it supported. This led to a precipitous drop in river herring numbers (900 fish in 2002) and years of international fisheries conflict. In 2013 the Maine legislature removed the last of its barriers at fishways as a result of scientific studies by, and support from, many interests on both sides of the border. The Milltown Dam fishway research trap is now documenting significant river herring re-colonization of the watershed with 157,743 river herring counted in 2017 and 270,659 counted in 2018. The capacity of the St. Croix river is estimated to be around 20 million river herring.
In 2018 passive integrated transponder (PIT) tagging was conducted to monitor the migration of river herring from the St. Croix River into upstream areas of Moosehorn National Wildlife Refuge (NWR). This included looking into the success of river herring passing through four potential barriers which have been modified to assist with fish passage.
The St. Croix International Waterway Commission (SCIWC), in conjunction with U.S. Fish and Wildlife Service (USFWS) are working together to assess upstream and downstream (postspawning) movements of river herring within the Magurrewock Stream in Moosehorn NWR, evaluate river herring movements between potential migratory barriers by recording the percentage of fish arriving at each structure, and assess passage performance of the potential barriers, and lastly, to investigate the effects of environmental conditions (temperature flow, etc.) on movements and passage performance.
This investigation follows a significant population increase in river herring recorded by the research trap at Milltown Dam in St. Stephen New Brunswick. This increase is a result of restoration efforts on both sides of the border, which includes the opening of fishways at the dams in Woodland and Grand Falls, allowing river herring to return to their spawning grounds.