Alewife, a species of river herring, are a vital part of the Somes Pond-Long Pond watershed food web. The Sanctuary has worked for 20 years to create and maintain better passageways around dams and other obstacles so that the resilient fish can make their way to their spawning grounds and then return to the ocean. The ponds, streams, and saltwater cove in Somesville are truly a wildlife spectacle zone when the fish are on the move! Eagles, osprey, gulls, loons, foxes, otters, mink, seals and many others try to get some of the alewife action while they can.
These fish are anadromous migrators, spending most of their lives in saltwater and coming to spawn in freshwater, hopefully, multiple times. They spawn in the ponds and lakes where they hatched for the first time as three- and four-year-olds and show very high fidelity to their natal water body. Adults return to the ocean within a couple of weeks of arrival and most of them are in and out in a month, although some stay longer. Consequently, what happens in our watershed throughout the year affects the ability of the fish to make it to their destination and get back out.
Drought conditions and stream blockages from debris are significant challenges for adults and the one- to three-inch-long juveniles, who emigrate from summer to late fall. A down year in the number of fish spawning in year one often means a down year three and/or four years later. We spend a lot of time making water flow adjustments to our fish ladders and removing blockages year-round.
In 2005 there were only 361 alewife counted at the Mill Pond (the blue line in the graph). Through fish ladder improvements, supplemental stocking to jump start the repopulation, and careful monitoring of stranding conditions and blockages the numbers have increased substantially on average, with dips along the way due to the effect of small cohorts showing up in later years. See the graph below for the variability of alewife counted (actual counts, not estimates) by Sanctuary staff and volunteers over the years at the Mill Pond and Long Pond. The Sanctuary started monitoring Long Pond alewife numbers in 2010 (red line).
Alewife per Year 2005 – 2025
Most of the alewife entering the Mill Pond in Somesville after climbing the first fish ladder from the salt water cove below spawn in Somes Pond (104 acres). To get there they have to ascend three ladders from saltwater to Somes Pond to avoid the dams, and their impassable spillways, created for various types of mills in the later 1700s. In most years, about 10-30% of those fish go all the way to Long Pond (900 acres) a mile and a half inland.
We had a multi-decade record number of adult alewife entering the Mill Pond in 2025 with 57,083 counted and 15,698 hand-netted into Long Pond. For a bigger picture of how our migration compares to others in the State of Maine and other northeastern states, visit the Shad and River Herring Data Portal managed by Manomet Conservation Science and supported by the Gulf of Maine River Herring Network, which we are a member of.
Due to the relatively low number of alewife that arrived during the migrations of 2022 (30,756) and 2023 (12,660) and the fact that three and four year old fish make up the vast majority of our run’s cohorts, we are not expecting a significant dropoff from last year’s high number of incoming fish. Factors affecting the low level of repeat spawners and young demographic profile include the possibilities of by-catch in the ocean while fishing for other species and low levels of escapement back to the ocean during drought years for juveniles.