March of the Migrating Fish

You don’t have to travel to Africa or the Arctic tundra to witness one of the world’s great migrations. Maine’s migratory fish provide a rare opportunity to see wildlife in abundance, and they are an excellent example of resilience and persistence.

Species such as salmon and shad, blueback herring, and alewives need distinct habitat at different stages of their life, and this necessitates migration. Using smell, they navigate from the nutrient rich waters of the sea, up rivers and tributary streams to the sweet waters of their birth.

Humans have put many obstacles in their way in the form of dams, culverts, and hydropower turbines. In late May, hundreds of shad and alewives were spotted bumping their noses against the Kesslen Dam on the Mousam River in Kennebunk. There has been a dam at that site since the 18th century, and the current structure dates from the 1950s. It has never been modified to accommodate fish passage, yet annually the fish return, attempting to access the ideal habitat that lies upstream. Perhaps one day the dam will be removed, and the fish can spawn, restoring the ecological health of the river.

Image by Alicia Heyburn

There are natural barriers to migration as well, such as rocky conditions during periods without rain and trees toppled into streams by winter storms. On May 23, I hiked along Mill Brook, a tributary of the Presumpscot River. The first alewives of the season were spotted the day before I arrived, and by noon the following day, tens of thousands were pooled below a rocky outcrop known as Goldilocks Falls (so named because the water must be “just right,” not too high and not too low, for them to ascend). Sharp dorsal fins pierced the surface of the shallow water, turning it silver. The sun beat down. Osprey, Bald Eagles, Great Blue Herons, and gulls would soon arrive to feast. Alewives are a healthier food source for fish-eating birds. Coming from the ocean, they are free from organic chemicals and heavy metals such as PCBs, dioxin, and mercury that can bioaccumulate in fish resident to polluted rivers.

Water levels in Mill Brook were low, yet the fish attempted to ascend the slick falls on their way to Highland Lake. They turned sideways and flapped their bodies, using their fins almost like claws to get purchase on the rock and slip over the top into a deeper channel.

Once in the lake, the fish spawn, releasing trillions of eggs, many of which become nourishment for other organisms. Hundreds of thousands of juveniles will swim out of Mill Brook in early fall and down the Presumpscot, arriving at Casco Bay near Maine Audubon’s Gilsland Farm. From here they enter the sea in schools to become the base of the marine food system in the Gulf of Maine.

Each barrier that is removed means more fish to reproduce, and their great migration can continue to replenish nutrients in our rivers and streams, and boost our commercial fisheries.