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Why Restore Anadromous Fish?

American Shadby Matthew Carpenter, NH Fish and Game Anadromous Fish Biologist

Similar to the bison of the great plains, the decline of diadromous fish marked a major shift in the ecosystem of the Northeast.


The inhabitants of freshwater were once far more connected to the ocean than they are today. Every year, fish would migrate by the thousands into New Hampshire's rivers, lakes, and ponds. Atlantic salmon, sturgeon, American eel, river herring, and sea lamprey were all harvested in abundance by native Americans and then by early European settlers. Dams, habitat loss, and overfishing have reduced these migratory runs to just a tiny fraction of their numbers prior to the industrial revolution.


The sheer quantity of these fish, and their offspring, must have had a profound effect on the ecosystem of the northeast. Research on the west coast shows a significant influx of nutrients from the ocean in the form of migrating salmon has a tremendous influence on the productivity of freshwater ecosystems far from the coast. In New England, predators benefit from diadromous fish species at each phase of their life cycle. The adults are preyed upon on their upstream and downstream migrations. Eggs and larvae are an important source of food for many fish species and shorebirds. Juvenile fish, from salmon in small streams to alewives in lakes and ponds, populate freshwater habitats in staggering numbers. Waterfowl, eagles, osprey, mink, otter, snapping turtles, and predaceous fish are just a few examples of species that benefit from this seasonally abundant source of food. Juvenile diadromous fish become increasingly vulnerable during their downstream migration. Imagine millions of juvenile shad migrating down the Merrimack and Connecticutt Rivers in late summer. Today, only a few thousand shad even have the chance to spawn in New Hampshire waters. There is a ripple effect on the ecosystem as predator populations grow and the shift in resource use takes the pressure off of other prey populations. There are many aspects to this ripple effect that we do not understand. In many ways, we do not know what we are missing. Most of these fish populations were already in serious decline before accurate records were kept. We are left with anecdotal stories of abundant harvest and a seemingly limitless aquatic resource. Similar to the bison of the great plains, the decline of diadromous fish marked a major shift in the ecosystem of the Northeast.


Each diadromous fish species has its own unique influence on the freshwater habitat that it uses. American eels migrate by the millions as tiny elvers originating in spawning grounds in the Sargasso Sea. hey can make there way into every available habitat type, from stream to pond to lake, where they influence the ecosystem as an abundant source of prey as juveniles and as voracious predator as they mature. uring their migration back to the ocean, as silver eels, they become a valuable food source for large predators like eagles and harbor seals. Alewives, blueback herring, and American shad spawn in incredible numbers in lake and pond, fast moving stream, and large river habitat, respectively. ea lamprey spawn and then die. They make redds in streams similar to those of salmon. Their offspring spend 5 years burrowed in the sediment, filter feeding and cleaning the water before they mature and migrate out to sea. Atlantic salmon are perhaps the most famous diadromous fish species in the region. Now extirpated, they once used their leaping ability to spawn in the furthest reaches of mountain streams.


According to [RSA 206], The New Hampshire Fish and Game Department (NHFGD), is responsible for "Conservation, protection, and management of wildlife populations and habitats, the collection of necessary scientific information, and the enforcement of fish and game laws for the purpose of sustaining healthy populations of fish, wildlife, and marine resources" in New Hampshire. The restoration of diadromous fish, including American shad, Atlantic salmon, river herring, sea lamprey, and American eel, has long been an important part of the mission of the NHFGD. Dams built in the late nineteenth century caused the extirpation of Atlantic salmon and significant declines in the populations of other migratory fish, which were once extremely abundant in New Hampshire waters. The primary goal for diadromous fish restoration in New Hampshire is to return migratory fish populations to healthy levels by improving access to historical spawning and nursery habitat. The state of New Hampshire has long been deprived of abundant diadromous fish runs, a natural resource that would be of great ecological and economic value for the region.

Conservation Strategies


There are many partners involved in diadromous fish restoration, with US Fish and Wildlife Service usually taking the lead. USFS, NOAA, NMFS, ASMFC, MA DEP, Maine DMC, Vermont Fish and Wildlife, and Connecticut DEP must all coordinate restoration efforts for diadromous fish, which often cross into multiple states during their migration.


Coordinating research at sea and managing offshore fisheries (including bycatch) are critical strategies for restoring diadromous fish populations. However, New Hampshire has little control over what happens at sea. All we can do is pay attention to the results of research and adjust strategies accordingly. What we do have some control over is improving access to spawning habitat and improving the quality of that habitat in terms of water quality and physical structure. While there are many factors contributing to the declines of diadromous fish, lack of access to historical spawning habitat is clearly one of the most important factors. Dam removals are the ideal solution, but with many active hydropower facilities in the state, removal is often not feasible. Improving fish passage during both upstream and downstream migration at hydropower dams is critical to the restoration of diadromous fish populations. Fish stocking is another strategy used to increase access to spawning habitat. In the case of shad, fish are trucked from fish lift and trapping facilities downstream to inaccessible spawning areas upstream of dams. In the case of herring, fish are sometimes captured from river with healthy runs, like the Kennebec River in Maine, and transferred into rivers with depressed runs, like the Merrimack.


People have been trying to restore diadromous fish along the Atlantic coast for over 300 years, since the first dams were built by settlers and fishing pressure increased dramatically. There are many hurdles and complicating factors. History is littered with failed attempts at diadromous fish reatoration. This does not mean we should stop trying. Abundant diadromous fish runs are a lost New Hampshire legacy. They are a natural wonder that is worth saving.