All of Maine’s rivers were once the home of vast schools of Atlantic salmon returning from the ocean to reproduce. It has been estimated that in the Kennebec River alone as many as 70,000 Atlantic salmon returned annually.

Sadly, those great runs were decimated by overfishing, water pollution and construction of dams without fish passage. In Maine, by 1950, only a few fish returned to the rivers of Washington County.

Since 1968, the U.S. government has had a program to restore Atlantic salmon in Maine’s rivers. The flagship river for restoration is the Penobscot, because that was the last river from which these fish were exterminated.

It is true that there was once an indiscriminate fishery for Atlantic salmon on the high seas. However, that has not been the case for almost 30 years.

In 1982, all of the Atlantic salmon producing nations in both North America and Europe entered into an international treaty to create the North Atlantic Salmon Conservation Organization (NASCO). After that, the only high seas fisheries for Atlantic salmon of North American origin were by Greenland and Canada.

Those fisheries were brought under control through painstaking negotiations in NASCO by the middle of the1990s. Natives in Greenland and Labrador each have a small food fishery, but the huge harvests of the 1970s are long since gone. There are no longer any mysterious factory fishing ships just offshore poaching on North American Atlantic salmon.

Nevertheless, recovery of Atlantic salmon has been slow. In 2011, there have been excellent returns, but it remains to be seen whether this is the beginning of a long-term trend, or is only a one-time event.

The Atlantic Salmon Federation, an international organization with offices both in Brunswick and St. Andrews, New Brunswick, and Dalhousie University in Halifax, Nova Scotia, are studying the survival of juvenile Atlantic salmon as they migrate from their home rivers to Greenland, where they feed and grow from a few inches to many pounds.

Electronic pingers are inserted in young fish and detected by an array of counters in river estuaries and ocean passages, such as the Cabot Straight and the Straight of Belle Isle. This is time-consuming and costly, but early results show that predation by fish, birds and mammals have a major impact on the numbers of surviving fish.

Historically, the numbers of juvenile fish were so large as to overwhelm predation. Until the stocks are restored to sufficient size, cropping by predators may continue to have a depressing effect for many years to come.

This is a phenomenon frequently seen in nature. It is one if the reasons why restoration of the full suite of anadromous fish that historically shared Maine’s rivers, including the prolific alewife, is a priority. The buffering effect of alewives can have a positive effect on survival of juvenile Atlantic salmon.

It took a long time to destroy the runs of Atlantic salmon. Restoration will also take a long time. We must think in terms of decades, not just a few years.

Clinton B. Townsend is a member of the board of directors of the Atlantic Salmon Federation and served as a United States commissioner to the North Atlantic Salmon Conservation Organization from 1990 to 1994.