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In December 2000, wild Atlantic salmon populations in small coastal rivers in Maine – the Dennys, East Machias, Machias, Pleasant, Narraguagus, Ducktrap, and Sheepscot rivers, and Cove Brook – were protected as endangered under the Endangered Species Act. This protection was expanded in 2009 to include salmon populations in several of the state’s larger rivers, including the Penobscot, Kennebec and Androscoggin. This dynamic map shows these rivers and the salmon’s migration in the Penobscot River to the sea.

Atlantic Salmon

Homepage photo of salmon embryo courtesy of Haruka Fujimaki, Mount Holyoke College, Mass., student. The image won 9th place in the Olympus Bioscapes Photomicroscopy Competition and was published in the Dec. 2009 issue of Scientific American.

Already on the brink, Atlantic salmon face a changing climate

Atlantic salmon are among the most imperiled species in the Northeast Region. While at one time hundreds of thousands of salmon made their epic migration from the oceans of Greenland to their natal rivers in Maine, now only remnant populations remain. Recovering this iconic species is a priority for the U.S. Fish and Wildlife Service.

The odds have been against the salmon. The construction of dams, overfishing, habitat loss, and water pollution have collectively caused their decline.

We are learning that climate change may be yet another threat to the species’ survival. Recent trends toward earlier snowmelt runoff, less river ice, and warming water temperatures are affecting areas in Maine where the Atlantic salmon is protected as an endangered species.

“Adult salmon appear to be returning to the Penobscot River about two weeks ahead of when they migrated historically. We collect these fish every year to reproduce them in a hatchery setting, and we’ve observed that they’re ready to spawn a little earlier each autumn,” says Paul Santavy, manager of the U.S. Fish and Wildlife Service’s Maine Fisheries Program Complex.

He explains that when spawning occurs in the hatchery on this accelerated schedule the young fish mature too early in the spring. In the incubators, they absorb their internal yolk sacs and need to be released into the river to feed before the water is warm enough to support the invertebrate animals they need as food.

Biologists at the Craig Brook National Fish Hatchery, part of the Atlantic salmon recovery operation that Santavy oversees, are experimenting with ways to delay the spawning cycle that they’re seeing in the Penobscot River salmon. By artificially manipulating water temperatures and length of daylight they are attempting to bring the captive fish more in sync with seasonal river conditions. Hatchery-raised fish comprise 95-percent of the salmon population in the Penobscot.

Adult Atlantic salmon. Credit: Bob Michelson
Adult Atlantic salmon. Credit: Bob Michelson

An additional concern in recovering the Atlantic salmon is that the rivers in the Downeast region of Maine are experiencing extreme fluctuations in water flows – heavier rainfalls happening less frequently, combined with earlier snow and melting river ice – that have been predicted in certain climate change models. During these weather events, the high-flowing water doesn’t have a chance to absorb into the river beds, which contain minerals that serve as a buffer to changing water chemistry. This lack of buffering can increase the acidity of the water. After the rainwater flushes out the system, river flows now tend to decrease rapidly unlike the gradual decline seen in the past.

According to Santavy, these shifting river conditions occur at a time in the spring when the juvenile salmon are very vulnerable. In the wild, the fish have just a one-month window of time to transform from parr into smolt before migrating to the sea.

Biological studies conducted by the U.S. Geological Survey (U.S.G.S.) show that it can take weeks for a salmon in this life stage to recover after being exposed to acidic conditions.  Acidic pH levels in the water lower than 6 (7 is neutral) can cause naturally occurring aluminum levels in the water to become reactive to the fish. This can prevent calcium from binding to the fishes’ gills – a necessary process for their transition to the marine environment. If this is the case, Santavy says it could help explain the reason smolt are being lost in the estuaries, and would be devastating to recovery efforts.

In an effort to better understand how changes in these river systems affect Atlantic salmon, a team of scientists from the U.S.G.S., Maine Cooperative Research Unit, National Oceanic and Atmospheric Administration, Maine Department of Marine Resources, National Research Council and the University of Maine is researching how changes in summer low stream flows and stream temperatures in the northeast may affect the populations. The study will also evaluate management options that may mitigate the effects of these conditions. The study is funded by the U.S.G.S. National Global Warming and Wildlife Science Center.

Atlantic Salmon Life Cycle

Atlantic salmon are anadromous, meaning that they travel from the sea to spawn in fresh water. Remarkably, the salmon imprint on their home rivers, recognizing their chemical fingerprint, and find their way back to them as adults to reproduce. The eggs hatch during winter and the tiny salmon, called fry, emerge from the gravel in spring. Juvenile salmon, called parr, remain in fresh water for up to three years before undergoing physiological changes to prepare for their migration into salt water. Salmon remain in the sea for two winters before they return to rivers and streams to spawn. The adult fish are about 2 ½ feet long and weigh about ten pounds.

To learn more about the Atlantic salmon life cycle, check out this video: FLV version (3:38 - 62.08MB - Video by Bob Michelson)

Salmon lifecycle in photos

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