As Alaska Glaciers Shrink, Salmon Populations May Also Decline

A view of the Kenai River (Source: Frank Kovalchek/Wikimedia Commons).

Alaska is experiencing some of the most rapid changes to glaciers and ice fields on Earth. Global warming is causing drier summers and wetter autumns, and changing the landscape through the melting of glaciers and the loss of wetlands and wildfires. The salmon population in the area will also likely be impacted from these environmental changes: some will benefit from the changes, while some will be negatively impacted. A new study in Fisheries Journal investigates the region’s Kenai River and the future climate change impacts on healthy salmon populations.

Salmon in the Kenai River

The Gulf of Alaska region produces a third of the world’s wild salmon; however, the Chinook Salmon (O. tshawytscha) population has declined. Environmental changes are likely to impact future populations. The Kenai River supports world-famous fisheries in the region and exemplifies the high social, economic, and ecological value of wild salmon as well as the complex changes they face. Its yield faces a serious threat with the area strongly influenced by glaciers that are losing mass. The Chinook Salmon may not recover, and the populations that are declining threaten the livelihoods of the dependent fishing communities. The fishing communities are diverse and include indigenous, sport and commercial fishers. The authors of the study wrote, “Kenai River salmon support several of Alaska’s largest recreational salmon fisheries, major commercial gill-net and personal-use dip-net fisheries, as well as small-scale subsistence and educational fisheries.”

The Salmon Life Cycle

Chinook Salmon (fry) (Source: USFWS Photo by Roger Tabor/Flickr).

Chinook salmon is an anadromous fish species. Individuals hatch in freshwater rivers, then the young fish swim out to the ocean to grow and mature, and later return up the river to spawn and then die. The cycle begins as the female lays eggs in the gravel on the bottom of the river (this nest is called redd). The redd is then fertilized by a male, and the eggs remain in the gravel throughout the winter as the embryos develop. As the eggs hatch in the spring, alevins emerge. Alevins are tiny fish with the yolk sac of the egg attached to their bellies. After they have consumed the yolk sac and grown larger, they emerge from the gravel and are then considered fry. Fry can spend up to a year or more in their natal stream. After, the fry begin to migrate downstream toward the ocean.

Eran Hood, professor of Environmental Science & Geography at the University of Alaska Southeast, told GlacierHub that glaciers provide an important source of streamflow during the late summer salmon spawning season. In addition, Hood added, “glaciers are important for moderating stream temperatures during warm periods when spawning salmon can become metabolically stressed by warm water temperatures and associated low levels of dissolved oxygen.”

Glacial Rivers

Glacially-fed rivers respond to weather and climate differently than non-glacial rivers. During hot and dry summers, the water in a typical non-glacial river will warm up and streamflows will drop. However, the same summer conditions will cause glaciers to melt faster and lead to more cold water input into glacial rivers. In Alaska, many important salmon rivers are fed by a mix of glacial and non-glacial streams. If one of the streams suffers from drought conditions, there is a chance that another stream in the same section of the watershed has a lot of deep and cold water. Schoen explained to GlacierHub, “This habitat diversity helps to stabilize salmon runs on a large scale, and lessen the risks of boom-and-bust dynamics in our fisheries.”

Jeffrey A. Falke, professor of fisheries and assistant leader at the Institute of Arctic Biology at the University of Alaska Fairbanks, explained to GlacierHub that the major concerns from a freshwater perspective are changing patterns in the timing and magnitude of stream flows, and increasing water temperatures. “Salmon are at the margin of their range in much of Alaska so the latter may be less of an immediate concern. However, changes to flows have already occurred and are projected to increase into the future,” he said.

Falke told GlacierHub that the glacial rivers are an important habitat for multiple species of salmon across Alaska. The river bottoms and banks are also important habitats for the fish. Glacier loss causes changes to the hydrology of these systems, which includes both the rivers and the habitats that they support. Climate change could make the glacier river systems more similar to surface water/snowmelt runoff systems, which would therefore reduce the diversity of habitats. By reducing or removing the habitats favored by specific salmon species and by specific stocks (sub-populations) within the species, it would also reduce the salmon biodiversity. Falke further stated, “I’m not sure we can do anything about glacial loss, but continuing to work to ensure that there is a broad array of intact habitats in other areas will be key.”

Perspective rendering showing retreat of the Skilak Glacier, a major source of glacial runoff to the Kenai River. Colored lines indicate ice extent for 1952, 1978, and 2013 derived from aerial photographs (Source: Future of Pacific Salmon in the Face of Environmental Change).

The author of the study, Erik Schoen, a postdoctoral fellow at Alaska Cooperative Fish and Wildlife Research Unit at the University of Alaska Fairbanks, told GlacierHub that climatic and landscape changes influence salmon ecosystems. These diverse ecosystems are large, varied, and interact with glaciers in different ways,. Thus, the changes will not necessarily be all negative or all positive. “Some of the salmon runs that Alaskans have relied on for generations are probably going to decline, but other runs may become more productive, and we have a chance to shape that with strong habitat protections,” he said.

Changing Environmental Factors

The authors of the study conclude that salmon rivers in this region face a complex set of climate-driven changes, including warmer summer stream temperatures, glacier retreat, and increasing streamflows during fall and winter when developing embryos are vulnerable to more rapid flow even in relatively sheltered areas where females deposit their eggs.

The overall results of climate change are likely going to cause winners and losers, the authors note. There are five species of Pacific salmon, and they each use a range of different life-history strategies and habitat types, so are likely to respond in different ways. Schoen explained to GlacierHub that hotter, drier summers will expose salmon to low oxygen levels which can cause die-offs. “This is a big concern in small, lowland streams, but less so in streams with a cooling glacial or snow-melt influence. Warmer winters are causing more rain-on-snow events, which can cause floods that kill salmon eggs in the streambed,” he said. It’s important to mention that some streams are more likely to be affected than others, he added. A positive outcome from glacier retreat is that it allows salmon to colonize new streams and lakes. Longer ice-free growing seasons allow the juvenile salmon to grow larger in certain habitats.

Economic Impacts

Schoen explained the economic importance of salmon in the region: “Salmon fishing is one of the main pillars of the Kenai Peninsula’s economy, and an important part of the overall Alaskan economy. This includes commercial fishing (and support industries) and recreational fishing, which is a major driver of the tourism industry.”

The study can help build resilience toward a changing climate. Schoen told GlacierHub, “Our goal was to highlight the rapid changes happening in the Gulf of Alaska region and explain what this means for salmon and the people who depend on them.” There is a large amount of research documenting these changes; however, the majority does not always allow for an easy understanding of the big picture. “We wanted to make the science more accessible to the general public, policy makers, and scientists in other fields,” Schoen added.

Increased Resilience

Chinook Salmon (Source: Pacific Northwest National Laboratory, Flickr).

Falke told GlacierHub that the best way to ensure robust salmon populations is to maintain and promote diverse habitats and life histories. “Luckily in Alaska there are mostly intact habitats, and the example of the Bristol Bay sockeye salmon fishery is the best to highlight how diversity equals both ecological and economic resilience,” Falke added.

Schoen explained to GlacierHub that prior research has shown that fishing communities can stabilize their revenue streams by diversifying their catch to include different fish species and stocks. A stock of fish is a population within a species that migrates together, breeds together, and is genetically distinct; one species will have a number of stocks, some of which could respond to climate impacts more favorably than others. However, many fishing communities have adopted strategies that are the reverse, concentrating their efforts on fewer stocks. “Diversifying the fishing sector (and overall economy) is an important goal to increase the resilience of Alaskan communities to rapid and unpredictable climate change,” Schoen further explained.

Hood told GlacierHub about the critical importance of more holistic research, which can provide an understanding of how glacier change is impacting the structure and function of food webs downstream rivers and estuaries. “This information will allow us to better project future impacts and understand how ecosystems services such as fisheries and tourism opportunities may change in the future,” Hood added.

This research show the complex effects of glacier retreat on salmon populations and the humans that depend on them. Though most salmon species face less favorable conditions in most of their range, some species are hit harder than others. And the impacts on the habitats, though generally negative, are less severe in some areas than others, and some new habitats are being created by glacier retreat. This article marks a major advance in this complex system, a topic of great importance for the fishing communities— indigenous, sport and commercial fishers.

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