Glacier meltwater into Southeast Alaska estuaries influences fish diets

Two researchers at waters edge pulling in seine net
Emily Whitney and Doug Duncan pull in a seine net. Photo by Anne Beaudreau.

Juneau, Alaska—In most estuaries, fish feed on ocean- and river-dwelling plants and animals.  However, fish living in some estuaries near Juneau also have the option to eat terrestrial and freshwater species delivered by glacier meltwater.

Estuaries are coastal water bodies that contain both saltwater from the ocean and freshwater from rivers or streams. The rivers and streams bring a variety of aquatic species to the coast. Southeast Alaska estuaries also get freshwater from glacier melt, which can introduce terrestrial species to the ecosystem. Currently, about 30 percent of the freshwater flowing into the Gulf of Alaska comes from glacier melt. 

Researchers at the University of Alaska Fairbanks School of Fisheries and Ocean Sciences are studying how estuary habitats and food webs are influenced by glacier meltwater. To date, few studies have looked at the effect of glacier melt on the species composition of estuaries. 

The study will reveal some of the links between glacier and estuary environments in Southeast Alaska. Understanding the effect of glacier melt on the whole estuary ecosystem is important because 95% of the glaciers in Southeast Alaska are thinning or retreating. As the glaciers continue to melt, this understanding will help researchers evaluate how estuary habitat and community composition are likely to change.

Approximately 70,000 people in Southeast Alaska rely on estuaries and nearshore marine ecosystems for food, recreation, and economic support. Changes in these nearshore species impact not only Southeast Alaska marine ecosystems, but also the communities that rely on them.

Glacier melt affects species that have low tolerance for changes in salinity or temperature, such as kelp, and the silt carried into estuaries blocks light, which makes it harder for algae to photosynthesize. But glacier runoff also delivers carbon and other nutrients downstream that could increase productivity in estuaries. 

Closeup of hands holding several sculpin fish
A handful of Pacific staghorn sculpin at one of the research sites. Photo by Emily Whitney.

SFOS assistant professor Anne Beaudreau and master’s student Emily Whitney are studying variability in diets of fish in estuaries that receive different amounts of glacier meltwater. “We’re using diet as a way of looking at the interactions and habitat linkages within the estuary environment,” Whitney said. “This will help us understand what food sources are coming in from the terrestrial and freshwater environments and how they are mixing with the food sources available in the estuary from marine sources.” The research, which is funded by the Alaska Experimental Program to Stimulate Competitive Research (EPSCoR), Alaska Sea Grant and other programs at UAF, will be completed over a period of three to four years.

Beaudreau and Whitney are studying diets of juvenile coho salmon, Dolly Varden, starry flounder and Pacific staghorn sculpin, which are abundant in Southeast Alaska estuaries with and without the influence of glacier meltwater. In addition, starry flounder and staghorn sculpin are in the middle of the food web—they are an important food source for predators like halibut, bears, otters and eagles, and they shape the environment by preying on smaller fish and insects. 

The researchers collected fish samples at estuaries near Juneau with varying levels of influence from glacier meltwater. Cowee Creek estuary is 13 percent glaciated (covered by glaciers). In contrast, the Eagle River estuary is 48 percent glaciated, and the Mendenhall River estuary is 63 percent glaciated. Samples were collected by seining along beaches in the estuaries near the mouths of the creeks and rivers from April to September 2014. 

The researchers identified all fish from the seine nets, and now they are looking at the stomach contents of a sample of fish to get a snapshot of the fish diets in each of the estuaries. They also plan to analyze carbon and nitrogen isotopes in fish muscles to get a long-term look at the food different fish have been eating. 

Beaudreau and Whitney have begun looking at trends in staghorn sculpin diets. “Staghorn sculpins are small fish with big mouths, and pretty much eat whatever fits inside their mouth,” Beaudreau said, making them a good indicator for prey species living in each estuary. So far, the researchers have found that staghorn sculpin have been eating both aquatic and terrestrial insects as well as a range of marine invertebrates and fish. This suggests that rivers and glacier melt play an important role in affecting the availability of prey for these fish.

young man and woman researchers closely examining a small fish
Doug Duncan and Emily Whitney work at one of the sampling sites. Photo courtesty of Emily Whitney.

They also found the staghorn sculpin had eaten a lot of species tolerant of low salinity environments, including mysid shrimp and polychaetes. This eating pattern may show that predators and prey are already adjusting to low salinity conditions created by the inflow of glacier meltwater, or may reflect that these estuaries have species that are tolerant of various levels of salinity.

The initial results suggest that the differences in diet over the season are greater than differences in diet between the various estuaries. The freshwater-tolerant species represent only a fraction of the total diet. More variation is occurring based on when species are available throughout the season.

The data collected during this study will help establish a baseline understanding of how glacier melt can influence food sources for estuary fish. As glaciers continue to recede in Southeast Alaska, this understanding will help scientists and communities better understand how to sustainably manage coastal ecosystems and resources.