“I grew up on the shores of Connecticut looking into tidal pools and wondering about the plants and animals living there: where they move to when the tide goes out, and from when the tide comes in, and why. Once I even tracked my cat out my 3rd floor window and onto the roof to see how she accessed my bedroom at night. So I’ve always been curious about movement patterns in nature. The whys and hows of nature are my passion and driving force.” ~Heidi Golden
Heidi Golden, Ecology and Evolutionary Biology researcher and Ph.D. candidate at the University of Connecticut, collects and preserves fin clip samples from Arctic grayling to gain genetic information about fish diversity and movement. She has collected from extremely remote spots, landing by helicopter and then walking kilometers to get appropriate samples from habitats ranging from the northern foothills of the Brooks Range north to the Arctic coastal plain.
The data she gains adds to a growing understanding of Arctic grayling collected by scientists including Dr. Linda Deegan, Marine Biological Laboratory (Woods Hole, MA), as well as fishery biologists with the U.S. Fish and Wildlife Service.
Thermokarsts and fish
Our world is not static. The Arctic grayling lives as long as 20 years and might travel as far as 100 miles [160 kilometers] during spring spawning and summer feeding times. Populations of this hardy fish rely on multiple interlinked habitats to survive, which means they are experiencing some of the impacts of our shifting world.
“Aside from changing hydrology and changing seasonality, another strong impact in the Arctic having repercussions for the Arctic freshwater system are features called thermokarsts. A thermokarst is an area of permafrost thaw.” ~Heidi Golden
Thawing permafrost slides downhill in a tumble of muddy previously-frozen organic matter. It exposes dark soil to the sun, collecting heat instead of reflecting it like snow or lighter-colored mosses and lichens. The thermokarst material moves by gravity, often into rivers and lakes. Golden says it can change the whole trophic structure of the lake or stream by adding unusual levels of nutrients, increasing turbidity and decreasing how much sunlight permeates through the water column. Thermokarsts can instigate major problems for Arctic grayling and other freshwater fish species, especially when they’re located upstream of the gravelly riverbeds, which Arctic grayling require as spawning habitat.
“For example, if there is site fidelity to a certain spawning location and a thermokarst occurs upstream, increased sediment and nutrients to that location might decrease survivorship of eggs and/or young. Altered vital rates might have strong repercussions for populations of grayling that need to use specific sites for their spawning, feeding and overwintering habitats.” ~Heidi Golden
“Arctic grayling life history cycle requires a landscape that has those different habitats available to them and the ability to get from one location to the other at specific times during the year. The only way they can do that is through water courses and only at certain periods of time. So if the river happens to go dry during a critical movement period, they will be unable to reach essential habitat.” ~Heidi Golden
The Arctic has been warming about twice as fast as the rest of the Northern Hemisphere. Amplified Arctic warming can impact weather patterns and increase the likelihood of extreme events like droughts, floods, and heat waves.
Arctic grayling are reliant on late fall rains to boost stream depth and allow them a path back to their overwintering habitat where deep water resists freezing and provides enough oxygen for the fish to survive all winter. Unpredictable drought events prove problematic, as individual populations of Arctic grayling require connectivity (accessible and interlinked waterways) not only for local population persistence, but in order to maintain the overall population’s genetic health and resilience.
“That’s the broader picture of connectivity across the landscape and why fragmentation could interfere with the ability of a locally extinct population to be recolonize. Local extinctions happen, but being able to recolonize those locations or others as conditions change is critical for overall population sustainability.” ~Heidi Golden
How do genes vary among different populations of Arctic grayling? What habitats do the groups use, and how do they travel between them? How are the populations connected? Do fish migrate between groups?
Golden hypothetically describes a panmictic population, in which all individuals exchange genes and form a large homogenous genetic group, in contrast to fully isolated populations, in which no genetic exchange is possible and local adaption creates very different spatially separate gene pools. Neither is ideal for a species to persist in a variable and changing climate.
“If you have some isolation and some interchange of genes, that’s the best scenario for resilience in terms of genetic variability because you get some local genetic adaptation, which causes genetic changes among local populations, but you get some exchange, too, resulting in a lot of genetic diversity across the landscape.” ~Heidi Golden
“From those samples, I was able to distinguish a number of different genetic populations, or genetic clusters, which means individuals within these clusters are exchanging genes more frequently than they are with other clusters. There is still a little bit of dispersal among local populations but mostly they are spawning and exchanging genes within these genetically distinct local populations.” ~Heidi Golden
Golden outlines the groups. Some genetic clusters appear to be enforced by watershed boundaries, or the physical paths of streams, rivers and lakes created approximately 10,000 years ago when glaciers retreated. Other groups, however, appear to be associated with river dry zones: stretches of waterway that sometimes dry up.
River dry zones
“The dry zones are not there all the time. They are ephemeral dry zones. In the spring-time water moves through so the fish could migrate from one location to another, but the dry zones seem to form strong barriers, nevertheless. Perhaps dry zones prevent grayling moving from one critical location to another. Fish that enter such risky areas end up dying, possibly leading to natural selection within the system.” ~Heidi Golden
When adventurous Arctic grayling end up trapped due to a drought and die, they can no longer pass on genes to the next generation. In contrast, fish prone to less travel might be rewarded with greater success as their genes persist in the population.
“As climate change causes more dry zones to occur, they might block off the more risky venturing fish. So the ones that go far, for example, might have a greater likelihood of getting trapped by a dry zone while returning to overwinter. And if those fish are weeded out of the population by natural selection what you end up with is a population that tends to migrate less, possibly leading to increased population isolation. If the same scenario plays out across the aquatic landscape, that then has implications for the whole meta-population.” ~Heidi Golden
Within each genetic cluster, Golden suspects that there might be further differentiation within populations. It may be that specific Arctic grayling individuals only spawn at certain spawning locations. She’s investigating genetics of ‘young- of- the- year’ Arctic grayling to test whether spawning site fidelity by adults within a genetic cluster might cause further genetic differentiation of subpopulations. Are all adults within predefined clusters spawning together to create homogenious local population? Or, do adults show spawning site fidelity and only breed with other adults at that particulate site, despite intermingling during the warm summer growing season and cold overwintering season?
Site fidelity to a spawning location could put populations even more at risk in the changing Arctic – what happens when spawning locations are disrupted by drought or by thermokarst events? What about habitat fragmentation due to human development? Golden talks about fish management, and where climate change knowledge comes into the equation.
“Studying Arctic systems, where habitat fragmentation is rapidly occurring, provides information that can be transferred to other species relying on multiple habitat types but that haven’t yet experienced climate impacts. We can’t change the course of climate change. Even if we stopped all greenhouse gas emissions today, we still have a lag in the climate system of at least 100 years. We can, however, make informed management decisions, such that when we build roads, for example, that bridges and/or culverts are properly placed to ensure connectivity among critical habitats. Additionally, knowing which populations are more or less susceptible to local extinction should influence where human infrastructure occurs in the future. It’s these other anthropogenic factors that might invoke a tipping point from some species, yet we can control them now and should manage them carefully.” ~Heidi Golden
Frontier Scientists: presenting scientific discovery in the Arctic and beyond
- Interview with Heidi Golden, Ph.D. candidate at the University of Connecticut, July 2014
- Interview with Jeff Adams, fishery biologist, U.S. Fish and Wildlife Service, July 2014
- ‘Arctic Grayling species profile’ Alaska Department of Fish & Game, Rocky Holmes, Andy Gryska (2007)
- ‘Evidence linking Arctic amplification to extreme weather in mid-latitudes’ Jennifer A. Francis, Stephen J. Vavrus, Geophysical Research Letters, DOI: 10.1029/2012GL051000 (March 2012)