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Climate change is occurring rapidly in the Arctic, and an improved understanding of the response of aquatic biota and ecosystems will be important for this data‐limited region. Here, we applied biochronology techniques and mixed‐effects modelling to assess relationships among growth increments found on lake trout (Salvelinus namaycush) otoliths (N = 49) captured from 13 lakes on the Arctic Coastal Plain of northern Alaska, observed and modelled climate patterns, and individual‐level fish and lake characteristics. We found that annual growth varied by year, fish growth slowed significantly as individuals aged, and females grew faster than males. Lake trout had higher growth in flow‐through lakes relative to lakes that were perennially or seasonally connected. Annual growth was positively correlated with observed air temperature measurements from a local weather station for the period 1998–2014, but no clear warming trend was evident for this period. Modelled August air temperatures from 1978–2014 predicted lake trout annual growth (root mean squared error = 0.045 mm) and indicated increasing temperatures and annual lake trout growth over the period 1950–2014. This study demonstrated that biochronology techniques can reconstruct recent climate patterns and provide a better understanding of trends in Arctic lake ecosystems under a changing climate. [ABSTRACT FROM AUTHOR]

Polymorphism facilitates coexistence of divergent morphs (e.g., phenotypes) of the same species by minimizing intraspecific competition, especially when resources are limiting. Arctic char (Salvelinus sp.) are a Holarctic fish often forming morphologically, and sometimes genetically, divergent morphs. In this study, we assessed the morphological and genetic diversity and divergence of 263 individuals from seven populations of arctic char with varying length‐frequency distributions across two distinct groups of lakes in northern Alaska. Despite close geographic proximity, each lake group occurs on landscapes with different glacial ages and surface water connectivity, and thus was likely colonized by fishes at different times. Across lakes, a continuum of physical (e.g., lake area, maximum depth) and biological characteristics (e.g., primary productivity, fish density) exists, likely contributing to characteristics of present‐day char populations. Although some lakes exhibit bimodal size distributions, using model‐based clustering of morphometric traits corrected for allometry, we did not detect morphological differences within and across char populations. Genomic analyses using 15,934 SNPs obtained from genotyping by sequencing demonstrated differences among lake groups related to historical biogeography, but within lake groups and within individual lakes, genetic differentiation was not related to total body length. We used PERMANOVA to identify environmental and biological factors related to observed char size structure. Significant predictors included water transparency (i.e., a primary productivity proxy), char density (fish·ha‐1), and lake group. Larger char occurred in lakes with greater primary production and lower char densities, suggesting less intraspecific competition and resource limitation. Thus, char populations in more productive and connected lakes may prove more stable to environmental changes, relative to food‐limited and closed lakes, if lake productivity increases concomitantly. Our findings provide some of the first descriptions of genomic characteristics of char populations in arctic Alaska, and offer important consideration for the persistence of these populations for subsistence and conservation. [ABSTRACT FROM AUTHOR]

In high‐latitude lakes, air temperature is an important driver of ice cover thickness and duration, which in turn influence water temperature and primary production supporting lake consumers and predators. In lieu of multidecadal observational records necessary to assess the response of lakes to long‐term warming, we used otolith‐based growth records from a long‐lived resident lake fish, lake trout (Salvelinus namaycush), as a proxy for production. Lake trout were collected from seven deep, oligotrophic lakes in Lake Clark National Park and Preserve on in southwest Alaska that varied in the presence of marine‐derived nutrients (MDN) from anadromous sockeye salmon (Oncorhynchus nerka). Linear mixed‐effects models were used to partition variation in lake trout growth by age and calendar‐year and model comparisons tested for a mean increase in lake trout growth with sockeye salmon presence. Year effects from the best mixed‐effects model were subsequently compared to indices of temperature, lake ice, and regional indices of sockeye salmon escapement. A strong positive correlation between annual lake trout growth and temperature suggested that warmer springs, earlier lake ice break‐up, and a longer ice‐free growing season increase lake trout growth via previously identified bottom‐up increases in production with warming. Accounting for differences in the presence or annual escapement of sockeye salmon with available data did not improve model fit. Collectively with other studies, the results suggest that productivity of subarctic lakes has benefitted from warming spring temperatures and that temperature can synchronise otolith growth across lakes with and without sockeye salmon MDN. [ABSTRACT FROM AUTHOR]

Despite low species diversity and primary production, trophic structure (e.g., top predator species, predator size) is surprisingly variable among Arctic lakes. We investigated trophic structure in lakes of arctic Alaska containing arctic char Salvelinus alpinus using stomach contents and stable isotope ratios in two geographically-close but hydrologically-distinct lake clusters to investigate how these fish may interact and compete for limited food resources. Aside from different lake connectivity patterns ('leaky' versus 'closed'), differing fish communities (up to five versus only two species) between lake clusters allowed us to test trophic hypotheses including: (1) arctic char are more piscivorous, and thereby grow larger and obtain higher trophic positions, in the presence of other fish species; and, (2) between arctic char size classes, resource polymorphism is more prominent, and thereby trophic niches are narrower and overlap less, in the absence of other predators. Regardless of lake cluster, we observed little direct evidence of arctic char consuming other fishes, but char were larger (mean TL = 468 vs 264 mm) and trophic position was higher (mean TP = 4.0 vs 3.8 for large char) in lakes with other fishes. Further, char demonstrated less intraspecific overlap when other predators were present whereas niche overlap was up to 100% in closed, char only lakes. As hydrologic characteristics (e.g., lake connectivity, water temperatures) will change across the Arctic owing to climate change, our results provide insight regarding potential concomitant changes to fish interactions and increase our understanding of lake trophic structure to guide management and conservation goals. [ABSTRACT FROM AUTHOR]

Mercury (Hg) is a widespread element and persistent pollutant, harmful to fish, wildlife, and humans in its organic, methylated form. The risk of Hg contamination is driven by factors that regulate Hg loading, methylation, bioaccumulation, and biomagnification. In remote locations, with infrequent access and limited data, understanding the relative importance of these factors can pose a challenge. Here, we assessed Hg concentrations in an apex predator fish species, lake trout (Salvelinus namaycush), collected from 14 lakes spanning two National Parks in southwest Alaska, U.S.A. We then examined factors associated with the variation in fish Hg concentrations using a Bayesian hierarchical model. We found that total Hg concentrations in water were consistently low among lakes (0.11–0.50 ng L−1). Conversely, total Hg concentrations in lake trout spanned a thirty-fold range (101–3046 ng g−1 dry weight), with median values at 7 lakes exceeding Alaska's human consumption threshold. Model results showed that fish age and, to a lesser extent, body condition best explained variation in Hg concentration among fish within a lake, with Hg elevated in older, thinner lake trout. Other factors, including plankton methyl Hg content, fish species richness, volcano proximity, and glacier loss, best explained variation in lake trout Hg concentration among lakes. Collectively, these results provide evidence that multiple, hierarchically nested factors control fish Hg levels in these lakes. [Display omitted] • Hg concentrations in lake trout vary widely within and across 14 remote Alaska lakes. • In half of the lakes, median concentrations exceed state health criteria. • Age and body condition control variation in Hg among fish within a lake. • Plankton methyl Hg and volcano proximity control variation in fish Hg among lakes. [ABSTRACT FROM AUTHOR]

Lake trout Salvelinus namaycush is a slow-growing, late-maturing, cold-water-adapted species whose native distribution is described by the glaciated regions of North America. Southwestern Alaska, the western limit of the species range, has undergone repeated episodes of glacial advance and retreat. In this study, lake trout were sampled from 15 lakes in six drainages on or near Togiak National Wildlife Refuge (TNWR) in southwestern Alaska. Data from 14 microsatellite loci were used to test for genetic evidence that lake trout survived the most recent glacial advance in a local refugium, and to test the effect of habitat size, elevation, and drainage on within- and among-population variation. We could not infer that unglaciated lakes within the TNWR served as a refugium during the most recent glacial maximum. In a neighbor-joining tree, populations largely clustered by drainage, suggesting a single founding source for each drainage. An exception occurred in the Nushagak and Kuskokwim rivers, where evidence for stream capture was detected. Among-population variation, as measured by FST, was 0.35; pairwise FST within drainages ranged from 0.05 to 0.47, indicative of low gene flow, even within drainages, though individual-based analysis detected movement of lake trout among lakes within the Kanektok, Goodnews, and Nushagak rivers. Lake surface area was positively associated with allele richness, and negatively associated with FIS, indicating that smaller lakes in the TNWR harbor less genetic diversity and may have more limited adaptive potential. [ABSTRACT FROM AUTHOR]

High-latitude ecosystems are among the most vulnerable to long-term climate change, yet continuous, multidecadal indicators by which to gauge effects on biology are scarce, especially in freshwater environments. To address this issue, dendrochronology (tree-ring analysis) techniques were applied to growth-increment widths in otoliths from lake trout ( Salvelinus namaycush) from the Chandler Lake system, Alaska (68.23°N, 152.70°W). All otoliths were collected in 1987 and exhibited highly synchronous patterns in growth-increment width. Increments were dated, the widths were measured, and age-related growth declines were removed using standard dendrochronology techniques. The detrended time series were averaged to generate an annually resolved chronology, which continuously spanned 1964-1984. The chronology positively and linearly correlated with August air temperature over the 22-year interval ( p < 0.01), indicating that warmer summers were beneficial for growth, perhaps by increasing fish metabolic rate or lake productivity. Given the broad distribution of lake trout within North America, this study suggests that otolith chronologies could be used to examine responses between freshwater ecosystems and environmental variability across a range of temporal and spatial scales. [ABSTRACT FROM AUTHOR]

We examined the effect of large (potentially piscivorous) lake trout ( Salvelinus namaycush) on the dietary habits of small lake trout in an arctic lake. We hypothesized that large lake trout constrain the foraging of small lake trout, thus, in the absence of large lake trout, small lake trout will shift their diet from littoral to more abundant prey offshore. We tested this hypothesis using samples from a removal experiment where all lake trout large enough to be susceptible to gill nets were removed from a small arctic Alaskan lake during 1988–1989. We examined size at age and conducted stomach content and stable isotope analyses of lake trout collected during removal (1988), early recovery (1990), and late recovery (1999) portions of the study. Lake trout grew more quickly following removal. All lake trout fed on a variety of prey, but stomach analyses provided little information on segregation of diet between size classes. δ15N and δ13C analyses showed that small lake trout shifted their diet after large lake trout were removed, apparently toward more reliance on offshore zooplankton, which also implies a habitat shift to open water. Thus, we conclude that large lake trout are restricting the dietary habits of small lake trout, a restriction that was removed in an exploited population. [ABSTRACT FROM AUTHOR]

Presents research on the fish communities in the foothills region of the North Slope of Alaska and presents a geomorphic-trophic model for landscape control of Arctic lake food webs. Why the arctic is suitable for understanding the relationships between geomorphic setting, fish distribution, and lake trophic structure; How lake trout serve as keystone predators; Food source of lake trouts; Geomorphic constraints in arctic Alaskan lakes; Importance of landscape features to lake food webs.