Migratory Arctic char (Salvelinus alpinus) as a prey pulse for Arctic marine predators

Abstract

Variation in resource distribution and phenology can necessitate that mobile animals move to track resource availability through space and time (Abrahms et al., 2021; Furey et al., 2018). Northern latitudes are characterized by extreme seasonality in temperature and food availability, making them rich in examples of species' capitalizing on resource waves or pulses (e.g., food and thermally suitable habitat) over the brief summer before either coping with or migrating to avoid resource limitations over winter. An iconic northern example is in Alaskan brown bears (Ursus arctos gyas) that track brief, asynchronous salmon migrations to extend their access to this vital resource (Schindler et al., 2013). Marine examples include the bowhead whale (Balaena mysticetus), various seabirds that time feeding and life history events with the recession of the annual sea ice and subsequent production blooms (Mallory & Forbes, 2007), and polar bears (Ursus maritimus) that exhibit hyperphagia (strongly elevated appetite) in spring when weaned ringed seal (Pusa hispida) pups are accessible and at peak fatness (Stirling & McEwan, 1975). However, given the challenges of northern research (Mallory et al., 2018), western scientific documentation of such events is lacking for many species. Here, we showcase images (Figures 1 and 2) of summer aggregations of migratory Arctic char (Salvelinus alpinus) with individual char being hunted and consumed by multiple marine mammal and seabird species. We hypothesize that these events may be examples of resource tracking by Arctic char and their marine predators.

The Arctic char are the most northerly distributed freshwater or anadromous fish whose range spans across coastal areas of the circumpolar Arctic (Reist et al., 2013; Weinstein et al., 2024). Anadromy is a common trait in many northern freshwater fishes that involves fish hatching in freshwater, living part of their life at sea, and returning to freshwater for reproduction (McDowall, 2008). Adult anadromous Arctic char overwinter and spawn in freshwater, where the young rear under nutrient-poor conditions for several years before their first marine migration (~3–8 years and ~200 mm; Gilbert et al., 2016; Johnson, 1980). First-time migrants and adults migrate to the ocean in spring, which is in part dictated by the timing of river ice breakup (Figure 1a; Dutil, 1986; Gilbert et al., 2016; Johnson, 1980; Menzies, 2024, 23:10). At sea, Arctic char spend the short summer foraging (~4–6 weeks) in a relatively high productivity environment (Harris et al., 2022; Johnson, 1980). In late summer, they return to freshwater to access spawning and overwintering habitats before freeze-up, avoiding the frigid winter in the Arctic Ocean (Johnson, 1980). After returning to freshwater, Arctic char generally cease feeding (Boivin & Power, 1990; Dutil, 1986). Thus, after the onset of annual marine migrations, nearly all of their required annual energy intake can occur during their 4- to 6-week marine feeding periods, providing a striking example of dependence on resource pulses.

During marine migrations, Arctic char form predictable aggregations that are spatially and temporally condensed (Figure 1a,b). Even so, some studies have shown that these events may not be periods of high predator-related mortality for Arctic char (Caza-Allard et al., 2021; Munaweera et al., 2022), nor have they been documented as significant contributors to the overall diet of higher trophic level predators (e.g., seabirds and marine mammals). Here, however, we document numerous seabird and marine mammal predators foraging on aggregations of Arctic char as they enter and forage in the marine environment (Figures 1 and 2). This documentation includes rare observations of the capture of relatively large, anadromous Arctic char by glaucous gulls (Larus hyperboreus), red-throated loons (Gavia stellata), ringed seals, and narwhal (Monodon monoceros). These predators undertake significant migrations which, in this case, may target migratory Arctic char.

These observations were made in person (A.L. Vail) and from video recorded using a long lens camera (50–1000 mm), drones, and static cameras deployed on land and underwater at Kuuganajuk, Nunavut (Somerset Island; 72°41′0″ N, 93°25′0″ W) in the Canadian Arctic in summer, 2022. Video was captured by an author (A.L. Vail) and colleagues for the documentary, “Our Oceans” (Menzies, 2024, 21:19–28:07). Observations of the Arctic char migration from the lake to the ocean and the associated predation events occurred from 25 June to 5 July 2022, during which time a crew filmed for ~14 h/day. Narwhal foraging was observed from 3 to 6 August 2022, during which time filming was limited to 2 days and ~8 h/day due to logistics of reaching the filming locations.

Arctic seabirds are typically predators with broad diets, but glaucous gulls are the quintessential generalists; they feed on terrestrial and marine prey and are also scavengers (Weiser & Gilchrist, 2020). Diets of Arctic loon species are poorly known, but most consume small fish along with freshwater and marine invertebrates during the breeding season (notably red-throated loons; Rizzolo et al., 2020). Although diet studies of both glaucous gulls and red-throated loons are limited, the fish they consume are thought to primarily include smaller species such as Arctic cod (Boreogadus saida), capelin (Mallotus villosus), sculpins, sandlance (Ammodytes sp.), and Atlantic herring (Clupea harengus; Elliott & Gaston, 2008).

In the documented interactions between predators and Arctic char (Figure 1d,e; Menzies, 2024, 23:57), loons can be seen swimming with numerous adult Arctic char. We used the typical bill length of these birds (60 and 50 mm, respectively) to estimate fish length from the footage. Many Arctic char were about the same length as the loon (~600 mm; Rizzolo et al., 2020), meaning that those fish were likely >2000 g (Dutil, 1986; Gilbert et al., 2016). While these fish are too large for gulls and loons to catch, the largest fish captured were >300 mm or >230 g with an estimated energy content of >1000 kJ (Figure 2c,d; Dutil, 1986). Consuming one fish that size would meet the daily maintenance energetic need for adult glaucous gulls (Weiser & Gilchrist, 2020) or the daily energy expenditure for post-natal growth of loon chicks (Rizzolo et al., 2020) and contain far more energy than the smaller fish typically consumed by these species in the Arctic (Elliott & Gaston, 2008). Thus, the pulse of migrating char provides an enormous opportunity for Arctic seabirds during the breeding season or pre-fall migration, periods of high energetic need.

The life history of ringed seals has been strongly shaped by sea ice dynamics where seals forage more intensively in the productive summer and fall periods to rebuild lost energy stores during the less productive overwintering period (McLaren, 1958). Ringed seals are generalist and opportunistic consumers with diets consisting of numerous invertebrate and fish species (Gjertz & Lydersen, 1986), which vary with latitude and resource availability (Yurkowski et al., 2016). It is general knowledge in some northern communities that ringed seals will consume migrating Arctic char, but char have rarely been documented in western scientific studies of seal stomach contents (e.g., Gjertz & Lydersen, 1986). The interactions documented herein depict a pursuit predation event where an individual Arctic char is isolated from a larger aggregation and consumed (Figure 2a,b; Menzies, 2024, 24:11 and 24:48). These observations provide additional support that opportunistic foraging by ringed seals on an Arctic char migration pulse may be a regional phenomenon that requires further study.

The narwhal is a deep-diving cetacean that is endemic to the Arctic (Laidre et al., 2015). The narwhal range covers the eastern Canadian high Arctic, West and East Greenland, Svalbard, and Franz Joseph Land (Heide-Jørgensen et al., 2013). Narwhals follow the formation and retreat of annual sea ice over the course of extensive annual migrations, and their feeding intensity is generally higher during the over-wintering period. In summer, narwhals spend their time in ice-free bays and fjords of the high Arctic or at glacial fronts (especially around Greenland) consuming shrimp (Pandalus spp.), Arctic cod, polar cod (Arctogadus glacialis), and capelin (Watt et al., 2013). In autumn, they migrate to overwintering areas that are deep, offshore, and ice-covered, usually along the continental slope (Laidre et al., 2003, 2004), where they feed more intensively on Greenland halibut (Reinhardtius hippoglossoides), boreoatlantic armhook squid (Gonatus fabricii), or capelin (Laidre & Heide-Jørgensen, 2005; Watt et al., 2013). The observations here confirm that narwhals may opportunistically feed on the summer pulse of Arctic char (Figure 2; Menzies, 2024, 25:56). However, the extent to which narwhals rely on Arctic char as an annual food source is unknown given the limited spatial and temporal availability, and that such pulses are not available to all summering subpopulations. The images also show narwhals using their tusks to stun Arctic char before consumption (Figure 2e), which agrees with contemporaneous observations by others in the same area (O'Corry-Crowe et al., 2025). Footage collected by researchers in the Eastern Canadian Arctic showed narwhals stunning Arctic cod with their tusks, and these independent observations suggest that this is a tactic used by male narwhals.

Of the four marine predators we describe here, only ringed seals have previously been described in the literature as active predators of anadromous Arctic char (Gjertz & Lydersen, 1986). While adult Arctic char are often too large for loons or gulls to consume whole, gulls may still opportunistically feed on large adult char that become stranded in shallow water during their upriver migration (e.g., Gilbert et al., 2016), an occurrence that is likely to change in frequency as climate change continues to alter hydrological regimes. Additionally, Arctic char have been anecdotally reported in the diet of loons in Greenland (Weiser & Gilchrist, 2020). Marine mammal predator-related mortality is assumed to be low for anadromous adult Arctic char, and such events have rarely been documented despite extensive overlap with piscivorous marine mammals (Finley & Gibb, 1982; Matley et al., 2015; Moore, 1975). For instance, separate studies examined ringed seal diets in spring and summer in the Cumberland Sound region of Nunavut, Canada, and in northwestern Spitsbergen, Svalbard, and found Arctic char in gut contents of only 8% and 4% of seals, respectively (Gjertz & Lydersen, 1986; Moore, 1975). We could not find any scientific documentation of narwhal hunting Arctic char. Recent studies using archival pop-off tags and traditional and local knowledge from hunters have documented predation events of Arctic char and the closely related Dolly Varden char (Salvelinus malma malma) by beluga whale (Delphinapterus leucas) in the western Canadian Arctic (Gallagher et al., 2021; Loseto et al., 2018). Cumulatively, these observations and those documented here suggest that large marine mammals, including whales and seals, do occasionally consume anadromous salmonids in the Arctic and are a source of mortality that could be important for some populations. In general, however, marine predation of Arctic salmonids is not well documented, and future studies should focus on populations that may be likely to experience these potentially brief but intense predation events to (1) determine how these events impact mortality rates and population dynamics and (2) understand the broad ecological importance of the fish in their ecosystem.

The images discussed here depict remarkable behaviors and trophic interactions shaped by the high spatiotemporal variation in resource availability that defines high-latitude environments. The Arctic char are themselves in pursuit of greater food availability as they migrate to the marine environment where they can become predictable prey for several marine predators ready to capitalize on the influx of food. The events discussed here may be examples of intentional resource tracking by these marine predators or may be examples of opportunistic foraging strategies. These possibilities and the relative paucity of western scientific research in the region raise several questions that warrant further investigation. For instance, do these predators track the timing of the Arctic char migration to sea? Does access to these resources influence predator habitat selection? Are these indeed rare events or are they just rarely documented? How important are these prey pulses to predator energy budgets? How are these events going to be shaped by shifts in species' ranges (e.g., killer whales; Orcinus orca) and the timing of seasonal events with ongoing environmental change? While relevant western scientific research in the north has been relatively limited, local people have lived in these regions for millennia, interacting closely with these animals. As such, studies of traditional and local knowledge are a natural starting point for further exploration.

The authors declare no conflicts of interest.