Introduction

The 9th International Charr Symposium gathered scientists with an interest in charr biology and management from the entire geographical range of the genus Salvelinus. Goals of the symposium, similar to previous Charr Symposiums, were to provide a continuing forum for sharing the latest research findings within a broad range of topics, networking among like-minded scientists, and developing future research collaborations. Furthermore, most presenters were invited to publish their work here in the proceedings of the symposium as a special issue of Hydrobiologia (2019). A few oral and poster presentations were invited to submit as chapters in a forthcoming book on the lake charr S. namaycush (Walbaum, 1792) (Muir et al. unpublished).

The charr symposium series has a long history (all of which were attended by Anders Klemetsen), from its origins in Winnipeg, Canada (1981), then moving to locations around the northern hemisphere, including Sapporo, Japan (1988), Trondheim, Norway (1994), Trois-Rivières, Canada (2000), Reykjavik, Iceland (2006), Stirling, Scotland (2009), Yuzhno-Sakhalinsk, Russia (2012), and Tromsø, Norway (2015; Table 1). Symposiums have drawn an average of 117 attendees; technical sessions have included an average of 66 oral and 25 poster presentations; and published proceedings have included an average of 29 articles (Table 1; Fig. 1). Geographic representation of those attending the nine symposiums generally favored the host nation, but with representatives from 21 countries, with the strongest contingents from Canada, Japan, Norway, and USA, and somewhat fewer participants from Iceland, Russia, Sweden, and the UK, and still fewer from Austria, Finland, France, Germany, and Switzerland (Fig. 2; top panel). Iran (1 representative at the 6th symposium), Mexico (1 at 6th), Netherlands (1 at 8th), Spain (1 at 8th), Taiwan (6 at 3rd), Turkey (1 at 6th), and Yugoslavia (1 at 3rd) were represented at only one symposium.

Table 1 Year, location, number of attendees, number of oral and poster presentations, venue for proceedings, and number of articles published for nine international charr symposiums (symposium number is referenced in Figs. 1, 2, 3)
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Fig. 1
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Numbers of attendees, oral and poster presentations, and articles published in proceedings of nine International Charr Symposiums (see Table 1 for locations and dates of each symposium)

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Fig. 2
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Country of affiliation for those attending (upper panel) and publishing in (lower panel) the proceedings of nine international charr symposiums (see Table 1 for locations and dates of each symposium). Participant list not available for the 5th symposium, so numbers reflect the distribution of presenters

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The idea to hold the 9th symposium in the USA was originally conceived by the first three authors of this introduction while they were attending the 7th symposium in Yuzhno-Sakhalinsk, Russia. The 9th symposium was convened for the first time in the United States on 18–21 June 2018, in Duluth, Minnesota, on the shores of Lake Superior, where the lake charr and brook charr (S. fontinalis, Garman 1885) thrive today and have supported prosperous fisheries for centuries. Oral and poster presentations during the symposium represented the entire geographic and taxonomic range of the genus and included studies from across the range of the Salvelinus genus. Geographic representation of those attending favored the home location (Canada and USA), with fewer participants from Iceland, Japan, Norway, and the UK (Fig. 2; top panel). Taxonomic representation of studies presented as oral and poster presentations also favored more studies of the three endemic North American species, lake charr, brook charr, and bull charr (S. confluentus, Suckley 1859), and fewer studies of the two holarctic species, Arctic charr (S. alpinus, Linnaeus 1758) and Dolly Varden (S. malma, Walbaum 1792), than previous symposiums (Fig. 3, top panel). Presentations about the white-spotted charr (S. leucomaenis, Pallas 1814) were similar in number to previous symposiums, which reflects a steady historical rate of contribution by Japanese researchers dedicated to the study of the species.

Fig. 3
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Species of interest for oral and poster presentations (upper panel) and publications of proceedings (lower panel) from nine international charr symposiums (see Table 1 for locations and dates of each symposium)

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The genus Salvelinus (charrs) contains the northernmost freshwater fish species on earth, which also show the most extensive polymorphisms of all vertebrate species (e.g., S. alpinus, Klemetsen 2013; S. namaycush, Muir et al. 2016). Knowledge and understanding of the evolution, ecology, and biology of charrs has increased greatly in recent decades, aided by growing research and development of new technologies, and communicated partly through the long history of the charr symposiums and their ensuing proceedings (Table 2; Fig. 3). The location of the 9th symposium in North America highlighted similar subject areas as previous symposiums, such as cultural value, genetics and evolution, behavior and habitat use, reproductive ecology and physiology, life history and population dynamics, trophic ecology (including the role of parasites), and management. Past and current proceedings have covered the entire geographical and taxonomic range of the Salvelinus genus, with an emphasis on studies in Canada and Norway, and to a lesser extent in Japan, Russia, UK, and USA, but also in Austria, Finland, France, Germany, Iceland, Sweden, and Switzerland, (Fig. 2, bottom panel). Articles published in the nine proceedings tended to focus on S. alpinus, with fewer about S. fontinalis, S. leucomaenis, S. malma, and S. namaycush, and fewer still about S. confluentus (Fig. 3, bottom panel). Below, we summarize the 9th symposium and published proceedings.

Table 2 Citations for proceedings of 8 International Charr Symposiums, in order of occurrence, from 1984 to 2015
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Cultural anthropology

Dr. Ian Winfield delivered a keynote address for a small session on cultural anthropology, which also included an oral presentation on the cultural importance of the coaster form of the brook charr in the Lake Superior basin. In 2017, the long-standing cultural importance of Arctic charr to Lake Windermere, the largest natural lake in the Lake District of northwestern England, culminated in a documentary film (‘Brass, Three Down’) that highlighted environmental issues facing the species and celebrating its role in cultural life (Winfield et al., 2019). The Lake District becomes a UNESCO World Heritage Site (Winfield et al., 2019). The keynote and associated article provided an interesting cultural connection between Windermere and the first charr symposium in Winnipeg in 1981. By special invitation from the convener, Lionel Johnson, Charlotte Kipling presented a 400-year history of Windermere charr fishing methods, management, and conservation, as one of the oldest-known charr fisheries (Kipling, 1984).

Genetics and evolution

Dr. Louis Bernatchez delivered a keynote address for multiple sessions organized thematically around the related subjects of genetics, evolution, taxonomy, and systematics, which included 26 oral and 9 poster presentations. The keynote highlighted 20 + years of research on brook charr across the species’ native range. Three articles were included here in the proceedings. First, Oleinik et al. (2019) resolved the origin of endemic stone charr (S. malma kuznetzovi, Taranetz 1933) in the Kamchatka River basin, Russia, wherein two divergent lineages, stone charr and white charr (S. albus, Bogutskaya & Naseka 2004), were carriers of haplotypes for a new group of common origin and separate from Northern Dolly Varden from the Chukchi Peninsula and Alaska. Second, Yamamoto et al. (2019) described spatial and temporal changes in genetic diversity of isolated white-spotted charr populations on Hokkaido Island, Japan, where dams fragmented populations and thereby caused those fish upstream of dams to differentiate from downstream, anadromous populations through loss of genetic diversity. Because of habitat fragmentation, isolated populations tended to cluster haphazardly within rivers and the effective population sizes were reduced to fewer than 50 individuals, thereby leaving them vulnerable to continuous loss of genetic diversity in the future. Third, Miller et al. (2019) described how removal of non-native brown trout (Salmo trutta, Linnaeus 1758) led to increased abundance and size structure of a native brook charr population in a stream in Minnesota, north-central USA. Surprisingly, brook charr effective population size and effective number of breeders did not change after brown trout removal, because increased numbers of reproductively successful adults were offset by highly skewed family sizes and the isolated population lacked genetic diversity.

Behavior, movement, and habitat

Dr. Thomas Binder delivered a keynote address for multiple sessions about movement, behavior, and habitat, which included 21 oral and 7 poster presentations. The keynote summarized insights gained over two decades of research on lake charr movements, behavior, and habitat use, and will be published as a chapter in the upcoming book about lake charr (Muir et al., unpublished). Four articles were included here in the proceedings. First, Keylor et al. (2019a) found that siscowet lake charr could visually forage on deep-water sculpin (Myoxocephalus thompsonii, Girard 1851) at any depth in Lake Superior based on available light levels. The reaction distance to sculpin may not decline until depths exceed 200 m in Lake Superior, but reaction distances were not affected by substrate type under any light intensity. Second, Keylor et al. (2019b) found that siscowet lake charr foraging success increased with light intensity and was higher for pelagic prey than benthic prey, so daytime foraging depths were deepest in summer and shallowest in winter. Nighttime foraging depths were deepest in winter and shallowest in summer. Third, Mulder et al. (2019) found that anadromous Arctic charr (1) were more active during day than night and at the beginning and end of each daylight period, in response to prey and light availability for foraging, and (2) activity increased with temperature during spawning and ice-breakup periods, but not in winter when increased activity was correlated with colder temperatures for overwintering in a lake in southern Labrador, Canada. Fourth, Miyamoto & Araki (2019) found in laboratory studies that survival and settlement of age-0 white-spotted charr were inversely related to water depth and structural complexity that would change in response to climate change and dam operation. Results suggested that declining water levels may lead to reduced abundance of white-spotted charr due to increasing predation risk and interference competition and that increased habitat structure complexity might mitigate reductions in white-spotted charr abundance.

Reproductive ecology and physiology

Dr. Skúli Skúlason delivered a keynote address for a session about reproductive ecology, developmental ontogeny, and physiology, which included 3 oral and 2 poster presentations. The keynote described an “eco-evo-devo” model that integrated environmental, ecosystem, and community processes (ecological), sorting and transmission of variation across generations (evolution), and the role of developmental ontogeny in phenotypic variation (developmental) (Skúlason et al., 2019). Three articles were included here in the proceedings. First, Alekseyev et al. (2019) described contrasting spawning patterns of parapatric and sympatric Arctic charr forms in Transbaikalia (Russia) noting that reproductive patterns varied greatly in time, duration, location, depth, and bottom substrate. Diversification of reproductive strategies was linked to the exceptionally high diversity among Transbaikalian charr, with differences in reproductive strategies being an important determinant of intraspecific diversification. Second, Nagler et al. (2019) studying a wild population of Dolly Varden from Florence Lake, Alaska, found that plasma insulin-like growth factor-1 (IGF-I) was related to body condition during the summer growth period, but was not a useful endocrine marker of reproductive state. Third, Smith & Ridgway (2019) described the results of a literature review of 53 publications estimating brook charr thermal habitat (fundamental thermal niche = 13–17°C; realized thermal niche 10–20°C), which matched the peak of the aerobic scope curve (Fry curve) published 70 years ago (Graham, 1949).

Life history and population dynamics

Dr. Michael Hansen delivered a lead-off presentation to multiple sessions organized thematically around related subjects of population ecology, population dynamics, and life history variation, which included 22 oral and 6 poster presentations. His presentation described variation in life history and population dynamics of lake charr across the species’ range in North America, which will be a chapter in the upcoming book about lake charr (Muir et al., unpublished). Five articles were included here in the proceedings. First, Chavarie et al. (2019) found that spring precipitation and summer air temperature were related to growth of anadromous Arctic charr captured in subsistence fisheries from the Hornaday River, NWT, but that the temperature effect decreased with age while the precipitation effect increased with age, likely as energy allocation shifted with the onset of maturity. Second, Kissinger et al. (2019) quantified the advantage of life history diversification by lake charr in the interconnected Husky Lakes of the Western Canadian Arctic, NWT, where semi-anadromous and brackish-water-resident lake charr grew faster in brackish water than freshwater residents, and brackish-water residents were significantly older at capture than the other two life histories. Third, Gallagher et al. (2019) found that rarely observed resident female Dolly Varden were among the largest-sized-at-age compared to resident males and pre-smolt anadromous females, while fecundity was similar to anadromous females, in three rivers of the western Canadian Arctic, North America. Fourth, Morita et al. (2019) documented competing effects of habitat fragmentation and mitigation for stream-dwelling white-spotted charr populations in 30 dammed streams of the Oshima Peninsula, Hokkaido Island, Japan, where three populations went extinct (probability of extinction was inversely related to watershed area), and one new population had colonized a dammed stream after a fish ladder was installed. Fifth, Mitro et al. (2019) used web-based stream temperature and fish distribution models to predict that suitable habitat will decline by 67.9% for brook charr and 31.7% for brown trout from the late-20th century to the mid-21st century in Wisconsin streams in response to climate change.

Trophic ecology

Dr. Kimmo K. Kahilainen delivered a lead-off presentation to multiple sessions organized thematically around the related subjects of trophic ecology, parasites, predators, toxicology, and pollution, which included 12 oral and 5 poster presentation. Seven articles were included here in the proceedings. First, Kahilainen et al. (2019) examined the role of Arctic charr in a comparative study of large sub-Arctic lakes. They found that Arctic charr increased the total area of community trophic niche space much more in monomorphic whitefish (Coregonus lavaretus, Linnaeus 1758) lakes, where charr were the top piscivore, than in polymorphic whitefish lakes with more predators. The effect was ascribed to differences in food-chain length that was longer in the monomorphic whitefish lakes, and the keystone predator role of Arctic charr in the monomorphic whitefish lakes. Second, Knudsen et al. (2019), examining the impacts of anthropogenic stressors on polymorphic Arctic charr, noted that introduction of the opossum-shrimp Mysis relicta (Lovén 1862) had reduced ancestral niche segregation between or among two deep-water morphs and thereby increased resource competition, and possibly, the risk of hybridization. Third, Henriksen et al. (2019) found that growth rate of Arctic charr was linearly related to abundance of a less harmful parasite, Crepidostomum spp., and non-linearly related to abundance of a more harmful parasite, Eubothrium salvelini (Schrank 1790), perhaps because the negative effect of a high-cost parasite was outweighed by the energy gained from feeding on the intermediate host. Fourth, Paterson et al. (2019) found that habitat-restricted Arctic charr and littoral brown trout populations were surprisingly similar in trophic niches and parasite communities in two interconnected southern Norwegian lakes that differed in altitude. Fifth, Moccetti et al. (2019) investigated parallelism in the trophic niches of polymorphic populations of Arctic charr in similar sub-Arctic lakes of northern Norway. Using both recent (habitat and diet) and time-integrated niche estimators (stable isotope analysis and trophic transmitted parasites), they showed a parallelism in habitat choice and external morphology for reproductive isolated Arctic charr morphs (small-sized deep-water and pelagic and littoral upper-water omnivorous morphotypes) but no parallelism in the pattern of diet choice between compared profundal morphotypes. These findings demonstrated how evolution can produce diverse outcomes among systems with apparently similar environmental and ecological conditions. Sixth, Boone & Quinlan (2019) found fish lice, Salmincola sp., on most brook charr in a remnant population in Tobin Harbor on Isle Royale, Lake Superior, and concluded that infestation had reached a level that could influence future population growth and sustainability. Last, Hoxmeier et al. (2019) reported, contrary to expectations, that native brook charr displaced non-native brown trout in a Minnesota stream when increasing base-flow discharge led to declining water temperature, despite increasing air temperature.

Management

Dr. Daniel Schill delivered a keynote address for multiple sessions about management of charr as both native and non-native species, which included 12 oral and 5 poster presentations. The keynote summarized how management of charr in the western USA has attempted to restore federally listed bull charr while simultaneously mitigating negative effects of non-native lake charr and brook charr. Five articles were included here in the proceedings. First, Dux et al. (2019) described how removal of adult and juvenile non-native lake charr by incentivized angling, gill netting, and trap netting caused a 65% decline in adult abundance and a 56% decline in recruitment in Lake Pend Oreille, Idaho, during 2006–2016. This mitigated effects on native bull charr and valuable fisheries for kokanee (Oncorhynchus nerka, Walbaum 1792) and rainbow trout (Oncorhynchus mykiss, Walbaum 1792). Second, Hansen et al. (2019) found that abundance of lake charr in Lake Pend Oreille, Idaho, could be suppressed by 90% within 7–13 years by continued fishing at peak historical (2006–2016) levels. Once suppressed, lake charr abundance targets could be maintained with 76–86% less fishing effort using an optimal allocation of gillnet mesh sizes, thereby facilitating the recovery of kokanee prey populations upon which native bull trout depend. Third, Corsi et al. (2019) described how suppression of top-down effects (predation) of lake charr and a natural reduction in bottom-up effects (competition) of Mysis diluviana (Audzijonyte & Väinölä, 2005) simultaneously led to increased kokanee production from 224 tonnes to 408 tonnes in Lake Pend Oreille, Idaho. Fourth, Rogers et al. (2019) found that the Klondike strain lake charr had lower survival, were heavier at length, grew to shorter length at maturity and asymptotic length, and had higher frequencies of fresh sea lamprey wounds at larger sizes than the Seneca strain of lean lake charr in Lake Erie. Fifth, Carlson et al. (2019) used precipitation- and groundwater-corrected stream temperature models to predict effects of climate change on brook charr populations in Michigan. Projected stream warming caused greater habitat losses for brook charr in surface runoff-dominated streams (i.e., streams with limited groundwater-driven thermal buffering) than in groundwater-dominated streams. However, even groundwater-dominated streams will not provide enough habitat for brook charr survival and growth if groundwater temperatures increase or groundwater inputs decline because of reduced precipitation.

Conclusion

Research reported here in the proceedings of the 9th International Charr Symposium has built upon the history of earlier symposiums and will be continued with the 10th symposium scheduled to convene in Nikko City, Tochigi Prefecture, Japan, in 2021. As this symposium has underlined, growing attention is being paid to the management and sustainability of charr species throughout the world because of both large-scale (e.g., climate change) and local-scale (e.g., development) anthropogenic stressors that are now affecting once-remote populations. Further, while knowledge about charr species is increasing, much remains to be discovered, with an ever-increasing demand for the practical knowledge needed to effectively mitigate and manage threats to charr species throughout their native ranges. As the North American work with lake charr central to this symposium highlighted, charr can be both threatened and a threat. Within their native range, charr may require substantial management and research to preserve endemic diversity (e.g., lake charr within the Great Lakes basin), while outside their native range, charr pose an invasive threat by reducing native species abundances, including other charr species, and restructuring food webs (e.g., lake charr introductions in western North America). Thus, active management of charr species is likely to become ever more important in the future, particularly in habitats at the extremes of their native ranges. Within North America, management and research continues to focus on apex predator populations of lake charr within the Great Lakes, with increasing concern for stream-resident brook charr populations facing extirpation or displacement through competition with other species and continuing or increasing concern for climate-related impacts on northern populations of all charr species. In Europe and Japan, flow regulation associated with river damming, species introductions, and climate change all pose ongoing or increasing concerns. One presumes a similar situation prevails in Russia. Thus, for charr biologists, continued study of basic ecological mechanisms provides a breadth of opportunity, although much of it likely will need to be completed within the context of understanding and managing vulnerability of charr species to ecosystem structural changes brought about by large-scale anthropogenic stressors.