ABSTRACT Lake charr (Salvelinus namaycush) head and body variation were compared among lakes across the species range in North America, to determine if variation was organized more strongly among morphs or lakes. For 3445 lake charr, head‐shape variation was > 17 times greater among morphs than lakes and body‐shape variation was > 16 times greater among morphs than lakes. Body depth, head depth, caudal peduncle length and depth, pectoral and pelvic fins lengths, and preorbital length varied more among morphs than lakes. Four morphs were delineated based on cluster analysis of combined head and body shape. The fat–fat (short head + fat body) morph had a shorter preorbital head length, deeper head and body, shorter and deeper caudal peduncle, and longer paired fins than the lean–lean (long head + slender body) morph, which had a longer preorbital length, shallower head and body, longer and narrower caudal peduncle, and shorter paired fins. The fat–lean (short head + slender body) morph was similar to the fat–fat morph in preorbital length, body depth, caudal peduncle length, and paired fin lengths, but not in head depth or caudal peduncle depth. The lean–fat (long head + fat body) morph was similar to the fat–fat morph in body depth, head depth at the eye, caudal peduncle length, and paired fin lengths, but was similar to the lean–lean morph in maximum head depth and preorbital length. The most extreme fat–fat morphs were from widely separated Great Bear Lake and Lake Superior, whereas the most extreme lean–lean morphs were from Skilak Lake (Alaska), Lake Superior, and Great Bear Lake. The lean–lean morph, present in every lake, was the common morph. A fat–fat morph was present in multiple lakes, but differed greatly in ecological function among lakes, which was more consistent with a hypothesis that multiple lake charr morphs differentiated in multiple lakes across the species' range, rather than were derived from a single already‐specialized morph that colonized multiple lakes from common glacial refugia.

Objective The Lake Trout Salvelinus namaycush diversified into multiple morphs in many lakes of northern North America. Four morphs remain in Lake Superior, of many forms that arose since the most recent glaciation of North America. Our objective was to determine if Lake Trout phenotypic diversity was greater within or among four morphs at six geographically distant locations in Lake Superior.Methods Lake Trout were sampled using standardized multi-mesh gill nets in three depth strata at six locations in Lake Superior that were known to have multiple morphs and were assigned to one of four morphs by reconciling statistical (geometric morphometric analysis of head and body shape) and visual (agreement among three experts) assignment rules. The relative importance of morphs, locations, the interaction between morphs and locations, and residual error for describing variation in head and body shape and associated linear traits were compared.Results The same four morphs were present at nearly all locations. Variation in head and body shape, and associated linear characteristics (head depth, preorbital length, body depth, caudal peduncle length, and caudal peduncle depth), was greater among morphs than among locations. This finding supports a hypothesis that Lake Trout morphs were consistent in shape and related linear traits across a large spatial scale within an environmentally diverse large lake, thereby possibly reflecting genetic differences among morphs. In contrast, variation in linear dimensions describing fin lengths, maxillary length, and other linear segments of head and body length was greater among locations than among morphs. This finding suggests that expressions of these morphometric measures was influenced by location-specific abiotic and biotic conditions.Conclusions We conclude that Lake Trout morphs appeared to have a genetic component to their head and body shape and some other linear morphometric characteristics that was differentially expressed in relation to local environmental conditions. Sources of three currently recognized nonlean Lake Trout morphs are present at 5-6 locations in Lake Superior to develop broodstocks for reintroductions elsewhere where fishery managers wish to diversfy genetic and phenotypic diversity of Lake Trout populations, if logistically accessable spawning aggregations can be located and propogated.

Variability in individual fish growth both within and among populations can interact with mortality to affect variation in size-at-age and other critical features of populations. Herein, we developed and applied mixed-effects, hierarchical growth models to back-calculated length-at-age data from six lake trout (Salvelinus namaycush) populations in Lake Superior to quantify how growth variation was attributable to persistent sources within and among populations and transient (short-term and not consistent over time for an individual) sources. Persistent variation in growth among individuals explained more variability in length-at-age than transient variation, and most of this variation was within populations rather than among populations. Simulations showed that the modeling approach could robustly estimate most growth function parameters, even with mismatches between true and assumed among-population covariation, although a higher number of populations enabled better estimation of certain population-level parameters. An implicit assumption to our interpretation was that lake trout populations in Lake Superior had not experienced substantial size-selective mortality, so their length-at-age patterns largely reflected growth variation rather than size-selective mortality. This assumption should be tested when interpreting future applications of this growth modeling approach.

Life-history variation among four lake trout Salvelinus namaycush morphs was quantified at six geographically distant locations in Lake Superior (similar to 30 to 250 km apart), one of the largest freshwater lakes in the world (82,100 km(2)). Lake trout were sampled using standardized multi-mesh gillnets in three depth strata at six locations in Lake Superior that were known or thought to have multiple morphs. Life-history traits were estimated using length-age analysis of back-calculated growth from sagittal otolith increments. Morphs, assigned using statistical and visual assignment rules, included 122 humpers, 646 leans, 86 redfins, and 1154 siscowets. Density (CPUE) varied 11-fold among morphs, 7-fold among locations, and 3-fold among depths. Morphs seemed to fill the same ecological niche at all locations, because life-history traits related to weight (body condition, buoyancy, mean weight), age, and growth rate varied more among morphs than locations. However, abiotic and biotic variation among locations also seemed to exert control over life-history variation, because life-history traits related to length, maturity, and early life history varied more among locations than morphs. We conclude that lake trout morphs appeared to have a genetic component to their life history that was differentially expressed along environmental gradients.

Historically, Cisco Coregonus artedi and deepwater ciscoes Coregonus spp. were the most abundant and ecologically important fish species in the Laurentian Great Lakes, but anthropogenic influences caused nearly all populations to collapse by the 1970s. Fishery managers have begun exploring the feasibility of restoring populations throughout the basin, but questions regarding hatchery propagation and stocking remain. We used historical and contemporary stock-recruit parameters previously estimated for Ciscoes in Wisconsin waters of Lake Superior, with estimates of age-1 Cisco rearing habitat (broadly defined as total hectares <= 80 m depth) and natural mortality, to estimate how many fry (5.5 mo posthatch), fall fingerling (7.5 mo posthatch), and age-1 (at least 12 mo posthatch) hatchery-reared Ciscoes are needed for stocking in the Great Lakes to mimic recruitment rates in Lake Superior, a lake that has undergone some recovery. Estimated stocking densities suggested that basin-wide stocking would require at least 6.41 x 10(8) fry, 4.69 x 10(8) fall fingerlings, or 3.43 x 10(8) age-1 fish for a simultaneous restoration effort targeting historically important Cisco spawning and rearing areas in Lakes Huron, Michigan, Erie, Ontario, and Saint Clair. Numbers required for basin-wide stocking were considerably greater than current or planned coregonine production capacity, thus simultaneous stocking in the Great Lakes is likely not feasible. Provided current habitat conditions do not preclude Cisco restoration, managers could maximize the effectiveness of available production capacity by concentrating stocking efforts in historically important spawning and rearing areas, similar to the current stocking effort in Saginaw Bay, Lake Huron. Other historically important Cisco spawning and rearing areas within each lake (listed in no particular order) include 1) Thunder Bay in Lake Huron, 2) Green Bay in Lake Michigan, 3) the islands near Sandusky, Ohio, in western Lake Erie, and 4) the area near Hamilton, Ontario, and Bay of Quinte in Lake Ontario. Our study focused entirely on Ciscoes but may provide a framework for describing future stocking needs for deepwater ciscoes.

To determine if otolith weight can be used to accurately and precisely estimate age-based life history metrics of Lake Trout Salvelinus namaycush populations, we quantified relationships between Lake Trout otolith age and weight sampled from a wide geographic and size range of North American lakes. Separate nonlinear age-weight models for each of 22 surveys (each year within each area or lake) described 82.8% of variation in the relationship between otolith age and weight, whereas separate age-weight models for each of 13 lakes or areas within lakes (years combined) described only 0.4% less variation. Over all surveys, age increased at an average rate of 1.13 years per milligram increase in otolith weight and the otolith weight-age relationship was significantly nonlinear. Age of individual fish could not be estimated accurately from otolith weight alone, although bias of mean age estimated from mean otolith weight was small, whether using survey-specific or general otolith age-weight relationships. Age at maturity and survival derived from indirect otolith age-weight relationships were estimated with less bias than growth parameters. We conclude that a general model for estimating Lake Trout age from otolith weight may be more useful for maturity and survival analyses that are less susceptible to estimation error of individual (often old) fish than for growth analysis that can be susceptible to estimation error of individual (often old) fish.

Pathways through which phenotypic variation among individuals arise can be complex. One assumption often made in relation to intraspecific diversity is that the stability or predictability of the environment will interact with expression of the underlying phenotypic variation. To address biological complexity below the species level, we investigated variability across years in morphology and annual growth increments between and within two sympatric lake charr Salvelinus namaycush ecotypes in Rush Lake, USA. A rapid phenotypic shift in body and head shape was found within a decade. The magnitude and direction of the observed phenotypic change were consistent in both ecotypes, which suggests similar pathways caused the variation over time. Over the same time period, annual growth increments declined for both lake charr ecotypes and corresponded with a consistent phenotypic shift of each ecotype. Despite ecotype-specific annual growth changes in response to winter conditions, the observed annual growth shift for both ecotypes was linked, to some degree, with variation in the environment. Particularly, a declining trend in regional cloud cover was associated with an increase of early-stage (ages 1-3) annual growth for lake charr of Rush Lake. Underlying mechanisms causing changes in growth rates and constrained morphological modulation are not fully understood. An improved knowledge of the biology hidden within the expression of phenotypic variation promises to clarify our understanding of temporal morphological diversity and instability.

Abstract In a polyphenic species, differences in resource use are expected among ecotypes, and homogeneity in resource use is expected within an ecotype. Yet, using a broad resource spectrum has been identified as a strategy for fishes living in unproductive northern environments, where food is patchily distributed and ephemeral. We investigated whether specialization of trophic resources by individuals occurred within the generalist piscivore ecotype of lake trout from Great Bear Lake, Canada, reflective of a form of diversity. Four distinct dietary patterns of resource use within this lake trout ecotype were detected from fatty acid composition, with some variation linked to spatial patterns within Great Bear Lake. Feeding habits of different groups within the ecotype were not associated with detectable morphological or genetic differentiation, suggesting that behavioral plasticity caused the trophic differences. A low level of genetic differentiation was detected between exceptionally large‐sized individuals and other piscivore individuals. We demonstrated that individual trophic specialization can occur within an ecotype inhabiting a geologically young system (8,000–10,000 yr BP), a lake that sustains high levels of phenotypic diversity of lake trout overall. The characterization of niche use among individuals, as done in this study, is necessary to understand the role that individual variation can play at the beginning of differentiation processes.

Historically, cisco Coregonus artedi Lesueur was the predominant prey fish and target of commercial fisheries throughout Lake Superior, but most spawning stocks collapsed by the mid-1900s. Stocks partially recovered by the early 1990s, but contemporary abundance is considered to be below historical levels and driven by intermittent recruitment. Stochastic, age-structured simulation models were used to determine whether historical (pre-1955) cisco yield in Lake Superior was consistent with contemporary (1992-2015) abundance, life-history characteristics and recruitment dynamics. When compared to contemporary stocks, the findings suggest historical stocks had: (1) similar recruits per spawner at low spawning stock sizes; (2) lower rates of compensatory density dependence; (3) similar or lower recruitment variation depending on the area and (4) higher median adult and age-1 density. These findings are consistent with the hypothesis that eutrophication during the historical period supported greater recruitment and adult abundance and that re-oligotrophication during the contemporary period may be limiting full recovery.

Historically, Cisco Coregonus artedi and Lake Whitefish Coregonus clupeaformis were abundant throughout the Laurentian Great Lakes, but overharvest, habitat degradation, and interactions with exotic species caused most populations to collapse by the mid-1900s. Strict commercial fishery regulations and improved environmental and ecological conditions allowed Cisco to partially recover only in Lake Superior, whereas Lake Whitefish recovered in all the upper Great Lakes (Superior, Michigan, and Huron). The differential responses of Cisco and Lake Whitefish to improved environmental and ecological conditions in lakes Michigan and Huron have led to questions about potential negative interactions between these species. To provide context for fishery managers, we tested for positive and negative correlations between historical (1929-1970) Cisco and Lake Whitefish commercial gill net catch per effort (CPE; kg/km of net) at a variety of spatial scales in Michigan waters of the upper Great Lakes. The three best-fit spatial models-LAKEWIDE, REGIONAL 10, and SIMPLE-all had similar levels of support (scaled second-order Akaike Information Criterion, 3.0), and we used these models to determine whether there was a significant correlation between Cisco and Lake Whitefish CPE (positive and negative). There was either no correlation between Cisco and Lake Whitefish CPE or a positive correlation for most (12 of 13) pairwise (Cisco-Lake Whitefish) comparisons. We identified no strong positive or negative correlations in the lakewide (LAKEWIDE) or reduced (SIMPLE) models. In the regional model (REGIONAL 10), we identified strong and positive correlations between Cisco and Lake Whitefish CPE in two regions (rho = 0.59-0.71) and a weak negative correlation in one region (rho = -0.45). Collectively, our findings suggest that Cisco and Lake Whitefish CPE were largely independent of each other; thus, these species likely did not interact to the detriment of one another in Michigan waters of the upper Great Lakes during 1929-1970.

Four lake trout, Salvelinus namaycush, Walbaum 1792 morphs occur in Lake Superior: lean, siscowet, humper, and redfin. Diets of lean and siscowet have been relatively well described. However, less is known about diets of humper and redfin, and overall few studies have been conducted at offshore shoals. We compared gut content data among mature (357–867 mm) sympatric lake trout morphs caught at two offshore shoals in Lake Superior, Stannard Rock and Superior Shoal, in 2013 and 2014 (total n = 416). All morphs were caught in shallow (<50 m), mid (50–100 m), and deep (>100 m) strata. Invertebrates made up a greater portion of the stomach contents than did fish for all morphs by both percent occurrence and proportional biomass, and Mysis was the primary invertebrate consumed by all morphs at both sites. Coregonus spp. and deepwater sculpin, Myoxocephalus thompsonii were the most commonly consumed fish. Humper had the highest average proportional biomass of deepwater sculpin and had no other identifiable species of fish in their guts. Biomass of fish in redfin guts was highest for Coregonus spp., followed by similar amounts of deepwater sculpin and burbot, Lota lota. Diet overlap among morphs was high, and differences in prey consumption between sites are likely related to prey availability. Additional study is needed to determine if differences in trophic ecology between humper and other morphs are sufficient to support concurrent stocking of multiple morphs, particularly in light of recent declines in native prey fishes, especially Coregonus spp., in the Laurentian Great Lakes.

Length of fish species with forked tails, such as the Lake Trout Salvelinus namaycush, can be measured as total (TL), fork (FL), or standard (SL) length, although individual studies of such species often rely on only one measurement, which hinders comparisons among studies. To determine if variation in the relationship between FL and TL among Lake Trout populations affected estimates of FL from TL, we compared length relationships within Lake Trout populations sampled in multiple years, among multiple locations within lakes, among lakes, and from all samples from across the species' range. Samples were from across the geographic range of the species and a wide range of lake sizes (1.31-82,100 km(2)) to represent the full range of variation in abiotic and biotic variables expected to influence the FL:TL relationship. The functional relationship for estimating FL (mm) from TL (mm) was FL = 0.91 x TL - 8.28 and TL from FL was TL = 1.09 x FL + 9.05. Error induced by length conversion was less when using a length relationship from a different year in the same lake than from a different area in the same lake or from a different lake. Estimation error was lowest when using an overall length conversion from across the species' range, which suggests the overall relationship could be used whenever a more accurate length conversion is not available for a population of interest. Our findings should be useful for providing a standardized model for converting FL to TL (and TL to FL) for Lake Trout, such as comparing published findings of different measurement units, converting measurement units by agencies or institutions that change sampling methods over time, or programs that use different sampling methods among areas.

A simulation model of lake trout Salvelinus namaycush (Walbaum 1792) population dynamics in Lake Pend Oreille, Idaho, was used to estimate (1) the optimal allocation of effort among gillnet mesh sizes that minimizes abundance in the shortest time; (2) the number of years needed to suppress the population to 90% of peak abundance; and (3) once suppressed, how much effort could be reduced to sustain abundance indefinitely. A density-dependent stochastic simulation model was parameterized from data in 2006–2016, including parameter uncertainty and implementation error. Time to suppression could be reduced by using more large-mesh gillnet than was used during 2007–2016. Continued fishing at the peak level of total gillnetting effort, but using an optimal effort allocation among meshes, would suppress abundance to the target level within 7–13 years. Once suppressed, gillnet effort could be reduced 76–86% (157,000 m, 95% CI 116,000–199,000 m) to sustain abundance at the target level. Our findings suggest that time to suppression of lake trout populations in other systems may be able to be reduced by optimizing gillnet effort allocation among mesh sizes, and that total effort can be greatly reduced to sustain abundance at the reduced level thereafter.

Research on Lake Pend Oreille, Idaho, has focused on the influence of two potential limiting factors for kokanee Oncorhynchus nerka (Walbaum, 1792): competition for food with Mysis diluviana (Loven, 1862, hereafter Mysis) and predation by lake trout Salvelinus namaycush (Walbaum, 1792). Population fluctuations of Mysis and lake trout have resulted in substantial heterogeneity in food web conditions, apparently altering both bottom-up and top-down dynamics. Therefore, relative importance of predation and competition were evaluated as drivers of kokanee abundance, biomass, and production. A series of general linear models was used to evaluate relative influences of Mysis and lake trout on kokanee. Kokanee production was a density-dependent process and the collapse of Mysis corresponded to an increase in the modeled maximum annual production of kokanee from 224 tonnes to 408 tonnes. Lake trout also negatively influenced kokanee biomass. A Mysis-mediated, predator-induced kokanee biomass collapse occurred when lake trout and Mysis abundances were both high. Sustainable management of this fishery requires recognition that competition with Mysis will define the scope of kokanee production and therefore the scope of sustainable predation.

The nonnative lake trout (Salvelinus namaycush Walbaum, 1792) population in Lake Pend Oreille, Idaho increased exponentially during 1999–2006. This led to an unsustainable level of predation mortality on kokanee (Oncorhynchus nerka Walbaum, 1792), increased the conservation threat to native bull trout (Salvelinus confluentus Suckley, 1859), and jeopardized the popular recreational fishery for kokanee and rainbow trout (Oncorhynchus mykiss Walbaum, 1792). In response, lake trout were suppressed since 2006 using incentivized angling, gill netting, and trap netting. From 2006 through 2016, 193,982 lake trout were removed (50% by gill netting; 44% by angling; 6% by trap netting). During this period, age-8 + (adult) lake trout abundance declined by 64%, age-3 (recruit) abundance declined by 56%, and mean total annual mortality (A) was 31.1%. Lake trout did not show evidence of a density-dependent response. Kokanee did not collapse and rebounded to abundances not observed since before lake trout expansion. Bull trout abundance declined during suppression, but the population was sustained. Lake trout suppression allowed a harvest fishery for kokanee and trophy fishery for rainbow trout to be restored. We conclude that suppression can be an effective management action for mitigating effects of nonnative lake trout in a large, deep lake.

The 9th International Charr Symposium convened on 18–21 June 2018, in Duluth, Minnesota, USA, to gather scientists with an interest in charr biology and management from the entire geographical range of the genus Salvelinus. The symposium was attended by 169 individuals from six countries, and included 99 oral and 32 poster presentations, 28 of which were published in the ensuing proceedings. Topic areas of presentations and publications included (1) cultural anthropology; (2) genetics, evolution, taxonomy, and systematics; (3) behavior, movement, and habitat; (4) reproductive ecology, developmental ontogeny, and physiology; (5) population ecology, dynamics, and life history; (6) trophic ecology, parasites, predators, toxicology, and pollution; and (7) management. Research reported in these proceedings 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.