Lake Trout Salvelinus Namaycush Research Articles

Life History Migrations of Adult Yellowstone Cutthroat Trout in the Upper Yellowstone River

AbstractKnowledge of salmonid life history types at the watershed scale is increasingly recognized as a cornerstone for effective management. In this study, we used radiotelemetry to characterize the life history movements of Yellowstone Cutthroat Trout Oncorhynchus clarkii bouvieri in the upper Yellowstone River, an extensive tributary that composes nearly half of the drainage area of Yellowstone Lake. In Yellowstone Lake, Yellowstone Cutthroat Trout have precipitously declined over the past 2 decades primarily due to predation from introduced Lake Trout Salvelinus namaycush. Radio tags were implanted in 152 Yellowstone Cutthroat Trout, and their movements monitored over 3 years. Ninety‐six percent of tagged trout exhibited a lacustrine–adfluvial life history, migrating upstream a mean distance of 42.6 km to spawn, spending an average of 24 d in the Yellowstone River before returning to Yellowstone Lake. Once in the lake, complex postspawning movements were observed. Only 4% of radio‐tagged trout exhibited a fluvial or fluvial–adfluvial life history. Low prevalence of fluvial and fluvial–adfluvial life histories was unexpected given the large size of the upper river drainage. Study results improve understanding of life history diversity in potamodromous salmonids inhabiting relatively undisturbed watersheds and provide a baseline for monitoring Yellowstone Cutthroat Trout response to management actions in Yellowstone Lake.Received July 21, 2016; accepted March 25, 2017 Published online June 12, 2017

Using Voice Recognition Software to Collect Fisheries Biodata: A Proof of Concept

Biological data collection and processing involves numerous steps, is time consuming, and is error prone. We developed the Voice Data Recording System (VDRS) that eliminates multiple steps in the traditional process by integrating voice recognition software with a normalized, relational database to capture data onto a laptop computer. We compared traditional and VDRS methods and report efficiency and error rates based on Lake Superior Lake Trout Salvelinus namaycush survey data collected during 2000–2016. Efficiency was measured by data collection rate and processing duration. Data collection rates were not different between methods. Processing duration for the traditional method averaged 200 staff hours per year, whereas the VDRS has no data processing. Error rates measured for the traditional data method averaged 5% and the VDRS had 0% error. We found that the VDRS is more efficient and less error prone than the traditional method of data collection and provides substantial cost and time savings.

Modeling Otolith Weight using Fish Age and Length: Applications to Age Determination

AbstractRelationships among otolith weight and age have been explored widely as cost‐effective means to predict age. However, otolith weight is influenced by both fish age and somatic growth, making it necessary to partition these confounding effects to best use otolith weight to predict age. We used several hatchery strains of Lake Trout Salvelinus namaycush that varied in capture season, year, location, and mean size at stocking to develop a maximum likelihood model of otolith weight as complementary but independent functions of age (mg/year) and fish length (mg/mm). Once these relationships were established, we determined age probabilities for each fish based on their measurements. Our best model included age and somatic growth components, with cumulative mean temperature as a variable to scale for seasonal differences in otolith accretion. For fish between 3 and 19 years of age, otolith weight increased by 1.16 mg/year, with an additional somatic growth component of 0.62 × exp(fish length × 0.0038) mg/mm. We correctly assigned age to 60% of the age‐3 fish, but this decreased to about 10% among the oldest age‐classes; however, predicted age residuals approximated normal distributions within each age‐class. The model was robust to seasonal and annual variation, but somatic growth differences among some strain and stocking‐size groupings resulted in modal aging biases of ±1 year. Given the stability in otolith weight components arising from fish age and somatic growth, we suggest that modeling otolith weight using reader age estimates offers a new quality assurance/quality control tool to assess whether reader ages support the widely documented properties of otolith growth. When otolith weight and fish length are more informative of age, our model may also be useful in estimating age composition data similar to an age–length key.Received December 19, 2016; accepted March 20, 2017 Published online May 26, 2017

Small-scale intraspecific patterns of adaptive immunogenetic polymorphisms and neutral variation in Lake Superior lake trout.

Many fishes express high levels of intraspecific variability, often linked to resource partitioning. Several studies show that a species' evolutionary trajectory of adaptive divergence can undergo reversals caused by changes in its environment. Such a reversal in neutral genetic and morphological variation among lake trout Salvelinus namaycush ecomorphs appears to be underway in Lake Superior. However, a water depth gradient in neutral genetic divergence was found to be associated with intraspecific diversity in the lake. To investigate patterns of adaptive immunogenetic variation among lake trout ecomorphs, we used Illumina high-throughput sequencing. The population's genetic structure of the major histocompatibility complex (MHC Class IIβ exon 2) and 18 microsatellite loci were compared to disentangle neutral and selective processes at a small geographic scale. Both MHC and microsatellite variation were partitioned more by water depth stratum than by ecomorph. Several metrics showed strong clustering by water depth in MHC alleles, but not microsatellites. We report a 75% increase in the number of MHC alleles shared between the predominant shallow and deep water ecomorphs since a previous lake trout MHC study at the same locale (c. 1990s data). This result is consistent with the reverse speciation hypothesis, although adaptive MHC polymorphisms persist along an ecological gradient. Finally, results suggested that the lake trout have multiple copies of the MHC II locus consistent with a historic genomic duplication event. Our findings indicated that conservation approaches for this species could focus on managing various ecological habitats by depth, in addition to regulating the fisheries specific to ecomorphs.

Recreational Postrelease Mortality of Lake Trout in Lakes Superior and Huron

AbstractThe effectiveness of fishing regulations that result in the release of some angler‐caught fish depends on accurate knowledge of the postrelease mortality of those individuals. In the Laurentian Great Lakes, Lake TroutSalvelinus namaycushare a major component of recreational fisheries; across large regions of the lakes, they are managed with length limit and daily quota regulations assuming a 15% postrelease mortality rate. Due to concerns regarding the accuracy of that rate, we conducted a tagging study to estimate Lake Trout postrelease mortality in Lakes Superior and Huron, and we examined environmental and fishing factors that influenced the return rates of tagged fish. The basic study design was to compare tag return rates between two groups: (1) a treatment group comprising fish that were caught and released by anglers; and (2) a control group comprising fish that were caught via trap net and released. Tag return rates for the angler‐caught group were evaluated in relation to depth of capture, surface temperature at release (ST), fishing method, anatomical hook site, play time, handling time, and barotrauma. Tag return rates for angler‐caught fish declined significantly with increasing ST; the other factors’ effects on tag return rates were generally small. For Lake Superior, model‐averaged (Akaike's information criterion) postrelease mortality estimates incorporating ST were 15.0% (SE = 5.6%) at STs less than 10°C, 42.6% (SE = 3.0%) at STs of 10–16°C, and 43.3% (SE = 3.6%) at STs greater than 16°C. Model‐averaged estimates for Lake Huron were 52.5% (SE = 26.8%) at STs less than 10°C, 45.2% (SE = 14.0%) at STs of 10–16°C, and 76.4% (SE = 5.4%) at STs greater than 16°C. Based on these findings, alternative fishery management regulations that limit recreational catch‐and‐release angling of Lake Trout in the Great Lakes may be prudent. Current management policies based on an assumed 15% postrelease mortality are likely underestimating the total numbers of Lake Trout that are removed by recreational anglers.Received January 26, 2017; accepted April 29, 2017 Published online June 26, 2017

Use of Electricity to Sedate Lake Trout for Intracoelomic Implantation of Electronic Transmitters

AbstractUse of telemetry data to inform fisheries conservation and management is becoming increasingly common; as such, fish typically must be sedated before surgical implantation of transmitters into the coelom. Given that no widely available, immediate‐release chemical sedative currently exists in North America, we investigated the feasibility of using electricity to sedate Lake Trout Salvelinus namaycush long enough for an experienced surgeon to implant an electronic transmitter (i.e., 180 s). Specifically, our study objectives were to determine (1) whether some combination of electrical waveform characteristics (i.e., duty cycle, frequency, voltage, and pulse type) could sedate Lake Trout for at least 180 s; and (2) whether Lake Trout that were sequentially exposed to continuous DC and pulsed DC had greater rates of spinal injury and short‐term mortality than control fish. A Portable Electrosedation System unit was used to sedate hatchery and wild Lake Trout. Dual‐frequency pulsed‐DC and two‐stage approaches successfully sedated Lake Trout and had similar induction and recovery times. Lake Trout sedated using the two‐stage approach did not have survival rates or spinal abnormalities that were significantly different from those of control fish. We concluded that electricity was a viable alternative to chemical sedatives for sedating Lake Trout before surgical implantation of an electronic transmitter, but we suggest that Lake Trout and other closely related species (e.g., Arctic Char Salvelinus alpinus) may require morphotype‐specific electrical waveforms due to their morphological diversity.Received January 28, 2017; accepted April 24, 2017 Published online June 20, 2017

Movement patterns and spatial segregation of two populations of lake trout Salvelinus namaycush in Lake Huron

Abstract Movement ecology is an important component of life history and population dynamics, and consequently its understanding can inform successful fishery management decision-making. While lake trout populations in Lake Huron have shown signs of recovery from near extinction in recent years, knowledge of their movement behavior remains incomplete. We used acoustic telemetry to describe and compare movement patterns of two Lake Huron lake trout populations: Drummond Island and Thunder Bay. Both populations showed high spawning site fidelity, with no evidence of co-mingling during non-spawning season. Detections between spawning periods were mainly limited to receivers within 100 km of spawning locations, and suggested that the two populations likely remained segregated throughout the year. Drummond Island fish, which spawn inside the Drummond Island Refuge, primarily dispersed east into Canadian waters of Lake Huron, with 79–92% of fish being detected annually on receivers outside the refuge. In contrast, Thunder Bay fish tended to disperse south towards Saginaw Bay. Large proportions (i.e., > 80%) of both populations were available to fisheries outside the management zone containing their spawning location. Thunder Bay fish moved relatively quickly to overwinter habitat after spawning, and tended to repeat the same post-spawning movement behavior each year. The consistent, predictable movement of both populations across management zones highlights the importance of understanding population dynamics to effective management of Lake Huron lake trout.

A Comparison of Two Mobile Electrode Arrays for Increasing Mortality of Lake Trout Embryos

AbstractConservation of sport fisheries and populations of several native fishes in the western United States is dependent on sustained success of removal programs targeting invasive Lake Trout Salvelinus namaycush. Gill-netting of spawning adults is one strategy used to decrease spawning success; however, additional complementary methods are needed to disrupt Lake Trout reproduction where bycatch in gill nets is unacceptable. We developed and tested two portable electrode arrays designed to increase Lake Trout embryo mortality in known spawning areas. Both arrays were powered by existing commercial electrofishing equipment. However, one array was moved across the substrate to simulate being towed behind a boat (i.e., towed array), while the other array was lowered from a boat and energized when sedentary (i.e., sedentary array). The arrays were tested on embryos placed within substrates of known spawning areas. Both arrays increased mortality of embryos (>90%) at the surface of substrates, but only the s...

An extremely sensitive nested PCR-RFLP mitochondrial marker for detection and identification of salmonids in eDNA from water samples.

BackgroundSalmonids are native from the North Hemisphere but have been introduced for aquaculture and sport fishing in the South Hemisphere and inhabit most rivers and lakes in temperate and cold regions worldwide. Five species are included in the Global Invasive Species Database: rainbow trout Oncorhynchus mykiss, Atlantic salmon Salmo salar, brown trout Salmo trutta, brook trout Salvelinus fontinalis, and lake trout Salvelinus namaycush. In contrast, other salmonids are endangered in their native settings.MethodsHere we have developed a method to identify salmonid species directly from water samples, focusing on the Iberian Peninsula as a case study. We have designed nested Salmonidae-specific primers within the 16S rDNA region. From these primers and a PCR-RFLP procedure the target species can be unequivocally identified from DNA extracted from water samples.ResultsThe method was validated in aquarium experiments and in the field with water from watersheds with known salmonid populations. Finally, the method was applied to obtain a global view of the Salmonidae community in Nalón River (north coast of Spain).DiscussionThis new powerful, very sensitive (identifying the species down to 10 pg DNA/ml water) and economical tool can be applied for monitoring the presence of salmonids in a variety of situations, from checking upstream colonization after removal of river barriers to monitoring potential escapes from fish farms.

Walleye Introduction Eliminates Predation Refuge for Adfluvial Cutthroat Trout and Rainbow Trout

AbstractA predation refuge allowed Yellowstone Cutthroat Trout Oncorhynchus clarkii bouvieri and Rainbow Trout O. mykiss to coexist with Lake Trout Salvelinus namaycush in Buffalo Bill Reservoir, Wyoming. Recently, Walleye Sander vitreus appeared prompting concern about the persistence of Oncorhynchus populations. Study objectives were to (1) understand how environmental conditions have contributed to the coexistence of Lake Trout and Oncorhynchus populations, and (2) determine habitat use, diet, and per capita consumptive demand of Walleyes. While Walleye population expansion is difficult to predict, we compared consumptive demand of Lake Trout to biomass and production of Oncorhynchus populations to evaluate if additional predation demand by Walleyes could be sustained by prey populations. We measured limnological variables and sampled fishes monthly during April–October in 2012 and 2013 to determine habitat use and collect growth and diet samples. Low Secchi depth (1.6 m) and warm (19°C) epilimnetic temperatures in summer appear to have inhibited Lake Trout predation and provided Oncorhynchus spp. with a thermal refuge from predation, but this refuge was eliminated with the introduction of Walleyes. Prior to thermal stratification, Lake Trout and Oncorhynchus spp. co‐occurred at depths less than 18 m, but during summer, Oncorhynchus spp. remained in shallow water and Lake Trout retreated to the hypolimnion. Unlike Lake Trout, Walleyes always co‐occurred with Oncorhynchus spp. Only large (≥600 mm TL) Lake Trout consumed Oncorhynchus spp. and only during the unstratified period; in contrast, 64% of Walleye diet was Oncorhynchus spp., regardless of season. Per capita consumption of Oncorhynchus spp. by Walleyes (13.43 kg) was similar to that by Lake Trout (17.46 kg), despite the longer Lake Trout life span. Thus, although Lake Trout consumed only about 18% (462 kg) of the August biomass (2,624 kg) and 9% of annual production (5,049 kg) of young Oncorhynchus spp., expansion of the Walleye population could threaten Oncorhynchus populations in the future.Received July 13, 2016; accepted October 19, 2016 Published online January 31, 2017

Reproduction of Lake Trout Morphotypes at Isle Royale in Northern Lake Superior

AbstractThe reproductive biology of male and female lean, siscowet, redfin, and humper Lake Trout Salvelinus namaycush was assessed by measuring the gonadosomatic index (GSI) and by histological analysis of the gonads over 2 years from spring to fall at sites surrounding Isle Royale National Park, Lake Superior. Whole livers, otoliths, and ovaries were also collected to measure hepatosomatic index (HSI), age, and fecundity. In general, GSIs increased from spring to summer in male and female siscowets, leans, redfins, and humpers, and this was accompanied by a large increase in the number of vitellogenic (>2.0 mm) follicles in females. Postspawned ovaries and regressing testes were predominant in all morphotypes by October but were also observed in September, indicating that some spawning occurred at least by September. Further, preovulatory siscowet females (GSIs > 10) and siscowet males with fully mature and even regressing testes were observed in the spring and summer, indicating that spawning occurs at least in the spring and fall in siscowets at Isle Royale. For all morphotypes, average male HSIs were significantly lower than female HSIs. Absolute fecundity was positively related to size and age for all morphotypes. Humpers had the lowest absolute fecundity. Leans, redfins, and siscowets had similar fecundity relationships. The results conclusively demonstrate that deepwater siscowet Lake Trout reproduce at least during the spring and fall at Isle Royale and that leans, humpers, and redfins reproduce during the fall. In lakes that exhibit sympatric populations of deepwater and shallow‐water ecotypes of Arctic Char Salvelinus alpinus, the deepwater form also spawns in both spring and fall. Our data and those on Arctic Char suggest that there is an adaptive strategy for maintaining alternative spawning times in the deepwater ecotypes in these populations.Received June 20, 2016; accepted November 2, 2016 Published online January 31, 2017

Progress Toward Lake Trout Rehabilitation at a Stocked and Unstocked Reef in Southern Lake Michigan

AbstractSustained efforts to restore naturally reproducing populations through the stocking of marked juvenile Lake Trout Salvelinus namaycush have occurred since the 1960s but have been largely unsuccessful in achieving that goal. Julian's Reef in southwestern Lake Michigan has been a targeted Lake Trout rehabilitation site since 1985 and was designated a first‐priority site in a revised management strategy for Lake Trout rehabilitation in 2011. We evaluated progress toward rehabilitation objectives for spawning Lake Trout at Julian's Reef and compared that progress to unstocked Waukegan Reef nearby in light of a recent increase to approximately 50% unmarked adults at both sites, as detected by annual fall spawning assessments. We observed significant progress toward meeting the rehabilitation objectives for adult Lake Trout in southern Lake Michigan. The spawning populations at Julian's Reef and Waukegan Reef did not differ in relative abundance, sex ratio, age structure, or spawner sources (stocking location; clipped or unclipped) except that the sex ratio at Waukegan Reef during the assessment period favored female spawners. These results suggest that the adult Lake Trout population is strong enough to permit natural recruitment in southern Lake Michigan, warranting the need to identify the source of natural reproduction so as to fully evaluate factors related to rehabilitation success. We propose that management agencies should consider investigating other unstocked sites where Lake Trout may be spawning and should revise their rehabilitation plans to include actions that could limit detrimental impacts on naturally reproducing populations where they exist.Received October 15, 2015; accepted July 22, 2016 Published online November 10, 2016

Genetic and phenotypic variation along an ecological gradient in lake trout Salvelinus namaycush.

BackgroundAdaptive radiation involving a colonizing phenotype that rapidly evolves into at least one other ecological variant, or ecotype, has been observed in a variety of freshwater fishes in post-glacial environments. However, few studies consider how phenotypic traits vary with regard to neutral genetic partitioning along ecological gradients. Here, we present the first detailed investigation of lake trout Salvelinus namaycush that considers variation as a cline rather than discriminatory among ecotypes. Genetic and phenotypic traits organized along common ecological gradients of water depth and geographic distance provide important insights into diversification processes in a lake with high levels of human disturbance from over-fishing.ResultsFour putative lake trout ecotypes could not be distinguished using population genetic methods, despite morphological differences. Neutral genetic partitioning in lake trout was stronger along a gradient of water depth, than by locality or ecotype. Contemporary genetic migration patterns were consistent with isolation-by-depth. Historical gene flow patterns indicated colonization from shallow to deep water. Comparison of phenotypic (Pst) and neutral genetic variation (Fst) revealed that morphological traits related to swimming performance (e.g., buoyancy, pelvic fin length) departed more strongly from neutral expectations along a depth gradient than craniofacial feeding traits. Elevated phenotypic variance with increasing water depth in pelvic fin length indicated possible ongoing character release and diversification. Finally, differences in early growth rate and asymptotic fish length across depth strata may be associated with limiting factors attributable to cold deep-water environments.ConclusionWe provide evidence of reductions in gene flow and divergent natural selection associated with water depth in Lake Superior. Such information is relevant for documenting intraspecific biodiversity in the largest freshwater lake in the world for a species that recently lost considerable genetic diversity and is now in recovery. Unknown is whether observed patterns are a result of an early stage of incipient speciation, gene flow-selection equilibrium, or reverse speciation causing formerly divergent ecotypes to collapse into a single gene pool.Electronic supplementary materialThe online version of this article (doi:10.1186/s12862-016-0788-8) contains supplementary material, which is available to authorized users.

A Lota lota Consumption: Trophic Dynamics of Nonnative Burbot in a Valuable Sport Fishery

AbstractUnintentional and illegal introductions of species disrupt food webs and threaten the success of managed sport fisheries. Although many populations of Burbot Lota lota are declining in the species’ native range, a nonnative population recently expanded into Flaming Gorge Reservoir (FGR), Wyoming–Utah, and threatens to disrupt predator–prey interactions within this popular sport fishery. To determine potential impacts on sport fishes, especially trophy Lake Trout Salvelinus namaycush, we assessed the relative abundance of Burbot and quantified the potential trophic or food web impacts of this population by using diet, stable isotope, and bioenergetic analyses. We did not detect a significant potential for food resource competition between Burbot and Lake Trout (Schoener's overlap index = 0.13), but overall consumption by Burbot likely affects other sport fishes, as indicated by our analyses of trophic niche space. Diet analyses suggested that crayfish were important diet items across time (89.3% of prey by weight in autumn; 49.4% in winter) and across Burbot size‐classes (small: 77.5% of prey by weight; medium: 76.6%; large: 39.7%). However, overall consumption by Burbot increases as water temperatures cool, and fish consumption by Burbot in FGR was observed to increase during winter. Specifically, large Burbot consumed more salmonids, and we estimated (bioenergetically) that up to 70% of growth occurred in late autumn and winter. Further, our population‐wide consumption estimates indicated that Burbot could consume up to double the biomass of Rainbow Trout Oncorhynchus mykiss stocked annually (>1.3 × 105 kg; >1 million individuals) into FGR. Overall, we provide some of the first information regarding Burbot trophic interactions outside of the species’ native range; these findings can help to inform the management of sport fisheries if Burbot range expansion occurs elsewhere.Received March 18, 2016; accepted August 15, 2016 Published online October 14, 2016

Managing lake trout lakes in a warming world: a paleolimnological assessment of nutrients and lake production at three Ontario sites

ABSTRACTNelligan C, Jeziorski A, Rühland KM, Paterson AM, Smol JP. 2016. Managing lake trout lakes in a warming world: A paleolimnological assessment of nutrients and lake production at three Ontario sites. Lake Reserve Manage. 32:315−328.Recent declines in hypolimnetic dissolved oxygen (DO) concentrations in many lakes throughout Ontario have prompted concern regarding the health of lake trout (Salvelinus namaycush) populations. We used diatom assemblages and spectrally inferred chlorophyll a from the dated sediments of 3 lake trout lakes to track trends in total phosphorus (TP) and whole-lake production over the past ∼150 years. Eagle Lake is considered a sensitive lake trout lake because DO concentrations have been lower than the provincial standard (>7 mg/L) since the early-2000s; however, a lack of monitoring data impedes the evaluation of current conditions within a historical context. For a regional perspective, we compared trends in Eagle Lake with those of 2 additional lake trout lakes ∼200 km away. The diatom assemblages of Eagle Lake underwent a shift in the early-1980s, marked by a decline in the relative abundance of Stephanodiscus minutulus and an increase in small Cyclotella (sensu lato) taxa. This change is indicative of nutrient reductions and is consistent with a decline in both monitored and diatom-inferred TP. Sedimentary chlorophyll a peaked after the 1980s, suggesting whole-lake production has increased independent of nutrient loading, likely due to regional warming and a longer growing season. Diatom compositional changes at the 2 other sites are indicative of climate warming, with increases in the relative abundance of Discostella stelligera and spectrally inferred chlorophyll a. Our data suggest climate is likely the dominant driver of the observed algal changes, highlighting that the effects of warming on habitat quality should be considered in lake management strategies.

Islands in the ice stream: were spawning habitats for native salmonids in the Great Lakes created by paleo‐ice streams?

AbstractLake trout Salvelinus namaycush, lake whitefish Coregonus clupeaformis and cisco Coregonus artedi are salmonid fishes native to the Laurentian Great Lakes that spawn on rocky substrates in the fall and early winter. After comparing the locations of spawning habitat for these species in the main basin of Lake Huron with surficial substrates and the hypothesized locations of fast‐flowing Late Wisconsinan paleo‐ice streams, we hypothesize that much of the spawning habitat for these species in Lake Huron is the result of deposition and erosion by paleo‐ice streams. This hypothesis may represent a new framework for the identification and protection of spawning habitat for these native species, some of which are currently rare or extirpated in some of the Great Lakes. We further suggest that paleo‐ice streams may have been responsible for the creation of native salmonid spawning habitat elsewhere in the Great Lakes and in other glaciated landscapes.

Seasonal and Diel Bathythermal Distributions of Lake Whitefish in Lake Huron: Potential Implications for Lake Trout Bycatch in Commercial Fisheries

AbstractDepths and temperatures recorded during 2003–2005 by archival tags implanted in Lake Whitefish Coregonus clupeaformis and in Great Lakes origin (GLO) and New York Finger Lakes origin (FLO) strains of Lake Trout Salvelinus namaycush were used to compare seasonal diel depth and temperature distributions in Lake Huron. Seasonal depth distributions were examined to determine if species differences could be exploited to reduce bycatch of Lake Trout in commercial Lake Whitefish gill‐net fisheries. Both GLO and FLO Lake Trout used deeper and colder waters than Lake Whitefish during daylight and dark. Temperature differences between species were greatest during periods of stratification when behavioral thermoregulation was possible. Other than during spawning periods, the greatest depth and temperature separation occurred in late July between FLO Lake Trout (37.0 m; 6.5 ºC) and Lake Whitefish (25.1 m; 10.2 ºC). If maximum depths of gill nets were regulated seasonally to between 25 and 35 m, ≥ 50% of the Lake Whitefish population would be vulnerable while avoiding 89% or more of Lake Trout. Lake Trout percentages targeted under such regulations would be lowest in late July (GLO = 8% and FLO = 11%) and early August (GLO = 4% and FLO = 7%). However, archival tags measure fish depth and not location or bottom depth, and modal depths of gill‐net effort for Lake Whitefish (38 to 57 m) exceeded those from the Lake Whitefish archival tag data. This discrepancy suggests that many Lake Whitefish might be pelagic above the reach of bottom‐set gill nets, so depth restrictions could be less effective in reducing Lake Trout bycatch than suggested by our data. A further implication is that use of gill nets suspended above the bottom to target pelagic Lake Whitefish could also reduce bycatch while potentially increasing Lake Whitefish harvest.Received August 19, 2015; accepted February 21, 2016 Published online June 28, 2016

Factors Affecting Prestocking Coded Wire Tag Loss in Lake Trout Tagged by an Automated System

AbstractTag loss is an important consideration in tagging studies. We used two approaches to describe prestocking coded wire tag (CWT) loss in Lake Trout Salvelinus namaycush tagged by an automated tagging system and released in the Laurentian Great Lakes. First, four strains of Lake Trout tagged by an automated system through the Great Lakes Mass Marking Program were observed for up to 254 d to describe how tag loss and adipose fin clip success relates to time posttagging. Second, we also evaluated final tag loss and fin clip success from 197 tag lots from 2011 to 2013 that were tagged with an automated system to explore the factors affecting tag loss and fin clip success, and to compare tag loss and fin clip success rates with those from 1,080 tag lots from 1985 to 2003 that were manually tagged. Coded wire tag losses from experimental lots of four strains of Lake Trout were low and ranged from 2.8% to 5.7%. Coded wire tag losses stabilized 150 d posttagging, when fish had a mean length of 131 mm, which is far longer than that reported for other salmonines (30 d). We developed a descriptive model to correct for time effects on tag loss; tag loss could be estimated with high confidence after 100 d posttagging. Coded wire tag losses varied by genetic strain, possibly due to differences in body size and shape. Fish that lost a CWT were significantly smaller than fish retaining a CWT. In our comparison of the automated system with manual tagging, CWT loss across 197 automated tag lots was highly variable (0.0–14.0%), but it was <5.5% in 86% of samples and less than the loss rates from manually tagged fish (<5.5% in 72% of samples). Our results provide important details for CWT studies on Lake Trout and other species.Received July 28, 2015; accepted February 7, 2016 Published online June 1, 2016

Genetic architecture and maternal contributions of early-life survival in lake trout Salvelinus namaycush.

The influences of additive, non-additive and maternal effects on early survival (uneyed embryo survival, eyed embryo survival, alevin survival and overall survival to first feeding) were quantified in lake trout Salvelinus namaycush using a 7 × 7 full-factorial breeding design. Maternal effects followed by non-additive genetic effects explained around one third of the phenotypic variance of the survival traits. Although the amount of additive genetic effects were low (<1%), suggesting a limited potential of the traits to respond to new selection pressures, how maternal and non-additive genetic effects may respond to selection under certain circumstances are discussed.

Feeding Ecology of Native and Nonnative Salmonids during the Expansion of a Nonnative Apex Predator in Yellowstone Lake, Yellowstone National Park

AbstractThe illegal introduction of Lake Trout Salvelinus namaycush into Yellowstone Lake, Yellowstone National Park, preceded the collapse of the native population of Yellowstone Cutthroat Trout Oncorhynchus clarkii bouvieri, producing a four‐level trophic cascade. The Yellowstone Cutthroat Trout population's collapse and the coinciding increase in Lake Trout abundance provided a rare opportunity to evaluate the feeding ecology of a native prey species and a nonnative piscivore species after the restructuring of a large lentic ecosystem. We assessed diets, stable isotope signatures, and depth‐related CPUE patterns for Yellowstone Cutthroat Trout and Lake Trout during 2011–2013 to evaluate trophic overlap. To evaluate diet shifts related to density, we also compared 2011–2013 diets to those from studies conducted during previous periods with contrasting Yellowstone Cutthroat Trout and Lake Trout CPUEs. We illustrate the complex interactions between predator and prey in a simple assemblage and demonstrate how a nonnative apex predator can alter competitive interactions. The diets of Yellowstone Cutthroat Trout were dominated by zooplankton during a period when the Yellowstone Cutthroat Trout CPUE was high and were dominated by amphipods when the CPUE was reduced. Lake Trout shifted from a diet that was dominated by Yellowstone Cutthroat Trout during the early stages of the invasion to a diet that was dominated by amphipods after Lake Trout abundance had increased and after Yellowstone Cutthroat Trout prey had declined. The shifts in Yellowstone Cutthroat Trout and Lake Trout diets resulted in increased trophic similarity of these species through time due to their shared reliance on benthic amphipods. Yellowstone Cutthroat Trout not only face the threat posed by Lake Trout predation but also face the potential threat of competition with Lake Trout if amphipods are limiting. Our results demonstrate the importance of studying the long‐term feeding ecology of fishes in invaded ecosystems.Received August 24, 2015; accepted December 23, 2015 Published online April 14, 2016