Diel timing of lake sturgeon (Acipenser fulvescens) activity revealed by satellite tags in the Laurentian Great Lake Basin

Lake sturgeon (Acipenser fulvescens) are slow-growing and long-lived residents of the Laurentian Great Lakes Basin, making them vulnerable to anthropogenic stressors. The behavior of lake sturgeon, and how it may result in conflict with human activities, remain incompletely understood. Our goal was to describe how environmental conditions, such as depth, temperature, and time of day, influence lake sturgeon activity. Data were recorded using pop-up archival satellite tags, which have provided insight into the ecology, physiology, and behavior of challenging to study marine megafauna, yet are rarely deployed within freshwater ecosystems despite parallel research needs. Adult lake sturgeon were fitted with satellite tags containing integrated accelerometers near spawning grounds in the Niagara River between Canada and the United States. We used the time-series of acceleration in recovered tags to test the hypothesis that lake sturgeon exhibit diel activity cycles in situ. We defined activity as acceleration greater than two standard deviations from a running average. We applied this classification to partial records obtained from satellite transmission, an approach that can render incomplete datasets a useable product for analysis. Lake sturgeon were most active nocturnally with a significant mean at 22:32 h. Generalized linear mixed-effect models showed temperature and time of day to be significant predictors of activity. Temporal cycles of fish activity should be understood and could be exploited to reduce human-fish conflict in the increasingly altered waterways of the globe.     

Seasonal use of two unregulated Lake Superior tributaries by lake sturgeon

Lake sturgeon movement in two adjacent unregulated Lake Superior tributaries, the Pic and White rivers, was assessed over several years to determine seasonal use, identify potential contributing factors for entry or exit migrations, and evaluate whether sturgeon using these tributaries constituted one or two populations. A total of 95 lake sturgeon implanted with radio transmitters were tracked using multiple stationary receivers augmented with boat-based manual surveillance during peak movement times. Both rivers were used by lake sturgeon during the open water (“ice off”) season. In general, spawning sturgeon moved to the first insuperable barrier (i.e., natural rapids) during the spawning season, and then moved downstream to deeper pools in mid- to late summer. Non-spawning sturgeon moved into the river concurrently but remained in lower portions of the river. Lake sturgeon emigrated from the Pic River and resided in Lake Superior during the winter season whereas a small portion of radio transmittered sturgeon, originally sampled in the Pic River, overwintered in the White River. River discharge and the interaction between discharge and water temperature were correlated with upstream movement, and river discharge was also correlated with outmigration. No genetic structuring was apparent between Sturgeon within the two rivers, consistent with telemetry data showing radio-tagged fish moving readily between the rivers. This study provided pertinent seasonal use information of unregulated Great Lake tributaries and may contribute to planning processes for future hydroelectric developments to minimize disruptions to lake Sturgeon populations.     

Multi-year evidence of unbiased sex ratios in hatchery and wild-reared age-0 lake sturgeon (Acipenser fulvescens)

Assessment of population sex ratios allows managers to forecast recruitment dynamics and loss of genetic diversity in natural populations and is important when the focal population is in low abundance and subject to demographic stochasticity. If levels of natural or artificial selection differ for males and females and levels of mortality likewise vary, lower levels of population recruitment, loss of genetic diversity, and genetic drift can occur. Lake sturgeon (Acipenser fulvescens) are a species of conservation concern, where restoration efforts increasingly rely on hatchery supplementation. Raising larvae to the juvenile stage can increase survival during important early life stages; however, knowledge is lacking concerning effects of artificial rearing environments on differential sex-specific survival before release. We genetically determined the sex of 1459 age-0 lake sturgeon from three cohorts (2016 through 2018) using PCR assays of the ALLWSex2 acipenserid sexing marker. Sexed individuals represented three groups: (1) wild-captured dispersing larvae that died during hatchery rearing, (2) wild-captured dispersing larvae that survived hatchery rearing to release, and (3) wild-captured, wild-raised age-0 individuals. Sex ratios of wild-captured larvae (dead + live) were nearly 50:50 in all years surveyed. We observed slight, but non-significant, directionality in sex ratios in the live and dead hatchery-reared larvae and in wild-captured age-0 individuals. Genetic sexing methods allow for analyses during prolonged pre-reproductive periods and associated variable environmental and demographic circumstances, in situations where physical determination of sex is not possible.     

Lipid levels and diet compositions in lake charr ecotypes at Isle Royale in northern Lake Superior

Lipid levels have been reported for lake charr Salvelinus namaycush ecotypes from Lake Superior, but have not been investigated for all four extant ecotypes concurrently in the same geographic area. In this study, a Distell fatmeter was used to measure lipid levels in the muscle in lean, humper, redfin, and siscowet lake charr collected seasonally during 2013 and 2014 on the north, east, and west sides of Isle Royale, Lake Superior from two depth strata: <80 and ≥80 m. Diet compositions and dietary similarity of lake charr were also measured. Differences in lipid level were analyzed by ANCOVA with length and age as covariates and ecotype, depth, sex, and season as factors. Lipid levels were found to be higher in lake charr inhabiting waters ≥80 m. In waters <80 m, mean lipid level was highest in large siscowets (≥703 mm; 41.2%) and lowest in old redfins (≥age 32; 11.8%). At depths ≥80 m, mean lipid levels were highest in large siscowets (64.4%) and lowest in young redfins (14.7%). In both depth strata, leans and humpers had intermediate lipid levels. Diet compositions were similar among ecotypes and did not explain the differences in lipid levels. Siscowets have the greatest depth range and vertical movement patterns among lake charr ecotypes in Lake Superior which is likely facilitated by high levels of intramuscular lipid. Lipid levels in leans and siscowets are heritable traits, but heritability of lipid levels in other ecotypes remains unknown.     

Winter Diet of Lake Herring (Coregonus artedi) in Western Lake Superior

Lake herring (Coregonus artedi) and zooplankton samples were simultaneously collected through the ice in the Apostle Islands region of western Lake Superior to provide information on the winter feeding ecology of lake herring. Zooplankton constituted the entire diet of the 38 lake herring collected for this study. We found no evidence of piscivory, although it has been reported by anglers. Diet selectivities were calculated using a Wilcoxon signed-ranks test and showed a preference of lake herring for larger zooplankton, especially Diaptomus sicilis, whereas the smaller copepod, Cyclops bicuspidatus thomasi, and immature copepod stages were selected against. These data document that overwintering copepods are food for a broad size range of lake herring in winter.     

Lake sturgeon (Acipenser fulvescens) spawn in the St. Marys River Rapids,Michigan

The St. Marys River connects Lake Superior to Lake Huron, comprising the international border between Michigan, United States, and Ontario, Canada. This Great Lakes connecting channel naturally encompasses various habitats including lakes, wetlands, islands, tributaries, side channels, and main channels. The St. Marys River Rapids are shallow rock areas with high flow velocities (>1 m/s) in the upper river adjacent to the navigation locks and electric power generating stations, while the Little Rapids are shallow, recently restored rocky areas with lower velocities located about 7 km downstream. The St. Marys River Rapids provide important spawning habitat for several native and introduced fishes, but spawning by lake sturgeon (Acipenser fulvescens) was not previously documented. We sampled for lake sturgeon eggs and larvae in both locations during June and July 2018–2019 using weekly benthic egg mat lifts and overnight D-frame larval fish drift nets. Viable lake sturgeon eggs (11 in 2018, 45 in 2019) were collected in the tailrace of a hydroelectric power facility adjacent to the St. Marys River Rapids. Larval lake sturgeon (21 in 2018, 1 in 2019) were collected in the same area as the eggs. Neither lake sturgeon eggs nor larvae were collected at Little Rapids in either year. Our results are the first documentation of successful lake sturgeon spawning and larval drift in the upper St. Marys River. While our observations showed spawning in a human-made tailrace area, the fate of larvae produced here is unknown and warrants further research.     

Genetic origins and diversity of lake sturgeon in the St. Louis River Estuary

Lake sturgeon Acipenser fulvescens were extirpated from the St. Louis River Estuary (SLRE) by the early 1900’s due to overfishing and habitat degradation. A restoration stocking program began in 1983, and continued almost annually until 2000. Lake sturgeon stocked into the SLRE were primarily obtained from the Wolf River (Lake Winnebago) genetic stock (n = 861,000) but some sturgeon were obtained from the Sturgeon River (Lake Superior) genetic stock (n = 61,380). Recently, spawning and natural recruitment has been documented near the Fond du Lac Dam, the upstream limit for lake sturgeon migrating from Lake Superior. However, the genetic origin of lake sturgeon spawning in the SLRE was unknown. Our objectives were to determine (1) the genetic origins and (2) genetic diversity of lake sturgeon spawning in the SLRE. Using both GENECLASS2 and ONCOR, a majority (79–81%) of lake sturgeon captured in the SLRE during spawning (2016–2018) assigned to the Wolf River genetic stock (Lake Winnebago) with greater than 80% probability using established microsatellites and a standardized genetic baseline. Other genetic stocks present (≥1%) included the Pic and Goulais rivers and possibly the Black Sturgeon River (identified using GENECLASS2, but not ONCOR); no fish assigned to the Sturgeon River using either method. Genetic diversity metrics showed that the SLRE lake sturgeon population was similar to other Lake Superior lake sturgeon populations. Overall, the SLRE Sturgeon population appears headed towards recovery. Adaptive management practices currently being employed should be continued to help guide further recovery of this population.     

Differences in pyloric caeca counts between lean and siscowet lake trout (Salvelinus namaycush) in southern Lake Superior

There are four documented morphotypes of lake trout (Salvelinus namaycush) in Lake Superior, with the two dominant forms being the shallow water lean and the deep water siscowet. These morphotypes are differentiated externally by morphometrics and meristics. Pyloric caeca counts have been used to distinguish closely related fish species including the Salvelinus genus. From samples collected in 2009, 2011–2013, and 2017, we counted and measured pyloric caeca from 116 lean and 119 siscowet lake trout from southern Lake Superior. For a subset of 22 leans and 19 siscowets between 575 and 625 mm, we measured individual pyloric caecum basal diameter, length, and estimated individual caecum and total caeca surface areas. Siscowets had significantly fewer and thicker pyloric caeca than leans, but caecum length did not differ between the morphotypes. Mean pyloric caeca count for siscowets and leans was 131 and 153, respectively. Mean individual caecum surface area was 31% higher in siscowets than in leans. When adjusted for mean total number of pyloric caeca, total caecum surface area for siscowets was 12% greater than leans. We postulate that greater pyloric caecum surface area in siscowets may be an adaption for greater lipid uptake because they have substantially higher lipid content than leans. Based on our findings, pyloric caeca counts can be used in addition to other meristic and morphometric characteristics to help distinguish lean and siscowet lake trout.     

Genetic variation at the mtDNA ND-1 locus among North American wild and hatchery brown trout (Salmo trutta)

Despite extensive knowledge of mitochondrial DNA (mtDNA) variation in European brown trout (Salmo trutta) populations, little is known about their nucleotide sequence variation in North America. The objective of this study was to quantify single nucleotide polymorphisms (SNPs) at the ND-1 mtDNA locus of 62 brown trout from hatcheries in Michigan and Wisconsin as well as Michigan streams and Lake Michigan. We identified 25 SNPs that characterized nine distinct mtDNA haplotypes in the Wild Rose, Gilchrist and Seeforellen brown trout strains. Although most SNPs were represented by synonymous nucleotide substitutions, three individuals of the Seeforellen strain had non-synonymous nucleotide changes. MtDNA haplotypes identified in North American brown trout in this study showed nucleotide similarity at the ND-1 locus to brown trout from northern Europe.     

Application of a Depletion-Based Stock Reduction Analysis (DB-SRA) to lake sturgeon in Lake Erie

Lake Erie supported the greatest yield of lake sturgeon within the Laurentian Great Lakes near the end of the 19th century with >2000 metric tons caught at the peak of the fishery. The fishery collapsed by the 1920s when <1% of the previous peak catch was removed. Despite closures of the fishery, lake sturgeon remain rare in Lake Erie. We applied a depletion-based stock reduction analysis (DB-SRA) to the catch of lake sturgeon from 1879 to 1929 to gain estimates of sustainable fishery reference points and the historic carrying capacity of Lake Erie for lake sturgeon. We also simulated population growth of lake sturgeon from 1929 to the present with varying assumptions of the current carrying capacity of the lake. The estimated historic carrying capacity of lake sturgeon was 22,652 metric tons. During the height of the fishery, exploitation was as high as 37% which was more than an order of magnitude greater than that required for maximum sustainable yield. Projections of the population from 1929 to 2016 suggest sufficient time has passed since the collapse of the fishery that the population should have recovered to levels that would support a fishery at maximum sustainable yield. However, lake sturgeon remain rare in Lake Erie indicating that other factors such as habitat availability may be limiting their recovery. Our estimates of carrying capacity will be informative when setting recovery targets which consider the amount of habitat loss.     

Evidence of lake trout (Salvelinus namaycush) spawning and spawning habitat use in the Dog River,Lake Superior

Lake trout spawn primarily in lakes, and the few river-spawning populations that were known in Lake Superior were believed to be extirpated. We confirmed spawning by lake trout in the Dog River, Ontario, during 2013–2016 by the collection of and genetic identification of eggs, and we describe spawning meso- and microhabitat use by spawning fish. Between 2013 and 2016, a total of 277 lake trout eggs were collected from 39 of 137 sampling locations in the river. The majority of eggs (220) were collected at the transition between the estuary and the river channel crossing the beach. Lake trout eggs were most often located near the downstream end of pools in areas characterized by rapid changes in depth or slope, coarse substrates, and increased water velocities, where interstitial flows may occur. Depths in wadeable areas where eggs were found averaged 0.9?m (range: 0.4 to 1.3?m) and substrate sizes consisted of large gravel, cobble, and boulder; comparable to spawning characteristics noted in lakes. Water velocities averaged 0.66?m·s^?1 (range: 0.33 to 1.7?m³·s^?1) at mid-depth. This information on spawning habitat could be used to help locate other remnant river-spawning populations and to restore river-spawning lake trout and their habitat in rivers that previously supported lake trout in Lake Superior. The Dog River population offers a unique opportunity to understand the ecology of a river spawning lake trout population.     

Lake Sturgeon (Acipenser fulvescens) in Goulais Bay,Lake Superior: Cohort strength determinants and population viability

Lake Sturgeon (Acipenser fulvescens) is a species of conservation concern throughout North America, and healthy populations are rare. Earlier sampling efforts identified the Goulais Bay population in Lake Superior as a potentially healthy population after three years of sampling. With seven additional years of sampling, we updated the earlier analysis and developed a matrix population model to conduct a population viability analysis (PVA). We identified a non-linear relationship between cohort strength and May river discharge rate which was incorporated into the population model to evaluate the influence of future discharge scenarios on population persistence. Population size was estimated, with an open-population mark-recapture model, at approximately 5,200 juvenile Lake Sturgeon. This estimate equates to approximately 440 mature females and 625 mature males in the population. A population of this size has a probability of extinction of 4 % and 18 % over 250 and 1000 years under status quo conditions. If the May river discharge were to decrease in the future, which may represent the most likely scenario under future climate conditions, our model predicts an increased risk of population extirpation. This indicates that increased management actions may be required to ensure this population remains resilient.     

Contemporary diets of Lake Superior lake whitefish off the Keweenaw Peninsula and changes in condition from the 1980s to 2010s

Over the last two decades, declines in lake whitefish (Coregonus clupeaformis) recruitment and growth in many areas of the Laurentian Great Lakes have raised concerns about the status of this important species. Although Lake Superior populations have been less affected than those in other Great Lakes, these populations still face multiple threats. We characterized lake whitefish diets collected off the Keweenaw Peninsula between 2015 and 2017 and compared results to previous Lake Superior studies. We additionally estimated length-weight relationships to determine whether lake whitefish body condition (i.e., expected weight-at-length) had changed since the 1980s. Diet diversity was low, although individual specialization was moderate to high. Fish transitioned from consuming Diporeia in the spring to Mysis and fish eggs during fall and winter; sphaeriids composed 20–30% of diets across all seasons. Compared to findings for other Lake Superior regions, lake whitefish diets comprised lower percentages of high energy items (e.g., Diporeia, Mysis) and higher percentages of low energy items (e.g., sphaeriids). Expected weights in the 2000s and 2010s were lower in the 400- and 500-mm length groups but similar in larger lengths groups compared to the 1980s; condition was highest across all lengths in the 1990s. The observed decline in condition since the 1990s in the 400- and 500-mm length groups, in combination with possibly greater consumption of less energetically profitable items, suggests that lake whitefish <600 mm or preferred prey resources in this lake region may be experiencing stressors leading to condition declines, although what these stressors are remain unknown.     

Spatial and Temporal Variation of Ammonium in Lake Superior

Many aspects of Lake Superior's nitrogen cycle are poorly described in spite of the fact that the lake's nitrate concentration has risen dramatically this past century. One important, yet underdescribed parameter is the concentration of ammonium. Here, we present data to resolve spatial and temporal variation along with vertical profiles of ammonium concentration in Lake Superior. Lake-wide average concentrations were low (0.21 μM, n =166) with considerable spatial and temporal variation. During the onset of summer, the western margin of the lake had higher average concentrations than open and eastern parts. Surface layer (<10 m) ammonium concentrations showed an increase from January to October. Relatively higher ammonium concentration in the near bottom waters at a number of sites during August indicated efflux from sediment to be an important process. Subsurface maxima near the thermocline were observed in late August and persisted until September-October suggesting that ammonium might be controlled by food web processes during warm, stratified conditions. The higher potential for ammonium uptake compared to external inputs suggested rapid turnover of ammonium in the lake.     

Distribution of the Amphipod Diporeia in Lake Superior: The Ring of Fire

Diporeia, formerly the dominant benthic macroinvertebrate in the Great Lakes, remains a keystone species in Lake Superior. Little is known, however, about fine scale amphipod distributions, especially as influenced by the production, transport and transformation of energy resources. Here, we document the distribution and abundance of Diporeia along 19 transects around the lake's perimeter. Regions of elevated density, averaging 958 ± 408 Diporeia/m² (mean ± S.D.) were observed along all transects, typically within slope habitat (depth of 30–125 m). Waters shoreward (shelf habitat, < 30 m) and lakeward (profundal habitat, > 125 m) of these regions supported significantly lower densities, averaging 239 ± 178/m² and 106 ± 59/m², respectively. Amphipods within regions of elevated density, termed here the Ring of Fire, account for two-thirds of the lakewide population while occupying only one-quarter of the benthic habitat. The Ring of Fire, observed lakewide as a band averaging 14.2 ± 9.4 km in width, is characterized as a region of transitional sediment deposition with gentle slope, proximate to nearshore locations of elevated primary production. Within the Ring of Fire exceptionally high densities are found in the south central region, where the Keweenaw Current and slope bathymetries serve to funnel production from adjoining regions of high production. Density measurements for the 173 stations sampled here are used to estimate lakewide Diporeia standing stock (22.5–37.7 trillion individuals, 4.4–7.4 Gg dry weight, 2.1–3.5 Gg C), individual and biomass density (274–460/m², 0.05–0.09 g DW/m², 0.03–0.04 gC/m²) and areal (0.02–0.03 g C/m²/yr) and total (1.6–2.6 Gg C/yr) production.     

Movement and growth indicators in resident and adfluvial coaster brook trout (Salvelinus fontinalis) in the Hurricane River,Lake Superior,Michigan, USA

Brook trout (Salvelinus fontinalis) are found throughout Lake Superior, Lake Nipigon, and their tributaries. Lacustrine and adfluvial life history variants were historically popular with anglers and were called coasters; coaster brook trout populations are now severely reduced and are of conservation concern. Coasters were known to grow larger and mature later than their stream resident counterparts. This study compared movement patterns, age, size, condition, and relative weight of wild coaster and resident brook trout from the Hurricane River, Pictured Rocks National Lakeshore, Michigan. Wild brook trout ≥ 100 mm from the Hurricane River downstream from Hurricane Falls were tagged with passive integrated transponder tags and monitored for stream-lake movement behavior from May 2003 to November 2007. During 2006 and 2007, brook trout were scale sampled and aged to construct a regression that was then used to calculate the age of all brook trout tagged from 2003 to 2007. Most brook trout movement took place in the fall with October the peak month of emigration with a secondary peak in late spring/early summer and some activity nearly year round. There were no differences found in age structure, size or condition between coasters and residents while in the stream. Our data suggest that a priori growth differences are not determining the expression of coaster outmigration and that stream-lake movements made by coasters, likely driven by habitat requirements, may be highly flexible and facultative.     

Microplastic contamination of sediments across and within three beaches in western Lake Superior

Microplastic pollution of the environment is ubiquitous, but the processes by which microplastics accumulate within beach sediments are not yet well understood. We isolate microplastic pollution from the sediments at three western Lake Superior beaches. Samples of both surface and subsurface sediments are considered. We find that the average microplastic contamination is 65 microplastic particles kg^?1 sediment across our sites with significant variability across beaches, but the microplastic composition is always dominated by polyester fibers. The variation across beaches does not seem to relate to the distance from suspected sources of microplastics to the lake. Within each beach, we find no significant variation of the mean microplastic concentration in the cross-shore direction or in the surface vs. subsurface sediments at the wrack line. We interpret this mean microplastic concentration in the sediment as a measure of the bulk microplastic concentration in the nearshore water at each beach. In subsurface sediments, we observe a significant difference in the variance of microplastic concentrations, and we attribute this variation to the intermittency of the extreme hydrodynamic conditions that deposit microplastics deeper into the sediment.     

Spatial genetic structure and recruitment dynamics of burbot (Lota lota) in Eastern Lake Michigan and Michigan tributaries

Burbot (Lota lota) are the only freshwater member of the Cod like (Lotidae) family that have a circumpolar distribution and occupy the widest geographic distribution of all Laurentian Great Lakes fish species. Information regarding burbot spatial genetic structure and recruitment dynamics is critical for the development of effective management strategies. Although burbot are a species of conservation concern throughout their range, little demographic or behavioral information exists. We estimated levels of genetic diversity within, and the degree of spatial population structure between samples collected from Lake Michigan and tributaries of the Manistee River, MI. Measures of genetic diversity across 10 microsatellite loci were moderately high. Disparities between adult groups sampled in Lake Michigan and the Manistee River were notable for observed heterozygosity (0.662 vs 0.488) and allelic richness (11.7 vs 6.6). Significant levels of inter-population variance in microsatellite allele frequencies (F_ST 0.154 to 0.208) were detected between Lake Michigan and the Manistee River samples. Results indicate reproductive isolation between what plausibly may be riverine and lacustrine spawning life history types. Pedigree analyses for three cohorts sampled in the Manistee River revealed that a sizeable number of adults contributed reproductively to multiple cohorts, indicating spawning philopatry. While data were collected from restricted areas in lacustrine and river habitats, analyses revealing microgeographic genetic structuring, potentially attributed to life history polymorphisms, have significant implications for burbot management in the Great Lakes.