Sediment nitrification and denitrification in a Lake Superior estuary

Inorganic nitrogen (N) transformations and removal in aquatic sediments are microbially mediated, and rates influence N-transport. In this study we related physicochemical properties of a large Great Lakes embayment, the St. Louis River Estuary (SLRE) of western Lake Superior, to sediment N-transformation rates. We tested for associations among rates and N-inputs, vegetation biomass, and temperature. We measured rates of nitrification (NIT), unamended base denitrification (DeNIT), and potential denitrification [denitrifying enzyme activity (DEA)] in 2011 and 2012 across spatial and depth zones. In vegetated habitats, NIT and DeNIT rates were highest in deep (ca. 2 m) water (249 and 2111 mg N m^− 2 d^− 1, respectively) and in the upper and lower reaches of the SLRE (> 126 and 274 mg N m^− 2 d^− 1, respectively). Rates of DEA were similar among zones. In 2012, NIT, DeNIT, and DEA rates were highest in July, May, and June, respectively. System-wide, we observed highest NIT (223 and 287 mg N m^− 2 d^− 1) and DeNIT (77 and 64 mg N m^− 2 d^− 1) rates in the harbor and from deep water, respectively. Amendment with NO₃⁻ enhanced DeNIT rates more than carbon amendment; however, DeNIT and NIT rates were inversely related, suggesting the two processes are decoupled in sediments. Average proportion of N₂O released during DEA (23–54%) was greater than from DeNIT (0–41%). Nitrogen cycling rates were spatially and temporally variable, but we modeled how alterations to water depth and N-inputs may impact DeNIT rates. A large flood occurred in 2012 which temporarily altered water chemistry and sediment nitrogen cycling.     

Establishment patterns of non-native fishes: Lessons from the Duluth-Superior harbor and lower St. Louis River, an invasion-prone Great Lakes coastal ecosystem

We conducted a 2-year, multi-gear survey in the lower St. Louis River, which includes the Duluth-Superior harbor, an international shipping port and non-native species invasion “hotspot.” Our objectives were to quantify the contribution of non-native species to the overall fish assemblage and assess their spatial distribution and abundance. We captured 10 non-native fishes; none, however, were first detections. Non-native fishes composed roughly one quarter of the total species richness, were found in 84% of samples, and composed 15% of the total abundance. The spatial distribution and abundance of non-native fishes was dependent on gear selection (and thereby habitat selection), sampling location, and abundance measure (individuals or biomass). We used a recently published non-native species establishment framework to integrate catch data from the different gears. Viewed in the context of this framework, we identified only two non-native fishes, Eurasian ruffe (Gymnocephalus cernuus) and round goby (Neogobius melanostomus), as both widespread and abundant, whereas we identified three as localized and rare. Moreover, the time since first detection of non-native fishes was not predictive of their frequency of occurrence across the study area, underscoring the importance of environmental and biological factors in controlling fish establishment success. Although non-native fishes constitute a considerable portion of the trawl catch in the river channel, from a multi-gear system-wide perspective, native fishes collectively make up the majority of the fish biomass and abundance in the lower St. Louis River.     

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.     

Reproduction and Early Life History of Ruffe (Gymnocephalus cernuus) in the St. Louis River,a Lake Superior Tributary

Reproduction and early life history of ruffe (Gymnocephalus ceriums) was investigated during April to July in 1993 and 1994 in the St. Louis River, a western Lake Superior tributary. This study was conducted to assist fishery managers in determining possible interactions among the early life stages of ruffe and other North American percids, and in obtaining information useful in developing control methods targeted at the early life stages of ruffe. Ruffe had a prolonged spawning period that extended from late April to late June with peak spawning in mid to late May when water temperatures were between 12 and 14°C. The majority of ruffe protolarva were captured 1 to 2 weeks after egg deposition between mid May and late June and most were captured in water 0.5-m deep. Onshore-offshore movements were not observed, but diel vertical movements of larval ruffe were observed on several occasions. The greatest chance of ballast water transport of pelagic larval ruffe is between mid May and July. Information on reproduction and early life history in this report will assist fishery mangers in development of ruffe control methods, and assist Great Lakes shipping in ballast water management to prevent the spread of ruffe.     

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.     

Spatial Distributions and Temporal Trends in Sediment Contamination in Lake St. Clair

The sediments of Lake St. Clair were surveyed in 2001 for a range of compound classes including metals (such as total mercury and lead), polychlorinated biphenyls, polychlorinated dibenzo-p-dioxins and dibenzofurans, organochlorine pesticides, polycyclic aromatic hydrocarbons, and short- and medium- chain chlorinated paraffins, in order to evaluate the spatial distribution and temporal trends of contamination. Concentrations of contaminants were generally low compared to the lower Great Lakes (Erie and Ontario), and were typically below the Canadian Sediment Quality Probable Effect Level (PEL) guidelines. The only exceptions were for mercury and DDE, where concentrations exceeded their respective PEL at one of the thirty-four sites sampled. With the exception of mercury, it was difficult to interpret spatial trends in contaminant concentrations due to these low levels, although relatively elevated concentrations of several contaminants were found in L’Anse Creuse Bay and at the outflow of the Thames River. In the case of mercury, historically-contaminated sediments in the St. Clair River associated with chlor-alkali production appeared to contribute to loadings to Lake St. Clair. There have been substantial reductions in sediment contamination in Lake St. Clair over the past three decades, as determined through sediment core profiles as well as through comparison of current data to those from historical surveys conducted in the early 1970s. These results indicate that management actions to reduce contaminant loadings to Lake St. Clair have been generally successful.     

Temporal and spatial trends of organochlorines and mercury in fishes from the St. Clair River/Lake St. Clair corridor,Canada

The temporal and spatial relationships of a suite of organochlorine contaminants and mercury were examined in various fish species of the St. Clair River/Lake St. Clair corridor, Canada, in order to evaluate the effectiveness of remediation efforts and to assess the risk to human and wildlife fish consumers. In Lake St. Clair, fish tissue concentrations of mercury, polychlorinated biphenyls (PCBs), octachlorostyrene (OCS), hexachlorobenzene (HCB), and dichlorodiphenyltrichloroethane (DDT) decreased consistently from the 1970s until the 1980s and 1990s, after which the rate of contaminant decline slowed or concentrations stabilized. This trend was consistent in up to 13 species (both young-of-the-year and adult fishes) comprising different trophic positions and dietary habits, suggesting that the changes were reflective of ambient conditions rather than food web processes. Elevated concentrations of mercury, PCBs, OCS, HCB, and DDT were detected in St. Clair River young-of-the-year spottail shiner compared with fish from Lake Huron, indicating that non-atmospheric inputs of these chemicals, likely originating from sediment, remain in the St. Clair River. Current concentrations of mercury and PCBs, and mercury, PCBs, and DDT remain of concern to human and wildlife fish consumers, respectively. Given that contaminant decreases have generally stabilized in fish, we suggest that further natural recovery of contaminants in St. Clair corridor fishes will be slow since contaminants will likely continue to be influenced by sediment levels.     

Assessment of fish mercury levels in the upper St. Lawrence River,Canada

The stretch of the St. Lawrence River near Cornwall, Ontario, Canada, between the Moses–Saunders power dam to the eastern outlet of Lake St. Francis in Québec, is currently listed as an Area of Concern (AOC), and has restrictive fish consumption advisories in place, largely due to high mercury levels. This study examined long-term temporal trends of mercury concentrations in northern pike, smallmouth bass, walleye and yellow perch from the St. Lawrence River, including the Cornwall AOC. In addition, differences in fish mercury concentrations among river sections were compared for each species using historical (1975–1979) and recent (2000–2011) data. Statistically significant declines in mercury concentrations were observed in all river sections for yellow perch (~ 40%), as well as in northern pike and walleye in at least one river section. For the river section encompassing the AOC, recent mercury concentrations are 33–59% lower than historical mercury concentrations for all four species. Further, a comparison of recent mercury concentrations between the AOC river section and other Ontario lakes/rivers suggests that AOC mercury values are within the 75th percentile of values for other Ontario water bodies in three of the four species considered. However, current fish mercury values for the AOC river section remain higher than one or more upstream river sections. These results indicate that recovery is still ongoing in this AOC.     

Life Cycle of a Mayfly Hexagenia Limbata in the St. Marys River Between Lakes Superior and Huron

Length-frequency distribution curves of Hexagenia limbata nymphs collected in May, August, and October 1974 and May 1975 in the St. Marys River between Lakes Superior and Huron were bimodal for each sampling period. These curves, combined with interpretation of nymphal emergence period and mean surface water temperatures, indicate that the population of Hexagenia nymphs in the St. Marys River is composed of two year classes or cohorts. One cohort emerges per season, 2 years after egg deposition.     

Bioassessment of Benthic Macroinvertebrates in Wetland Habitats of Lake Saint-Pierre (St. Lawrence River)

We evaluated the potential of vegetation and sediment habitats in wetlands of the St. Lawrence River for developing a macroinvertebrate bioassessment program with reference conditions. During September 2004, we collected macroinvertebrates in emergent vegetation and sediment in both fluvial sites (reference) and tributary-plume sites (impacted) in waters of the north and south shores of Lake Saint-Pierre (St. Lawrence River). In each habitat, we compared taxa richness, abundance, and community structure of macroinvertebrates between reference and impacted sites, and used multivariate models to relate macroinvertebrate community to environmental conditions. Each habitat was suitable for discriminating reference fluvial sites from impacted tributary-plume sites based on macroinvertebrate communities. In emergent vegetation, macroinvertebrates were dominated by epibenthic fauna such as crustaceans (Gammaridae, Asellidae) and molluscs (Valvatidae) at fluvial sites, and insect larvae (Chironomidae, Caenidae) at tributary-plume sites. In sediment, macroinvertebrates comprised a greater proportion of endobenthic fauna such as Oligochaeta and Sphaeridae. Crustaceans and molluscs were still dominant at fluvial sites and Oligocheata and Chironomidae at tributary-plume sites. No strong difference was depicted in macroinvertebrate composition between north and south shore water masses. Environmental variables explained a higher proportion of variance in macroinvertebrate community composition in emergent vegetation than in sediment (68% versus 44%). Macroinvertebrate composition in sediment was more related to metal contamination, whereas macroinvertebrate composition in emergent vegetation was related primarily to vegetation type and water quality. Relevance of the study for bioassessment of macroinvertebrates in the St. Lawrence River using the reference condition approach is discussed.     

Temporal trends in near-shore sediment contaminant concentrations in the St. Clair River and potential long-term implications for fish tissue concentrations

Near-shore surface sediment of the St. Clair River (Ontario) extending along 9 km of waterfront downstream of local industries were contaminated with mercury (Hg), hexachlorobenzene, hexachlorobutadiene and octachlorostyrene based on data collected in 2006 and 2008. However, unlike Hg, concentrations of organic compounds were low at most stations suggesting improvements in sediment quality since 2001 (post sediment remediation projects). Concentrations of Hg were greater than Provincial Sediment Quality Guidelines and ranged as high as 41 μg/g in the surficial 0–5 cm sediment. Measured (2001–2004 data) and estimated (2006 data) concentrations of methyl Hg in invertebrates were greater than concentrations in invertebrates collected from upstream reference stations. Predicted methyl Hg concentrations in walleye using food chain multipliers, benthic invertebrate Hg tissue concentrations and current sediment concentrations exceeded human health consumption guidelines and were consistent with measured sport fish data supporting assumptions about Hg biomagnification. Historical data showed that for fish collected from the lower St. Clair River and Lake St. Clair, Hg concentrations have declined since 1978, but have remained stable since the mid 1980s, consistent with the sediment Hg data. The consistency in the sediment and fish datasets and the absence of a known significant local point source of Hg suggests that the sediment may be an important source of Hg to the St. Clair River food-chain.     

Density,production, and survival of walleye (Sander vitreus) eggs in the Muskegon River,Michigan

Walleye (Sander vitreus) is an important sport fish in the Great Lakes that is experiencing low reproductive success after severe population declines starting in the late 1940s. In the Muskegon River, Michigan, natural reproduction of walleyes remains low and is largely supplemented by stocking. To determine factors influencing walleye reproductive success in the Muskegon River, we estimated walleye egg survival using insitu egg incubators covered with nitex screening (2003–2004) and estimated density and survival of fertilized eggs caught on furnace filter traps across different substrate types (2005–2006). We compared physical habitat suitability for walleye eggs under high and low flow scenarios. Density of walleye eggs was highest in regions of gravel/cobble substrates. Egg survival was higher in egg incubators (24–49.5%) than on furnace filter traps (2.0%), suggesting predation is an important source of walleye egg mortality in the Muskegon River. Cold water temperatures that extended developmental stage durations may also be an important source of egg mortality. The dynamic habitat suitability model predicted low suitability for eggs due to poor temperature and velocity conditions. Despite low egg survival rates, 40 million to 1 billion eggs were estimated to hatch. The low natural reproduction of walleyes in the Muskegon River is likely due to a combination of low walleye egg survival and failure of walleye larvae to reach their nursery grounds in Muskegon Lake.     

Dietary niche and growth rate of the nonnative tubenose goby (Proterorhinus semilunaris) in the Lake Superior basin

The tubenose goby (Proterorhinus semilunaris) entered the Great Lakes in the 1990s via ballast water, but remains poorly studied within North America, making it difficult to predict its effects on native ecosystems. Dietary breadth and somatic growth rate have important ramifications for survival, competitiveness, and dispersal ability of a fish species, and thereby its ecological impact. We studied diet and growth of age-0 tubenose goby within the St. Louis River, a tributary to Lake Superior that contains the largest population within the Lake Superior basin. We sampled tubenose gobies from shallow, vegetated habitats during summer and fall. Stomach contents were identified and weighed to measure fullness and dietary breadth between seasons and several locations. We aged fish based on otolith daily increments to model somatic growth. Diet was dominated by isopods and amphipods, and dietary breadth was low and not significantly different between locations and seasons. Tubenose goby diet strongly overlapped with that of tadpole madtom (Noturus gyrinus), a native, demersal species. We tested several candidate growth models; the Gompertz growth function was the most parsimonious model among those examined. The model demonstrates that tubenose goby obtains a small maximum size and is short-lived. We conclude that tubenose goby presents a unique risk to the Great Lakes and other freshwater bodies because their life history is typical of invasive species, their diet overlaps with native fish, and because they occupy shallow, vegetated habitat which functions as both nursery and foraging habitat for many native fishes.     

Contaminant concentrations in bald eagles nesting on Lake Superior,the upper Mississippi River,and the St. Croix River

We measured concentrations of DDE, total PCBs, and mercury in bald eagle (Haliaeetus leucocephalus) nestlings at three locations in the upper Midwest: Lake Superior, the upper Mississippi River, and the St. Croix River, 2006–2008. We also analyzed trends in concentrations of these contaminants for eagles on the southern shore of Lake Superior, from 1989 to 2008, using the current and previously published data. Concentrations of DDE in nestling blood plasma samples were greatest on Lake Superior (geometric mean: 16.2 μg/kg, n = 29), whereas concentrations of total PCBs were highest in Mississippi River samples (88.6 μg/kg, n = 51). Mercury concentrations were highest along the upper St. Croix River (6.81 μg/g wet weight in feathers, n = 19). For Lake Superior, DDE concentrations declined significantly in nestling blood plasma samples from 1989 to 2008, an average of 3.0% annually. Similarly, total PCBs in Lake Superior eaglets decreased 4.0% annually from 1989 to 2008, and mercury concentrations in nestling feathers from Lake Superior nests also decreased significantly from 1991 to 2008, 2.4% per year. With the possible exception of mercury on the upper St. Croix River, mean concentrations in 2006–2008 of all three compounds were below levels associated with significant impairment of reproduction for all sites, and reproductive rates at all three sites averaged > 1.2 young per occupied territory, which is greater than the rate indicative of a healthy population.     

A Bioenergetics Modeling Evaluation of Top-down Control of Ruffe in the St. Louis River,Western Lake Superior

Ruffe (Gymnocephalus cernuus), were accidentally introduced into the St. Louis River estuary, western Lake Superior, in the mid 1980s and it was feared that they might affect native fish through predation on eggs and competition for forage and habitat. In an effort to control the abundance of ruffe and limit dispersal, a top-down control strategy using predators was implemented in 1989. We used bioenergetics modeling to examine the efficacy of top-down control in the St. Louis Riverfront 1991 to 1994. Five predators––northern pike (Esox lucius), walleye (Stizostedion vitreum vitreum), small-mouth bass (Micropterus dolomieui), brown bullhead (Ictalurus nebulosus), and yellow perch (Perca flavescens)––were modeled to determine their consumption of ruffe and four other native prey species––spottail shiner (Notropis hudsonius), emerald shiner (Notropis atherinoides), yellow perch (Perca flavescens), and black crappie (Pomoxis nigromaculatus). Although predators ate as much as 47% of the ruffe biomass in 1 year, they were not able to halt the increase in ruffe abundance. The St. Louis River is an open system that allows predators to move freely out of the system, and the biomass of managed predators did not increase. A selectivity index showed all five predators selected the native prey and avoided ruffe. The St. Louis River has several predator and prey species creating many complex predator-prey interactions; and top-down control of ruffe by the predators examined in this study did not occur.     

Assessment of fish consumption beneficial use impairment at the Great Lakes Thunder Bay and St. Marys River Areas of Concern,Canada

The North American Great Lakes were designated with 43 locally degraded Areas of Concern (AOCs) in the 1980s. Remediation activities geared towards restoring beneficial use impairments (BUIs) at the AOCs have been conducted by both the American and Canadian governments. Here we examine if the fish consumption BUI has been restored at the Thunder Bay Harbour and St. Marys River AOCs within the Canadian waters by applying a three-tier assessment framework using the fish contaminant data collected by the Government of Ontario, Canada. Fish consumption advisories published by the government as well as simulated advisories based on the post-2005 data were examined in Tiers 1 and 2. The results highlighted that the restrictions advised on eating fish from the AOCs are mild and are typically similar to other non-AOC areas of lakes Superior and Huron. Temporal trend analyses of three contaminants of concern, mercury, PCB and dioxins/furans, generally showed substantial improvements over the last 30+ years and mostly continued declining trends in the recent years. These findings support a re-designation of the fish consumption BUI to “not impaired” at the two AOCs. As a follow up, it is recommended to confirm improvements in the dioxin/furan/dioxin-like PCB levels in fish at the Thunder Bay AOC. It is also advisable to conduct a survey to properly define “beneficial use” of fish consumption for the AOCs (i.e., which fish and in what quantity do people eat), and thereby validate the critical assumption of 8+ meals/month as a non-restrictive advice used in this assessment.     

Genomics reveals identity,phenology and population demographics of larval ciscoes (Coregonus artedi,C. hoyi,and C. kiyi) in the Apostle Islands,Lake Superior

We demonstrate, for the first time, the ability to reliably assign an assemblage of larval coregonines [Salmonidae Coregoninae] to shallow and multiple deepwater species. Larval coregonines from the Apostle Islands, Lake Superior, were genotyped using restriction site-associated DNA sequencing (RADseq) and were assigned to species using reference genotypes from adult corgonines from the same region. Of the 193 genotyped larvae, 101 were assigned as Coregonus artedi (average assignment probability = 97.6%), 57 were assigned as C. kiyi (average assignment probability = 95.5%), and 28 were assigned as C. hoyi (average assignment probability = 89.0%). Coregonus artedi were collected earliest in the season, followed by C. kiyi and then C. hoyi. Estimates of genetic diversity within each species provide a baseline for future monitoring in the Apostle Islands. Our success with species assignment indicates the promise of leveraging genomic data for larval coregonine identification, which could enable assessing and evaluating early life history dynamics and recruitment processes at the species level to the benefit of ongoing coregonine restoration and management efforts.     

Erosion of glacial till from the St. Clair River (Great Lakes basin)

Variations in water level observed in Lakes Michigan and Huron during the last few decades have motivated a comprehensive study involving climatic, hydrologic and hydraulic factors organized by International Joint Commission of the Great Lakes. It has been submitted, among other possible causes, that changes in conveyance in the St. Clair River could be contributing to the lowering of the upper Great Lakes water level. Sediment transport processes, in particular bed scour and erosion, can affect significantly a river's conveyance, thus creating the need to assess the erodibility properties of the river bed. To this end, laboratory tests were performed in order to obtain the value of the critical shear stress needed to erode the cohesive fraction of the bed sediment material, known as glacial till, from the St. Clair River. Different flows with increasing velocities were run up to the point where initial sediment erosion could be observed. Through detailed near-bed velocity measurements using a laser Doppler velocimetry (LDV) system, a value of 4.2 N/m² was obtained as the critical shear stress for the erosion of glacial till. A threshold for the critical shear stress for erosion of similar cohesive sediments was also found and expressed in dimensionless form. These results could be used in combination with mathematical models to estimate the risk of scour and erosion at locations where the glacial till is exposed to both strong currents and flow forces induced by the large navigation vessels commonly observed along the course of the St. Clair River.     

Relationships between wind-driven and hydraulic flow in Lake St. Clair and the St. Clair River Delta

A three-dimensional hydrodynamic forecasting model of the Great Lakes Huron-Erie Corridor is used to investigate mixing and the relationship between hydraulic and wind-induced currents in a shallow lake system in which lake inflows come through several channels of a river delta. The hydrodynamics in Lake St. Clair and the channels of the St. Clair River Delta are evaluated for (1) a one-year simulation from 1985 including water age calculation, (2) 8 different wind direction scenarios, and (3) a storm event. Observations and model simulations show distinct regions in the lake in which currents are forced by either hydraulic flow from the river system or from wind stress over the lake. However, during severe storm events, these regions are found to shift or even disappear due to changes in the delta channel inputs into the lake. These changes underscore the need for realistic, unsteady river flow boundary conditions at interfaces between a shallow lake and river delta. Steady inflow conditions will not allow for potential shifting of these current zones, and will also fail to resolve flow retardation or reversals during storm events.     

Asymmetric Hybridization of Pink (Oncorhynchus gorbuscha) and Chinook (O. tshawytscha) Salmon in the St. Marys River,Michigan

Natural hybridization between pink salmon (Oncorhynchus gorbuscha) and Chinook salmon (O. tshawytscha) has been observed only in the Great Lakes, where both species have been introduced. The direction of hybridization between these species is poorly understood, thus the present study analyzed mitochondrial DNA (mtDNA) to determine whether the maternal parents were pink or Chinook salmon. During annual salmon population surveys on the St. Marys River (1998–2002), fifty putative hybrids of pink salmon and Chinook salmon were identified from meristic, morphometric and color characters. We designed primers to amplify a fragment of the mitochondrial D-loop control region that included both a variable and a control restriction site (BstNI). Polymerase chain reaction, restriction endonuclease digestion and capillary electrophoresis of mtDNA were used in order to identify the maternal parent of each hybrid salmon. In addition, the amplified fragments from the three fish species were sequenced to further verify the results. All hybrid salmon specimens were found to possess Chinook salmon mtDNA, indicating that hybridization between Chinook salmon and pink salmon is asymmetrical and likely unidirectional, occurring only between male pink salmon and female Chinook salmon. Influences contributing to the hybridization of these salmonid species could include limited spatial and temporal segregation of spawners, sexual selection, a limiting number of Chinook males, and/or physiological factors.