Molecular Evidence for Divergent Breeding Groups of Walleye (Stizostedion vitreum) in Tributaries to Western Lake Erie

Molecular evidence from mitochondrial DNA (mtDNA) restriction fragment length polymorphisms (RFLPs) supports the existence of genetically different spawning groups of walleye (Stizostedion vitreum) among the Maumee, Sandusky, and Grand rivers that flow into Lake Erie. Three separate regions of the mtDNA molecule (the D-loop region, the NADH dehydrogenase 3, 4, and 4L genes, and the 12S and 16S rRNA genes) from female walleye eggs were amplified using the polymerase chain reaction technique. Amplifications were digested with nine different four base cutting restriction endonucleases and were resolved on 9% polyacrylamide gels. The D-loop and the NADH genes produced RFLPs, revealing a total of eight different haplotypes. The frequencies of haplotypes within the Maumee River spawning walleye were significantly different from the haplotypes within the Sandusky River. Frequencies of the haplotypes within the Grand River spawning walleye were not statistically different from those of the Maumee or Sandusky rivers. These results support the hypothesis that the Maumee and Sandusky River spawning walleye are members of two different breeding groups. The molecular identification of these walleye breeding groups may help in future fisheries management decisions.     

Predation on Walleye Eggs by Fish on Reefs in Western Lake Erie

We examined diets of fishes from gillnet and egg pump collections conducted on reefs in western Lake Erie during walleye (Sander vitreus) egg incubation periods from 1994–1999 and 2004 to assess incidence of walleye eggs in fish diets. We collected no potential egg predators in samples taken in 1994 but from 1995–1999 and in 2004 we caught 22 different species of fish on reefs in addition to spawning walleye. In most years, white perch (Morone americana) stomachs contained more walleye eggs than any other species on the reefs averaging 253 eggs per stomach. We also found lower numbers of walleye eggs in the stomachs of channel catfish (Ictalurus punctatus; 53 eggs/stomach), johnny darter (Etheostoma nigrum; 2 eggs/stomach), logperch (Percina caprodes; 10 eggs/stomach), quillback (Carpiodes cyprinus; 184 eggs/stomach), rock bass (Ambloplites rupestris; 3 eggs/stomach), round goby (Neogobius melanostomus; 4 eggs/stomach), sculpin (Cottidae; 21 eggs/stomach), silver chub (Macrhybopsis storeriana; 3 eggs/stomach), spottail shiner (Notropis hudsonius; 14 eggs/stomach), trout-perch (Percopsis omiscomaycus; 30 eggs/stomach), white sucker (Catastomus commersonii; 20 eggs/stomach), and yellow perch (Perca flavescens; 181 eggs/stomach). Similar to other studies of predation on walleye eggs, our results indicate that prolonged incubation periods increase the potential for egg loss due to predation.     

Comparison of Five Ageing Methodologies Applied to Walleye (Stizostedion Vitreum) in Burt Lake,Michigan

Age estimations were performed on 25 walleyes (Stizostedion Vitreum) from Burt Lake, Michigan, by sectioning pectoral and pelvic fin rays, dorsal fin spines, and otoliths and by impressing scales. Age groups 4 to 11 were found using these methods. Paired t-tests showed no significant differences among all pairs (p > 0.05), except the pelvic fin ray and otolith combination. Variability exists among the age estimations for individual fishes, however, statistically all methods were of equal precision. A subjective merit analysis showed otoliths and pectoral fin rays had the highest rating, while pelvic fin rays had the lowest. In areas of the Great Lakes which have walleye populations that exhibit similar age patterns as Burt Lake walleye, otolith or pectoral fin ray ageing methods should be used as an occasional check for the reliability of scale age estimations.     

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 Herring (Coregonus artedi) and Rainbow Smelt (Osmerus mordax) Diets in Western Lake Superior

We describe the diets of lake herring (Coregonus artedi) and rainbow smelt (Osmerus mordax) in western Lake Superior during the summers of 1996 and 1997. Both species consumed predominantly (> 71% by number) zooplankton, showing a preference for larger taxa. Diet overlap between the two species was low (Schoener's index = 0.42). Mysis was most important in rainbow smelt diets, whereas Diaptomus sicilis was most important in lake herring diets. Rainbow smelt selected larger taxa, and larger individuals within a taxon when compared to lake herring, although rainbow smelt tended to be smaller fish. Fish diets have changed relative to previous studies and may be reflecting changes in the zooplankton community. Continued changes in the fish and zooplankton community will alter predatorprey and energetic pathways, ultimately affecting growth and production of the ecosystem.     

Genetic Assessment of Walleye (Sander vitreus) Restoration Efforts and Options in Nipigon Bay and Black Bay,Lake Superior

Walleye (Sander vitreus) stocks in Nipigon Bay and Black Bay historically numbered as the largest stocks in Lake Superior, but collapsed in the 1960s due to overfishing, habitat loss, and other pressures. We used microsatellite DNA analyses to assess the success and relative contributions of past rehabilitation stocking to walleye in Nipigon Bay, and to investigate the relationship between historical and contemporary populations in Black Bay. Based on the genetic data, juvenile stocking and adult transfers from four source populations into Nipigon Bay differed substantially in their contributions to the reestablished population. The genetic data also indicated that natural reproduction was occurring and identified survivors from the former Nipigon Bay population. Similar genetic analysis of scale samples from the historical Black Bay fishery and present-day walleye from a major tributary (Black Sturgeon River) showed that the historical and contemporary samples comprise one genetic stock, which is significantly different from neighboring native and introduced populations. These findings suggest that walleye restoration efforts in Lake Superior are working, and highlight the utility of and options for adaptive management approaches for restoring extirpated populations.     

Postspawning Mortality of Rainbow Smelt in Western Lake Superior

Extensive die-offs of rainbow smelt, Osmerus mordax (Mitchell), have occurred in the western end of Lake Superior during each of the past several springs. Sex, age composition, and incidence of fungus infection in smelt from the 1977 die-off were compared with the same characteristics of smelt of the extant spawning population to determine the extent and possible causes of mortality. The die-off in 1977 began about 3 weeks after the end of the spawning run and continued for approximately 2 weeks. The occurrence of dead rainbow smelt was greatest near the Superior-Duluth harbor and decreased as distance from the harbor increased. Age structure of smelt that died was similar to that of the spawning population. Some factor or factors related to spawning may have caused the mortality. The most probable cause of the die-off was temperature stress on spawning smelt in the spawning areas which increased the susceptibility of smelt to the fungus Saprolegnia sp. and may have promoted osmoregulatory imbalance. Male smelt were more vulnerable to the cause of the die-off than female smelt; young fish, especially females, were more resistant than older fish.     

Relationship of Lithology and Geomorphology to Erosion of the Western Lake Superior Coast

Coastal erosion/accretion rates measured from aerial photos taken in the 1930s and 1960s-1970s were compared with lithologic and geomorphic properties of the northern and southern coasts of western Lake Superior. The coasts were divided into four lithologic types based on Quaternary geology: bedrock, clay, sandy till, and water-laid sand and gravel. Average erosion of the southern coast was significantly higher than that of the northern coast for clay (0.78 vs. 0.14 m/yr), and for water-laid sand and gravel (1.89 vs. 0.16 m/yr). Sandy till on the southern coast eroded an average of 0.12 m/yr; till units were negligible along the northern coast. Bedrock constitutes 57% of the northern coast and has eroded at an average rate of 0.08 m/yr; bedrock exposures are rare along the southern coast. Clay coasts facing north or northwest eroded significantly faster than those facing south, and water-laid sand and gravel coasts facing north eroded significantly faster than those facing southeast. This is probably because major storm winds and waves come from the northeast, with a greater impact on the north- and northeast-facing shores. Offshore slope and bluff slope were not significantly related to erosion rates.     

Kinetics of Red Clay Bluff Dissolution in Western Lake Superior

The dissolution of red clay bluff samples from the southwestern Lake Superior shoreline area in Lake Superior water or deionized water is studied by following the aqueous concentrations of reactive silica over a three month period. The dissolution process is initially rapid, followed by a first-order dissolution process (k = 9.4 × 10^?7 sec^?1) up to about thirty days. After about 30 days, the rate of dissolution of the bluff material follows linear kinetics (k = 5.4 × 10^?8 mg SiO₂/gram of bluff per second).     

Total gaseous mercury (TGM) concentration over Lake Superior and Lake Michigan

Mercury-contaminated fish are a serious problem in the Great Lakes basin, because mercury is a potent neurotoxin that poses a danger to both humans and wildlife. Lake Superior lake trout and walleye have the highest mercury concentrations of the five Great Lakes. Because the atmosphere is the major source of mercury to the Great Lakes, information on the over-water mercury concentration is essential to model the mercury biogeochemical cycle. For the first time in the peer-reviewed literature, this paper presents total gaseous mercury (TGM) measurements made over Lake Superior and Lake Michigan. The Lake Superior aircraft measurements were made at an altitude of 300 m, and the Lake Michigan aircraft measurements at a variable altitude of 30–300 m. The over-water Lake Superior TGM of 1.02 ± 0.34 ng/m³ is much lower than the TGM from nine stations in the Canadian Atmospheric Mercury Measurement Network (CAMNet) and six stations in the Atmospheric Mercury Network (AMNet). The land-based TGM concentrations average range from 1.25 to 1.75 ng/m³ which are in good agreement with current global average values of 1.3–1.6 ng/m³. The over-water Lake Michigan TGM is 1.65 ± 0.61 ng/m³. We also present Lake Superior over-water measurements of volatile organic compounds (VOC), ozone (O₃), nitrogen oxide (NO_y), and particulate matter. Elemental carbon (EC) is a tracer for mercury because mercury is released during the combustion of coal. EC is significantly correlated with TGM over both Lake Superior and Lake Michigan. TGM over Lake Michigan is also significantly correlated with organic carbon, sulfate, nitrate and ammonium.     

Evaluation of Bottom Trawls as Compared to Acoustics to Assess Adult Lake Herring (Coregonus artedi) Abundance in Lake Superior

We compared density estimates from day bottom trawl tows against night midwater trawl tows and acoustic gear to test the hypothesis that adult lake herring (≥ 250 mm) are underestimated by day bottom trawl tows during the annual USGS spring fish community survey in Lake Superior. We found average density at nine nearshore stations was significantly higher at night (21.3 adult fish/ha) compared to day (1.0 adult fish/ha; p = 0.0119). At nine offshore stations, no lake herring were captured during the day but density averaged 39.6 adult fish/ha at night. At a lakewide scale (n = 18 stations), precision (relative standard error) was much better using night midwater trawls and acoustic gear (37%) compared to day bottom trawls (100%). Moderate sample size increases using the former methodology would likely bring precision within recommended levels (≤ 30%) for stock-recruit data sets. Our results suggest that 1) population abundances of adult lake herring in Lake Superior are much higher than previously considered, 2) the annual spring fish community survey may not provide a relative index of abundance of adult lake herring, 3) night midwater trawls and acoustic gear are necessary for assessing adult lake herring abundance, and 4) previous studies using lake herring data from the annual spring fish community survey need to be re-evaluated in light of these results. Lake herring appear to become progressively more pelagic and less susceptible to bottom trawling as they mature. Day bottom trawls appear to be an adequate tool for estimating relative density of age-1 recruits, although this method still suffers from relatively poor precision.     

Spatial Patterns Emphasize the Importance of Coastal Zones as Nursery Areas for Larval Walleye in Western Lake Erie

Lake Erie walleye Sander vitreus exhibits significant interannual variability in year-class strength. Recent research revealed the importance of larval growth and survival rates in determining walleye year-class strength in western Lake Erie, indicating that spatial and temporal overlap of larvae with good habitat conditions (e.g., abundant prey, warm waters) promoted walleye growth and survival. To assess the spatial overlap between walleye larvae and habitat parameters (water depth, temperature, water clarity, prey density) in western Lake Erie, we evaluated the spatial distribution of walleye larvae and these habitat parameters with intensive sampling at 30 to 36 sites during spring 1994–1999. We analyzed spatial relationships among pelagic walleye larvae and various habitat attributes using a geographic information system and principal components analysis. Larval walleye density was consistently highest at nearshore sites during all years and showed a high degree of spatial overlap with high ichthyoplankton density, and warm water temperatures. Larval walleye density was negatively associated with water depth and water clarity. Two principal components represented 79.6% of the total variability in site attributes. Principle components analysis supported our spatial analysis by graphically separating sites into distinct groups based on larval walleye density and habitat attributes. These analyses indicated that similar relationships between larval distribution and habitat attributes occur each year, emphasizing the importance of nearshore coastal zones as nursery areas for walleye.     

A 2001 Survey of Crustacean Zooplankton in the Western Arm of Lake Superior

This study cataloged the current taxonomic composition and distribution of crustacean zooplankton at 12 locations in the western arm of Lake Superior during May and August 2001. Zooplankton were last surveyed to this taxonomic level in the western arm in 1973, prior to a number of biotic changes to the lake. Notable differences in the 2001 zooplankton assemblage compared to historical conditions include the establishment of the exotic Bythotrephes longimanus, declines in the density of Daphnia retrocurva and Bosmina longirostris, and an increase in density of Holopedium gibberum. These changes are consistent with an increase in invertebrate predation and similar to changes reported after B. longimanus invasions elsewhere in North America, but a cause and effect relationship is correlative. Densities and body lengths of Lake Superior B. longimanus were within the ranges reported for other Great Lakes.     

Some Notes on Water Color in Keweenaw Bay (Lake Superior)

Spectral subsurface irradiance reflectance—intrinsic water color—was derived from above-water radiance measurements using a hand-held spectroradiometer along a transect on Keweenaw Bay, Lake Superior. The reflectance spectra were typical of oligotrophic lacustrine waters. The reflectance peak wavelength shifted from 484 nm at stations farthest offshore to 540 nm near the head of the bay. This change coincided with a decrease in Secchi-disk depth from 16 to 8 m, and an increase in concentrations of chlorophyll a and total suspended matter from about 0.45 to 0.60 mg m⁻³ and from 0.3 to 0.5 g m⁻³, respectively. The concentration of chromophoric dissolved organic matter (gilvin), expressed as the absorption of filtrate at 440 nm, varied between 0.11 and 0.2 m⁻¹. Like almost all inland waters, Keweenaw Bay should be classified as a Case 2 water due to the concentrations of gilvin and inanimate particles relative to plankton biomass. A model for chlorophyll-a estimation from spectral reflectance that adequately predicted concentrations in mesotrophic to highly eutrophic Case 2 waters elsewhere gave negative values when applied to the Keweenaw Bay transect. Evidently, there is a need of algorithm development for oligotrophic lacustrine waters. Estimates improved using a modified blue to green band ratio algorithm previously applied for remote sensing of oceanic waters. In optimization of semi-empirical algorithms for estimation of plankton biomass in Lake Superior, absorption by gilvin as well as by inanimate particles merits special consideration.     

Organochlorine Insecticides and PCB in the Surficial Sediments of Lake Superior (1973)

Analysis of 405 Lake Superior surficial sediments (0-3 cm depth) collected in 1973 were found to contain low levels of DDE, HEOD, and PCB, but no detectable mirex, heptachlor expoxide, endrin, endosulfan, or chlordane. p,p’-DDE was detected in 50%, HEOD in 9%, and PCB in 44% of the sediment samples at trace or low measurable levels. Mean residues for the whole lake were p,p’-DDE 0.71 ± 1.65 ng/g, HEOD below the detection level of 0.25 ng/g, and PCB 3.3 ± 5.7 ng/g. p,p’-DDE was more concentrated in depositional basins than in non-depositional zones while PCB levels were similar for the two zones.     

Hydroacoustic Estimates of Abundance and Spatial Distribution of Pelagic Prey Fishes in Western Lake Superior

Lake herring (Coregonus artedi) and rainbow smelt (Osmerus mordax) are a valuable prey resource for the recovering lake trout (Salvelinus namaycush) in Lake Superior. However, prey biomass may be insufficient to support the current predator demand. In August 1997, we assessed the abundance and spatial distribution of pelagic coregonines and rainbow smelt in western Lake Superior by combining a 120 kHz split beam acoustics system with midwater trawls. Coregonines comprised the majority of the midwater trawl catches and the length distributions for trawl caught fish coincided with estimated sizes of acoustic targets. Overall mean pelagic prey fish biomass was 15.56 kg ha⁻¹ with the greatest fish biomass occurring in the Apostle Islands region (27.98 kg ha⁻¹), followed by the Duluth Minnesota region (20.22 kg ha⁻¹), and with the lowest biomass occurring in the open waters of western Lake Superior (9.46 kg ha⁻¹). Biomass estimates from hydroacoustics were typically 2–134 times greater than estimates derived from spring bottom trawl surveys. Prey fish biomass for Lake Superior is about order of magnitude less than acoustic estimates for Lakes Michigan and Ontario. Discrepancies observed between bioenergetics-based estimates of predator consumption of coregonines and earlier coregonine biomass estimates may be accounted for by our hydroacoustic estimates.     

Distribution and Abundance of Rainbow Smelt in Western Lake Superior as Determined from Acoustic Sampling

Distribution and abundance of rainbow smelt (Osmerus mordax, Mitchill) in the western arm of Lake Superior were described through an acoustic sampling program conducted from June 1978 through early June 1980. Density and abundance were highest in shallow water (bottom depth <50 m), where rainbow smelt underwent vertical movements from the bottom and concentrated in warmer (11 to 16° C) near-surface water (2- to 30-m depth) at night. Rainbow smelt densities and abundance were extremely low in the deeper waters (>50 m) along the north shore and in the open lake. Abundance of rainbow smelt in the western area of the lake was estimated to be 286 X 10⁶ fish (4.35 X 10⁶ kg) in 1978. Abundance declined to 164 X 10⁶ fish (2.5 X 10⁶ kg) in 1979. Comparison of rainbow smelt abundance estimates with commercial yields suggested that annual fishing mortality is 15 to 27%. Predation by salmonid populations is considered a potentially important factor influencing smelt abundance levels. Rainbow smelt distribution and changes in abundance described by this study indicate smelt production is not high enough to support the fisheries or predator populations previously supported by native lake herring (Coregonus artedii, Lesueur) stocks.     

Food of Young-of-the-Year Lake Trout (Salvelinus Namaycush) in Presque Isle Harbor,Lake Superior

The food habits of young lake trout (Salvelinus namaycush) were studied by examining the digestive tracts of 293 young-of-the-year collected in Presque Isle Harbor, Lake Superior. Lake trout in the 25 to 27-mm length range started to eat food organisms before all of their yolk material was absorbed. Organisms consumed by the 25 to 27-mm young-of-the-year included Chironomidae, Copepoda (Harpacticoida, Calanoida, Cyclopoida), and Cladocerea (Daphnia spp., Bosmina sp., Chydorus sp.). Chironomid pupae and chironomid larvae accounted for 74% and 5%, respectively, of the total volume of food eaten by the young lake trout in Presque Isle Harbor. Although copepods, cladocerans, and mysids were present in many stomachs, their contribution to the total volume of food was only 15%. Some lake trout in the 32 to 54-mm length range had consumed fry of sculpin (Cottus spp.) or rainbow smelt (Osmerus mordax), but the overall contribution of fish fry to the total volume of food was only 4% (frequency of occurrence, 10%). The lake trout in Presque Isle Harbor fed heavily on planktonic organisms, sparingly on benthic organisms, and were opportunistic feeders that appeared to prey on whatever forage organisms were available in the shallow nearshore waters.     

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.     

Annual and Offshore Changes in Bacterioplankton Communities in the Western Arm of Lake Superior during 1989 and 1990

A shallow site in the western arm of Lake Superior near Duluth, Minnesota was sampled bimonthly from May to October during 1989 and 1990 to identify seasonal and annual changes in bacterioplankton communities. The greatest change in bacterioplankton abundance was between 1989 (1.48 × 10⁹/L ± 0.06 SE) and 1990 (1.14 × 10⁹/L ± 0.06 SE). The majority of bacterial cells (65%) were cocci. Individual cells were larger during 1989 (0.067 μm³ ± 0.007 SE) than 1990 (0.025 μm³ ± 0.002 SE). Although the rate of thymidine incorporation varied from 0.2 to 47.0 pmol/L/h over both years (mean = 12.1 pmol/L/h ± 1.3 SE), no consistent temporal or spatial changes were detected. Bacteria were more abundant (∼2×) and productive (∼10×) at the mouth of the Lester River than offshore of this site. During July and August, a benthic nepheloid layer (BNL) formed at shallow offshore sites but bacterioplankton abundance and production in this BNL were usually similar to values measured in the hypolimnion. Three additional sites from the Duluth basin northeast to the Chefswet basin were sampled during late summer (Aug-Sept) 1990 to identify spatial differences in bacterioplankton communities. Although the number of bacteria was often greater at shallower sites compared to deeper sites further offshore, a strong gradient was not found and bacterial production was similar at all sites. These results may be due in part to the lake basin morphology in this region of Lake Superior, as well as the time when these additional offshore sites were investigated.