Comparison of Otolith and Scale Ages for Yellow Perch from Lake Michigan

The age composition of the yellow perch (Perca flavescens) population in Lake Michigan is assessed annually by five management agencies, but all agencies do not use the same structure to estimate ages. The reliability of the most commonly used structure, scales, has not been formally evaluated for this population. We compared ages estimated by three readers from scales and sagittal otoliths for 150 yellow perch from southwestern Lake Michigan. The maximum age of yellow perch determined from scales and otoliths was 12. Otoliths had better precision (reproducibility) and usually had more annuli than scales for all three readers. Scale ages were usually younger than otolith ages when otolith ages were ≥ 7. Chi-square tests revealed significant differences (P < 0.05) between the age distributions determined from scales and otoliths for two of the three readers. We recommend use of otoliths for aging Lake Michigan yellow perch greater than 150 mm in length because of greater precision, easier readability, and detection of more annuli.     

Energy content of young yellow perch and walleye in Saginaw Bay

We evaluated seasonal energy content of age-0 yellow perch Perca flavescens and walleye Sander vitreus in Saginaw Bay, Lake Huron in 2009 and 2010. We also determined the energy content of age-1 fish from the 2009 and 2010 cohorts the following spring (i.e., for fish that had survived one winter) to evaluate overwinter energy losses. As expected, larger fish within each species had disproportionately higher energy content (i.e., slope relating length and energy > 3.0) than smaller conspecifics. By contrast to expectations, allometric slopes were > 3.0 in nearly all months, not just the fall, and were higher for age-0 yellow perch than for walleye, even though increased allocation to growth would have seemingly been beneficial to even the largest yellow perch during summer. Seasonal energy allocation patterns differed between years. In 2009, length specific energy content increased from late summer to fall for both species. However, for the 2010 cohorts of fish, length specific energy content decreased between late summer and fall for yellow perch and did not change for walleye. There were 13–17% overwinter declines in length specific energy content between the fall (October or November) and the spring (May) with no major differences between cohorts within a species or between species for a given year. Because young yellow perch and walleye are similar physiologically but differ in size (i.e., yellow perch are smaller), it is possible that overwinter energy losses are more important for yellow perch than for walleye.     

Energetic considerations for managing double-crested cormorants on Lake Champlain

We studied foraging distribution, activity budgets, fish consumption, and energetics of double-crested cormorants (Phalacrocorax auritus) at two breeding colonies on Lake Champlain. Our objective was to determine if fish consumption and distribution of predation changed with movements of cormorants associated with efforts to reduce numbers of cormorants on one of the colonies. Wildlife managers reduced populations of cormorants on Young Island, Vermont by oiling their eggs, which resulted in dispersal of breeding cormorants 35 km south to Four Brothers Islands, New York. We found that as cormorants shifted from Young Island to the colony on Four Brothers Islands, energy demands, foraging distribution, and total fish consumption increased. Birds on Four Brothers Islands foraged a greater distance from the colony compared to birds on Young Island. Additionally, consumption of yellow perch (Perca flavescens) shifted to rainbow smelt (Osmerus mordax) when more birds bred on Four Brothers Islands. The dispersal of cormorants from Young Island to Four Brothers Islands reduced predation on yellow perch but increased overall fish consumption. Our estimates of fish consumption ranged from 322,000–425,000 kg of fish per year at Young Island to 899,000–1,086,000 kg of fish per year at Four Brothers Islands. Results from this study demonstrate secondary impacts of management on Young Island to unmanaged areas.     

Distribution of Larval Yellow Perch (Perca Flavescens) in Nearshore Waters of Southeastern Lake Michigan

Spatial and temporal distribution of larval yellow perch (Perca flavescens) in southeastern Lake Michigan was described from samples collected during 1973–1981 with plankton nets and benthic sleds in water 0.5 to 21 m deep. Yellow perch larvae ≤ 7.5 mm total length were numerous from mid-May to early or mid-July. Larvae taken before mid-June hatched from eggs spawned in inland waters, rather than in Lake Michigan. Those larvae appearing early occurred chiefly in shallow water (≤1.5 m), whereas larvae caught later were chiefly distributed in deeper water (≥ 6 m). Larval perch abundance was similar at all bottom depths ≥ 6 m, was low in surface water during the day, and low near bottom at night. At 0.5- and 1.5-m stations, significantly more larvae were caught at night than during the day.     

Measurement of Diporeia respiration rate for Lake Superior

The freshwater amphipod Diporeia is a dominant macroinvertebrate species in Lake Superior’s benthic community and an important prey item for many fish. A capacity to predict growth and production rates of Diporeia using a bioenergetics model requires information on physiological processes of the species. The objective of this study is to quantify oxygen consumption of Lake Superior Diporeia and to determine if respiration rate changes with body length. Diporeia were collected from Lake Superior and kept over natural sediment maintained at 4 °C. Dissolved oxygen levels for groups of immature (2 mm), juvenile (4 mm), and adult (6 mm) Diporeia in 20 ml microcosms were measured using a polarographic microelectrode. Mass-specific respiration rates for Lake Superior Diporeia ranged from 32.0 to 44.7 mg O₂ g DW^− 1 day ⁻¹. A significant relationship between body length and mass-specific respiration rate (p > 0.1) was not found. The estimate of Diporeia respiration presented here is significantly higher (p < 0.05) than previous findings from populations in Lakes Michigan and Ontario. This study provides new data on respiration rates of Lake Superior Diporeia and compares findings to studies for other connecting Great Lakes.     

Estimation of Zooplankton Mean Length for Use in an Index of Fish Community Structure and its Application to Lake Erie

Mills et al. (1987) developed an index of zooplankton mean size to assess the state of fish communities. The use of this index was evaluated in an assessment of the fish community structure in 1993 at nearshore and offshore sites in the three Lake Erie basins. Mills et al.’s index was developed using a 153-μm mesh net, while the samples in this study have been collected with 64-μm and 110-μm mesh size nets. Two methods were used to convert the data to 153-μm equivalent collections: (a) regression relationships based on simultaneous collections with three mesh sizes, and (b) elimination of smaller organisms that would have passed through the 153-μm mesh by determining the minimum length of inclusion (MLI). The regressions employed for the conversion of zooplankton mean length (ZML) between the nets were: ZML₁₅₃ = 0.137 + 0.988 ZML₁₁₀ (mm) (r²= 0.804) (n = 10) and ZML₁₅₃ = 0.042 + 1.330 ZML₆₄ (mm) (r² = 0.931) (n = 9). The MLI that resulted in the same mean length as the 153-μm sample averaged (± 1 SE) 0.267 ± 0.016 mm (n =19).The comparison between zooplankton mean length and fish community structure in the western basin of Lake Erie in 1993 showed good agreement with Mills et al.’s index. However, the same was not true for the 1988 to 1990 data. Reasons for this discrepancy are discussed.     

Estimating the Size of Fish Consumed by Double-crested Cormorants: Considerations for Better Understanding Cormorant-Fish Interactions

We measured 926 smallmouth bass (Micropterus dolomieu), 6,935 yellow perch (Perca flavescens), 6,416 rock bass (Ambloplites rupestris), and 4,852 pumpkinseed (Lepomis gibbosus) otoliths recovered from double-crested cormorant (Phalacrocorax auritus) pellets to determine the sizes (total lengths) of these fish consumed by cormorants. Otoliths were recovered from cormorant pellets collected from 1993 to 2002 at six colonies along the eastern Lake Ontario–St. Lawrence River corridor. Otolith – length fish length regressions were used to estimate the length of fish species consumed by cormorants. Only 1.5% of these otoliths had no visible erosion, 33.3% had minor erosion, and 65.2% had moderate erosion. We found that the exclusive use of uneroded otoliths severely limited the sample size available for estimating fish size and likely would cause an overestimation of fish size. Species-specific differences were evident when using erosion criteria to determine fish size and could result in bias when estimating length, especially for species such as smallmouth bass whose otoliths possess a rostrum that is readily eroded. Using a random sample (n = 100) of all intact otoliths recovered in pellets provided a conservative estimate of fish length that was smaller than that derived from uneroded or minimally eroded otoliths. Annual variation in the size of fish consumed by cormorants was more pronounced than seasonal variation for most species. We describe and recommend a new technique that incorporates both chick regurgitant and pellet samples for estimating the size of fish consumed by cormorants.     

Neonatal Behavioral Assessment Scale Performance in Humans Influenced by Maternal Consumption of Environmentally Contaminated Lake Ontario Fish

Behavioral effects in neonates of their mothers’ consumption of Lake Ontario fish were examined using the Neonatal Behavioral Assessment Scale (NBAS). Newborns of women who had consumed > 40 equivalent pounds of fish were placed in a high-exposure group (n = 152), those of women who had consumed < 40 equivalent pounds of fish were placed in a low-exposure group (n = 243), and offspring of women who reported having never eaten Lake Ontario fish comprised the control group (n = 164). Assessments were given at 12–24 hours after birth, and again at 25–48 hours after birth. Despite relatively low levels of fish consumption, newborns in the high-exposure group scored more poorly than those in both the low-exposure and control groups on the Reflex, Autonomic, and Habituation clusters of the NBAS. No significant group differences were found on the Orientation, Range of State, Regulation of State, or Motor clusters of the NBAS, nor did birth weight or head circumference differ between groups. These results represent the first replication and extension of the neonatal results of the Lake Michigan Maternal Infant Cohort study (Jacobson et al. 1984).     

Spatial and temporal comparisons of double-crested cormorant diets following the establishment of alewife in Lake Champlain,USA

Increasing numbers of double-crested cormorants (Phalacrocorax auritus) on Lake Champlain have caused concerns related to potential impacts on the yellow perch (Perca flavescens) population. However, with the establishment of alewife (Alosa pseudoharengus) in 2003, cormorant foraging may have changed. We examined cormorant diets from four areas of Lake Champlain to assess past, current, and potential future impacts of cormorants on the changing fish community. During the breeding seasons of 2001–2002 and 2008–2009, we observed spatial and temporal differences in cormorant diets. Yellow perch dominated diet composition during 2001–2002 at Young Island (73% and 90% yearly weight totals) during all reproductive periods. Four Brothers Islands diet composition in 2002 varied according to reproductive period. In 2008 and 2009, alewife were predominant in diets at Four Brothers Islands (56% and 71%) and the South site (65% and 62%), with yellow perch comprising a high proportion of diets at Young Island (44% and 56%). Results from a MANOVA confirmed differences among sites, reproductive period, and the interaction of these factors (P < 0.0001) when describing diet compositions for the post-alewife years. PCA results denoted a general shift in cormorant diets from 2001–2002 to 2008–2009. Our study demonstrated that the diet of piscivorous birds may shift with a new forage species and may vary significantly within a single large water body. Accordingly, efforts to manage piscivorous birds with the intent to decrease mortality of specific fish species should be site specific when possible.     

A Comparison of Photosynthate Allocation in Lakes

We compared the relationships between photosynthate allocation to protein, carbohydrate, lipid and low molecular weight (LMW) fractions and the variables daylength and water temperature in Lakes Huron, Michigan, and Ontario as well as three smaller headwater lakes in the Lake Ontario drainage. In all lakes investigated the allocation of recently produced photosynthate to carbohydrate was strongly related to daylength (% carbohydrate = −3.5 * daylength (hr) + 72.8; n = 59, r² = 0.56). The percentage of photosynthate allocated to protein was a function of water temperature in all lakes although the y-intercept for the protein-temperature relationship was much lower in the three headwater lakes and Lake Ontario (% protein = 0.50 * temperature (°C) + 6.1; n = 37, r² = 0.52) than in Lake Huron and Lake Michigan (% protein = 0.68 * temperature (°C) + 24.2; n = 23, r² = 0.49). The increase in allocation to protein was related to a decrease in allocation to low molecular weight material (% LMW = −1.1 * % protein + 57.13; n = 60, r² = 0.72). The percentages of photosynthate in lipid and LMW material were not related to any of the environmental variables measured. Assuming that photosynthate allocation is related to biochemical composition, the phytoplankton in Lakes Huron and Michigan were more protein rich for a given temperature than those in Lake Ontario and in the smaller inland lakes. The protein deficit was due to an increase in allocation to LMW material.     

Development of a Bioenergetics Model for the Round Goby (Neogobius melanostomus)

The round goby (Neogobius melanostomus) was introduced into the Great Lakes in the 1990s through ballast water transfer. Gobies are potential vectors for material transfer between the benthic and pelagic zones. A bioenergetics model was developed for round gobies to enable us to quantify the flow of energy, contaminants, and nutrients from the benthos to pelagic fishes. Weight- and temperature-dependent coefficients for metabolism and consumption were derived. Food consumption increased with temperature up to 26°C before sharply decreasing, and weight-specific consumption decreased with increasing fish weight (allometric coefficient = −0.256, SE = 0.160). Specific oxygen consumption was inversely related to body mass (allometric coefficient = −0.157, SE = 0.025) and increased exponentially with temperature. Estimated Q₁₀ for respiration was 1.84. Additional parameters were obtained from the literature to describe specific dynamic action, egestion, excretion, and reproductive losses. The bioenergetics model explained growth of round gobies in Lake Erie and in their native range.     

Age and growth of round gobies in Lake Michigan,with preliminary mortality estimation

The round goby (Neogobius melanostomus) is a prevalent invasive species throughout Lake Michigan, as well as other Laurentian Great Lakes, yet little information is available on spatial variation in round goby growth within one body of water. Age and growth of round goby at three areas of Lake Michigan were studied by otolith analysis from a sample of 659 specimens collected from 2008 to 2012. Total length (TL) ranged from 48 to 131 mm for Sturgeon Bay, from 50 to 125 mm for Waukegan, and from 54 to 129 mm for Sleeping Bear Dunes. Ages ranged from 2 to 7 years for Sturgeon Bay, from 2 to 5 years for Waukegan, and from 2 to 6 years for Sleeping Bear Dunes. Area-specific and sex-specific body–otolith relationships were used to back-calculate estimates of total length at age, which were fitted to von Bertalanffy models to estimate growth rates. For both sexes, round gobies at Sleeping Bear Dunes and Waukegan grew significantly faster than those at Sturgeon Bay. However, round goby growth did not significantly differ between Sleeping Bear Dunes and Waukegan for either sex. At all three areas of Lake Michigan, males grew significantly faster than females. Based on catch curve analysis, estimates of annual mortality rates ranged from 0.79 to 0.84. These relatively high mortality rates suggested that round gobies may be under predatory control in Lake Michigan.     

Reproductive Potential and Fecundity of Lake Trout Strains in Southern and Eastern Waters of Lake Ontario, 1977–1994

We assessed the reproductive potential of various genetic strains of hatchery lake trout (Salvelinus namaycush) in southern and eastern Lake Ontario from indices of fecundity and indices of male abundance. Indices were constructed from catches of mature lake trout in gill nets during September 1980 to 1994 after correcting for mortality from sea lampreys (Petromyzon marinus) which occurred between September sampling and late fall spawning. Strain and age were assigned to individual lake trout based on clipped fins and maxillary bones or coded wire tags. Fecundity-length relationships for fish of the same age, determined from mature females collected in 1977 to 1981 and 1994, were not different (P > 0.05) among genetic strains. For all strains combined, fecundity-length relationships in 1977 to 1981 were not different among fish of various ages but in 1994, age-5 and -6 fish had fewer eggs (P < 0.003) than age-7 fish, and age-7 fish had fewer eggs (P < 0.003) than fish of age 8, 9, or 10. Annual indices of fecundity varied 19 fold and indices of mature males varied 11 fold; both indices were low in the early 1980s, increased sharply in the mid 1980s, and peaked in 1993. The strain which dominated fecundity and mature male indices shifted during the study from Seneca Lake strain to Lake Superior strain and then back to Seneca Lake strain. However, changes in either reproductive potential or genotypes do not appear responsible for the abrupt appearance of naturally-produced yearling lake trout throughout southern and eastern Lake Ontario in 1994–1995, the first widespread occurrence of juveniles produced by hatchery lake trout in Lake Ontario.     

Determining the efficacy of microsatellite DNA-based mixed stock analysis of Lake Michigan's Lake Whitefish commercial Fishery

Management of commercially exploited fish should be conducted at the stock level. If a mixed stock fishery exists, a comprehensive mixed stock analysis is required for stock-based management. The lake whitefish Coregonus clupeaformis comprises the primary commercial fishery across the Great Lakes. Recent research resolved that six genetic stocks of lake whitefish were present in Lake Michigan, and long-term tagging data indicate that Lake Michigan's lake whitefish commercial fishery is a mixed stock fishery. The objective of this research was to determine the usefulness of microsatellite data for conducting comprehensive mixed stock analyses of the Lake Michigan lake whitefish commercial fishery. We used the individual assignment method as implemented in the program ONCOR to determine the accuracy level at which microsatellite data can reliably identify component populations or stocks. Self-assignment of lake whitefish to their population and stock of origin ranged from > 96% to 100%. Evaluation of genetic stock discreteness indicated a moderately high degree of correct assignment (average = 75%); simulations indicated supplementing baseline data by ～ 50 to 100 individuals could increase accuracy by up to 4.5%. Simulated mixed stock commercial harvests with known stock composition showed a high degree of correct proportional assignment between observed and predicted harvest values. These data suggest that a comprehensive mixed stock analysis of Lake Michigan's lake whitefish commercial fishery is viable and would provide valuable information for improving management.     

Energy Density of Three Fishes from Minnesota Waters of Lake Superior

Energy densities of three salmonine prey species in Lake Superior were determined by bomb calorimetry to examine seasonal trends, differences due to gender and to establish a relation with percent dry weight. These results from April through October for 1992 and 1993 were compared with other studies from the Great Lakes region. Across the 2-year sampling period rainbow smelt Osmerus mordax energy density decreased from April to June and July then significantly increased into September. Lake herring Coregonus artedii energy density was highest in June, decreased significantly by August, and remained stable through October. Bloater C. hoyi energy density was lowest in July, then increased in August and early September before decreasing in late September. There were no significant differences in energy density between males and females for rainbow smelt and lake herring collected on the same date across sampling dates. However, bloater energy density was significantly higher for females than for males on two dates. Energy density estimates for bloater in Lake Superior were lower than those reported from Lake Michigan. Energy density values of rainbow smelt and lake herring fell within the range reported in Lake Michigan and Lake Ontario (rainbow smelt), and from two inland lakes in Minnesota and Lake Mendota, Wisconsin (lake herring). Percent dry weight was a good predictor of energy density (P < 0.001, r² = 0.92–0.99) for all three species for both years. Percent dry weight vs energy density was significantly different between years for bloater and lake herring, and was significantly different for all three species compared with populations in other lakes in the Great Lakes Region.     

Association of Low Reproductive Rates and High Contaminant Levels in Bald Eagles on Green Bay,Lake Michigan

Bald eagles (Haliaeetus leucocephalus) nesting on the shores of Green Bay, Lake Michigan, had reproductive rates significantly lower than those of neighboring eagles nesting in inland Wisconsin (0.55 vs. 1.1 young per occupied territory). This study investigated effects of two factors which have depressed eagle reproductive rates at other locations: exposure to organochlorine contaminants and low food availability. Levels of dichloro-diphenyl-dichloroethylene (DDE) and total polychlorinated biphenyls (PCBs) in addled eggs and eaglet blood samples from Lake Michigan and inland Wisconsin reference sites were measured. An index to food availability, the food delivery rates by parent eagles to nestlings, and other behavioral indices that have been associated with food availability, were measured and compared to inland reference data. Mean contaminant concentrations in addled eggs from Green Bay were 8.3 μg/g wet weight DDE and 31.3 μg/g wet weight total PCBs (1987 to 1996, n = 9). Mean concentrations in nestling blood plasma were 53 μg/kg wet weight DDE and 207 μg/kg wet weight total PCBs (1987 to 1995, n = 8). Indices of food availability were generally normal in comparison to inland reference data. Mean food delivery rate to the nestlings was 2.55 items per nestling per day (n = 7 nests). None of the behavioral measures differed significantly from inland reference values, suggesting that prey was adequate. It was concluded that organochlorine contaminants caused all or most of the depression in reproductive rates of Green Bay bald eagles.     

Differential habitat use patterns of yellow perch Perca flavescens in eastern Lake Michigan and connected drowned river mouth lakes

Understanding stock structure and habitat use is important for sustainable fisheries management and conservation of genetic and phenotypic diversity. In eastern Lake Michigan, yellow perch Perca flavescens is found in both the nearshore region of Lake Michigan proper and drowned river mouth (DRM) lakes, small estuary-like systems directly connected to Lake Michigan. Recent genetic analyses suggest complex stock structure between these habitats and the potential migration of Lake Michigan yellow perch into the profundal zone of DRM lakes. We quantified carbon (δ¹³C) and oxygen (δ¹⁸O) stable isotope ratios of yellow perch otolith cores to index natal origins, and measured muscle δ¹³C, nitrogen (δ¹⁵N), δ¹⁸O, and hydrogen (δ²H) isotope ratios to reflect recent diet and habitat use. Stable isotope ratios of otolith cores and muscle samples support the existence of resident populations in nearshore Lake Michigan and DRM lakes, as well as Lake Michigan migrants using DRM lakes. Most fish caught in DRM lakes that had natal and recent stable isotope values similar to Lake Michigan fish were collected during fall in DRM lake profundal zones. Comparison of otolith core and muscle stable isotope ratios of individual yellow perch suggest that individuals that recently migrated to DRM lakes also spent early life in Lake Michigan. Differential habitat use patterns of yellow perch in eastern Lake Michigan may have important implications for harvest estimates and fishing regulations. Migration by Lake Michigan fish into DRM lakes does not appear to be related to reproduction, and the underlying benefits of these migrations remain unclear.     

Dreissenid mussels are not a “dead end” in Great Lakes food webs

Dreissenid mussels have been regarded as a “dead end” in Great Lakes food webs because the degree of predation on dreissenid mussels, on a lakewide basis, is believed to be low. Waterfowl predation on dreissenid mussels in the Great Lakes has primarily been confined to bays, and therefore its effects on the dreissenid mussel population have been localized rather than operating on a lakewide level. Based on results from a previous study, annual consumption of dreissenid mussels by the round goby (Neogobius melanostomus) population in central Lake Erie averaged only 6 kilotonnes (kt; 1 kt = one thousand metric tons) during 1995–2002. In contrast, our coupling of lake whitefish (Coregonus clupeaformis) population models with a lake whitefish bioenergetics model revealed that lake whitefish populations in Lakes Michigan and Huron consumed 109 and 820 kt, respectively, of dreissenid mussels each year. Our results indicated that lake whitefish can be an important predator on dreissenid mussels in the Great Lakes, and that dreissenid mussels do not represent a “dead end” in Great Lakes food webs. The Lake Michigan dreissenid mussel population has been estimated to be growing more than three times faster than the Lake Huron dreissenid mussel population during the 2000s. One plausible explanation for the higher population growth rate in Lake Michigan would be the substantially higher predation rate by lake whitefish on dreissenid mussels in Lake Huron.     

Spatial and temporal distribution of the cyanotoxin microcystin-LR in the Lake Ontario ecosystem: Coastal embayments,rivers, nearshore and offshore,and upland lakes

Cyanotoxins, a group of hepatotoxins and neurotoxins produced by cyanobacteria, pose a health risk to those who use surface waters as sources for drinking water and for recreation. Little is known about the spatial and seasonal occurrence of cyanotoxins in Lake Ontario and other lakes and ponds within its watershed. Within the embayments, ponds, rivers, creeks, shoreside, and nearshore and offshore sites of Lake Ontario, microcystin-LR concentrations were low in May, increased through the summer, and reached a peak in September before decreasing in October. Considerable variability in microcystin-LR concentrations existed between and within habitat types within the Lake Ontario ecosystem. In general, the average microcystin-LR concentration was two orders of magnitude lower in embayment (mean = 0.084 μg/L), river (mean = 0.020 μg/L), and shoreside (mean = 0.052 μg/L) sites compared to upland lakes and ponds (mean = 1.136 μg/L). Concentrations in the nearshore sites (30-m depth) and offshore sites (100-m depth) were another order of magnitude lower (mean = 0.006 μg/L) than in the creek/river, bay/pond, and shoreside habitats. Only 0.3% (2 of 581) of the samples taken in Lake Ontario coastal waters exceeded the World Health Organization (WHO) Drinking Water Guideline of 1 μg microcystin/L for humans. In contrast, 20.4% (20 of 98) of the samples taken at upland lakes and ponds within the watershed of Lake Ontario exceeded WHO Guidelines. No significant relationship between nitrate and microcystin-LR concentrations was observed in Lake Ontario even though a significant positive relationship existed between phosphorus and phycocyanin and microcystin-LR concentrations. At an upland lake site (Conesus Lake) in the Ontario watershed, the development of a littoral Microcystis population was not observed despite high nutrient loading (P and N) into the nearshore zone, well-developed nearshore populations of filamentous Spirogyra and Zygnema, the occurrence of Dreissena spp., and the known occurrence of Microcystis and microcystin production in the pelagic waters of Conesus Lake.     

Contaminant Concentrations and Reproductive Rate of Lake Superior Bald Eagles, 1989–2001

We investigated the trend in contaminant concentrations in Lake Superior bald eagles (Haliaeetus leucocephalus) from 1989–2001, and examined the relationship of contaminant concentrations to eagle reproductive rate during that time. Concentrations of dichloro-diphenyl-dichloroethylene (DDE) and total polychlorinated biphenyls (PCBs) in nestling blood plasma samples decreased significantly from 1989-2001 (p = 0.007 for DDE, p = 0.004 for total PCBs). Mean contaminant concentrations in eaglet plasma, 21.7 μg/kg DDE (n=51) and 86.7 μg/kg total PCBs (n = 54), were near or below the estimated threshold levels for impairment of reproduction as determined in other studies. A preliminary assessment of polybrominated diphenyl ether (PBDE) concentrations indicated a mean of 7.9 μg/kg total PBDEs in Lake Superior eaglet plasma (n = 5). The number of occupied bald eagle nests along the Wisconsin shore of Lake Superior increased from 15 to 24 per year, between 1989 and 2001 (p < 0.001, r² = 0.70, n = 13 years). Eagle reproductive rate did not increase or decrease significantly between 1989 and 2001 (p = 0.530, r² = 0.037, n = 13 years, mean productivity = 0.96 young per occupied nest). The lack of correlation between reproductive rate and contaminant concentrations, as well as the comparison of contaminant concentrations to the estimated thresholds for impairment of reproduction, suggest that DDE and PCBs no longer limit the reproductive rate of the Lake Superior eagle population in Wisconsin.