Trends in the distribution and abundance of Hexagenia spp. in Saginaw Bay,Lake Huron, 1954–2012: Moving towards recovery?

Since at least the 1940s, multiple anthropogenic disturbances to the Laurentian Great Lakes have had detrimental effects on benthic habitats and biota including decimating the environmentally sensitive burrowing mayfly genus Hexagenia around the mid-1950s. While remediation efforts have facilitated recovery of some populations, benthic surveys in Saginaw Bay, Lake Huron in the last 50 years have only occasionally discovered Hexagenia nymphs. Recently, adult Hexagenia swarms have been reported near the bay; therefore, we corroborated the local presence of Hexagenia adults and evaluated the current status of Saginaw Bay Hexagenia nymphs. We quantified adults during mayfly emergence events in 2010 at three Tawas City, Michigan, USA area locations, and found > 17 Hexagenia/m²/site. We quantified nymphs from Ponar grab samples collected at 57 sites in Saginaw Bay between 2009 and 2012, and found 1.5 nymphs/m² overall with nymphs present at 15.8% of sites sampled, their greatest documented distribution in Saginaw Bay since 1956. Additionally, we mapped bay sediment composition and related sampling site abiotic conditions with both Hexagenia presence and abundance using Zero-Inflated Poisson regression. Model results indicate that the probability of observed Hexagenia absence being true absence is positively related to both sediment sandiness and surficial dissolved oxygen concentration while Hexagenia abundance is greatest where surficial temperatures are ~ 18.6 °C and is also related to sediment sand content. The documentation of nearby adults and in bay nymphs may indicate the beginning of a Hexagenia return to Saginaw Bay, and, therefore, a possible improvement of the ecosystem's benthic health.     

Distribution and Abundance of Burrowing Mayflies (Hexagenia spp.) in Lake Erie, 1997–2005

Burrowing mayflies (Hexagenia limbata and H. rigida) recolonized sediments of the western basin of Lake Erie in the 1990s following decades of pollution abatement. We predicted that Hexagenia would also disperse eastward or expand from existing localized populations and colonize large regions of the other basins. We sampled zoobenthos in parts of the western and central basins yearly from 1997–2005, along the north shore of the eastern basin in 2001–2002, and throughout the lake in 2004. In the island area of the western basin, Hexagenia was present at densities ≤ 1,278 nymphs/m² and exhibited higher densities in odd years than even years. By contrast, Hexagenia became more widespread in the central basin from 1997-2000 at densities ≤ 48 nymphs/m² but was mostly absent from 2001-2005. Nymphs were found along an eastern basin transect at densities≤ 382/m² in 2001 and 2002. During the 2004 lake-wide survey, Hexagenia was found at 63 of 89 stations situated throughout the western basin (≤ 1,636 nymphs/m², mean = 195 nymphs/m², SE = 32, N = 89) but at only 7 of 112 central basin stations, all near the western edge of the basin (≤ 708 nymphs/m²), and was not found in the eastern basin. Hexagenia was found at 2 of 62 stations (≤ 91 nymphs/m²) in harbors, marinas, and tributaries along the south shore of the central basin in 2005. Oxygen depletion at the sediment-water interface and cool temperatures in the hypolimnion are probably the primary factors preventing successful establishment throughout much of the central basin. Hexagenia can be a useful indicator of lake quality where its distribution and abundance are limited by anthropogenic causes.     

Evaluation of lower Green Bay benthic fauna with emphasis on re-ecesis of Hexagenia mayfly nymphs

The last historic Hexagenia specimen in lower Green Bay was officially recorded in 1955. Field surveys and Hexagenia viability studies were completed to determine if lower Green Bay could support Hexagenia re-ecesis and where in the bay egg stocking could best be accomplished. The invertebrate field data were compared with historical population data based on earlier published studies in the 1950s, 1970s and 1990s to determine the bay's ecological trajectory to better understand the re-ecesis success potential of Hexagenia. No native Hexagenia were observed during this study. Deep water invertebrate diversity within the upper lower bay appears to be improving, whereas the diversity along the lower mid-bay may be deteriorating. Shallower, nearshore samples indicated a better condition with Caenis mayflies sparsely present, amphipods, isopods, gilled snails, odonates, oligochaetes, chironomids, and meiofauna present. These results suggested improved conditions shoreward versus degraded conditions deeper. Hexagenia egg viability and neonate growth indicated Hexagenia could successfully inhabit in situ Green Bay nearshore (<2?m) substrates; however, deep substrates were generally inhospitable probably due to hypoxia and unstable fluid substrates. As an outcome of the field surveys and studies of Hexagenia viability in Green Bay mud, Hexagenia stocking began in 2014 with the first adults since 1955 emerging in 2016 at several lower bay nearshore locations. Improved water quality from remediation efforts in the watershed could facilitate the return of Hexagenia to deeper water.     

The relative importance of anammox and denitrification to total N2 production in Lake Erie

Production of dinitrogen gas via microbially mediated anaerobic ammonium oxidation (anammox) and denitrification plays an important role in removal of fixed N from aquatic ecosystems. Here, we investigated anammox and denitrification potentials via the ¹⁵N isotope pairing technique in the helium flushed bottom water (~0.2 m above the sediment) of Sandusky Bay, Sandusky Subbasin, and Central Basin in Lake Erie in three consecutive summers (2010?2012). Potential rates of anammox (0–922 nM/day) and denitrification (1 to 355 nM/day) varied greatly among sampling sites during the 3 years we studied. The relative importance of anammox to total N₂ production potentially ranged from 0 to 100% and varied temporally and spatially. Our study represents one of the first efforts to measure potential activities of both anammox and denitrification in the water column of Lake Erie and our results indicate the Central Basin of Lake Erie is a hot spot for N removal through anammox and denitrification activities. Further, our data indicate that the water column, specifically hypolimnion, and the surface sediment of the Lake Erie Central Basin are comparatively important for microbially mediated N removal.     

Downflux of Sediment,Organic Matter,and Phosphorus in the Niagara River Area of Lake Ontario

Vertical arrays of sediment traps were installed near the mouth of the Niagara River and at nearshore and offshore sites. Surveys of temperature, turbidity, and conductivity were used to establish that the traps nearest the river mouth were under the river outflow plume. A single offshore array in 1980 indicated downfluxes of 1-2 g · m^?2 · d^?1 of dry matter in the mid-water column. These fluxes were similar to those of small lakes but much higher fluxes were found near the bottom of Lake Ontario. More extensive work in 1981 confirmed the presence of a near-bottom nepheloid layer and the effect it has on increasing downflux extimates. Sediment traps near shore and at the river mouth caught substantially more material at all depths than did traps at the offshore stations. Downfluxes of phosphorus and sediment were consistent with independent estimates of loading and retention.     

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.     

Annual Production of Burrowing Mayfly Nymphs (Hexagenia spp.) in U.S. Waters of Lake St. Clair

Burrowing mayfly nymphs (Hexagenia spp.) were sampled monthly, September through October 1995 and April through August 1996, with a standard Ponar grab (538 cm² jaw opening) at 16 stations in U.S. waters of Lake St. Clair. Annual production (production, P) was 0 to 477 mg dry weight/m² at three stations where pollution and sediment grain-size distribution limited the population, and was 738 to 5,255 mg dry weight/m² at the other 13 stations. The highest production value measured for Hexagenia in Lake St. Clair was about three times higher than the highest value reported for other areas in the northern United States and Canada (39° to 53° North latitude). The production-mean annual biomass (biomass, B) ratio (P/B) for Hexagenia in Lake St. Clair in 1995–96 was described by the straight line P = 2.4 B (R² = 0.94). Adding published P/B data for other North American populations changed the relation only slightly to P = 2.5B (R² = 0.96). A P/B ratio of 2.5 is consistent with the expected value for an aquatic insect with a 2-year life cycle and overlapping cohorts, and these data suggest this relation has general applicability for estimating production of Hexagenia in the northern United States and Canada. Size-class and seasonal partitioning of Hexagenia biomass and production were evident in the data. Both biomass and production were highest among nymphs 16.0 mm and larger, and biomass was highest in October and again in June, immediately before the annual emergence of subimagos. The large size of the mature nymphs and the concentration of biomass and production among the larger nymphs in the population is consistent with their importance in the diets of many fishes in the northern United States and Canada.     

Diets,population structure,and seasonal activity patterns of mudpuppies (Necturus maculosus) in an urban,Great Lakes coastal habitat

Freshwater species native to the Laurentian Great Lakes region face numerous environmental stressors, and the conservation status and ecological relationships of many remain poorly understood. One such species, the mudpuppy (Necturus maculosus), is declining, but better information on their natural history and development of more effective population monitoring techniques is needed. We assessed seasonal variation in capture success, biases in capture techniques, and feeding ecology of mudpuppies in Wolf Lake, a highly perturbed and urban former estuarine wetland complex to Lake Michigan. Trapping periods of ≥ 3 consecutive nights occurred from January to May 2015, and October 2015 to March 2016. Overall trapping success differed among trapping periods (p = 0.01) and declined precipitously at water temperatures above 14.1 °C (p < 0.001). Mudpuppies in traps (mean 26.9 ± 0.5 cm) were larger than those caught with hand nets (mean 14.7 ± 0.8 cm, p < 0.0001), suggesting that multiple methods may be needed to accurately assess demographics. Stomach contents obtained through gastric lavage included mollusks, leeches, insects, isopods, amphipods, crayfish, fish, a frog, and a juvenile conspecific. Invasive species, including rusty crayfish (Orconectes rusticus), round gobies (Neogobius melanostomus), and zebra/quagga mussels (Dreissena spp.) were present in guts, suggesting mudpuppy foraging has changed along with aquatic communities in the region. Prey community analyses revealed differences in overall diet among size classes of mudpuppies (p = 0.001), but relatively weak similarity within size classes. Results suggest that mudpuppies in lake ecosystems occupy a broad niche that changes as they grow.     

Variability of light absorption properties in optically complex inland waters of Lake Chaohu,China

Absorption coefficients of phytoplankton, colored detrital matter (CDM), non-algal particles (NAP), colored dissolved organic matter (CDOM), and their relative contributions to total non-water absorption (a_t ? w) are essential variables for bio-optical and radiative transfer models. Light absorption properties showed large range and variability sampled at 194 stations throughout Lake Chaohu between May 2013 and April 2015. The a_t ? w was dominated by phytoplankton absorption (a_ph) and NAP absorption (a_d). The contribution of CDOM absorption to a_t ? w was lower than 30%. Phytoplankton and NAP were the primary sources of spatial and vertical variability in absorption properties. Light absorption by CDOM, though significant in magnitude, was relatively constant. CDM absorption (a_dg) was dominated by NAP. The spatial variation of the absorption coefficients from each of the optically active constituents were driven by several main inflow rivers in the western and middle part of Lake Chaohu. Algal blooms and bottom resuspension contributed to vertical variability as observed by phytoplankton and NAP profiles. Specific absorption of phytoplankton had significant spatial and seasonal variations without vertical variation. The spectral slope of absorption showed no significant spatial variability (p > 0.05). Variations of absorption affected different ranges of remote sensing reflectance (R_rs) spectrum, thereby increasing the difficulty of applying the remote sensing algorithm in optically complex waters. Parameters and relationships presented in this study provide useful information for bio-optical models and remote sensing of lakes similar to Lake Chaohu in terms of optical properties.     

Sedimentation Rates and Depositional Processes in Lake Superior from 210Pb Geochronology

Sedimentation rates range from 0.01 to 0.32 cm/yr in 17 sediment box cores from Lake Superior, as determined by ²¹⁰Pb geochronology. Shoreline erosion and resuspension of nearshore sediments causes moderate to high (0.05–0.11 cm/yr) sedimentation rates in the western arm of Lake Superior. Sedimentation rates are very high (> 0.15 cm/yr) in marginal bays adjoining Lake Superior; and moderate to very high (0.07–0.19 cm/yr) in open lake regions adjacent to marginal bays. Resuspension of nearshore and shoal top sediments in southern and southeastern Lake Superior by storms is responsible for depositional anomalies in ²¹⁰Pb profiles corresponding to 1905, 1916–1918, and 1940 storms. Sedimentation rates are very low (0.01–0.03 cm/yr) in the central basins due to isolation from sediment sources. These data indicate that sedimentation rates and processes vary significantly in different regions of Lake Superior. The sedimentation rates provided by this study, in conjunction with previously-reported sedimentation rates, yield a better understanding of the Lake Superior depositional environment.     

Silica Regeneration Processes in Nearshore Southern Lake Michigan

The seasonal depletion of dissolved silica to levels that limit diatom production is particularly critical in Lake Michigan's nearshore zone where diatom biomass is greatest, and where silica regeneration from sediments is not well-understood. In our study, intact, medium-fine sand cores, collected from an 11 m deep site in nearshore Lake Michigan during July-August 1980, released soluble reactive silica (SRS) at a mean rate of 2,707 ± 122 (SE) μg Si cm^?2 yr^?1 when incubated in darkness and at 12° C. This measured SRS release was greater than a diffusive flux (270 ± 49 (90% C.I.) μg Si cm^?2 yr^?1) estimated from SRS pore water profiles and physical sediment properties. SRS release from individual cores was not correlated with abundance of most macroinvertebrates (chironomids, pisidiid clams, or oligochaetes). However, a significant (P < 0.05) and inverse relationship between SRS release and Pontoporeia hoyi densities implied that amphipods suppressed SRS release through mixing and burial of a surficial floe layer, where most dissolution of biogenic silica occurs. Moreover, SRS release rates measured from our coarse-grained nearshore sediments were comparable to rates reported for fine-grained offshore material and further implicate dissolution of surficial biogenic silica as the source of remineralized SRS. Because nearshore areas of Lake Michigan undergo strong seasonal variations in temperature and diatom production, and because significant riverine silica inputs exist, we cannot extrapolate our results on a lakewide, or season-long basis. The data, however, strongly imply that nearshore sediments are an important participant in the Lake Michigan silica cycle.     

Historical Changes in PCB Patterns in Lake Ontario and Green Bay,Lake Michigan, 1971 to 1982, from Herring Gull Egg Monitoring Data

Patterns of PCB congener bioaccumulation were examined in archived herring gull (Larus argentatus) eggs collected from Big Sister Island in Green Bay, Lake Michigan, and Scotch Bonnet Island in Lake Ontario from 1971 to 1982 as part of the Canadian Wildlife Service's Great Lakes Herring Gull Monitoring Program. Concentrations of 97 PCB congeners were measured. From 1971 to 1982, ecological half-lives of most congeners, particularly the tri- through hexachlorobiphenyls, were greater in eggs from Green Bay than Lake Ontario. Comparing sum PCB levels in eggs collected in 1971 and 1982, concentrations declined 80% at Scotch Bonnet Island and 74% at Big Sister Island. PCB congener patterns were different in eggs from the two colonies. Principal components analysis showed that inter-site differences in congener patterns became more apparent after 1976. This indicated that regional PCB sources were the most influential in determining patterns of biologically-available PCBs during the 1971 to 1982 period in these two lakes, via recycling of historical PCBs from sediments or gradually decreasing loading. Trend analysis of selected congeners specific to Aroclors 1242, 1254, and 1260 revealed that the rapid decline of less chlorinated congeners, observed from 1971 to 1976 in Lake Ontario, was explained by a decrease in loading of Aroclor 1242 to the lake. At both colonies, ecological half life of the congeners was significantly (p < 0.001) correlated with log K_ow and with –log HLC. Changes in PCB composition, after 1976 in Lake Ontario and from 1971 to 1982 in Green Bay, could be explained by differences in the physical behavior of individual congeners affecting removal by volatilization and sedimentation.     

Combined Effects of Cadmium and Zinc on a Lake Mighican Zooplankton Community

Two 3-week experiments with small enclosures were conducted in situ in northeastern Green Bay, Lake Michigan, to compare the effects of separate and combined additions of cadmium and zinc on the zooplankton community. The radioisotopes ¹⁰⁹Cd and ⁶⁵Zn were used as tracers to determine the sorption of cadmium and zinc by different particle size fractions. Separate additions of 2 μg Cd/L or 100 μg Zn/L, as well as combined additions of 2 μg Cd/L + 100 μg Zn/Lor 1 μg Cd/L + 50 μg Zn/L, caused significant reductions of total crustacean density, species diversity, two community similarity indices, and final dissolved oxygen concentration. The effects of separate additions of 100 μg Zn/L were generally more pronounced than those of 2 μg Cd/L, but were similar to those of ～5 μg Cd/L. The effects of combined additions of 2 μg Cd/L + 100 μg Zn/L were not significantly different from those of 100 μg Zn/Lalone and were primarily due to zinc because it reduced cadmium uptake by the plankton (10–85 μm and >85 μm fractions). The effects of combined additions of l μg Cd/L + 50 μg Zn/L probably were also mainly due to zinc. The overall results of this study suggest that less than a ten-fold increase in the concentration of zinc in Lake Michigan (presently ～5 μg/L) could have pronounced effects on the plankton community.     

Volatile Halocarbon Contaminants in the Niagara River and in Lake Ontario

Water samples from 95 stations in Lake Ontario and 16 stations in the lower Niagara River Were analyzed for volatile halocarbons and carbon disulfide. The following contaminants were observed at many stations with their lake-wide means and standard deviations: trichloro-fluoromethane (Freon 11), 249 ± 882 ng · L^?1; methylene chloride, 572 ± 1,826 ng · L^?1; chloroform, 18 ± 92 ng · L^?1; bromodichloromethane, 3 ± 9 ng · L^?1; and tetrachloroethylene, 9 ± 65 ng · L^?1. Eleven other compounds, including carbon disulfide, 1, 1–dichloroethylene, 1,1,1–trichloroethane, carbon tetra-chloride, and tetrachloroethylene were observed at trace levels or absent at most stations. Six compounds were observed in virtually all Niagara River samples and were traceable into the lake. As apparent from the large relative standard deviations for the above, contaminant concentrations varied strongly between stations, indicating areas of contaminant sources. Both industrialized and urban areas, such as Toronto, Hamilton, and the Niagara River, as well as comparatively small tributaries, such as Twelve Mile, Eighteen Mile, and Oak Orchard Creeks, Black River, and the Welland Canal, appear to be sources for several of the observed contaminants.     

Recovery of Burrowing Mayflies (Ephemeroptera: Ephemeridae: Hexagenia) in Western Lake Erie

Burrowing mayflies (Hexagenia spp.) are native to western Lake Erie and were abundant until the 1950s, when they disappeared due to degraded water and sediment quality. Nymphs were absent from the sediments of most of western Lake Erie after the 1950s, although small, widely disjunct populations apparently persisted near shore. Sediment samples collected in 1993 revealed several small populations near the western and southern shores and beyond the mouths of the Detroit and Maumee rivers. A larger population was found in the southern island area, but nymphs were absent in the middle of the basin. By 1995, nymphs had spread throughout the western half and eastern end of the basin but remained absent from the middle of the basin. These data indicate that Hexagenia began recolonizing nearshore areas before offshore areas. Increasingly large swarms of winged Hexagenia on shore and over the lake between 1992 and 1994 further indicate that mayflies are recolonizing the basin. Factors that have permitted Hexagenia recovery in western Lake Erie probably include improved sediment and water quality attributed to pollution abatement programs implemented after the early 1970s, and perhaps environmental changes in the early 1990s attributed to effects of the exotic zebra mussel (Dreissena polymorpha)     

Quantifying the predatory effect of round goby on Saginaw Bay dreissenids

Invasive dreissenid mussels (D. polymorpha and D. r. bugensis) have fundamentally altered Laurentian Great Lake ecosystems, however in many areas their abundances have declined since the mid-1990s. Another invader, the benthic fish round goby (Neogobius melanostomus), is morphologically adapted to feed on dreissenids and likely affects dreissenid populations; however, the degree of this predatory effect is variable. In 2009 and 2010, we examined round goby abundances, size distributions, diet contents, and diet selectivity in Saginaw Bay, Lake Huron; a shallow bay that has been subjected to numerous anthropogenic stressors. We further used a consumption model to estimate dreissenid consumption by three different size classes of round goby. Round gobies were found throughout the bay and most were smaller than 80 mm total length. Round gobies of all sizes consumed dreissenids (including fish as small as 30 mm total length), though dreissenids were rarely preferred. The relative proportion of dreissenids (by biomass) present in diets of round gobies increased with fish size, but also throughout the year for all size classes. Despite this, overall consumptive effects of round gobies on dreissenids in Saginaw Bay were low. Many dreissenids present in the bay were larger than those consumed by round gobies. Bioenergetics-based model estimates suggest that the smallest round gobies are responsible for the majority of dreissenid consumption. While our findings are limited to soft substrates and influenced by sampling restrictions, our study design allowed us to put bounds on our estimates based upon these multiple sources of uncertainty.     

Burrowing mayfly (Ephemeroptera: Ephemeridae: Hexagenia spp.) bioturbation and bioirrigation: A source of internal phosphorus loading in Lake Erie

Traditional lake eutrophication models predict lower phosphorus concentrations with decreased external loads. However, in lakes where decreased external phosphorus loads are accompanied by increasing phosphorus concentrations, a seeming “trophic paradox” exists. Western Lake Erie is an example of such a paradox. Internal phosphorus loads may help explain this paradox. We examined bioturbation and bioirrigation created from burrowing mayfly, Hexagenia spp., as a possible source of internal phosphorus loading. Phosphorus concentrations of experimental microcosms containing lake sediments, filtered lake water, and nymphs (417/m²) collected from western Lake Erie were compared to control microcosms containing sediments and lake water over a 7-day period. Phosphorus concentrations in microcosms containing Hexagenia were significantly greater than microcosms without nymphs. Further, we estimate the soluble reactive phosphorus flux from the sediments due to Hexagenia is 1.03 mg/m²/day. Thus, Hexagenia are a source of internal phosphorus loading. High densities of Hexagenia nymphs in western Lake Erie may help explain the “trophic paradox.” Furthermore, Hexagenia may be a neglected source of internal phosphorus loading in any lake in which they are abundant. Future studies of phosphorus dynamics in lakes with Hexagenia must account for the ability of these organisms to increase lake internal phosphorus loading.     

Contaminant-associated health effects in fishes from the Ottawa and Ashtabula Rivers,Ohio

The health of resident fishes serves as a biologically relevant barometer of aquatic ecosystem integrity. Here, the health of the Ottawa River and Ashtabula River (both within the Lake Erie Basin) were assessed using morphological and immunological biomarkers in brown bullheads (Ameiurus nebulosus) and largemouth bass (Micropterus salmoides). Biomarker metrics were compared to fish collected from a reference site (Conneaut Creek). Data utilized for analyses were collected between 2003 and 2011. Fish collected from all three river systems had markedly different contaminant profiles. Total PCBs were the dominant contaminant class by mass. In bullhead, PCBs were highest in fish from the Ashtabula River and there were no differences in fish collected pre- or post-remediation of Ashtabula Harbor (median = 4.6 and 5.5 mg/kg respectively). Excluding PCBs, the Ottawa River was dominated by organochlorine pesticides. Liver tumor prevalence exceeded the 5% trigger level at both the Ashtabula (7.7%) and Ottawa Rivers (10.2%), but was not statistically different than that at the reference site. There was no statistically significant association between microscopic lesions, gross pathology and contaminant body burdens. Collectively, contaminant body burdens were generally negatively correlated with functional immune responses including bactericidal, cytotoxic-cell and respiratory burst activity in both species. Exceptions were positive correlations of HCB and heptachlor epoxide with respiratory burst activity in largemouth bass, and HCB with respiratory burst activity in bullhead and ΣBHC for all three functional assays in bullhead. Data here provide additional support that organochlorine contamination is associated with immunomodulation, and that species differences exist within sites.     

Growth,Reproduction, Mortality,Distribution, and Biomass of Freshwater Drum in Lake Erie

Predominant age-groups in the Lake Erie freshwater drum Aplodinotus grunniens population were 3, 4, and 5 as determined from gill net, trap net, bottom trawl, and midwater trawl samples. Age and growth calculations indicated that females grew faster than males. However, the length-weight relation did not differ between sexes and was described by the equation: log W = ?5.4383 + 3.1987 log L. Some males became sexually mature at age 2 and all were mature by age 6. Females matured 1 year later than males. Three sizes of eggs were present in ovaries; the average total number was 127,000 per female for 20 females over a length range of 270 to 478 mm. Seasonal analysis of the ovary-body weight ratio indicated that spawning extended from June to August. A total annual mortality rate of 49% for drum aged 4 through 11 was derived from catch-curve analysis. Freshwater drum were widely distributed throughout Lake Erie in 1977–1979, the greatest concentration being in the western basin. They moved into warm, shallow water (less than 10 m deep) during summer, and returned to deeper water in late fall. Summer biomass estimates for the western basin, based on systematic surveys with bottom trawls, were 9,545 t in 1977 and 2,333 t in 1978.