Characteristics of a refuge for native freshwater mussels (Bivalvia: Unionidae) in Lake St. Clair

The Lake St. Clair delta (∼ 100 km²) provides an important refuge for native freshwater mussels (Unionidae) wherein 22 of the ∼ 35 historical species co-occur with invasive dreissenids. A total of 1875 live unionids representing 22 species were found during snorkeling surveys of 32 shallow (∼ 1 m) sites throughout the delta. Richness and density of unionids and zebra mussel infestation rates varied among sites from 3 to 13 unionid species, 0.02 to 0.12 unionids/m², and < 1 to 35 zebra mussels/unionid, respectively. Zebra mussel infestation of unionids in the delta appears to be mitigated by dominant offshore currents, which limit densities of zebra mussel veligers in nearshore compared to offshore waters (13,600 vs. 28,000/m³, respectively). Glycogen concentrations in the tissues of a common and widespread species in the delta (Lampsilis siliquoidea) suggest that zebra mussels may be adversely affecting physiological condition of unionids in a portion of the Lake St. Clair delta. Physiological condition and community structure of unionids within the delta may also be influenced by differences in food quantity and quality resulting from the uneven distribution of water flowing from the St. Clair River. The delta likely supports the largest living unionid community in the lower Great Lakes and includes several species that have been listed as Endangered or Threatened in Canada and/or the state of Michigan, making it an important refuge for the conservation of native unionids.     

Lakewide dominance does not predict the potential for spread of dreissenids

In recent years, quagga mussels (Dreissena rostriformis bugensis) have almost completely replaced zebra mussels (Dreissena polymorpha) in the Lower Great Lakes. As recreational boats are the main vector of spread for dreissenids in North America, this study examined whether lakes Erie and Ontario could still be sources for the spread of zebra mussels. In the summer–fall of 2010, the abundance of each species of Dreissena on 196 boats from 5 marinas in lakes Erie and Ontario was examined. Additional samples of Dreissena in 2010–2012 were collected in tributaries, bays, and in the upper littoral zones of these lakes. A total of 77 boats were fouled by Dreissena, and of those 61 were fouled by both species, 13 were fouled just by zebra mussels, and only 3 were fouled solely by quagga mussels. Although quagga mussels compose ~ 99% of dreissenids in eastern Lake Erie and in Lake Ontario, on boats at most marinas sampled, zebra mussels were usually more abundant and significantly larger than quagga mussels. Refugia for zebra mussels were found in bays, tributaries, and upper littoral zones with high wave activity. Thus, although quagga mussels are now more abundant than zebra mussels within the Lower Great Lakes, these waterbodies still have the potential to be a source for the spread of zebra mussels, and for some vectors, the propagule pressure from zebra mussels is likely greater than that from quagga mussels.     

Unionid mussels from nearshore zones of Lake Erie

Concern exists that the introduction of dreissenid mussels following long-term effects of pollution may have completely eliminated native mussel species from Lake Erie. Natural seiche events were used to facilitate surveys for live unionids on five occasions in the western basin of Lake Erie and Sandusky Bay between 2007 and 2009, and beach and estuary surveys were conducted at numerous additional sites between 2004 and 2009. Sixteen unionid species were found living in or near Lake Erie, including six sites in the nearshore zone of the lake. Each community consisted of live individuals from two to eight species, and evidence included live and/or fresh dead material from several state listed species at multiple sites. Where estimated, the mean overall density was low at 0.09 unionids/m², although similar to other known unionid refuges in the lower Great Lakes. While the ephemeral nature of seiche events makes them a limited survey tool, their application combined with increasing numbers of fresh shells washing ashore over the past few years indicates that unionids are extant in the western basin of Lake Erie, and may further suggest that conditions may be improving for native mussel species.     

Assessment of remnant unionid assemblages in a selection of Great Lakes coastal wetlands

Populations of native unionids have been in steady decline over the past century. The invasion of dreissenid mussels (Dreissena polymorpha and Dreissena rostriformis bugensis) in the mid-1980's impacted already imperiled unionid populations by greatly increasing their regional extinction rates. A selection of Great Lakes coastal wetlands around Michigan was surveyed to locate remnant populations of native unionids. Physical and chemical parameters were measured in coastal wetlands to evaluate the importance of these habitat parameters to remnant unionid assemblages. We assessed fouling rates by dreissenids on unionids and used artificial substrates to estimate dreissenid colonization densities. Live unionids were found in coastal wetlands of the Les Cheneaux Islands, the Lake St. Clair delta, and North Maumee Bay with significantly higher unionid fouling in the Les Cheneaux Islands compared to the other two sampling areas (F_2,76 = 4.97, p = 0.0095). No live unionids were documented in Beaver Island, Garden Island, Grand Traverse Bay, or Saginaw Bay wetlands. Dreissena colonization densities on artificial substrates averaged 19,213 m^− 2 at one site in North Maumee Bay, and 10,425 m^− 2 in Saginaw Bay, but no colonization occurred in the wetlands of Beaver Island, Garden Island, the Les Cheneaux Islands, or Grand Traverse Bay while Dreissena presence in the open water of these regions was evident. Dreissena colonization densities on artificial substrates increased with measures of anthropogenic disturbance and decreased with higher water level fluctuations and aerial exposure. Specific conductance, turbidity, and magnitude of water level fluctuations were important predictors of Dreissena colonization on artificial substrates.     

Spatial-temporal patterns of recolonizing adult mayflies in Lake Erie after a major disturbance

This 12-year study of Hexagenia male imagos documents the recovery of two species of burrowing mayflies, Hexagenia limbata and Hexagenia rigida in western Lake Erie after a 30-year absence due to hypoxia, resulting from cultural eutrophication. Annual adult mayfly collections were made at night during the peak emergence period at four sites along the north shore of the western basin of Lake Erie, 1997 to 2008. H. rigida, the dominant species in upstream riverine waterbodies, was the early colonizer, representing about 90% of all male imagos sampled in 1997. In 2000, when the two species were co-dominant, both inland aerial dispersal (5.5 km) and lakeward (0.25 to 4 km) oviposition patterns confirmed species co-existence. Twice weekly collections throughout the extended emergence period at one site confirmed that H. rigida was the dominant species in 1997, H. limbata and H. rigida were co-dominant in 2000, and H. limbata was dominant in 2002. Once H. limbata became the dominant species (> 90%) in 2000 to 2002 (depending on the site), it remained so. Both species followed a similar inland dispersal pattern, decreasing in density with increasing distance from shore; most mayflies were present within 1 km from shore. There was no significant difference in mean egg density of the two species among the sites extending lakeward in 2000 when the two species were equally abundant. The transition from the dominance of H. rigida to H. limbata may have resulted from several factors, including differential competition and growth between species or predation effects.     

The effects of dreissenid mussels on the survival and condition of burrowing mayflies (Hexagenia spp.) in western Lake Erie

Burrowing mayflies (Hexagenia limbata and H. rigida) are once again prominent members of the benthic community in western Lake Erie. However, this community is now dominated by dreissenid mussels. We conducted a laboratory experiment and field sampling to investigate whether survival and condition of Hexagenia were affected by the presence, density, and quality of dreissenid mussels. In a laboratory experiment, Hexagenia survival was higher in microcosms without dreissenid mussels. We also found Hexagenia density to be higher at field sites with low dreissenid density, suggesting that Hexagenia survival is higher in these areas as well. In microcosm treatments with low dreissenid density, Hexagenia survival was higher in treatments with live dreissenids than in treatments containing only dreissenid shells. These findings suggest that while dreissenid shells degrade the quality of soft sediments for Hexagenia, some of the negative effect is offset by the presence of live dreissenids. The positive effect of live dreissenids is likely due to additional food resources made available to Hexagenia by the filtering activity of dreissenids. Neither dreissenid density nor shell “type” (shells alone or live dreissenids in shells) had an effect on Hexagenia condition. Thus, the interactions between these dominant benthic invertebrates are complex. Recovery of Hexagenia populations in western Lake Erie is likely affected by both changing environmental conditions due to anthropogenic activities and the introduction of exotic species into the benthic community. The results are likely to be continued instability of the benthic food web and unpredictable consequences for human utilization of this ecosystem.     

Algal pigments in Lake Erie dreissenids,pseudofeces and sediments,as tracers of diet,selective feeding and bioaccumulation

Algal class biomarkers revealed the importance of diatoms and chlorophytes in the diet of dreissenids (n 18) collected from eastern Lake Erie (2–20 m depths; hard and soft substrate), during summer and fall, from 2003 to 2005. The most prominent biomarkers in dreissenids, typical of August, were fatty acid esters of carotenoids derived from diatoms (monoesters of fucoxanthinol, average 61%) and chlorophytes (diesters of mactraxanthin, average 30%), while non-esterified biomarkers from diatoms, chlorophytes, cryptophytes and cyanobacteria were below 4% of the total. At cool temperatures (June 2003, 13 °C), dreissenids had 84%:12% diatom:chlorophyte biomarkers, but with unseasonably warm temperatures (June 2004, 18 °C) and a nearshore chlorophyte bloom, dreissenids had a biomarker distribution similar to August. Bioconcentration factors in dreissenids relative to phytoplankton from Lake Erie were largest for the biomarkers from diatoms (21 L/g ww) and chlorophytes (29 L/g ww), compared to those from cryptophytes (2 L/g ww) and cyanobacteria (3 L/g ww). Unlike dreissenids (2003), matching pseudofeces (n 6) and sediments (n 16) contained a relatively large percentage of biomarkers for cryptophytes (20% June, 27% August) and cyanobacteria (31% August), suggesting that their low levels in dreissenids represent ingested feces, which are being rejected. Increased shell size (12–27 mm; surrogate for age; 3 sets) corresponded to an increase in diatom biomarkers (from 60 to 73%) but a decrease in chlorophyte biomarkers (from 31 to 17%); biomarker concentrations also decreased significantly (−3000 pmol/g ww/mm) in the most offshore collection, near Buffalo (Oct. 2005).     

Size-selective predation by round gobies (Neogobius melanostomus) on dreissenid mussels in the field

Although numerous studies have shown that round gobies (Neogobius melanostomus) prey on dreissenid mussels (Dreissena polymorpha and Dreissena bugensis), there is an apparent shortage of detailed field studies on the subject. The 5-month field study described here quantifies predation by round gobies on dreissenids in Presque Isle Bay, Lake Erie. Dreissenids dominated the diet of round gobies, composing 92% of the prey items recovered. Over half of the 3870 valves (1935 mussels) recovered from 155 round gobies were crushed, while the remainder were swallowed whole. Crushed dreissenids were larger than those swallowed whole, and the tendency to crush dreissenids did not vary among three length classes of round goby. Round goby length was positively related to average size of dreissenids consumed, average size of whole and crushed dreissenids, largest whole dreissenid consumed, and largest crushed dreissenid consumed. Indices of selectivity revealed similarly shaped curves for three length classes of round gobies, a shift toward larger dreissenid size classes with an increase in round goby length, and peak preferences for 8-11 mm dreissenids. Factors such as gape limitation, availability and accessibility of differently sized dreissenids, forces generated while removing mussels from the substrate and crushing them, and caloric content of dreissenids all likely play roles in the observed size-selectivity and differential processing of dreissenids. Although factors influencing size-selectivity are not completely understood, the observed preference of round gobies for dreissenids near the size when they are first reproductive could impact the demography of dreissenid populations.     

Occurrence of the Asiatic Clam Corbicula Fluminea in the Maumee River and Western Lake Erie

The Asiatic clam was found in only two locations along the southern shore of the western basin of Lake Erie: the thermal plume areas of two coal-fired power plants with once-through condenser cooling systems. Three sewage treatment plant outfalls were sampled, but Corbicula were found only in the sewage treatment plant outfall that was within the thermal plume of a power plant. No Corbicula were found in the vicinity of the nuclear power plant which has a closed-cycle natural draft cooling tower, and hence no significant thermal plume.From the age of some of the specimens, this clam has probably resided in the region since 1978. The Great Lakes are farther north than the natural range of this clam, and prolonged cold temperatures in this region may be responsible for the confinement of this clam to baseload power plants with continuous thermal plumes.     

Decadal changes in the benthic invertebrate community in western Lake Erie between 1981 and 2004

Faunal surveys of western Lake Erie showed a dramatic change in the species composition and abundance of the invertebrate mud-bottom community. Abundances of formerly dominant ostracods, tubificid and naidid oligochaetes, and chironomids declined 85%, 87%, 80%, and 72%, respectively, between 1982 and 2003. The majority of the declines occurred between 1982 and 1993, when dreissenid abundances were increasing rapidly. Hexagenia nymphs became important members of the infaunal community after 1993. Faunal declines were uncorrelated to weather-related variables. The frequency and magnitude of thermal stratification with accompanying hypoxic conditions during the study period remained within the normal range of variation observed in western Lake Erie. Significant wind events and storm driven seiches that might cause benthos mortality were not correlated with the decadal patterns of faunal decline. Nor were the faunal declines associated with the historical decrease in organic loading, because most of the decrease occurred prior to the study period. Nearly all abundant species declined significantly between 1982 and 1993, but deposit-feeding fauna, eutrophic indicator species, small organisms, and nearshore mud-bottom stations adjacent to dreissenid mussel habitat were most severely affected. These patterns suggest that a systemic change in the trophic structure of western Lake Erie occurred, due mostly to the invasion of dreissenid mussels nearshore and an accompanying diversion of organic matter away from deeper offshore muds. Trophic group amensalism may operate differently in marine and freshwater habitats, although dreissenids may exert ecosystem effects in the Great Lakes similar to oysters in Chesapeake Bay.     

Twenty five years of changes in Dreissena spp. populations in Lake Erie

Lake Erie has the longest history of colonization by both Dreissena polymorpha and Dreissena rostriformis bugensis in North America and is therefore optimal for the study of long-term dynamics of dreissenid species. In addition, the morphometry of Lake Erie basins varies dramatically from the shallow western to the deep eastern basin, making this waterbody a convenient model to investigate patterns of Dreissena distribution, as well as interspecies interactions among dreissenids. We compare our data on the distribution, density and wet biomass of both dreissenid species in Lake Erie collected in 2009 and 2011–2012 with previous data. We found that Dreissena spp. distribution in Lake Erie varied depending on the time since the initial invasion, collection depth, and lake basin. In 2009–2012, zebra mussels were smaller than in 1992 and were consistently smaller than quagga mussels. During 2009–2012, quagga mussels were found at all depths and in all basins, while zebra mussels were common in the western basin only, and in the central and eastern basins were limited to shallow depths, resulting in an almost complete replacement of D. polymorpha with D. rostriformis bugensis. In the shallowest western basin of Lake Erie, zebra mussels represented > 30% of the combined dreissenid density even after more than 20 years of coexistence, providing strong evidence that, even in lakes as large as Lake Erie (or at least its western basin), D. polymorpha may sustain a significant presence for decades without being displaced by quagga mussels.     

Thermal and hydrologic suitability of Lake Erie and its major tributaries for spawning of Asian carps

Bighead carp Hypophthalmichthys nobilis, silver carp H. molitrix, and grass carp Ctenopharyngodon idella (hereafter Asian carps) have expanded throughout the Mississippi River basin and threaten to invade Lakes Michigan and Erie. Adult bighead carp and grass carp have been captured in Lake Erie, but self-sustaining populations probably do not exist. We examined thermal conditions within Lake Erie to determine if Asian carps would mature, and to estimate time of year when fish would reach spawning condition. We also examined whether thermal and hydrologic conditions in the largest tributaries to western and central Lake Erie were suitable for spawning of Asian carps. We used length of undammed river, predicted summer temperatures, and predicted water velocity during flood events to determine whether sufficient lengths of river are available for spawning of Asian carps. Most rivers we examined have at least 100 km of passable river and summer temperatures suitable (> 21 C) for rapid incubation of eggs of Asian carps. Predicted water velocity and temperature were sufficient to ensure that incubating eggs, which drift in the water column, would hatch before reaching Lake Erie for most flood events in most rivers if spawned far enough upstream. The Maumee, Sandusky, and Grand Rivers were predicted to be the most likely to support spawning of Asian carps. The Black, Huron, Portage, and Vermilion Rivers were predicted to be less suitable. The weight of the evidence suggests that the largest western and central Lake Erie tributaries are thermally and hydrologically suitable to support spawning of Asian carps.     

Coexistence of the native benthic amphipod Diporeia spp. and exotic dreissenid mussels in the New York Finger Lakes

Populations of the benthic amphipod Diporeia spp. have sharply declined since the early 1990s in all North America's Great Lakes except Lake Superior. The onset and continued decline coincides with the invasion of these lakes by zebra (Dreissena polymorpha) and quagga (Dreissena rostriformis bugensis) mussels and the spread of quagga mussels to deep habitats. The six deepest Finger Lakes of central New York (Seneca, Cayuga, Skaneateles, Canandaigua, Keuka, and Owasco) have historically been Diporeia habitat and have had dreissenids for more than a decade. These lakes represent a wide range of trophic state, maximum depth, and dreissenid invasion history. We hypothesized that Diporeia abundance would be negatively impacted by dreissenid mussel expansion in the Finger Lakes. During 2006–2010, we sampled Diporeia and mussel populations in these six lakes. Diporeia was present in all six lakes, and was abundant (2000/m²) in Owasco Lake that has only zebra mussels and in Cayuga and Seneca Lakes that have had zebra and quagga mussels since 1994. Diporeia abundance was lowest (1000/m²) in Skaneateles, Canandaigua, and Keuka Lakes where quagga mussels have recently expanded. Productivity indicators explained much of the variability of Diporeia abundance. The persistence of Diporeia with quagga mussels in these lakes may be because of available alternative food resources. Fatty acid tracers indicate that Diporeia from Owasco Lake, the lake without quagga mussels, utilize diatoms, but Diporeia from Cayuga Lake that coexist with abundant quagga mussels also use food resources associated with terrestrial detritus that cannot be intercepted by dreissenids.     

Re-engineering the eastern Lake Erie littoral food web: The trophic function of non-indigenous Ponto-Caspian species

The trophic roles of key Ponto-Caspian invaders (quagga mussels Dreissena bugensis, amphipods Echinogammarus ischnus and round goby Apollonia melanostomus) within the littoral food web of eastern Lake Erie were quantified using stable isotopes (δ¹³C, δ¹⁵N). A dual stable isotope parameter search with a mass balance component was used to assess the isotopic importance of quagga mussels and amphipods as dietary items to two size classes of round goby. The utility of the mass balance simulation was also evaluated as a tool to approximate isotopic contributions of feasible prey and identify gaps incurred by “missing” prey items not included in the sampling. The mass balance dietary simulation, confirmed by stomach content data, indicated that isotopically important prey to small round goby (< 11.2 cm) were chironomids and Ponto-Caspian amphipods, while large round goby (≥ 11.2 cm) showed strong preference for quagga mussels. The dietary mass balance simulation output also supported the isotopic importance of round goby to the somatic growth of smallmouth bass, rock bass and freshwater drum. The isotopic mass balance output for yellow perch was more ambiguous, which may be in line with their known broadly omnivorous diet. The white bass output was in line with published data indicating increasing consumption of round goby for this species, while the brown trout output strongly favoured alewife isotopic contributions. However for white perch and walleye, the mass balance simulations were not in line with their known published diets in Lake Erie, probably due to a lack of key prey items in the sample set (e.g. zooplankton for white perch and shiner species for walleye). As expected, the Ponto-Caspian species have integrated themselves into the littoral food webs, and the “quagga mussel–round goby–smallmouth bass” food chain forms one of the key components within the trophodynamics of Lake Erie.     

Is reduced benthic flux related to the Diporeia decline? Analysis of spring blooms and whiting events in Lake Ontario

Benthic monitoring by USGS off the southern shore of Lake Ontario from October 1993 to October 1995 provides a detailed view of the early stages of the decline of the native amphipod Diporeia. A loss of the 1994 and 1995 year classes of Diporeia preceded the disappearance of the native amphipod at sites near Oswego and Rochester at depths from 55 to 130 m. In succeeding years, Diporeia populations continued to decline in Lake Ontario and were nearly extirpated by 2008. Explanations for Diporeia's decline in the Great Lakes include several hypotheses often linked to the introduction and expansion of exotic zebra and quagga mussels (Dreissena sp.). We compare the timeline of the Diporeia decline in Lake Ontario with trends in two sources of organic matter to the sediments — spring diatom blooms and late summer whiting events. The 1994–95 decline of Diporeia coincided with localized dreissenid effects on phytoplankton in the nearshore and a year (April 1994 to May 1995) of decreased flux of organic carbon recorded by sediment traps moored offshore of Oswego. Later declines of profundal (> 90 m) Diporeia populations in 2003 were poorly associated with trends in spring algal blooms and late summer whiting events.     

Distribution and abundance of freshwater polychaetes, Manayunkia speciosa (Polychaeta), in the Great Lakes with a 70-year case history for western Lake Erie

Manayunkia speciosa has been a taxonomic curiosity for 150 years with little interest until 1977 when it was identified as an intermediate host of a fish parasite (Ceratomyxa shasta) responsible for fish mortalities (e.g., chinook salmon). Manayunkia was first reported in the Great Lakes in 1929. Since its discovery, the taxon has been reported in 50% (20 of 40 studies) of benthos studies published between 1960 and 2007. When found, Manayunkia comprised < 1% of benthos in 70% of examined studies. In one extensive study, Manayunkia occurred in only 26% of 378 sampled events (1991–2009). The taxon was found at higher densities in one area of Lake Erie (mean = 3658/m²) and Georgian Bay (1790/m²) than in five other areas (mean = 60 to 553/m²) of the lakes. A 70-year history of Manayunkia in western Lake Erie indicates it was not found in 1930, was most abundant in 1961 (mean = 8039, maximum = 67,748/m²), and decreased in successive periods of 1982 (3529, 49,639/m²), 1993 (1876, 25,332/m²), and 2003 (79, 2583/m²). It occurred at 48% of stations in 1961, 58% in 1982, 52% in 1993, and 6% of stations in 2003. In all years, Manayunkia was distributed primarily near the mouth of the Detroit River. Causes for declines in distribution and abundance are unknown, but may be related to pollution-abatement programs that began in the 1970s, and invasion of dreissenid mussels in the late-1980s which contributed to de-eutrophication of western Lake Erie. At present, importance of the long-term decline of Manayunkia in Lake Erie is unknown.     

Beach sediment magnetism and sources: Lake Erie,Ontario, Canada

Measurements were made along the northwestern shore of Lake Erie, Canada to determine whether grain magnetic properties can be used to identify and distinguish sources of beach sediment. Although surface magnetic susceptibilities were highly variable, ranging from 56 to 9867 × 10⁻⁵ SI (Bartington MS2D), there was generally a gradual increase from the low beach (near the waterline) towards the high beach; there were also narrow, shore-parallel bands with high susceptibility at various points on the beach surface. Magnetic mineralogy on the beaches was dominated by low-Ti magnetite (570° < Tc < 580 °C), and the effective grain-size varied from pseudosingle domain in the low beach to multidomain on the high beach. Sandy bluff sediments in the eastern part of the study area had magnetic properties (e.g. S-ratios, hysteresis loops, thermomagnetic curves) that were similar to those on the beaches, whereas the magnetic properties of the extensive till bluffs and river basin sediments were quite different. The data suggested that, whereas the beaches in the western part of the study area are supplied with sediment from bluffs several tens of kilometres to the east, the source of the high magnetic concentrations on the eroding beaches of eastern Point Pelee remains to be determined.     

Suitable habitat model for walleye (Sander vitreus) in Lake Erie: Implications for inter-jurisdictional harvest quota allocations

Models predicting habitat distributions can give insight into species–habitat requirements and anticipate how populations respond to environmental change. Despite the economic and ecological importance of walleye (Sander vitreus) in Lake Erie, no preferred-habitat model exists and the spatial extent of suitable habitat is poorly understood. Empirical species-habitat models for three groups of walleye (juveniles, adults, and all walleye) was developed using records from a long term gill net data base (21 years). We examined the degree to which habitat suitability varies with vertical stratum for each group and whether the new model yields different estimates of available walleye habitat when compared to the current depth-based approach. Walleye occurrence in gill nets was positively related to water temperature, negatively related to water depth and water clarity, and unrelated to dissolved oxygen concentration. A model that incorporated interaction terms among the independent variables performed better than the linear, quadratic, and cubic generalized linear models (GLMs) for all three groups. Our results indicate that the extent of suitable habitat varies spatially in Lake Erie and is greatest in the West basin. Weighted Habitat Suitability Areas (WHSA), a combination of habitat quality and quantity, differed significantly among basins and vertical strata in Lake Erie. The current quota allocation strategy for Lake Erie walleye is based on the proportional amount of preferred habitat by jurisdiction. However, the current depth-based definition of preferred habitat may not be an adequate representation of walleye suitable habitat shared by each jurisdiction.     

Summer Food Habits of Pumpkinseeds (Lepomis gibbosus) and Bluegills (Lepomis macrochirus) in Presque Isle Bay,Lake Erie

Zebra mussels (Dreissena polymorpha) and quagga mussels (D. bugensis) have received much attention since they were first reported in the Great Lakes. Predation by fishes may be an important factor in regulating dreissenid populations, but the extent to which fish prey on them is not entirely clear. Pumpkinseeds (Lepomis gibbosus) are known to be effective predators of mollusks, but bluegills (Lepomis macrochirus) do not generally prey heavily on mollusks. Analysis of stomach contents of pumpkinseeds and bluegills collected from Presque Isle Bay of Lake Erie (Erie, Pennsylvania) revealed considerable differences in the diets of the two species. Specifically, dreissenids were the most important food item in the diet of pumpkinseeds and composed 63% of the volume of their diet. In contrast, dreissenids were relatively unimportant in the diet of bluegills and composed only 2.3% of the volume of their diet. Although bluegills and pumpkinseeds differed considerably in their consumption of dreissenids and other prey items, no clear differences in PCB concentrations were detected between the two species. Thus bluegills must acquire most of their PCBs from prey other than dreissenids.