Metal accumulation in Lake Michigan prey fish: Influence of ontogeny,trophic position,and habitat

Developing an understanding of factors that influence the accumulation and magnification of heavy metals in fish of the Laurentian Great Lakes is central to managing ecosystem and human health. We measured muscle tissue concentrations of heavy metals in Lake Michigan prey fish that vary in habitat use, diet, and trophic position, including alewife, bloater, deepwater sculpin, round goby, rainbow smelt, and slimy sculpin. For each individual, we measured tissue concentrations of four metals (chromium [Cr], copper [Cu], manganese [Mn], and total mercury [THg]), stable isotope ratios for trophic position (δ¹⁵N and δ¹³C), and individual fish attributes (length, mass). Total mercury concentration was positively related to total length and δ¹⁵N. Of all species, round goby had among the greatest increases in mercury per unit growth and was most isotopically distinct from other species. Profundal species (bloater, deepwater sculpin, slimy sculpin) had similar high THg tissue concentrations, possibly due to slower growth due to cold temperatures, whereas other species (alewife, round goby, rainbow smelt) showed more variation in THg. In contrast, other metals (Cr, Cu, Mn) had either a negative or no relationship to total length and δ¹⁵N, suggesting no bioaccumulation or biomagnification. Potential incorporation of mercury by sportfish may thus be related to species, age, diet, trophic position, and habitat of prey fish. Our findings serve as a foundation for understanding how heavy metals accumulate in Lake Michigan food webs and highlight the continued need for management of metal input and cycling in Lake Michigan.     

Feeding selectivity of slimy sculpin Cottus cognatus and deepwater sculpin Myoxocephalus thompsonii in southeast Lake Michigan: Implications for species coexistence

Feeding selectivity was compared between slimy sculpin Cottus cognatus and deepwater sculpin Myoxocephalus thompsonii collected from southeast Lake Michigan during 1999-2002 to evaluate the hypothesis that differential prey selection contributes to long-term coexistence of these species. Study results indicated that slimy and deepwater sculpin select different prey types and sizes. Selection for the shrimp-like crustacean Mysis diluviana was consistently greater for deepwater sculpin than for slimy sculpin, whereas selection for the amphipod Diporeia spp. was higher for slimy sculpin than for deepwater sculpin when Mysis was the only other available prey type. Slimy sculpin also exhibited higher selectivity for chironomids (order Diptera, family Chironomidae) compared with deepwater sculpin. Patterns in food resource partitioning were consistent between sampling periods covering different locations, seasons and depths, as well as between locations with varying levels of Diporeia availability. This consistency suggests (1) that differences in food use by the two species are associated with intrinsic differences in food preferences or feeding behaviors and (2) that Diporeia declines had not fundamentally altered the resource partitioning dynamics of the two species as of 2002. The results also indicated that slimy and deepwater sculpin can partition food resources on the basis of prey size since deepwater sculpin tended to select larger Diporeia than slimy sculpin. Differences in prey selection may mediate competitive interactions between slimy and deepwater sculpin directly by reducing diet overlap in areas of sympatry or indirectly by causing these fish to select different depth habitats.     

Occurrence of Cyathocephalus truncatus (Cestoda) in Fishes of the Great Lakes with Emphasis on its Occurrence in Round Gobies (Neogobius melanostomus) from Lake Huron

Cyathocephalus truncatus is a pathogenic cestode that is common in many Laurentian Great Lakes fish species, but the depth distribution of this cestode has not been studied. Cyathocephalus truncatus has been reported from 21 fish species and one hybrid representing seven orders and nine families in Lakes Superior, Michigan, Huron, and Ontario. We examined the intestinal contents of six species of fish collected in Michigan waters of Lake Huron, from DeTour to Harbor Beach, in 2001 to 2003 for the presence of this cestode species. Cyathocephalus truncatus was found in five native fish species and the exotic round goby. Prevalence (52%) and mean intensity (4.6 cestodes per infected fish) were highest in bloaters. This is the first reported occurrence of this cestode in round gobies from the Great Lakes. None of the gobies trawled from Lake Huron at depths of 27 to 46 m were infected, but prevalence and intensity of infection in round gobies increased significantly with depth from 55 to 73 m. Our diet study of round gobies indicated that they preyed heavily on amphipods (Diporeia hoyi) at depths of 55 to 73 m. Cyathocephalus truncatus was found in eight of 605 D. hoyi obtained by Ponar grab sampling. This suggests that C. truncatus eggs may be released from infected gobies and sink to deep basins with silt bottoms where D. hoyi occur.     

Additional deepwater sampling of prey fish in Lake Michigan annual bottom trawl survey reveals new insights for depth distribution dynamics

Beginning in 2013, sites at the 128-m bottom depth contour were added to the sampling design of the annual Lake Michigan bottom trawl survey for prey fish, which has been conducted by the U.S. Geological Survey Great Lakes Science Center (GLSC) each fall since 1973, to better assess fish depth distributions in a changing ecosystem. The standard sampling design included bottom depths from 9 to 110 m, although the GLSC also sporadically conducted bottom trawl tows at the 128-m bottom depth contour during 1973–1988. Enactment of this new sampling design in 2013 revealed that mean biomass density of deepwater sculpins (Myoxocephalus thompsonii) at the 128-m depth exceeded the sum of mean biomass densities at shallower depths, indicating that the bulk of the deepwater sculpin population is residing in waters deeper than 110 m. Thus, our findings supported the hypothesis that the depth distribution of the deepwater sculpin population had shifted to deeper waters beginning in 2007, thereby explaining, at least in part, the marked decline in deepwater sculpin abundance since 2006 based on the standard sampling design. In contrast, our results did not support the hypothesis that the slimy sculpin (Cottus cognatus) population had shifted to deeper waters sometime after 2000. A portion of the burbot (Lota lota) population may have also shifted in depth distribution to waters deeper than 110 m after 2007, based on our results. Our findings have served as an impetus to further expand the range of depths sampled in our bottom trawl survey.     

Round Goby and Mottled Sculpin Predation on Lake Trout Eggs and Fry: Field Predictions from Laboratory Experiments

The accidental introduction of round gobies (Neogobius melanostomus) into the North American Great Lakes has raised concerns about their potential impacts on local fauna. Gobies have similar habitat and spawning requirements to mottled sculpins (Cottus bairdi) and slimy sculpins (C. cognatus), and may already be displacing sculpins where the ranges of the species overlap. Like sculpins, gobies are capable of penetrating interstitial spaces to acquire food, and therefore may become predators of interstitially incubating lake trout eggs. Laboratory experiments were conducted to compare egg consumption rates and critical size (the minimum size at which a fish was capable of ingesting an egg) between round gobies and mottled sculpins. Predation by both species on lake trout eggs and fry was also examined in two grades of substrate (cobble and gravel). Mottled sculpins consumed larger numbers of eggs than round gobies of similar size, and were capable of ingesting eggs at smaller sizes than gobies. Both gobies and sculpins had lower foraging success on smaller substrates (gravel) than on cobble. Gobies are currently present at higher densities than sculpins in areas where they are established in the Great Lakes. The similar predation of lake trout eggs by round gobies and mottled sculpin and high densities the goby has achieved at some Great Lakes sites leads to the prediction that the round goby may negatively affect lake trout reproduction and therefore rehabilitation.     

Sculpin Community Dynamics in Lake Michigan

Two hypotheses have been proposed to explain the dynamics of sympatric populations of deepwater sculpin (Myoxocephalus thompsonii) and slimy sculpin (Cottus cognatus). The first hypothesis is that slimy sculpins negatively affect survival of deepwater sculpins, and therefore deepwater sculpins coexist with slimy sculpins only when a keystone predator, lake trout (Salvelinus namaycush), is abundant. According to the second hypothesis, changes in the abundances of the sculpins are driven by interactions with fishes other than sculpins. To evaluate both hypotheses, we applied regression analyses to long-term observations on abundances of both sculpin populations in Lake Michigan during 1973–2002. For slimy sculpin abundance, we considered the predation effect by lake trout and the effect of deepwater sculpins on slimy sculpins. For deepwater sculpin abundance, we considered the effect of alewife (Alosa pseudoharengus) on deepwater sculpins, the predation effect by burbot (Lota lota), and the effect of slimy sculpins on deepwater sculpins. An information criterion was used to select the best regression model explaining the temporal trends. The best model to explain trends in slimy sculpin abundance was the model that included the lake trout predation term only. The best model to explain trends in deepwater sculpin abundance was a model including the alewife and burbot predation terms. Thus, a negative effect of slimy sculpins on deepwater sculpins was not essential in capturing the sculpin community dynamics. Therefore, our results supported the second hypothesis. Further, our results supported the contention that control of the alewife population was a prerequisite for restoration of deepwater sculpin populations.     

Gastric evacuation rate,index of fullness,and daily ration of Lake Michigan slimy (Cottus cognatus) and deepwater sculpin (Myoxocephalus thompsonii)

Accurate estimates of fish consumption are required to understand trophic interactions and facilitate ecosystem-based fishery management. Despite their importance within the food-web, no method currently exists to estimate daily consumption for Great Lakes slimy (Cottus cognatus) and deepwater sculpin (Myoxocephalus thompsonii). We conducted experiments to estimate gastric evacuation (GEVAC) and collected field data from Lake Michigan to estimate index of fullness [(g prey/g fish weight)100%) to determine daily ration for water temperatures ranging 2–5 °C, coinciding with the winter and early spring season. Exponential GEVAC rates equaled 0.0115/h for slimy sculpin and 0.0147/h for deepwater sculpin, and did not vary between 2.7 °C and 5.1 °C for either species or between prey types (Mysis relicta and fish eggs) for slimy sculpin. Index of fullness varied with fish size, and averaged 1.93% and 1.85% for slimy and deepwater sculpins, respectively. Maximum index of fullness was generally higher (except for the smallest sizes) for both species in 2009–2010 than in 1976 despite reductions in a primary prey, Diporeia spp. Predictive daily ration equations were derived as a function of fish dry weight. Estimates of daily consumption ranged from 0.2 to 0.8% of their body weight, which was within the low range of estimates from other species at comparably low water temperatures. These results provide a tool to estimate the consumptive demand of sculpins which will improve our understanding of benthic offshore food webs and aid in management and restoration of these native species in the Great Lakes.     

Spatial Patterns in PCB Concentrations of Lake Michigan Lake Trout

Most of the PCB body burden in lake trout (Salvelinus namaycush) of the Great Lakes is from their food. PCB concentrations were determined in lake trout from three different locations in Lake Michigan during 1994–1995, and lake trout diets were analyzed at all three locations. The PCB concentrations were also determined in alewife (Alosa pseudoharengus), rainbow smelt (Osmerus mordax), bloater (Coregonus hoyi), slimy sculpin (Cottus cognatus), and deepwater sculpin (Myoxocephalus thompsoni), five species of prey fish eaten by lake trout in Lake Michigan, at three nearshore sites in the lake. Despite the lack of significant differences in the PCB concentrations of alewife, rainbow smelt, bloater, slimy sculpin, and deepwater sculpin from the southeastern nearshore site near Saugatuck (Michigan) compared with the corresponding PCB concentrations from the northwestern nearshore site near Sturgeon Bay (Wisconsin), PCB concentrations in lake trout at Saugatuck were significantly higher than those at Sturgeon Bay. The difference in the lake trout PCB concentrations between Saugatuck and Sturgeon Bay could be explained by diet differences. The diet of lake trout at Saugatuck was more concentrated in PCBs than the diet of Sturgeon Bay lake trout, and therefore lake trout at Saugatuck were more contaminated in PCBs than Sturgeon Bay lake trout. These findings were useful in interpreting the long-term monitoring series for contaminants in lake trout at both Saugatuck and the Wisconsin side of the lake.     

Factors governing the distribution and fish-community associations of the round goby in Michigan tributaries of the Laurentian Great Lakes

The round goby (Neogobius melanostomus) is increasingly being reported in tributaries of the Laurentian Great Lakes where these fish have been shown to adversely impact native stream biota. Determining the characteristics and distribution of invaded streams are the first steps toward effective round goby management. We sampled 30 tributaries in the Great Lakes basin and characterized each in terms of nine physical reach-scale attributes. Round goby were detected in 14 streams where abundances ranged from 4% to 53% of the fish sampled in each stream. Round goby was the single most abundant fish species sampled, constituting 14% of all fish encountered across all sites, and 30% of individuals in round goby-present sites. Round goby-present sites were larger, had lower channel slopes, less large wood, and less canopy cover than round goby-absent sites, suggesting that these attributes may promote round goby establishment. Mottled sculpin, cyprinids, brook stickleback, white sucker and rainbow trout were associated with goby absence while centrarchids, percids, yellow bullhead, and mud minnow were associated round goby presence. Collectively these results demonstrate that round goby are widespread in eastern Michigan tributaries to the Great Lakes, present in streams with a range of physical habitat characteristics, and that round goby presence is associated with certain fish species.     

A short-term look at potential changes in Lake Michigan slimy sculpin diets

Diporeia hoyi and Mysis relicta are the most important prey items of slimy sculpins (Cottus cognatus) in the Great Lakes. Slimy sculpins were collected from dreissenid-infested bottoms off seven Lake Michigan ports at depths of 27–73 m in fall 2003 to study their lake-wide diets. Relatively large dreissenid biomass occurred at depths of 37- and 46-m. Quagga mussels (Dreissena bugnesis) composed at least 50% of dreissenid biomass at Manistique, Saugatuck, and Sturgeon Bay. Mysis accounted for 82% of the sculpin diet by dry weight at eastern Lake Michigan while Diporeia composed 54–69% of the diet at western Lake Michigan and dominated the diets of slimy sculpins at all sites deeper than 46 m. In northern Lake Michigan, this diet study in new sites showed that slimy sculpin consumed more prey with low energy contents, especially chironomids, than Mysis and Diporeia in shallow sites (depth <55 m). We recommend diet studies on sedentary benthic fishes to be conducted along perimeters of the Great Lakes to observe changes in their diets that may be impacted by changing benthic macroinvertebrate communities.     

Use of wetland versus open habitats by round gobies in lakes Michigan and Huron: Patterns of CPUE,length, and maturity

It has been suggested that some Great Lakes coastal wetlands may be resistant to invasion by several non-indigenous species including round goby, Neogobius melanostomus. However, there is inconclusive evidence regarding how susceptible exposed fringing coastal wetlands, in particular, are to round goby invasion. Therefore, we quantified round goby catch per unit effort (CPUE) using fyke nets in the Beaver Archipelago of Lake Michigan, and the Les Cheneaux islands and Saginaw Bay regions of Lake Huron. In addition, we examined the influence of body size and maturity on round goby habitat use. Catch per unit effort from fyke nets was highest in the Beaver Archipelago, where wetlands were dominated by small, immature round gobies and open water habitats were dominated by large adults. Fyke net catches within Les Cheneaux sites were similar between habitats and differences in size and maturity were not observed. Conversely, very few round goby were captured in wetlands of Saginaw Bay where CPUE was moderate in open water. This indicates that some exposed fringing wetlands in the Great Lakes, specifically those with high productivity, could have a higher degree of resistance to round goby invasion.     

Reappearance of Deepwater Sculpin in Lake Ontario: Resurgence or Last Gasp of a Doomed Population?

Deepwater sculpin (Myoxocephalus thompsonii) were abundant in Lake Ontario in the 1920s and at least common into the 1940s. By the 1960s they were rare and, thereafter, some considered the population extirpated even though a synoptic survey of the lake in 1972 produced three, relatively large (148–165 mm total length, TL), and presumably old, specimens from the northern half of the lake. Deepwater sculpin were absent from annual survey catches in the 1980s and did not reappear until 1996, when three were caught in northern Lake Ontario. Isolated collections of deepwater sculpin continued during 1998–2004. Catches during 1996–2004 included five smaller individuals, 89-118 mm TL. In 2005, catches increased sharply, with 18 deepwater sculpin collected from southern waters and one from northern waters. Moreover, young, small sculpin were dominant in 2005–16 of the 19 sculpins averaged 68 ± 12 mm total length (± 1 s.d.). The young fish observed since 1996 could have originated from reproduction by the small in-lake population, from downstream drift of planktonic larvae from Lake Huron, or both. The presence of juveniles is a clear sign that conditions for survival of young deepwater sculpin are becoming more favorable, perhaps because of reduced abundance of alewife (Alosa pseudoharengus), a pelagic planktivore linked to depression of deepwater sculpin in Lake Michigan, and also low bundances of burbot (Lota lota) and lake trout (Salvelinus namaycush), benthic piscivores     

Trends in Mysis diluviana abundance in the Great Lakes, 2006–2016

With the large Diporeia declines in lakes Michigan, Huron, and Ontario, there is concern that a similar decline of Mysis diluviana related to oligotrophication and increased fish predation may occur. Mysis density and biomass were assessed from 2006 to 2016 using samples collected by the Great Lakes National Program Office's biomonitoring program in April and August in all five Great Lakes. Summer densities and biomasses were generally greater than spring values and both increased with bottom depth. There were no significant time trends during these 10–11 years in lakes Ontario, Michigan, or Huron, but there was a significant increase in Lake Superior. Density and biomass were highest in lakes Ontario and Superior, somewhat lower in Lake Michigan, and substantially lower in Lake Huron. A few Mysis were collected in eastern Lake Erie, indicating a small population in the deep basin of that lake. On average, mysids contributed 12–18% (spring-summer, Michigan), 18–14% (spring-summer, Superior), 30–13% (spring-summer, Ontario), and 3% (Huron) of the total open-water crustacean biomass. Size distributions consisted of two peaks, indicating a 2-year life cycle in all four of the deep lakes. Mysis were larger in Lake Ontario than in lakes Michigan, Superior, and Huron. Comparisons with available historic data indicated that mysid densities were higher in the 1960s–1990s (5 times higher in Huron, 2 times higher in Ontario, and around 40% higher in Michigan and Superior) than in 2006–2016.     

First documentation of spawning by deepwater sculpins in the Great Lakes and potential impacts of round gobies

Deepwater sculpins Myoxocephalus thompsonii, formerly fourhorn sculpin M. quadricornis (marine form), inhabit all five Great Lakes. Deepwater sculpin is prey for lake trout Salvelinus namaycush and feeds on benthos. Populations have been declining with possible reasons being oligotrophication, quagga mussels (Dreissena bugensis), predation, and impacts of round goby Neogobius melanostomus competition or predation on eggs. Deepwater sculpins are thought to spawn during winter based on gravid females and appearance of larval fish in spring, but a male guarding a nest has not been documented. We used remotely operated vehicle technology during December 2015, March 2017, and March 2021 in 70–191 m in Grand Traverse Bay, Lake Michigan to identify the distribution of deepwater sculpins, round gobies, and nests with a guarding male. Round gobies were much more abundant (observations/hr = 139) in water < 125 m than were deepwater sculpins (11/hr), while in water > 166 m, deepwater sculpins were much more common (39/hr) than at shallow sites. At deep sites, round gobies were more abundant (51/hr) than deepwater sculpins (39/hr). On 16 March 2021 in 190 m of 1.9 C surface water we discovered several presumptive, unoccupied nests, but one nest in a depression next to a branch contained a deepwater sculpin and a clump of eggs. This is first evidence of where and how deepwater sculpins spawn. We also documented round gobies in the same area with one within 2 m of the nest, creating the possibility for consumption of eggs potentially affecting their long-term survival.     

Metazoan Parasites of Introduced Round and Tubenose Gobies in the Great Lakes: Support for the “Enemy Release Hypothesis”

Recent invasion theory has hypothesized that newly established exotic species may initially be free of their native parasites, augmenting their population success. Others have hypothesized that invaders may introduce exotic parasites to native species and/or may become hosts to native parasites in their new habitats. Our study analyzed the parasites of two exotic Eurasian gobies that were detected in the Great Lakes in 1990: the round goby Apollonia melanostoma and the tubenose goby Proterorhinus semilunaris. We compared our results from the central region of their introduced ranges in Lakes Huron, St. Clair, and Erie with other studies in the Great Lakes over the past decade, as well as Eurasian native and nonindigenous habitats. Results showed that goby-specific metazoan parasites were absent in the Great Lakes, and all but one species were represented only as larvae, suggesting that adult parasites presently are poorly-adapted to the new gobies as hosts. Seven parasitic species are known to infest the tubenose goby in the Great Lakes, including our new finding of the acanthocephalan Southwellina hispida, and all are rare. We provide the first findings of four parasite species in the round goby and clarified two others, totaling 22 in the Great Lakes—with most being rare. In contrast, 72 round goby parasites occur in the Black Sea region. Trematodes are the most common parasitic group of the round goby in the Great Lakes, as in their native Black Sea range and Baltic Sea introduction. Holarctic trematode Diplostomum spathaceum larvae, which are one of two widely distributed species shared with Eurasia, were found in round goby eyes from all Great Lakes localities except Lake Huron proper. Our study and others reveal no overall increases in parasitism of the invasive gobies over the past decade after their establishment in the Great Lakes. In conclusion, the parasite “load” on the invasive gobies appears relatively low in comparison with their native habitats, lending support to the “enemy release hypothesis.”     

Laboratory Estimates of Salmonine Egg Predation by Round Gobies (Neogobius melanostomus), Sculpins (Cottus cognatus and C. bairdi), and Crayfish (Orconectes propinquus)

The continued lack of natural reproduction by lake trout (Salvelinus namaycush) in the Great Lakes has prompted development of models focused on the potential negative effects of interstitial predators. To aid in parameterization of such models we investigated the effect of temperature (1–2°C, 4–5°C, 7–8°C, and 10–11°C), predator group size, interspecific competition, egg density (60–6000 eggs m^–2), and an alternate food source on egg consumption by slimy (Cottus cognatus) and mottled (Cottus bairdii) sculpins, round gobies (Neogobius melanostomus), and the native crayfish (Orconectes propinquus) in the laboratory. Egg consumption by the sculpins and round goby all increased over the range of temperatures investigated (ca 0.5 eggs·day^–1 at 1–2°C to 1.5·eggs day^–1 at 10–11°C). Predator group size affected consumption; fewer eggs were eaten per individual slimy sculpin and round goby at densities of 5 or 10 per tank than 1 individual per tank. There was no effect of interspecific competition on egg consumption by slimy sculpins or round gobies at a density of 10 individuals per tank for various species combinations (10:0, 9:1, 5:5, 1:9, 0:10). A type II functional response to egg density was observed for sculpins, gobies, and crayfish although at extreme densities per capita consumption by crayfish and gobies declined. The presence of zebra mussels (Dreissena polymorpha) did not affect the number of rainbow trout (Oncorhynchus mykiss) eggs eaten by round gobies whose mussel consumption averaged 75% BW·day^–1. Of the four species examined, round gobies appeared to be the most effective egg predator while crayfish were the least.     

Phenology and species diversity in a Lake Huron ichthyoplankton community: Ecological implications of invasive species dominance

Ichthyoplankton communities are dynamic and vary spatiotemporally based on factors such as wind, water currents, and phenology. Nonetheless, ichthyoplankton are an indicator of spawning success in fish populations and examining their community diversity and composition can serve to provide information on ecosystem integrity. Although some ichthyoplankton species may be transient, understanding their distribution in space and time provides information on species composition, abundance, and habitat use during critical early life stages. We sampled the spring-summer ichthyoplankton community during 2008 and 2009 in northern Lake Huron to determine species succession, abundance, and species diversity along physical and environmental gradients. Seasonal succession of species was similar during both years, indicating well-defined patterns in spawning by northern Lake Huron fish populations. Invasive alewife, rainbow smelt, and round goby were the dominant species during both years, with native stickleback species also abundant. Shannon Entropy (H′) increased with increasing water temperature until late summer when H′ declined. H′ decreased with increasing bottom depth and distance to tributary mouth indicating the important ecological role of these habitat features during early life stages. Although ichthyoplankton diversity was comparable to or higher than that reported for other areas of the Great Lakes, the prominence of invasive species in our study is reflective of the disturbed state of the Lake Huron fish community, despite large reductions in invasive planktivorous fish since 2004. Continued monitoring of ichthyoplankton communities will be important for measuring the impacts of species invasions or other ecosystem stressors on fish community structure in the Great Lakes.     

Density and habitat use by the round goby (Apollonia melanostoma) in the Bay of Quinte,Lake Ontario

We assessed round goby (Apollonia melanostoma) density and size structure in two sections of the Bay of Quinte (Lake Ontario) that had been invaded by this species two years apart. Round goby density was assessed with 50 m linear transects, recorded with an underwater video recording apparatus developed for this study that included a depth sounder for maintaining a fixed distance above the substrate. The highest mean round goby densities were observed in the shallowest depth zone (1.5–3 m) at both sites, but there were differences between the sites in the habitat types where the highest densities occurred and there were no significant density differences among habitat types at either site (rock with sparse vegetation, mud with sparse vegetation, sand/mud with moderate vegetation cover). In the upper bay, mean body length of round gobies declined with depth, whereas in the lower bay, mean round goby length was greatest in the deepest zone. Mean body length of round gobies did not differ significantly by habitat type in either section of the bay.     

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.     

Survey of four essential nutrients and thiaminase activity in five Lake Ontario prey fish species

Thiamine deficiency is an impediment to salmonine reproduction in the Great Lakes, but little is known about other measures of dietary quality, such as lipid-soluble vitamins or fatty acids in prey fish. The objective of the present research was to measure selected essential nutrients and thiaminase activity in five Lake Ontario prey fish species (alewife Alosa psuedoharengus, rainbow smelt Osmerus mordax, slimy sculpin Cottus cognatus, threespine stickleback Gasterosteus aculeatus and round goby Neogobius melanostomus). Total thiamine was greater in alewife (13.6 nmol/g) than in the other species (6.2–9.0 nmol/g). In 2006, thiaminase activity was unexpectedly high in goby (12.49 nmol/g/min), sculpin (1.99 nmol/g/min) and smelt (9.24 nmol/g/min). In 2007, thiaminase activity in goby (0.99 nmol/g/min) and smelt (4.94 nmol/g/min) was low compared to 2006, whereas sculpin thiaminase activity was greatest (6.01 nmol/g/min). The causes for this variability are unknown. Thiaminase activity was within the expected range for alewife (4.31–6.31 nmol/g/min) and stickleback (0.06 nmol/g/min). Concentrations of retinoids, carotenoids, vitamin E (tocopherol) and fatty acids also differed among prey fish species. Tocopherol concentrations in goby (12.74 ng/mg), sculpin (25.29 ng/mg), and smelt (22.81 ng/mg) were greater than in alewife (1.59 ng/mg). Goby had the lowest ∑ ω-3 to ∑ ω-6 fatty acid ratio (1.44) when compared to sculpin (2.97) and smelt (2.85). Thiaminase concentrations in alewife and smelt (and possibly goby) suggest that they have the potential to adversely affect natural reproduction in salmonines. Concentrations of carotenoids, retinoids and tocopherol in prey fish appear to be lower than salmonine dietary requirements.