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Energy content is an important determinant of an individual fish's condition and a key input variable for bioenergetics applications. Energy content of young fish can change rapidly during ontogeny; given the numerical abundance and high mass-specific metabolic rates of young fish, it is particularly important to obtain detailed information on their energy content. To this end, we quantified the total body energy content and energy density of economically and ecologically important young (age-0–1) nonnative alewives Alosa pseudoharengus collected during 1998–2003 in eastern Lake Michigan and Muskegon Lake, a connected drowned river mouth lake. Our analysis demonstrates that energy content of young alewives varies across years, seasons, lakes, and ontogeny. Consistent with previous observations of size-dependent overwinter mortality, young alewives deplete a large amount of body energy between late fall and late spring (i.e., declines in length-specific energy content and energy density occur over...

Fish maturation schedules vary greatly among systems and over time, reflecting both plastic and adaptive responses to ecosystem structure, physical habitats, and mortality (natural and fishing). We examined maturation schedules of commercially exploited lake whitefish ( Coregonus clupeaformis ) in the Laurentian Great Lakes (Lakes Michigan, Huron, and Superior) by estimating ages and lengths at 50% maturity, age-specific maturity ogives (age-specific probability of being mature), and probabilistic maturation reaction norms (PMRNs; a metric that accounts for effects of growth and mortality). Collectively, these estimates indicated variation in maturation schedules between sexes (i.e., males tend to mature at younger ages and shorter lengths than females) and among systems (midpoint estimates of PMRNs were smallest for Lake Michigan fish, intermediate for fish in the main basin of Lake Huron, and largest for fish in Lake Huron’s Georgian Bay and Lake Superior). Temporally, recent increases in age at 50% maturity in Lakes Huron and Michigan may primarily reflect plastic responses to decreased growth rates associated with ecosystem changes (e.g., declines of the native amphipod, Diporeia spp.). As plastic and adaptive changes in maturation schedules of fish stocks may occur simultaneously and require different management considerations, we recommend the concomitant analysis of multiple maturation indices.

We evaluated bioelectrical impedance analysis (BIA) as a nonlethal means of predicting energy density and percent lipids for three fish species: Yellow perch Perca flavescens, walleye Sander vitreus, and lake whitefish Coregonus clupeaformis. Although models that combined BIA measures with fish wet mass provided strong predictions of total energy, total lipids, and total dry mass for whole fish, including BIA provided only slightly better predictions than using fish mass alone. Regression models that used BIA measures to directly predict the energy density or percent lipids of whole fish were generally better than those using body mass alone (based on Akaike's information criterion). However, the goodness of fit of models that used BIA measures varied widely across species and at best explained only slightly more than one-half the variation observed in fish energy density or percent lipids. Models that combined BIA measures with body mass for prediction had the strongest correlations between pred...

The identification of important spawning and nursery habitats for fish stocks can aid fisheries management, but is complicated by various factors, including annual variation in recruitment success. The alewife ( Alosa pseudoharengus ) is an ecologically important species in Lake Michigan that utilizes a variety of habitats for spawning and early life growth. While productive, warm tributary mouths (connected to Lake Michigan) may contribute disproportionately more recruits (relative to their habitat volume) to the adult alewife population than cooler, less productive nearshore habitats, the extent of interannual variation in the relative contributions of recruits from these two habitat types remains unknown. We used an individual-based bioenergetics simulation model and input data on daily temperatures to estimate alewife recruitment to the adult population by these different habitat types. Simulations suggest that nearshore lake habitats typically produce the vast majority of young alewife recruits. However, tributary habitats may contribute the majority of alewife recruits during years of low recruitment. We suggest that high interannual variation in the relative importance of habitats for recruitment is a common phenomenon, which should be considered when developing habitat management plans for fish populations.

During the summer of 2006, the western basin of Lake Erie experienced a bloom of the toxigenic cyanobacterium Microcystis aeruginosa . Across 11 sites, intracellular, particulate-bound microcystin levels in the seston increased to levels that exceeded World Health Organization guidelines for drinking water exposure (1 µg toxin·L–1). In contrast, toxin concentrations in yellow perch ( Perca flavescens ) muscle tissue (n = 68) declined from June to August, were negatively related to algal toxin levels, and never exceeded a conservative chronic exposure concentration estimated using proposed United States Environmental Protection Agency (US EPA) guidelines. Microcystin concentrations in yellow perch liver exceeded US EPA chronic exposure guidelines, were on average 125 times higher than muscle toxin concentrations per unit dry weight, and varied little throughout the summer. With current guidelines, humans do not appear to be at risk when consuming the muscle tissue of Lake Erie yellow perch collected during large-scale cyanobacterial blooms. However, this study highlights the need for a better understanding of the trophic transfer of cyanobacterial toxins through aquatic food webs in diverse ecosystems with an emphasis on understanding if these compounds could accumulate sufficiently to affect human health.

Elucidation of important nursery habitats for young fish can aid in the management and assessment of fish stocks. Herring (Clupea harengus) in the Baltic Sea primarily spawn in coastal areas, but larvae are also present in off-shore, open sea areas. To investigate if sheltered coastal habitats provide a better growth environment for larval herring, we compared short-term growth (as indexed by whole body RNA:DNA ratios) of larval herring from three habitat types of the northwest Baltic proper (sheltered inner bay, exposed outer bay, and open sea). In addition, we compared individual RNA content of adult female Eurytemora affinis (a common Baltic copepod) among these different habitats. High RNA levels in these copepods indicate high production of nauplii, which are important food for larval herring. Both RNA:DNA ratios of larval herring and RNA content of E. affinis were significantly greater in embayment habitats, suggesting that the sheltered coastal areas are high quality nursery habitats for young Baltic herring.

Alewives Alosa pseudoharengus are key components of Laurentian Great Lakes ecosystems and spawn in multiple habitat types. Exploration of alewife early life history dynamics within these different habitats should help identify important recruitment processes. During 2001-2003, we quantified physical (temperature, transparency) and biotic (chlorophyll a, zooplankton densities) habitat factors and collected age-0 alewives (using ichthyoplankton nets and trawls) in a nearshore region of Lake Michigan and Muskegon Lake, Michigan (a drowned river mouth lake connected to Lake Michigan). We characterized alewife hatch dates, individual condition, growth, mortality, and size-dependent overwinter survival to infer differences in habitat-specific recruitment success. Temperature, turbidity, chlorophyll-a concentrations, and densities of zooplankton prey were consistently higher in Muskegon Lake than in nearshore Lake Michigan. On average, young alewives in Muskegon Lake hatched earlier, grew faster, were i...

Natural and human activities can result in both high temporal and spatial variability in water temperature. Rapid temperature changes have the potential to dramatically affect physiological processes in aquatic organisms and, due to their limited mobility, fish early life stages are particularly vulnerable to ambient temperature fluctuations. In this study, we examined how the magnitude and frequency of temperature fluctuations affect survival, growth, development, expression of thermoresponsive genes, and gonadal differentiation in fathead minnows, Pimephales promelas. We exposed individuals (0 to 4 days post fertilization) of known genotypic sex to fluctuations of Δ4 °C over 12-h, Δ8 °C over 12- and 24-h, and three stable temperatures (21, 25, and 29 °C) for up to 45 d. Expression of hsp70 in fish exposed to the highest-magnitude, highest-frequency fluctuating treatment cycled in concert with temperature and was upregulated initially during exposure, and may have contributed to temperature fluctuations having little effect on time to and size at hatching (whole-organism responses). This treatment also caused fish to undergo nondirectional sex reversal. These results indicate that hsp70 may be involved in mediating thermal stress from subdaily temperature fluctuations and that sex determination in fathead minnows can be influenced by cycling temperatures.

The large percids, including Perca and Sander species, are economically and ecologically important species that inhabit large temperature regions of the Northern Hemisphere. In this chapter, we provide an overview of the environmental biology of the Perca (including yellow perch P. flavescens and Eurasian perch P. fluviatilis) and Sander (including walleye S. vitreus, pikeperch S. lucioperca, and sauger S. canadensis) genera, on which the majority of fisheries and aquaculture practices are focused. Through a comprehensive literature review, we discuss how individual- and population-level vital rates, including growth, foraging, reproduction, recruitment, and mortality, are mediated by biotic (e.g., density dependence, resource availability) and abiotic (e.g., temperature, light) environmental variables. As fisheries exploitation is a major source of size-selective mortality in many percid populations, we also examine the potential impacts of fishing mortality on both population metrics and individual vital rates, and identify several research areas that require further investigation. Through this review we aim to identify the major environmental drivers of variation in percid vital rates and thereby inform management practices for both wild and cultured percid populations.

We apply a robotic micromilling technique to the sampling of young-of-the-year (YOY) and 1+ otoliths from alewives (Alosa pseudoharengus) captured in different habitat types of Lake Michigan during 2001–2003. δ18O values of otolith cores of YOY alewives from Muskegon Lake (a sheltered, drowned river mouth lake connected to Lake Michigan), Muskegon Channel (which connects Muskegon Lake to Lake Michigan), and Lake Michigan proper are compared with δ18O values of ambient water from different potential early life habitats. Otolith core δ18O values used in conjunction with δ13C values serve as good discriminators of nursery areas. The majority of YOY alewives captured in Muskegon Lake emerge and grow in this habitat, whereas the majority of YOY alewives captured in Lake Michigan emerge and grow in Lake Michigan. In addition, early-life movements of alewives between the two lakes are documented but limited. Even if drowned river mouth lakes are more favorable for alewife growth and survival, their contribution to the Lake Michigan population could be limited because all of the 1+ alewives were individuals that spent their early life in Lake Michigan. The application of high-resolution isotope analysis of small otoliths of forage fish for fish population dynamics studies appears promising.

We used a long-term series of observations on alewife Alosa pseudoharengusabun- dance that was based on fall bottom-trawl catches to assess the importance of various abiotic and biotic factors on alewife recruitment in Lake Michigan during 1962-2002. We first fit a basic Ricker spawner-recruit model to the lakewide biomass estimates of age-3 recruits and the cor- responding spawning stock size; we then fit models for all possible combinations of the following four external variables added to the basic model: an index of salmonine predation on an alewife year-class, an index for the spring-summer water temperatures experienced by alewives during their first year in the lake, an index of the severity of the first winter experienced by alewives in the lake, and an index of lake productivity during an alewife year-class's second year in the lake. Based on an information criterion, the best model for alewife recruitment included indices of salmonine predation and spring-summer water temperatures as external variables. Our analysis corroborated the contention that a decline in alewife abundance during the 1970s and early 1980s in Lake Michigan was driven by salmonine predation. Furthermore, our findings indicated that the extraordinarily warm water temperatures during the spring and summer of 1998 probably led to a moderately high recruitment of age-3 alewives in 2001, despite abundant salmonines. A key problem in fisheries research is predicting recruitment from a given level of spawning stock size (Sissenwine et al. 1988; Myers et al. 2001; Kehler et al. 2002). Fish recruitment can be strong- ly influenced by many abiotic and biotic factors, including water temperature, water movements, predation, and spawning stock size (Sissenwine 1984; Hilborn and Walters 1992). Although im- portant factors affecting recruitment may vary across ecosystems (Madenjian et al. 1996), inter- esting patterns may emerge by comparing recruit- ment analyses for populations of a species across ecosystems (Myers 1998). An invasion of alewives Alosa pseudoharengus during the 1940s proved to be an important stressor to the Lake Michigan ecosystem (Wells and

Salmonines support valuable recreational fisheries and are the predominant predators in the open waters of the Great Lakes, yet the spatial distributions of salmonines in these systems have not been fully documented. We analyzed the horizontal distributions of steelhead Oncorhynchus mykiss in Lake Michigan from 1992 to 1997 and related these distributions to mean surface temperature and temperature variation. We used angler catch rate data from Lake Michigan natural resources agencies to index the spatial and temporal distributions of steelhead and obtained surface water temperature data from advanced very-high-resolution radiometer satellite imagery through the National Oceanic and Atmospheric Administration's CoastWatch Program. During most months, steelhead catch rates were negatively related to surface temperature and were highest in areas of high temperature variation (i.e., vertical thermal fronts and upwelling zones) where thermal conditions and prey densities may have been optimal for gro...

The relative quality of a habitat can influence fish consumption, growth, mortality, and production. In order to quantify habitat quality, several authors have combined bioenergetic and foraging models to generate spatially explicit estimates of fish growth rate potential (GRP). However, the capacity of GRP to reflect the spatial distributions of fishes over large areas has not been fully evaluated. We generated landscape scale estimates of steelhead (Oncorhynchus mykiss) GRP throughout Lake Michigan for 1994-1996, and used these estimates to test the hypotheses that GRP is a good predictor of spatial patterns of steelhead catch rates. We used surface temperatures (measured with AVHRR satellite imagery) and acoustically measured steelhead prey densities (alewife, Alosa pseudoharengus) as inputs for the GRP model. Our analyses demonstrate that potential steelhead growth rates in Lake Michigan are highly variable in both space and time. Steelhead GRP tended to increase with latitude, and mean GRP was much higher during September 1995, compared to 1994 and 1996. In addition, our study suggests that landscape scale measures of GRP are not good predictors of steelhead catch rates throughout Lake Michigan, but may provide an index of interannual variation in system-wide habitat quality.

It is a mainstay of community ecology that local exclusion of species will result if competitive pressures become too large. The pattern of exclusion may be complicated, but the qualitative orthodoxy has changed little since the pioneering work of Lotka, Volterra, and Gause—no two species can occupy the same niche. Stated in a more precise form, the higher the intensity of interspecific competition in an assemblage of species, the fewer the number of species that can coexist in perpetuity. We suggest that this orthodoxy results from “linear” thinking, and that if the classical equations are formulated more realistically with attendant nonlinearities, the orthodoxy breaks down and higher levels of competition may actually increase the likelihood that species will avoid competitive exclusion. Furthermore, this increased probability of coexistence at higher levels of competition is accompanied by characteristic dynamic patterns: ( i ) at lower levels of competition, after all extinction events have occurred, remaining species follow irregular chaotic patterns; ( ii ) at higher levels of competition, when most species coexist, all species are entrained in a single large limit cycle; ( iii ) the transient behavior appears to correspond to a special case of chaos, uniform phase chaotic amplitude.

In the original article lacks one institution of Daisuke Goto and incorrectly list the institution of Tomas O. Hook.

Aquatic food webs that incorporate multiple energy channels (e.g., nearshore benthic and pelagic) with varying productivity and turnover rates convey stability to biological communities by providing independent energy sources. Within the Lake Michigan food web, invasive dreissenid mussels have caused rapid changes to food web structure and potentially altered the channels through which consumers acquire energy. We used stable C and N isotopes to determine how Lake Michigan food web structure has changed in the past decade, coincident with the expansion of dreissenid mussels, decreased pelagic phytoplankton production, and increased nearshore benthic algal production. Fish and invertebrate samples collected from sites around Lake Michigan were analyzed to determine taxa-specific 13C:12C (delta13C) and 15N:14N (delta15N) ratios. Sampling took place during two distinct periods, 2002-2003 and 2010-2012, that spanned the period of dreissenid expansion, and included nearshore, pelagic and profundal fish and invertebrate taxa. The magnitude and direction of the delta13C shift indicated significantly greater reliance upon nearshore benthic energy sources among nearly all fish taxa as well as profundal invertebrates following dreissenid expansion. Although the mechanisms underlying this delta13C shift likely varied among species, possible causes include the transport of benthic algal production to offshore waters and increased feeding on nearshore prey items by pelagic and profundal species. delta15N shifts were more variable and of smaller magnitude across taxa, although declines in delta15N among some pelagic fishes suggest a shift to alternative prey resources. Lake Michigan fishes and invertebrates appear to have responded to dreissenid-induced changes in nutrient and energy pathways by switching from pelagic to alternative nearshore energy subsidies. Although large shifts in energy allocation (i.e., pelagic to nearshore benthic) resulting from invasive species appear to affect total production at upper trophic levels, changes in trophic structure and utilization of novel energy pathways may help to stabilize food webs following species invasions.

Great Lakes coastal marshes serve as spawning areas for adult and nurseries for young-of-year fishes, but the capacity of these habitats to facilitate fish reproduction is threatened due to their continued destruction and degradation. In order to appreciate the consequences of marsh loss and degradation, we collected fish larvae with icthyoplankton nets during the summers of 1997 and 1998 in three coastal marsh bays in Les Cheneaux, northern Lake Huron. In addition, we obtained several metrics of human activities and local habitat features (vegetation, water temperature, and substrate slope) and evaluated the importance of these metrics in structuring local larval fish assemblages. Our study indicated that local habitat features strongly and directly affected local larval fish assemblages in Les Cheneaux, while human activities did not. However, human activities may have altered local habitats in Les Cheneaux, thus indirectly impacting local larval fish assemblages.

The benthic macroinvertebrate Diporeia spp. have been extirpated from many areas of the Laurentian Great Lakes, but the mechanisms underlying such declines are not fully understood. Diporeia declines coinciding with the invasion of exotic dreissenid mussels (zebra and quagga) have led to the hypothesis that Diporeia declines are a result of decreased food availability from increasing competition with dreissenids for diatoms. There is additional evidence that Diporeia are negatively affected when in close proximity to dreissenids, probably because of exposure to toxins present in the mussels' pseudofeces. Diporeia are also known to be sensitive to anthropogenic contaminants (such as polychlorinated biphenyls [PCBs]) present in Great Lakes sediments. To better understand the physiological responses of Diporeia to diverse stressors, we conducted three 28-d experiments evaluating changes in the metabolomes of Diporeia (1) fed diatoms (Cyclotella meneghiniana) versus starved, (2) exposed (from Lake Michigan and Cayuga Lake) to quagga mussels (Dreissena bugensis), and (3) exposed to sediments contaminated with PCBs. The metabolomes of samples were examined using both two-dimensional gas and liquid chromatography coupled with mass spectrometry. Each stressor elicited a unique metabolome response characterized by enhanced citric acid cycle, fatty acid biosynthesis, and protein metabolism in diatom-fed Diporeia; impaired glycolysis, protein catabolism, and folate metabolism in Diporeia from Lake Michigan irrespective of quagga mussel exposure, suggesting lake-specific adaptation mechanisms; and altered cysteine and phospholipid metabolism during PCB exposure. Subsequent comparisons of these stressor-specific metabolic responses with metabolomes of a feral Diporeia population would help identify stressors affecting Diporeia populations throughout the Great Lakes.

Water mass movement within the Great Lakes may rapidly transport fish larvae from favorable nursery areas to less favorable habitats, thereby affecting recruitment success. During 2001 and 2002, we released satellite-tracked drifting buoys in eastern Lake Michigan to follow discrete water masses, and used ichthyoplankton nets to repeatedly sample larval fish within these water masses. Observed nearshore water currents were highly variable in both direction and velocity. Current velocities far exceeded potential larval fish swimming speeds, suggesting that currents can potentially rapidly advect fish larvae throughout the lake. Evidence suggests that while paired drifters released during 2002 were able to track relatively small alewife (Alosa pseudoharengus) and yellow perch (Perca flavescens) larvae within an alongshore coastal current, paired drifters released during 2001 failed to track larger alewife larvae when flow was more offshore and highly variable. These results are consistent with the decorrelation scales associated with alongshore and offshore transport.

Relieving phosphorus loading is a key management tool for controlling Lake Erie eutrophication. During the 1960s and 1970s, increased phosphorus inputs degraded water quality and reduced central basin hypolimnetic oxygen levels which, in turn, eliminated thermal habitat vital to cold-water organisms and contributed to the extirpation of important benthic macroinvertebrate prey species for fishes. In response to load reductions initiated in 1972, Lake Erie responded quickly with reduced water-column phosphorus concentrations, phytoplankton biomass, and bottom-water hypoxia (dissolved oxygen 2) requires cutting total phosphorus loads by 46% from the 2003–2011 average or reducing dissolved reactive phosphorus loads by 78% from the 2005–2011 average. Reductions to these levels are also protective of fish habitat. We provide potential approaches for achieving those new loading targets, and suggest that recent load reduction recommendations focused on western basin cyanobacteria blooms may not be sufficient to reduce central basin hypoxia to 2000 km2.