Tomas O. Höök

Recruitment Variability of Alewives in Lake Michigan

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-classs 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

The re-eutrophication of Lake Erie: Harmful algal blooms and hypoxia

Lake Erie supplies drinking water to more than 11 million consumers, processes millions of gallons of wastewater, provides important species habitat and supports a substantial industrial sector, with >

Increased competition may promote species coexistence

50 billion annual income to tourism, recreational boating, shipping, fisheries, and other industries. These and other key ecosystem services are currently threatened by an excess supply of nutrients, manifested in particular by increases in the magnitude and extent of harmful planktonic and benthic algal blooms (HABs) and hypoxia. Widespread concern for this important international waterbody has been manifested in a strong focus of scientific and public material on the subject, and commitments for Canada-US remedial actions in recent agreements among Federal, Provincial and State agencies. This review provides a retrospective synthesis of past and current nutrient inputs, impairments by planktonic and benthic HABs and hypoxia, modelling and Best Management Practices in the Lake Erie basin. The results demonstrate that phosphorus reduction is of primary importance, but the effects of climate, nitrogen and other factors should also be considered in the context of adaptive management. Actions to reduce nutrient levels by targeted Best Management Practices will likely need to be tailored for soil types, topography, and farming practices.

Feeding ecology of emerald shiners and rainbow smelt in central Lake Erie

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.

Nearshore energy subsidies support Lake Michigan fishes and invertebrates following major changes in food web structure

ABSTRACT To better understand the feeding ecology of two important Laurentian Great Lakes prey species, rainbow smelt Osmerus mordax and emerald shiners Notropis atherinoides, we quantified the diet composition, selectivity, daily ration, and diet overlap of both species in offshore central Lake Erie during May through October 2005, which spanned a period of severe hypolimnetic hypoxia (<2 mg O2/L). Rainbow smelt fed upon a variety of prey taxa, including zooplankton, chironomid pupae and larvae, and fish, whereas emerald shiners primarily consumed cladocerans, if available. In turn, diet overlap between rainbow smelt and emerald shiners was low except during September when hypolimnetic hypoxia reduced rainbow smelt access to benthic prey. Rainbow smelt most frequently selected chironomid pupae, while emerald shiners generally selected pupae or large predatory cladocerans (Leptodora or Bythotrephes). Daily ration and individual consumption by rainbow smelt were 54–68% less during hypoxia than at the same site during stratified pre-hypoxic or mixed post-hypoxic conditions. Although emerald shiner daily ration and individual consumption decreased between pre-hypoxic and hypoxic periods, it continued to decrease during the post-hypoxic period, suggesting that reduced consumption may not have been linked to hypoxic conditions. Ultimately, our findings suggest that emerald shiners are as important regulator of zooplankton abundance in the Great Lakes as rainbow smelt, given their potentially high mass-specific consumption rates, selectivity and diet patterns, and current high abundance.

Eco-genetic model to explore fishing-induced ecological and evolutionary effects on growth and maturation schedules

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.

HABITAT AND HUMAN INFLUENCES ON LARVAL FISH ASSEMBLAGES IN NORTHERN LAKE HURON COASTAL MARSH BAYS

Eco‐genetic individual‐based models involve tracking the ecological dynamics of simulated individual organisms that are in part characterized by heritable parameters. We developed an eco‐genetic individual‐based model to explore ecological and evolutionary interactions of fish growth and maturation schedules. Our model is flexible and allows for exploration of the effects of heritable growth rates (based on von Bertalanffy and biphasic growth patterns), heritable maturation schedules (based on maturation reaction norm concepts), or both on individual‐ and population‐level traits. In baseline simulations with rather simple ecological trade‐offs and over a relatively short time period (<200 simulation years), simulated male and female fish evolve differential genetic growth and maturation. Further, resulting patterns of genetically determined growth and maturation are influenced by mortality rate and density‐dependent processes, and maturation and growth parameters interact to mediate the evolution of one another. Subsequent to baseline simulations, we conducted experimental simulations to mimic fisheries harvest with two size‐limits (targeting large or small fish), an array of fishing mortality rates, and assuming a deterministic or stochastic environment. Our results suggest that fishing with either size‐limit may induce considerable changes in life‐history trait expression (maturation schedules and growth rates), recruitment, and population abundance and structure. However, targeting large fish would cause more adverse genetic effects and may lead to a population less resilient to environmental stochasticity.

Long-Term and Interannual Dynamics of Walleye and Yellow Perch in Saginaw Bay, Lake Huron

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.

Reliability of Bioelectrical Impedance Analysis for Estimating Whole-Fish Energy Density and Percent Lipids

Abstract Walleye Sander vitreus and yellow perch Perca flavescens are well-studied, ecologically important fish species that co-occur in a wide range of systems and experience complex interactions; on the one hand, they are physiologically and ecologically similar and therefore may respond analogously to environmental variation, while on the other hand they interact antagonistically as competitors, predators, and prey. In Saginaw Bay, Lake Huron, walleyes and yellow perch have supported a combination of commercial and recreational fisheries and have been exposed to a series of ecosystem-level stressors and management actions that may have impacted these populations via multiple pathways. We used dynamic factor analysis and correlation analysis of walleye and yellow perch annual fall trawl catch data to elucidate the overall trends in walleye and yellow perch populations in Saginaw Bay. The results suggest that walleyes and yellow perch trend differently; while the relative abundance of age-0–2 walleyes ge...

Short-term Water Mass Movements in Lake Michigan: Implications for Larval Fish Transport

Abstract 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 Akaikes 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...