Habitat fragmentation impedes dispersal of aquatic fauna, and barrier removal is increasingly used to increase stream network connectivity and facilitate fish dispersal. Improved understanding of fish community response to barrier removal is needed, especially in fragmented agricultural streams where numerous antiquated dams are likely destined for removal. We examined post-removal responses in two distinct fish communities formerly separated by a small aging mill dam. The dam was removed midway through the 6 year study, enabling passage for downstream fishes affiliated with a connected reservoir into previously inaccessible habitat, thus creating the potential for taxonomic homogenization between upstream and downstream communities. Both communities changed substantially post-removal. Two previously excluded species (white sucker, yellow perch) established substantial populations upstream of the former dam, contributing to a doubling of total fish biomass. Meanwhile, numerical density of pre-existing upstream fishes declined. Downstream, largemouth bass density was inversely correlated with prey fish density throughout the study, while post-removal declines in bluegill density coincided with cooler water temperature and increased suspended and benthic fine sediment. Upstream and downstream fish communities became more similar post-removal, represented by a shift in Bray-Curtis index from 14 to 41 % similarity. Our findings emphasize that barrier removal in highly fragmented stream networks can facilitate the unintended and possibly undesirable spread of species into headwater streams, including dispersal of species from remaining reservoirs. We suggest that knowledge of dispersal patterns for key piscivore and competitor species in both the target system and neighboring systems may help predict community outcomes following barrier removal.

Altered thermal regimes under climate change may influence host‐parasite interactions and invasive species, both potentially impacting valuable ecosystem services. There is considerable interest in how parasite life cycle rates, growth, and impacts on hosts will change under altered environmental temperatures. Likewise, transformed thermal regimes may reduce natural resistance and barriers preventing establishment of invasive species or alter the range and impacts of established exotic species. The Laurentian Great Lakes are some of the most invaded ecosystems and have been profoundly shaped by exotic species. Invasion by the parasitic sea lamprey (Petromyzon marinus) contributed to major declines in many Great Lakes fish populations. In Lake Superior, substantial progress has been made towards controlling invasive sea lamprey and rehabilitating native fish populations. Surface water temperatures in Lake Superior have been increasing rapidly since 1980 presenting a new challenge for management. Here we test how thermal changes in Lake Superior have impacted the feeding and growth of the parasitic sea lamprey. Sea lamprey have increased in size corresponding with longer durations of thermal habitat (i.e., longer growing seasons) for their preferred hosts. To compare regional differences in sea lamprey feeding and growth rates, we used a bioenergetics model with temperature estimates from a lake‐wide hydrodynamic model hindcast from 1979–2006. Spatial differences in patterns of warming across the lake result in regionally different predictions for increases in sea lamprey feeding rates and size. These predictions were matched by data from adult sea lamprey spawning in streams draining into these different thermal regions. Larger sea lampreys will be more fecund and have increased feeding rates, thus increasing mortality among host fishes. Resource management should consider these climate driven regional impacts when allocating resources to sea lamprey control efforts. Under new and evolving thermal regimes, successful management systems may need to be restructured for changing phenology, growth, and shifts in host‐parasite systems towards greater impacts on host populations.

Critical transitions between alternate ecosystem states are often preceded by increased variance and autocorrelation in time series of ecosystem properties. Analogous changes may occur in spatial statistics as ecosystems approach thresholds for critical transitions. Changes in spatial statistics near thresholds have been described using models, laboratory experiments, and remotely sensed data, but there have been no tests using deliberate manipulations of whole ecosystems in the field. We previously documented a whole-lake manipulation resulting in a transition to predator dominance, a type of critical transition. The food web of an experimental lake was forced via cascading trophic interactions from a stable state characterized by abundant prey fish, small zooplankton, and high chlorophyll concentrations to an alternative state dominated by predatory fish, large zooplankton and low chlorophyll concentrations. Time series of zooplankton and chlorophyll concentrations provided early warning of the regime shift. Here we test if similar early warning signals were present in space by applying spatial variance and the discrete Fourier transform to spatially distributed prey fish catch data from this regime shift. Prey fish spatial distributions were monitored daily using minnow traps deployed around the lake perimeter. Added predators reduced prey fish populations and altered their spatial distributions. Increases in spatial variance and shifts to low frequency spatial variance were observed up to a year in advance of the shift. There was no response in an adjacent reference lake. Our results demonstrate that spatial signals of approaching thresholds can be detected at the ecosystem scale.

Climate change is expected to alter species distributions and habitat suitability across the globe. Understanding these shifting distributions is critical for adaptive resource management. The role of temperature in fish habitat and energetics is well established and can be used to evaluate climate change effects on habitat distributions and food web interactions. Lake Superior water temperatures are rising rapidly in response to climate change and this is likely influencing species distributions and interactions. We use a three-dimensional hydrodynamic model that captures temperature changes in Lake Superior over the last 3 decades to investigate shifts in habitat size and duration of preferred temperatures for four different fishes. We evaluated habitat changes in two native lake trout (Salvelinus namaycush) ecotypes, siscowet and lean lake trout, Chinook salmon (Oncorhynchus tshawytscha), and walleye (Sander vitreus). Between 1979 and 2006, days with available preferred thermal habitat increased at a mean rate of 6, 7, and 5 days per decade for lean lake trout, Chinook salmon, and walleye, respectively. Siscowet lake trout lost 3 days per decade. Consequently, preferred habitat spatial extents increased at a rate of 579, 495 and 419 km2 per year for the lean lake trout, Chinook salmon, and walleye while siscowet lost 161 km2 per year during the modeled period. Habitat increases could lead to increased growth and production for three of the four fishes. Consequently, greater habitat overlap may intensify interguild competition and food web interactions. Loss of cold-water habitat for siscowet, having the coldest thermal preference, could forecast potential changes from continued warming. Additionally, continued warming may render more suitable conditions for some invasive species.

Lakeshore residential development (LRD) reduces coarse woody habitat (CWH) in lakes resulting in negative effects on fishes. We tested whether the addition of CWH could reverse those effects. We added CWH to Camp Lake, a lake with CWH abundances similar to developed lakes, following 2 years of study of the fish populations in the reference and treatment basins. Both basins were monitored for 4 years following the manipulation. Specifically, we tested for changes in the population dynamics (densities, size-structure, growth), diet, and behavior (habitat use) of bluegill (Lepomis macrochirus) and largemouth bass (Micropterus salmoides). CWH addition had no discernible effect on fish population dynamics. Diet and behavioral responses were more pronounced in the treatment basin. Prey diversity and availability increased. Piscivory increased, with decreased reliance upon terrestrial prey, for largemouth bass. Habitat use was positively correlated with CWH branching complexity and abundance. Our study suggests that negative effects observed in fish populations through CWH reductions cannot be reversed in the short-term by adding CWH. We recommend that regulations governing the LRD process be protective of CWH.

Catch-and-release angling is gaining popularity worldwide and plays an increasingly important role in both fisheries management and conservation. Mortality from catch-and-release angling is well documented across species, but the sublethal effects have not been evaluated in a natural setting. Laboratory studies have yielded mixed results regarding catch-and-release impacts on fish growth. These studies do not adequately capture the scales of stress and variability of a natural system. We used a 27-year mark–recapture study of 1050 individually tagged largemouth bass (Micropterus salmoides) to determine the effects of catch-and-release angling on the growth in a natural setting. Individual bass were angled one to six times per season. Recapture intervals ranged from 1 to 98 days. Largemouth bass exhibited a post-release period (~6 days) of weight loss. Following this weight loss, we observed a subsequent period of compensatory growth facilitating recovery to normal weight. We found that catch-and-release angling had little impact on the overall seasonal growth patterns of largemouth bass and therefore should have limited adverse effects on growth-dependent ecological functions.

Invasive species can have severe effects on aquatic ecosystems. After invasions occur, eradication should be considered whenever the potential loss of ecosystem services outweighs the cost of the eradication method. Here we evaluate the possibility of destratifying Crystal Lake, Wisconsin, USA, to eradicate the invasive fish rainbow smelt (Osmerus mordax). We modeled the effects of three destratification scenarios (non-, low-, and high-mixing) using both physical and biological models. Field observations were used to calibrate the models. Water temperatures estimated from 18 unique DYRESM simulations were used in a bioenergetics model to estimate growth of five age classes of rainbow smelt under normal and destratified conditions. Our simulations indicate that destratification can eliminate optimal rainbow smelt thermal habitat resulting in mortality. Destratified lake temperatures also surpassed several physiological critical temperatures. Bioenergetics simulations predicted a weight loss of 45-55% in yearling and adult rainbow smelt. We found that destratification is potentially effective for eradicating cold-water species in temperate lakes.

The ecological consequences of widespread fisheries-induced reductions of large pelagic predators are not fully understood. Tropical tunas are considered a main component of apex predator guilds that include sharks and billfishes, and thus may seem unsusceptible to secondary effects of fishing top predators. However, intra-guild predation can occur because of size-structured interactions. We compiled existing data of apex predator diets to evaluate whether skipjack tuna Katsuwonus pelamis and yellowfin tuna Thunnus albacares might be vulnerable to top-down control by large pelagic predators in the eastern tropical Pacific Ocean. We identified potentially important predators of tunas by the frequency, quantity, and size and age of tunas in their diets and considered the degree that predated tunas could have potentially contributed to the reproductive output of the population. Our results indicate that the proportion of predator diets consisting of skipjack and yellowfin tunas was high for sharks and billfishes. These predators also consumed a wide size range of tunas, including subadults of the size capable of making a notable contribution to the reproductive output of tuna populations. Our study suggests that in the eastern tropical Pacific Ocean tropical tunas act as mesopredators more so than apex predators. Sharks and billfishes have the potential to play an important role in regulating these tuna populations. This study sets the stage for future efforts that could ascertain whether diminished levels of large predators have enhanced the production of tuna stocks and whether the trophic interactions of skipjack and yellowfin tunas should be explicitly accounted for when their population dynamics are assessed.

Stable isotopes are widely employed to describe energy flow in aquatic communities, though interpretation of results can be confounded by the fact that organisms integrate over vastly different time scales. We used results from a 56-day whole-lake 13C addition and a bioenergetic modeling approach to estimate dorsal muscle carbon turnover rates in a natural setting for three sizes of bluegill ( Lepomis macrochirus ), largemouth bass ( Micropterus salmoides ), and yellow perch ( Perca flavescens ). Generally, dynamic δ13C models with a metabolic tissue replacement term were better supported than models predicting isotopic change from growth alone, except when relative growth rates were highest (age 0 bluegill). Across species and size classes, the percentage of carbon change due to tissue replacement was variable (2%–80%) and independent of fish size. The half-life of δ13C in age 0 fishes was similar and ranged from 8 to 18 days. In contrast, adult tissue half-lives were much longer (116–173 days). Based on these and previously published estimates, fish mass (g) was a strong predictor of fish carbon turnover rates, λ: log(λ) = –3.65 – 0.20 log(mass), r2 = 0.71.

A simple energy budget equation is developed to yield a bioenergetics model designed to simulate fish growth. Parameters for the model are estimated from the literature for application to yellow perch (Perca flavescens) and walleye (Stizostedion vitreum vitreum). Simulations are presented that demonstrate model output as functions of body size, activity level, ration level, food quality, and environmental temperature. Sensitivity analyses identify the importance of food consumption, activity, and excretion as biological processes represented in the parameters. On the basis of temperature conditions in selected lakes and specified feeding levels, simulations are presented to quantify the importance of year-to-year variation of temperature in determining growth. In heterothermal systems, temperature selection by percids can have a significant effect on growth. For walleye on fixed rations, annual growth can vary from zero to twofold increments due entirely to differences in summer temperatures. Variations in food quality have lesser effects. Key words: Perca, Stizostedion, bioenergetics model, growth, sensitivity, simulations

Predator-prey interactions are a primary structuring force vital to the resilience of marine communities and sustainability of the world's oceans. Human influences on marine ecosystems mediate changes in species interactions. This generality is evinced by the cascading effects of overharvesting top predators on the structure and function of marine ecosystems. It follows that ecological forecasting, ecosystem management, and marine spatial planning require a better understanding of food web relationships. Characterising and scaling predator-prey interactions for use in tactical and strategic tools (i.e. multi-species management and ecosystem models) are paramount in this effort. Here, we explore what issues are involved and must be considered to advance the use of predator-prey theory in the context of marine fisheries science. We address pertinent contemporary ecological issues including (1) the approaches and complexities of evaluating predator responses in marine systems; (2) the 'scaling up' of predator-prey interactions to the population, community, and ecosystem level; (3) the role of predator-prey theory in contemporary fisheries and ecosystem modelling approaches; and (4) directions for the future. Our intent is to point out needed research directions that will improve our understanding of predator-prey interactions in the context of the sustainable marine fisheries and ecosystem management.

Several hypotheses have been proposed to explain diel vertical migration (DVM); however, they have generally been applied to DVM behavior of a single trophic level. We evaluate the costs (predation risk) and benefits (foraging rate and growth rate potential) of different hypothetical and observed DVM trajectories for a three-level pelagic food chain in Lake Superior containing opossum shrimp (Mysis relicta), deepwater ciscoes (Coregonus spp.), and lake trout (Salvelinus namaycush). Lake trout appear to be maximizing foraging and growth rates by tracking vertically migrating ciscoes, while the DVM trajectories of ciscoes suggests a trade-off between predation risk and growth. For ciscoes, two alternative DVM trajectories both minimize the ratio of risk to growth: a shallow trajectory that follows low light levels down to 80 m during the day and a deep trajectory (below 150 m) that tracks highest Mysis densities. Observed cisco DVM trajectories appear to follow the shallow high risk - high growth trajectory in 2001, but switch to the deep, low risk - low growth trajectory in 2004 when lake trout density was higher and the density of ciscoes was lower.

Catastrophic ecological regime shifts may be announced in advance by statistical early warning signals such as slowing return rates from perturbation and rising variance. The theoretical background for these indicators is rich, but real-world tests are rare, especially for whole ecosystems. We tested the hypothesis that these statistics would be early warning signals for an experimentally induced regime shift in an aquatic food web. We gradually added top predators to a lake over 3 years to destabilize its food web. An adjacent lake was monitored simultaneously as a reference ecosystem. Warning signals of a regime shift were evident in the manipulated lake during reorganization of the food web more than a year before the food web transition was complete, corroborating theory for leading indicators of ecological regime shifts.

Diel vertical migration (DVM) of young-of-the-year (YOY) herring Clupea harengus and one of their major predators, pikeperch Sander lucioperca, was examined using bottom-mounted hydroacoustics in Himmerfjärden, a brackish bay of the Baltic Sea, in summer. In contrast to previous studies on DVM of C. harengus aggregated across size and age classes, YOY C. harengus showed a reverse DVM trajectory, deeper at night and, on average, shallower during the day. This pattern was observed consistently on five acoustic sampling occasions in 3 years and was corroborated by two out of three trawl surveys. Large acoustic targets (target strength >-33 dB, probably piscivorous S. lucioperca >45 cm) showed a classic DVM trajectory, shallow at night and deeper during the day. Variability in YOY C. harengus vertical distribution peaked at dawn and dusk, and their vertical distribution at midday was distinctly bimodal. This reverse DVM pattern was consistent with bioenergetic model predictions for YOY C. harengus which have rapid gut evacuation rates and do not feed at night. Reverse DVM also resulted in low spatial overlap with predators.

Differences in the preferred thermal habitat of Lake Superior lake trout morphotypes create alternative growth scenarios for parasitic sea lamprey (Petromyzon marinus) attached to lake trout hosts. Siscowet lake trout (Salvelinus namaycush) inhabit deep, consistently cold water (4–6°C) and are more abundant than lean lake trout (Salvelinus namaycush) which occupy temperatures between 8 and 12°C during summer thermal stratification. Using bioenergetics models we contrasted the growth potential of sea lampreys attached to siscowet and lean lake trout to determine how host temperature influences the growth and ultimate size of adult sea lamprey. Sea lampreys simulated under the thermal regime of siscowets are capable of reaching sizes within the range of adult sea lamprey sizes observed in Lake Superior tributaries. High lamprey wounding rates on siscowets suggest siscowets are important lamprey hosts. In addition, siscowets have higher survival rates from lamprey attacks than those observed for lean lake trout which raises the prospect that siscowets serve as a buffer to predation on more commercially desirable hosts such as lean lake trout, and could serve to subsidize lamprey growth.

Tetzlaff JC, Roth BM, Weidel BC, Kitchell JF. Predation by native sunfishes (Centrarchidae) on the invasive crayfish Orconectes rusticus in four northern Wisconsin lakes. Ecology of Freshwater Fish 2011: 20: 133–143. © 2010 John Wiley & Sons A/SAbstract – Introduced rusty crayfish (Orconectes rusticus) often establish abundant populations in lakes where they negatively impact native food webs. However, rusty crayfish do not become abundant in all lakes. Fish predation has been posed as one potential mechanism limiting rusty crayfish density, but there is limited empirical evidence to support this hypothesis. Using a case study approach, we quantified the extent and timing of predation on nonnative rusty crayfish by three native centrarchids in four northern Wisconsin lakes with contrasting crayfish densities. We estimated population‐level predation effects on rusty crayfish using bioenergetics models in combination with fish and crayfish abundance estimates. Bluegill (Lepomis machrochirus) and pumpkinseed (Lepomis gibbosus) consumed young‐of‐year crayfish exclusively whereas rock bass (Ambloplites ruspestris) consumed a wide size range of crayfish. Rusty crayfish were found in Lepomis diets primarily in late June and early July while rock bass consumed crayfish throughout the summer. Per individual, rock bass consumed more crayfish biomass than Lepomis, but at the population‐level, higher Lepomis densities resulted in greater numbers of crayfish and more crayfish biomass consumed by Lepomis than rock bass. Lepomis populations consumed 8 × , 14 × , 22 × and 120 × more individual crayfish than rock bass populations in the four study lakes. Bioenergetics simulations revealed that dense populations of Lepomis were capable of consuming a large percentage of the juvenile crayfish population in two of our study lakes. In systems with high Lepomis densities, Lepomis consumption of juvenile rusty crayfish may play a significant role in the invasion and establishment dynamics of rusty crayfish.