Recreational angling opportunities in lakes are distributed across landscapes and attract anglers based on the combination of angling quality, travel distance, and availability of facilities. The relationship between angler density and fishing quality, as measured by catch rate, represents a numerical response that is analogous to a predator numerical response to variability in prey abundance. We quantified this numerical response of anglers to rainbow trout, Oncorhynchus mykiss, populations distributed over a large lake district in south-central British Columbia, Canada. We developed a harvest dynamics model by linking this empirical description of the spatial numerical response of anglers to a logistic population growth rate model. The model was parameterized for rainbow trout and simulated spatial patterns of angler density and catch rates over a landscape. At locations distant from urban centers, angler density is low and catch rate high, suggesting near pristine conditions; at intermediate distances angler density is higher while catch rates are lower and approximate maximum sustainable levels; and at short distances angler density is sufficiently high to harvest to local extirpation. We extrapolated the model to other lake districts varying in human population size using an empirically derived angling participation rate relationship. Extrapolation to lake districts with one-tenth the human population maintained viable fisheries close to the urban area, and districts with 10 times the human populations could not maintain viable fisheries across much of their lake district. Landscape-scale spatial patterns differed quantitatively for species varying in rates of intrinsic population growth and carrying capacity, but the qualitative spatial patterns were consistent among species, demonstrating the pervasive impacts of the angler numerical response. To achieve a management goal of sustaining fisheries across landscapes, a change in management perspective is necessary, from that of individual lakes to one of dynamic harvest processes across landscapes. This new approach makes it clear that a one-size-fits-all management approach must be replaced with a mosaic of approaches cognizant of landscape-scale processes.

We analyze a stage-structured biomass model for size-structured consumer-resource interactions. Maturation of juvenile consumers is modeled with a food-dependent function that consistently translates individual-level assumptions about growth in body size to the population level. Furthermore, the model accounts for stage-specific differences in resource use and mortality between juvenile and adult consumers. Without such differences, the model reduces to the Yodzis and Innes (1992) bioenergetics model, for which we show that model equilibria are characterized by a symmetry property that reproduction and maturation are equally limited by food density. As a consequence, biomass production rate exactly equals loss rate through maintenance and mortality in each consumer stage. Stage-specific differences break up this symmetry and turn specific stages into net producers and others into net losers of biomass. As a consequence, the population in equilibrium can be regulated in two distinct ways: either through total population reproduction or through total population maturation as limiting process. In the case of reproduction regulation, increases in mortality may lead to an increase of juvenile biomass. In the case of maturation regulation, increases in mortality may increase adult biomass. This overcompensation in biomass occurs with increases in both stage-independent and stage-specific mortality, even when the latter targets the stage exhibiting overcompensation.

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Prey in natural communities are usually shared by many predator species. How predators coexist while competing for the same prey is one of the fundamental questions in ecology. Here, we show that competing predator species may not only coexist on a single prey but even help each other to persist if they specialize on different life history stages of the prey. By changing the prey size distribution, a predator species may in fact increase the amount of prey available for its competitor. Surprisingly, a predator may not be able to persist at all unless its competitor is also present. The competitor thus significantly increases the range of conditions for which a particular predator can persist. This ''emergent facilitation'' is a long-term, population-level effect that results from asymmetric increases in the rate of prey maturation and reproduction when predation relaxes competition among prey. Emergent facilitation explains observations of correlated increases of predators on small and large conspecific prey as well as concordance in their distribution patterns. Our results suggest that emergent facilitation may promote the occurrence of complex, stable, community food webs and that persistence of these communities could critically depend on diversity within predator guilds. emergent facilitation ͉ food-dependent prey development ͉ predator coexistence ͉ prey stage ͉ stage-specific predation

Engelhard, G. H., Peck, M. A., Rindorf, A., Smout, S. C., van Deurs, M., Raab, K., Andersen, K. H., Garthe, S., Lauerburg, R. A. M., Scott, F., Brunel, T., Aarts, G., van Kooten, T., and Dickey-Collas, M. Forage fish, their fisheries, and their predators: who drives whom? – ICES Journal of Marine Science, 71: . The North Sea has a diverse forage fish assemblage, including herring, targeted for human consumption; sandeel, sprat, and Norway pout, exploited by industrial fisheries; and some sardine and anchovy, supporting small-scale fisheries. All show large abundance fluctuations, impacting on fisheries and predators. We review field, laboratory, and modelling studies to investigate the drivers of this complex system of forage fish. Climate clearly influences forage fish productivity; however, any single-species considerations of the influence of climate might fail if strong interactions between forage fish exist, as in the North Sea. Sandeel appears to be the most important prey forage fish. Seabirds are most dependent on forage fish, due to specialized diet and distributional constraints (breeding colonies). Other than fisheries, key predators of forage fish are a few piscivorous fish species including saithe, whiting, mackerel, and horse-mackerel, exploited in turn by fisheries; seabirds and seals have a more modest impact. Size-based foodweb modelling suggests that reducing fishing mortality may not necessarily lead to larger stocks of piscivorous fish, especially if their early life stages compete with forage fish for zooplankton resources. In complex systems, changes in the impact of fisheries on forage fish may have potentially complex (and perhaps unanticipated) consequences on other commercially and/or ecologically important species.

Bottom fishing such as trawling and dredging may pose serious risks to the seabed and benthic habitats, calling for a quantitative assessment method to evaluate the impact and guide management to develop mitigation measures. We provide a method to estimate the sensitivity of benthic habitats based on the longevity composition of the invertebrate community. We hypothesize that long‐lived species are more sensitive to trawling mortality due to their lower pace of life (i.e., slower growth, late maturation). We analyze data from box‐core and grab samples taken from 401 stations in the English Channel and southern North Sea to estimate the habitat‐specific longevity composition of the benthic invertebrate community and of specific functional groups (i.e., suspension feeders and bioturbators), and examine how bottom trawling affects the longevity biomass composition. The longevity biomass composition differed between habitats governed by differences in sediment composition (gravel and mud content) and tidal bed‐shear stress. The biomass proportion of long‐lived species increased with gravel content and decreased with mud content and shear stress. Bioturbators had a higher median longevity than suspension feeders. Trawling, in particular by gears that penetrate the seabed >2 cm, shifted the community toward shorter‐lived species. Changes from bottom trawling were highest in habitats with many long‐lived species (hence increasing with gravel content, decreasing with mud content). Benthic communities in high shear stress habitats were less affected by bottom trawling. Using these relationships, we predicted the sensitivity of the benthic community from bottom trawling impact at large spatial scale (the North Sea). We derived different benthic sensitivity metrics that provide a basis to estimate indicators of trawling impact on a continuous scale for the total community and specific functional groups. In combination with high resolution data of trawling pressure, our approach can be used to monitor and assess trawling impact and seabed status at the scale of the region or broadscale habitat and to compare the environmental impact of bottom‐contacting fishing gears across fisheries.

Climate change is currently one of the main driving forces behind changes in species distributions, and understanding the mechanisms that underpin macroecological patterns is necessary for a more predictive science. Warming sea water temperatures are expected to drive changes in ectothermic marine species ranges due to their thermal tolerance levels. Here, we develop a mechanistic tool to predict size‐ and season‐specific distributions based on the physiology of the species and the temperature and food conditions in the sea. The effects of climate conditions on physiological‐based habitat utilization was then examined for different size‐classes of two commercially important fish species in the North Sea, plaice, Pleuronectes platessa, and sole, Solea solea. The two species provide an attractive comparison as they differ in their physiology (e.g. preferred temperature range). Combining dynamic energy budget (DEB) models with the temperature and food conditions estimated by an ecosystem model (ERSEM), allowed spatial differences in potential growth (as a proxy for habitat quality) to be estimated for 2 years with contrasting temperature and food conditions. The resulting habitat quality maps were in broad agreement with observed ontogenetic and seasonal changes in distribution as well as with the recent changes in distribution which could be attributed to an increase in coastal temperatures. Our physiological‐based model provides a powerful tool to explore the effect of climate change on the spatio‐temporal fish dynamics, predict effects of local or broad‐scale environmental changes and provide a physiological basis for observed changes in species distributions.

1. Invasions of top predators may have strong cascading effects in ecosystems affecting both prey species abundance and lower trophic levels. A recently discussed factor that may enhance species invasion is climate change and in this context, we studied the effects of an invasion of northern pike into a subarctic lake ecosystem formerly inhabited by the native top predator Arctic char and its prey fish, ninespined stickleback. 2. Our study demonstrated a strong change in fish community composition from a system with Arctic char as top predator and high densities of sticklebacks to a system with northern pike as top predator and very low densities of sticklebacks. A combination of both predation and competition from pike is the likely cause of the extinction of char. 3. The change in top predator species also cascaded down to primary consumers as both zooplankton and predator-sensitive macroinvertebrates increased in abundance. 4. Although the pike invasion coincided with increasing summer temperatures in the study area we have no conclusive evidence that the temperature increase is the causal mechanism behind the pike invasion. But still, our study provides possible effects of future pike invasions in mountain lakes related to climate change. We suggest that future pike invasions will have strong effects in lake ecosystems, both by replacing native top consumers and through cascading effects on lower trophic levels.