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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Modern approaches to Ecosystem-Based Management and sustainable use of marine resources must account for the myriad of pressures (interspecies, human and environmental) affecting marine ecosystems. The network of feeding interactions between co-existing species and populations (food webs) are an important aspect of all marine ecosystems and biodiversity. Here we describe and discuss a process to evaluate the selection of operational food-web indicators for use in evaluating marine ecosystem status. This process brought together experts in food-web ecology, marine ecology, and resource management, to identify available indicators that can be used to inform marine management. Standard evaluation criteria (availability and quality of data, conceptual basis, communicability, relevancy to management) were implemented to identify practical food-web indicators ready for operational use and indicators that hold promise for future use in policy and management. The major attributes of the final suite of operational food-web indicators were structure and functioning. Indicators that represent resilience of the marine ecosystem were less developed. Over 60 potential food-web indicators were evaluated and the final selection of operational food-web indicators includes: the primary production required to sustain a fishery, the productivity of seabirds (or charismatic megafauna), zooplankton indicators, primary productivity, integrated trophic indicators, and the biomass of trophic guilds. More efforts should be made to develop thresholds-based reference points for achieving Good Environmental Status. There is also a need for international collaborations to develop indicators that will facilitate management in marine ecosystems used by multiple countries.

We review and compare four broad categories of spatially-explicit modelling approaches currently used to understand and project changes in the distribution and productivity of living marine resources including: 1) statistical species distribution models, 2) physiology-based, biophysical models of single life stages or the whole life cycle of species, 3) food web models, and 4) end-to-end models. Single pressures are rare and, in the future, models must be able to examine multiple factors affecting living marine resources such as interactions between: i) climate-driven changes in temperature regimes and acidification, ii) reductions in water quality due to eutrophication, iii) the introduction of alien invasive species, and/or iv) (over-)exploitation by fisheries. Statistical (correlative) approaches can be used to detect historical patterns which may not be relevant in the future. Advancing predictive capacity of changes in distribution and productivity of living marine resources requires explicit modelling of biological and physical mechanisms. New formulations are needed which (depending on the question) will need to strive for more realism in ecophysiology and behaviour of individuals, life history strategies of species, as well as trophodynamic interactions occurring at different spatial scales. Coupling existing models (e.g. physical, biological, economic) is one avenue that has proven successful. However, fundamental advancements are needed to address key issues such as the adaptive capacity of species/groups and ecosystems. The continued development of end-to-end models (e.g., physics to fish to human sectors) will be critical if we hope to assess how multiple pressures may interact to cause changes in living marine resources including the ecological and economic costs and trade-offs of different spatial management strategies. Given the strengths and weaknesses of the various types of models reviewed here, confidence in projections of changes in the distribution and productivity of living marine resources will be increased by assessing model structural uncertainty through biological ensemble modelling.

Growth in body size during ontogeny often results in changes in diet, leading to life-history omnivory. In addition, growth is often dependent on food density. Using a physiologically structured population model, we investigated the effects of these two aspects of individual growth in a system consisting of two size-structured populations, an omnivorous top predator and an intermediate consumer. With a single shared resource for both populations, we found that life-history omnivory decreases the likelihood of coexistence between top predator and intermediate consumer in this intraguild predation (IGP) system. This result contrasts with previous unstructured models and stage-structured models without food-dependent development. Food-dependent development and size-dependent foraging abilities of the predator resulted in a positive feedback between foraging success on the shared resource at an early life stage and foraging success on the intermediate consumer later in life. By phenomenologically incorporating this feedback in an unstructured IGP model, we show that it also demotes coexistence in this simple setting, demonstrating the robustness of the negative effect of this feedback.