Yunne-Jai Shin

Impacts of Fishing Low-Trophic Level Species on Marine Ecosystems

High harvest levels of low–trophic level fishes may have cascading marine ecosystem effects. Low–trophic level species account for more than 30% of global fisheries production and contribute substantially to global food security. We used a range of ecosystem models to explore the effects of fishing low–trophic level species on marine ecosystems, including marine mammals and seabirds, and on other commercially important species. In five well-studied ecosystems, we found that fishing these species at conventional maximum sustainable yield (MSY) levels can have large impacts on other parts of the ecosystem, particularly when they constitute a high proportion of the biomass in the ecosystem or are highly connected in the food web. Halving exploitation rates would result in much lower impacts on marine ecosystems while still achieving 80% of MSY.

Ecosystem oceanography for global change in fisheries

Overexploitation and climate change are increasingly causing unanticipated changes in marine ecosystems, such as higher variability in fish recruitment and shifts in species dominance. An ecosystem-based approach to fisheries attempts to address these effects by integrating populations, food webs and fish habitats at different scales. Ecosystem models represent indispensable tools to achieve this objective. However, a balanced research strategy is needed to avoid overly complex models. Ecosystem oceanography represents such a balanced strategy that relates ecosystem components and their interactions to climate change and exploitation. It aims at developing realistic and robust models at different levels of organisation and addressing specific questions in a global change context while systematically exploring the ever-increasing amount of biological and environmental data.

End-To-End Models for the Analysis of Marine Ecosystems: Challenges, Issues, and Next Steps

Abstract There is growing interest in models of marine ecosystems that deal with the effects of climate change through the higher trophic levels. Such end-to-end models combine physicochemical oceanographic descriptors and organisms ranging from microbes to higher-trophic-level (HTL) organisms, including humans, in a single modeling framework. The demand for such approaches arises from the need for quantitative tools for ecosystem-based management, particularly models that can deal with bottom-up and top-down controls that operate simultaneously and vary in time and space and that are capable of handling the multiple impacts expected under climate change. End-to-end models are now feasible because of improvements in the component submodels and the availability of sufficient computing power. We discuss nine issues related to the development of end-to-end models. These issues relate to formulation of the zooplankton submodel, melding of multiple temporal and spatial scales, acclimation and adaptation, behavioral movement, software and technology, model coupling, skill assessment, and interdisciplinary challenges. We urge restraint in using end-to-end models in a true forecasting mode until we know more about their performance. End-to-end models will challenge the available data and our ability to analyze and interpret complicated models that generate complex behavior. End-to-end modeling is in its early developmental stages and thus presents an opportunity to establish an open-access, community-based approach supported by a suite of true interdisciplinary efforts.

Exploring fish community dynamics through size-dependent trophic interactions using a spatialized individual-based model

An individual-based model named OSMOSE (Object-oriented Simulator of Marine Ecosystems Exploitation) is used to investigate the dynamics of exploited marine fish communities. It allows the representation of age- and size-structured populations comprised of groups of individuals that interact within a spatialized food web. Within each group, which constitutes the basic interaction entity (the ‘super-individual’ in individual-based modelling terminology), fish belong to the same species, have similar biological parameters and behaviour rules. Somatic growth, reproduction, predation and starvation processes are modelled. Two rules apply for the predation process: for a given fish group, prey selection depends both on the spatial and temporal co-occurrence of the predator and its prey, and on the respective lengths of the prey versus the predator. Thus, fish feed regardless of the taxonomy of their prey. The strength of both predation and competition relationships therefore vary according to changes in relative species abundance. Preliminary investigations are conducted on a theoretical community comprising seven interacting species. The simulation results show how community stability can emerge from variability in species biomass. It is thus suggested that size-based trophic interactions, along with the existence of multiple weak links and species redundancy, favour community persistence and stability.

The functioning of marine ecosystems: a fisheries perspective.

There is considerable evidence that environmental variability plays a major role in controlling abundance and distribution of marine populations and that fisheries alter ecosystem functioning and state. This overviewdocuments emergent, i.e. visible to us as observers, ecosystem-level ecological patterns and addresses important questions regarding the exploitation of marine resources. Do marine ecosystems function differently from terrestrial systems? Do multiple stable states exist in marine ecosystems? Does removal of top predators in marine ecosystems result in fundamental changes in the plankton communities (top-down ‘trophic cascades’), as observed in lakes? Alternatively, are marine ecosystems characterized by bottom-up control such that fishing predatory fish does not disturb community structure and function? Does heavy exploitation of forage species, such as anchovies and sardines, cause changes in the functioning of upwelling ecosystems? The key to answering these questions and exploring whether general principles apply lies in understanding the energy flow within the ecosystems. The chapter reviews different types of energy flow in marine ecosystems, i.e. bottom-up control (control by primary producers), top-down control (control by predators) and wasp-waist control (control by numerically dominant species). No general theory can yet be ascribed to the functioning of marine ecosystems. Ecological understanding and models of ecosystem functioning are provisional and subject to change, and common sense is not sufficient when studying complex dynamic systems. However, tentative and partial generalizations are proposed, namely that bottom-up control predominates; top-down control plays a role in dampening ecosystem-level fluctuations; trophic cascades seldom occur; and wasp-waist control is most probable in upwelling systems. Moreover, alternation and large-scale synchronized fluctuations in fish stocks, stability of fish communities and emergent features such as size spectra are potentially important patterns when assessing states and changes in marine ecosystems. New and meaningful indicators, derived from our current understanding of marine ecosystem functioning, can be used to assess the impact of fisheries and to promote responsible fisheries in marine ecosystems.

Exploring fish community dynamics through size-dependent trophic interactions using a spatialized individual-based modelInteractions trophiques fondées sur la taille et dynamiques des communautés de poissons marins : exploration à l’aide d’un modèle spatial individus-centré.

An individual-based model named OSMOSE (Object-oriented Simulator of Marine Ecosystems Exploitation) is used to investigate the dynamics of exploited marine fish communities. It allows the representation of age- and size-structured populations comprised of groups of individuals that interact within a spatialized food web. Within each group, which constitutes the basic interaction entity (the ‘super-individual’ in individual-based modelling terminology), fish belong to the same species, have similar biological parameters and behaviour rules. Somatic growth, reproduction, predation and starvation processes are modelled. Two rules apply for the predation process: for a given fish group, prey selection depends both on the spatial and temporal co-occurrence of the predator and its prey, and on the respective lengths of the prey versus the predator. Thus, fish feed regardless of the taxonomy of their prey. The strength of both predation and competition relationships therefore vary according to changes in relative species abundance. Preliminary investigations are conducted on a theoretical community comprising seven interacting species. The simulation results show how community stability can emerge from variability in species biomass. It is thus suggested that size-based trophic interactions, along with the existence of multiple weak links and species redundancy, favour community persistence and stability.

SIMULATIONS OF FISHING EFFECTS ON THE SOUTHERN BENGUELA FISH COMMUNITY USING AN INDIVIDUAL-BASED MODEL: LEARNING FROM A COMPARISON WITH ECOSIM

By applying an individual-based model (OSMOSE) to the southern Benguela ecosystem, a multispecies analysis is proposed, complementary to that provided by the application of ECOPATH/ECOSIM models. To reconstruct marine foodwebs, OSMOSE is based on the hypothesis that predation is a size-structured process. In all, 12 fish species, chosen for their importance in terms of biomass and catches, are explicitly modelled. Growth, reproduction and mortality parameters are required to model their dynamics and trophic interactions. Maps of mean spatial distribution of the species are compiled from published literature. Taking into account the spatial component is necessary because spatial co-occurrence determines potential interactions between predatory fish and prey fish of suitable size. To explore ecosystem effects of fishing, different fishing scenarios, previously examined using ECOSIM, are simulated using the OSMOSE model. They explore the effects of targeting fish species in the southern Benguela considered to be predators (Cape hake Merluccius capensis and M. paradoxus) or prey (anchovy Engraulis encrasicolus, sardine Sardinops sagax, round herring Etrumeus whiteheadi). Simulation results are compared and are generally consistent with those obtained using an ECOSIM model. This cross-validation appears to be a promising means of evaluating the robustness of model outputs, when separate validation of marine ecosystem models are still difficult to perform.

Global in scope and regionally rich: an IndiSeas workshop helps shape the future of marine ecosystem indicators

This report summarizes the outcomes of an IndiSeas workshop aimed at using ecosystem indicators to evaluate the status of the world’s exploited marine ecosystems in support of an ecosystem approach to fisheries, and global policy drivers such as the 2020 targets of the Convention on Biological Diversity. Key issues covered relate to the selection and integration of multi-disciplinary indicators, including climate, biodiversity and human dimension indicators, and to the development of data- and model-based methods to test the performance of ecosystem indicators in providing support for fisheries management. To enhance the robustness of our cross-system comparison, unprecedented effort was put in gathering regional experts from developed and developing countries, working together on multi-institutional survey datasets, and using the most up-to-date ecosystem models.

Combined fishing and climate forcing in the southern Benguela upwelling ecosystem: an end-to-end modelling approach reveals dampened effects

The effects of climate and fishing on marine ecosystems have usually been studied separately, but their interactions make ecosystem dynamics difficult to understand and predict. Of particular interest to management, the potential synergism or antagonism between fishing pressure and climate forcing is analysed in this paper, using an end-to-end ecosystem model of the southern Benguela ecosystem, built from coupling hydrodynamic, biogeochemical and multispecies fish models (ROMS-N2P2Z2D2-OSMOSE). Scenarios of different intensities of upwelling-favourable wind stress combined with scenarios of fishing top-predator fish were tested. Analyses of isolated drivers show that the bottom-up effect of the climate forcing propagates up the food chain whereas the top-down effect of fishing cascades down to zooplankton in unfavourable environmental conditions but dampens before it reaches phytoplankton. When considering both climate and fishing drivers together, it appears that top-down control dominates the link between top-predator fish and forage fish, whereas interactions between the lower trophic levels are dominated by bottom-up control. The forage fish functional group appears to be a central component of this ecosystem, being the meeting point of two opposite trophic controls. The set of combined scenarios shows that fishing pressure and upwelling-favourable wind stress have mostly dampened effects on fish populations, compared to predictions from the separate effects of the stressors. Dampened effects result in biomass accumulation at the top predator fish level but a depletion of biomass at the forage fish level. This should draw our attention to the evolution of this functional group, which appears as both structurally important in the trophic functioning of the ecosystem, and very sensitive to climate and fishing pressures. In particular, diagnoses considering fishing pressure only might be more optimistic than those that consider combined effects of fishing and environmental variability.

A model for the phenotypic plasticity of North sea herring growth in relation to trophic conditions

An adaptation of the von Bertalanffy growth model is formulated to describe the phenotypic plasticity of fish somatic growth in relation to trophic conditions. The model is developed for the North sea Downs herring (Clupea harengus). It suggests that annual growth variability during 1974-1990 was mainly due to the combined effects of herring abundance and wind-induced turbu- lence (coincident with the spring stratification of the water column). Springtime turbulences cause reduced and delayed planktonic blooms preceding the annual foraging period of Downs herring. The negative relation observed between herring abundance and growth is hypothesized to be due to intra-specific competition for trophic resources. Incorporated into the calculation of yield per recruit, the established growth model provides slightly more optimistic diagnoses while dropping the classic assumption of constant weight at age. 0 Ifremer/Blsevier, Paris