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Modelling the effects of climate change, species interactions and fisheries - towards Ecosystem-based Fisheries Management in the Central Baltic Sea

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Downloaded from orbit.dtu.dk on: Sep 05, 2018 Modelling the effects of climate change, species interactions and fisheries - towards Ecosystem-based Fisheries Management in the Central Baltic Sea Lindegren,
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Downloaded from orbit.dtu.dk on: Sep 05, 2018 Modelling the effects of climate change, species interactions and fisheries - towards Ecosystem-based Fisheries Management in the Central Baltic Sea Lindegren, Martin Publication date: 2010 Document Version Publisher's PDF, also known as Version of record Link back to DTU Orbit Citation (APA): Lindegren, M. (2010). Modelling the effects of climate change, species interactions and fisheries - towards Ecosystem-based Fisheries Management in the Central Baltic Sea. Copenhagen, Denmark: Faculty of Science, Copenhagen University and National Institute of Aquatic Resources, Technical University of Denmark (DTU). General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights. Users may download and print one copy of any publication from the public portal for the purpose of private study or research. You may not further distribute the material or use it for any profit-making activity or commercial gain You may freely distribute the URL identifying the publication in the public portal If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim. Modelling the effects of climate change, species interactions and fisheries towards Ecosystem-based Fisheries Management in the Central Baltic Sea. Martin Lindegren Ph.D. Thesis 2010 The Graduate School of Science, Faculty of Science, University of Copenhagen, Denmark National Institute of Aquatic Resources, Danish Technical University, DTU-Aqua, Denmark FACULTY OF SCIENCE UNIVERSITY OF COPENHAGEN Modelling the effects of climate change, species interactions and fisheries towards Ecosystem-based Fisheries Management in the Central Baltic Sea. Martin Lindegren Academic dissertation By due permission of the Faculty of Science, University of Copenhagen, Denmark to be defended at Riddersalen, Charlottenlund Castle (DTU-Aqua) on Friday the 5 th of February, 2010, at Ph.D. Committee Associate Professor Lars Kamp Nielsen, Department of Biology, Copenhagen University (chair) Principal scientist Geir Ottersen, Department of Biology, University of Oslo (opponent) Senior scientist Michele Casini, Institute of Marine Research, Swedish Board of Fisheries (opponent) Academic advisors Professor Christian Möllmann, Institute for Hydrobiology and Fisheries Science, University of Hamburg Associate Professor Michael Olesen, Department of Biology, Copenhagen University Submitted: 08/01/2010 Abstract Marine ecosystem and the services which they supply are under threat from a wide range of human activities. In order to achieve sustainability, an ecosystem-based approach to fisheries management (EBFM), i.e., integrating multiple drivers in a common framework is therefore needed. The overarching aim of this thesis is to develop a decision-support tool fit for achieving EBFM in the Central Baltic Sea, an ecosystem heavily impacted by overfishing and climate change. To that end, a theoretical approach for modelling multispecies population dynamics was combined with advanced statistical methods in order to develop a stochastic food-web model integrating species interactions, between cod and the forage fish species herring and sprat, with external forcing through commercial fishing, zooplankton and climate effects. Furthermore, by linking models across sectors, i.e., with climate and bio-economical models, we were able to account for management consequences over a wide range of policy objectives and define overall ecologically and economically optimal management solutions. To that end, our coupled modelling tool demonstrates how by adopting an ecosystem approach we may quantitatively forecast the response of Baltic fish stocks to climate change and take appropriate management actions to mitigate negative effects on future fisheries production. Furthermore, by presenting the ecological need and economic advantage of our ecosystem-based approach we may establish the institutional and political will necessary for successful implementation of EBFM in the Baltic Sea, a vital first step towards achieving long-term sustainability in marine fisheries worldwide. 1 2 Contents Summary.. 4 Resumé (Summary in Danish). 6 List of original publications Introduction and background 9 2. Motivation and objectives of the study The Central Baltic Sea an overview of ecosystem characteristics Background of material, methods and modelling approaches 4.1. Data requirements Ecosystem-based modelling Multivariate Autoregressive models (MAR) The BALMAR food-web model fitting and selection Regional climate modelling and forecasts for the Baltic Sea Bio-Economic modelling of the Baltic fishery Integrated Ecosystem assessment and regime shift analysis Summary of main results and findings 5.1. Food-web model diagnostics and validation Recreating the variability of Central Baltic cod, herring and sprat Adaptive management and cod stock forecasts under climate change Food-web interactions and multi-species management in the Baltic Sea Bio-economic considerations and optimal management Overfishing and the Resilience of Marine Ecosystems Conclusions and future perspectives Acknowledgements.. 36 References Appendix (Paper I to IV) Summary Marine ecosystem and the services which they supply are under threat from a wide range of human activities, including overfishing, climate change and indirect alterations in species interactions. The dramatic collapses of Atlantic cod stocks illustrate clearly the failure of past management efforts and the need for developing new practises. In order to achieve sustainable marine development, an ecosystem approach to management, i.e., integrating multiple drivers in a common framework is therefore needed, but presently lacking due mainly to methodological shortcomings. The overarching aim of this thesis is to develop a decision-support tool fit for ecosystem-based fisheries management (EBFM) in the Central Baltic Sea, an ecosystem heavily impacted by human exploitation. In the late 1980s, the Central Baltic Sea underwent large-scale alterations in the structure and functioning of the ecosystem. This so-called regime shift was likely caused by overfishing and changes in the physical environment (i.e., temperature, salinity and oxygen conditions), eventually influencing the entire food-web dynamics from primary producers to top-predators. The recognition of the ecosystem context therefore calls for the integration of both external and internal drivers in model development. To that end, a theoretical approach for modelling multi-species population dynamics was combined with advanced statistical methods in order to develop a food-web model integrating species interactions, between cod and the prey species herring and sprat, with external forcing through commercial fishing, zooplankton and climate effects on the Central Baltic ecosystem. Furthermore, by linking models across sectors of interest, i.e., including ecological, climate and bio-economical models, we were able to account for management consequences over a wide range of objectives and define overall optimal management solutions. Based on the derived patterns of species interactions and the external forcing through fishing, zooplankton and climate, our food-web model clearly recreates and explains the past variability in Central Baltic fish stocks. As such, the so-called gadoid outburst (i.e., the strong and synchronous increase in several Atlantic cod stocks during the late 1970s) of Baltic cod could be explained by a period of historically low exploitation in combination with record high salinities, enabling successful reproduction due to high egg and larvae survival in the entire Baltic Sea. 4 Furthermore, we used our coupled climate-food-web model to explore whether given the present knowledge on climate effects, fishing and species interactions, we in hindsight could have managed the Baltic cod stock in ways such as to avoid the collapse. To that end, we formulated an adaptive management strategy in setting precautionary exploitation levels. By adapting fishing pressure to climate conditions and species interactions, our simulations show that we could indeed have prevented the cod stock from collapsing and promoted a recovery above ecologically safe levels. In an effort to adopt a holistic management perspective, we finally coupled the food-web model to a simple bio-economic model aiming to compare revenues of the Baltic cod fishery across management strategies. Based on our results, we argue that our adaptive management strategy would not only be ecologically but economically profitable due to increased landings and reduced fishing costs as the stock and hence the catchability is allowed to increase. These results support the importance of investing in natural capital (i.e., in future stock size) as a long-term management strategy for the Baltic cod. In summary, by improving the methodological development towards increasing ecological realism in fisheries modelling and providing cross-disciplinary exchange of scientific concepts and ideas, this Ph.D. contributes to the implementation of regionalised ecosystembased management in the Central Baltic Sea, a vital first step towards achieving long-term sustainability in marine fisheries worldwide. 5 Resumé Marine økosystemer og de goder, de leverer, er truet af en bred vifte af menneskelige aktiviteter, herunder overfiskning, klimaforandringer og indirekte ændringer i samspillet mellem arterne. Det dramatiske sammenbrud af Atlanterhavets torskebestandene påviser klart tidligere tiders fejlslagne forvaltning og behovet for at udvikle ny praksis. For at opnå en bæredygtig marine udvikling, er en økosystembaseret forvaltning (dvs. integration af flere samspillene faktorer i en fælles ramme) nødvendigt, men i øjeblikket fraværende på grund af især metodologiske mangler. Det overordnede mål med denne afhandling er at udvikle et operativt værktøj egnet til rådgivning af en økosystembaseret og bæredygtig fiskeriforvaltning (EBFM) i den centrale del af Østersøen, et økosystem, stærkt påvirket af menneskelig udnyttelse. I slutningen af 1980'erne gennemgik den centrale Østersø store ændringer i struktur og funktion af økosystemet. Denne tilstandsændring, der kan opfattes som et slags regimeskifte, var sandsynligvis forårsaget af overfiskning og ændringer i det fysiske miljø (dvs. temperatur, saltholdighed og iltindhold), som i sidste ende påvirkede hele fødenettet fra primærproducenter til toppredatorer. Erkendelsen af økosystemets sammenhængende måde at virke på, opfordrer derfor til integration af både eksterne og interne faktorer i en model udvikling. Med henblik herpå kombineredes en teoretisk populationsdynamisk model med avancerede statistiske metoder. Herved blev det muligt at udvikle en fødenetmodel for den centrale Østersø der integrerer samspillet mellem arterne, mellem torsk og byttedyr sild og brisling, med eksterne faktorer som kommerciel fiskeri, dyreplankton og klima virkninger. Desuden, ved at koble modeller på tværs af interesser og fagområder, herunder økologiske, klimatiske og bio-økonomiske modeller, blev det muligt at redegøre for forvaltningen konsekvenser over en bred vifte af mål og definere de overordnede optimale løsninger. Baseret på de afledte mønstre i samspillet mellem arterne og de ydre faktorer gennem fiskeri, dyreplankton og klima, har fødenetmodellen været i stand til at genskabe og forklare den tidligere variation i fiskebestande i Østersøen. Som sådan, kan den såkaldte gadoid outburst (dvs. de stærke og synkrone stigning i flere Atlanterhavs torskebestandene, herunder Østersøen i slutningen af 1970'erne) for Østersøens 6 vedkommende forklares ved en periode med historisk lav fiskeritryk i kombination med rekordhøje saltholdigheder, der betinger en succesfuld reproduktion i hele Østersøen i sær på grund af en høj æg- og larveoverlevelse. Endvidere er den koblede klima-fødenet model brugt til at undersøge, om man i lyset af den nuværende viden om klimaeffekter, fiskeri og samspillet mellem arterne, i bakspejlet kunne have undgået sammenbrud af torskebestanden i Østersøen. Til dette formål formuleres en adaptiv forvaltning strategi for fastsættelse af præventive udnyttelsesniveauer. Ved at tilpasse fiskeritrykket til klimaforhold og samspillet mellem arterne, viste simulationer, at vi faktisk kunne have forhindret et sammenbrud af torskebestanden ved at sikre en bestand over økologisk sikkert niveau. Endelig, i et forsøg på at indføre et mere sammenhængende og holistisk forvaltningsperspektiv, er fødenetmodellen koblet til en simpel bio-økonomisk model, der sigter mod at sammenligne indtægterne for torskefiskeriet i Østersøen på tværs af forvaltningsstrategier. Baseret på disse resultater, hævdes det at en mere adaptive forvaltning strategi ikke kun vil være økologisk forsvarlig, men økonomisk rentabel som følge af øgede landinger og reduceret. Disse resultater understøtter vigtigheden af at investere i naturlige kapital (dvs. sikring af fremtiden bestandsstørrelse) som en langsigtet forvaltningsstrategi for torsk i Østersøen. Sammenfattende ved at forbedre den metodologiske udvikling i retning af stigende inddragelse af en økologisk realisme i fiskerimodellering og udbygning af tværfaglig udveksling af videnskabelige begreber og ideer, er det mit håb at denne Ph. D. kan bidrage til en mere regionspecifik og økosystembaseret forvaltning i den centrale del af Østersøen. Dette kunne være et vigtigt første skridt hen imod at opnå langsigtet bæredygtighed i havfiskeri i fremtiden. 7 List of original publications This dissertation is based on the following four papers, referred throughout the text by their Roman numerals: I. Lindegren M, Möllmann C, Nielsen A, Stenseth N. C. (2009) Preventing the Collapse of the Baltic Cod Stock through an Ecosystem-based management approach. Proceedings of the National Academy of Sciences of the United States of America 106: II. Lindegren M, Möllmann C, Nielsen A, Brander K, MacKenzie B, Stenseth N. C. Ecological Forecasting under Climate Change the case of Baltic cod. Proceedings of the Royal Society B-Biological Sciences (in revision) III. Lindegren M, Möllmann C, Hansson L. A. Biomanipulation - a tool in Marine Ecosystem Management and Restoration? Ecological Applications (in revision) IV. Lindegren M, Diekmann R, Möllmann C. Regime shifts, Resilience and Recovery of a Local Cod Stock. Marine Ecological Progress Series (in press) 8 1. Introduction and background Marine ecosystems are in a rapid state of deterioration, as evidenced by the poor conditions of commercially important fish stocks worldwide (Hutchings & Reynolds 2004; Worm et al., 2009). Whether caused by overfishing (Myers et al., 1997; Jackson et al., 2001), climate change (Beaugrand et al., 2003; Lilly et al., 2008) or a combination of several factors (Ottersen et al., 2006; Anderson et al., 2008) policy makers, managers and the general public are rightly concerned that marine ecosystem and the services which they supply are under threat from a wide range of human activities. In order to achieve future ecological and economical sustainability, a new holistic paradigm for managing marine resources is therefore needed (Hilborn 2007). To that end, an ecosystem approach has been adopted as the fundamental principle for sustainable marine management regionally (i.e., within EU) and internationally (FAO 2003; EC 2009). Though several definitions exist, all of these can be considered as expressions of integrated measures that, to varying degrees, are embracing the concept of ecosystem-based management (UNEP 2006). While spatial scales and issues addressed differ between areas and ecosystems, the fundamental principles: (i) integrates ecological, social, and economic goals; (ii) recognize humans as key components of the ecosystem; (iii) incorporates understanding of ecosystem processes and how ecosystems respond to environmental perturbations; and (iv) addresses the complexity of natural processes and social systems by using an adaptive management approach in the face of uncertainty (see and MacLeod et al. (2005) for details). Ecosystem-based fisheries management (EBFM), i.e., integrating multiple drivers and services in a common fisheries management framework (Pikitch et al., 2004; Marasco et al., 2007), forms a central part of ecosystem-based marine management and focuses primarily on sustaining healthy marine ecosystems and the fisheries they support (Pikitch et al., 2004). However, despite the recent conceptual developments (Marasco et al., 2007), an EBFM-approach is far from operational, due mainly to methodological shortcomings in terms of incorporating ecological detail, representing climate effects and providing estimates of uncertainty in fisheries models (deyoung et al., 2004; Brander 2007). Hence, single-sector (species) based management practices still remain the basic means of resource exploitation in marine ecosystems worldwide. 9 The dramatic declines of North Atlantic cod stocks (Gadus morhua) provide a striking and often cited example of the failure of fisheries management during the past (Murawski et al., 1997; Hutchings 2000). These stock collapses mainly resulted from overfishing (Myers et al., 1997; Jackson et al., 2001) and climate driven declines in productivity, generally caused by hydrographic changes (e.g., temperature and salinity) and indirect alterations in food composition and abundance (i.e., zooplankton) impairing growth and survival of early lifehistory stages and eventually reproductive output, i.e., recruitment (Beaugrand et al., 2003; Lilly et al., 2008). In accordance with this, the recruitment failure of Eastern Baltic cod was mainly caused by high egg and larval mortalities due to climate-induced hydrographic changes in salinity and oxygen conditions in the Central Baltic Sea (Köster et al., 2005; Möllmann et al., 2008). In several areas the collapse of cod stocks was part of or one of the major factors inducing large-scale reorganization of ecosystems (Frank et al. 2005, Möllmann et al., 2009). These so-called ecosystem regime shifts are frequently caused by climatic changes (Hare & Mantua 2000, Scheffer et al., 2001) and/or by overexploitation resulting in cascading trophic interactions (Frank et al., 2005; Scheffer et al., 2005). Similarly to other areas, the Central Baltic Sea underwent both regime shifts and trophic cascades (Casini et al., 2008; Möllmann et al., 2009). Such alterations in ecosystem structure typically affect species interactions, eventually influencing food-web dynamics through both positive and negative feedback loops (Ives 1995). The recognition of the ecosystem context in the collapse of fish stocks calls for the integration of external and internal drivers, e.g., commercial fishing, climate change and species interaction, in the development and implementation of ecosystem-based modelling tools fit for achieving sustainable development within the framework of EBFM. 2. Motivation and objectives of the study The overarching aim of this thesis is to develop an ecosystem-based modelling tool capable of incorporating the effects of climate change, species interactions and fisheries in a common EBFM framework for the Central Baltic ecosystem. To that end, a theoretical approach for modelling multi-species population dynamics (Ives 1995; Ives et al., 2003; Ripa & Ives 2003) was combined with advanced statistical methods (Harvey 1989) in order to develop a stochastic food-web model integrating species interactions, between cod and the forage fish species herring (Clupea harengus) and sprat (Sprattus sprattus), with 10 external forcing through commercial fishing,
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