Optimal and Precautionary Management of the German Bight in the Presence of Thresholds

Abstract

The fishery in the German Bight and the greater North Sea are part of a highly interconnected socio-ecological system (SES). This SES consists of an ecosystem consisting of multiple species which are harvested by a number of different fishing métiers and sold to consumers. In this thesis I investigate the importance of the various interconnections between the dynamics of different species and human activities, the implications for the vulnerability of the fisheries. Managing the fisheries at the heart of this SES is made more complicated by the existence of tipping points in the economic viability of the fisheries and within the ecosystem, which can be triggered by a wide variety of drivers.

In Chapter 2 the results of a targeted literature review on empirical assessments of SES and tipping points in the marine realm and their use in ecosystem-based management are presented. The literature contains a wide variety of terminologies and definitions of these concepts, making it difficult to discern guiding principles for the practical application. Furthermore, existing work in empirical assessment of SES vulnerabilities (to tipping points or otherwise) tends to over-simplify the behaviour of human actors. This is the case, even while human actors tend to be one of the largest pressures on marine ecosystems with their behaviour significantly impacting stock dynamics and ecosystem composition.

Chapter 3 gives an overview of the history of the coastal fishery targeting mainly plaice and sole in the North Sea and the challenges faced due to climate change. Climate change causes changes in the spatial distribution of fish stocks due to a changing temperature gradient in the North Sea as well as changes in the productivity of the stocks. This in turn has consequences for the ecosystem and socio-ecological system the stocks are a part of. This chapter describes the challenges these changes pose for the fishery in relation to the fishery's adaptive capacity. As the fisheries are at the bottom with regards to the order of decision making on European fisheries their leeway to adapt to these changes is severely limited. Consequently, changes in management practices may be necessary if this fishery is to be maintained.

Chapter 4 presents a model that includes interaction between species dynamics within the ecosystem, through simultaneous harvests, and through consumer substitution preferences. The main results, besides replicating the finding that bycatch can increase harvesting mortality, are that simultaneous harvest properties may have no effects on stocks, and that the harvesting economy may change dramatically if discards are banned. Understanding the interrelation of simultaneous harvests and market forces is essential in designing overarching policy where economic effects, such as changing employment, need to be considered while also ensuring sustainable use of the ecosystem.

In Chapter 5 a novel framework to assess sensitivity, adaptive capacity, and vulnerability of an actor to a driver is developed based on derivatives of equations describing their behaviour. The framework is then applied to the bio-economic model developed in Chapter \ref{cpt:modellingInteraction}. The results indicate that fishers' profits are most vulnerable to changes in plaice prices, returns to effort, stock harvesting efficiency of plaice, plaice stocks, and wages. These results provide additional context for decision makers setting harvest policies, especially if the existence of the fishery and not only the protection of the stock is the management goal.

Finally the optimal harvest quantities are derived based on maximizing the intertemporal welfare of the representative household in Chapter 6. I find that the interaction effects play an important role in specifying optimal harvests. Furthermore, their importance in the design of total allowable catch and quantity tax based management is investigated. While it may superficially seem that quota based management would cancel out all interactions besides those within the ecosystem, the incentive of fishers to violate the quota or a discard ban will depend on the specifics of the simultaneous catch properties and substitution behaviour in consumer demand. Consequently, this incentive can be minimized by taking these properties into account while designing the management policy.