Evaluating the spatio-temporal impacts of environmental, political and economic changes on the stocks of Northeast Atlantic mackerel and North Sea herring and fisheries

Abstract

The future availability of fisheries resources is highly dependent on international coordination of management decisions, environmental conditions as well as fleet behaviour. Managing straddling and transboundary stocks (i.e. crossing Exclusive Economic Zone boundaries into adjacent international waters) is currently especially difficult due to many changes. Two such stocks, that are highly valuable for European fleets and provide many job opportunities, are the Northeast Atlantic (NEA) mackerel (Scomber scombrus) and North Sea autumn spawning (NSAS) herring (Clupea harengus) stocks. Current conflicts resulting from lack of management coordination or changes in environmental conditions seem, however, to threaten the status of those two stocks. These, in turn, can massively impair the livelihood of European fleets and corresponding employees targeting NEA mackerel and NSAS herring. It is therefore important to evaluate the effects of new management measures and other major impact factors on both the resource availability as well as the fleet behaviour as they are interdependent. Dynamic bio-economic models are now more commonly used as tools for fisheries management. They incorporate anthropogenic as well as natural processes to generate a better understanding of feedback mechanisms between the two systems. This thesis addresses the need to evaluate the impacts of current environmental, economic and political issues on the highly valuable NEA mackerel and NSAS herring stocks and the corresponding fisheries by applying and further developing the FishRent model. It is an age-structured simulation and optimization model that incorporates detailed stock and fleet dynamics on a short- to mid-term time-frame. By identifying the optimal effort allocation under a set of constraints, it can determine the equilibrium state that optimises a certain target variable, i.e. net profit. In the first chapter, the structure of eight pelagic fleets was investigated in order to understand the underlying data and illustrate possible differences. Furthermore, an existing version of FishRent was adapted from a demersal fishery to the pelagic and impacts of external factors (i.e. changes in recruitment, fish and fuel prices and an adaptation of the quota repartition key) on fleet net profit and stock biomass availability were determined on a temporal scale. In all scenarios, the Irish and German fleets were most vulnerable due to being very close to the economic break-even point. The implementation of a new quota repartition key according to biomass distribution of NEA mackerel and NSAS herring had the largest effect on all fleets, leading to losses of the German, Dutch and Danish fleets. The UK and Irish fleets, on the other hand, more than doubled their profit within a year. Alterations in fish and fuel prices had the second largest impacts on the eight fleets and the UK, Icelandic and large-scale Irish fleets had to disinvest nearly half of their fleet due to reporting losses. Reduced recruitment and a continued NSAS herring recruitment failure had the least influence on fleet profit compared to the other scenarios tested within this Chapter.