Effects of and on the food-web when fishing for maximum sustainable yields in the southern North Sea

• Main Author: Stäbler, Moritz
• Other Authors: Temming, Axel (Prof. Dr.)
• Format: Doctoral or Postdoctoral Thesis
• Language: English
• Published: Staats- und Universitätsbibliothek Hamburg Carl von Ossietzky 2017
• Subjects: Maximaler Dauerertrag; Nahrungsnetz-Modell; Ecopath; Mehrarten-Fischerei; Gemischte Fischerei; Fisheries; North Sea; Food-web modelling; Ecosystem-based management; Maximum sustainable yield; Multispecies fisheries; Mixed Fisheries; 333.7 Natürliche Ressourcen; Energie und Umwelt; 42.94 Meeresbiologie; 48.67 Fischerei; Fischerei; Nordsee; Nahrungskette; Ökosystemmanagement; Scholle; Seezunge; Kabeljau; Nordseegarnele; Ökosystem; ddc:333.7; kabeljau
• Online Access: http://nbn-resolving.de/urn:nbn:de:gbv:18-87554; https://ediss.sub.uni-hamburg.de/handle/ediss/7375

A main objective of fisheries management for the southern part of the North Sea under the European Commission’s Common Fisheries Policy is the achievement of maximum sustainable yields (MSY) from each stock. However, the stocks are not exploited in isolation, but are linked through biological and technical interactions, and the food-web and ecosystem they are part of. This PhD thesis describes the functioning of the southern North Sea’s food-web. It evaluates if, given this functioning, optimum yields of plaice, sole, cod, and brown shrimp can be achieved simultaneously. It also tests if such optimum solutions can be aligned with proxies of good environmental status. Potential changes in the catchabilities of sole and plaice, and their impacts upon MSY fishing are addressed. With the southern North Sea being subject to important ecosystem changes, such as increases in marine mammals and decreased nutrient loads through de-eutrophication measures, this thesis also investigates these processes’ consequences for fishing yields and strategies. Main tool of this thesis is the parametrization and use of a food-web model of ICES areas IVb and c (Figure I on page VII), using the Ecopath with Ecosim approach and software. The time-static Ecopath model represents the structure and functioning of the southern North, which turned out to be a highly connected, mature food-web (Chapter 1). In Chapter 2, the time-dynamic Ecosim model is subjected to a range of different fishing effort regimes to seek solutions leading to concurrent optimum yields of plaice, sole, cod and brown shrimps; and to test for their compliance with proxies of good environmental status. Its results show that direct and indirect trophic interactions between the four species considerably impair these endeavours. Running empirical data analyses, drivers of changes in the catchabilities of sole and plaice were identified, and their consequences upon MSY fishing and bycatch examined using Ecosim in Chapter 3. The results indicate density-dependent changes in ...