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In the Baltic Sea and North Sea, the sustainability of bottom-trawl fisheries is threatened by many issues, such as the impact of climate change on exploited demersal stocks, long-term overfishing, and the European Landing Obligation for quoted species. This thesis identifies challenges to bycatch and harvesting patterns of targeted species in four relevant Baltic Sea and North Sea bottom-trawl fisheries, and presents six recent research papers that investigate species and size selectivity of fishing gears that can contribute to the mitigation of fishery-specific issues. The bycatch of flatfish species, such as flounder (Platichthys flesus), dab (Limanda limanda), and plaice (Pleuronectes platessa), is an issue in the Baltic Sea otter-trawl fisheries targeting cod. Paper I investigates if the application of a sorting grid, originally proposed by the fishing industry, can reduce the bycatch of flatfish in these fisheries. The results revealed a large reduction in flatfish bycatch with the added advantage of providing an additional escape possibility for undersized cod without compromising the catchability of marketable sizes. Paper II evaluates an alternative to the grid system proposed in Paper I: the exploitation of fish behaviour to address the problem of flatfish bycatch in the Baltic cod-directed trawl fisheries. The results demonstrate that a simple flatfish excluder in the lower panel of the extension piece of the trawl can effectively reduce the bycatch of flatfish while maintaining the catches of the targeted cod. Paper II also introduces a novel methodology, based on video recordings, for quantitative evaluation of fish behaviour in relation to selection devices. The method produces behavioural tree diagrams representing and quantifying behavioural patterns in relation to the selection device being assessed. Double bootstrapping is used to account for the uncertainty caused by the limited number of fish observations and natural variation in fish behaviour. In the frame of the balanced harvesting paradigm, Paper III explores the feasibility of achieving alternative harvesting patterns for Baltic cod. The intended alternative harvesting pattern targets medium-sized cod and avoids catches of juvenile and the largest, most productive cod. Paper III demonstrates experimentally that a bell-shaped retention probability curve, usually associated with gillnet fisheries, can also be achieved in trawl gears by combining standard grid and codend devices. The current poor status of Baltic cod stocks has led to drastic quota reductions to historical minima. In conformity with the Landing Obligation for quoted species, cod has become a choke species to the emergent flatfish fisheries in the Baltic Sea. Based on the insights obtained in Paper II, Paper IV investigates if the behavioural patterns observed for flatfish and cod in the trawl gear could be used to limit cod catches in flatfish-directed trawl fisheries. Paper IV demonstrates experimentally that a large reduction in cod catches can be achieved by removing a section of the top panel of the extension piece. As for Baltic cod, drastic reductions in fishing quotas for North Sea and Kattegat cod have occurred in recent years as a management response to the poor status of these populations. Consequently, cod is a choke species in the North Sea otter-trawl fisheries targeting Nephrops. An efficient separation of Nephrops from fish species in the trawl could lead to better management of the available quotas. Paper V investigated the potential of square-mesh sieve panels to separate Nephrops from fish species. Results from experimentally testing four different sieve panels revealed that most fish species were efficiently separated from Nephrops. However, the sieving efficiency (probability to pass through the meshes of the sieve panel) for the largest, most valuable Nephrops remained too low. Therefore, the resulting separation rates of fish species and marketable Nephrops was found suboptimal and not suitable for commercial fisheries. Recent shifts in North Sea ecosystems have reduced the abundance of natural fish predators in the fishing grounds of brown shrimp (Crangon crangon), making the North Sea beam-trawl fishery the major source of mortality for the targeted shrimp. This new role has also caused concern over the sustainability of the harvesting patterns in this fishery. In the search for optimal harvesting patterns of brown shrimp, Paper VI delivers a predictive framework for codend size selection of brown shrimp. The framework that was created is based on a large selectivity dataset and allows predictions of codend size selection considering the effect of mesh size and mesh orientation. The predictive framework presented in Paper VI could aid fishery modellers to explore population dynamics of brown shrimp under a wide range of predicted exploitation pattern scenarios. Finally, the work presented in this thesis provides technological advances and a knowledge base that suggests how to reduce the bycatch of unwanted species and generate alternative harvest patterns in different Baltic Sea and North Sea trawl fisheries. |
