Discarding Discards: Identification of influential factors and possible mitigation tools in demersal trawl fisheries

Discarding of aquatic organisms is a global problem in the world’s fisheries, where more than 7 million tonnes are caught and subsequently discarded each year. The understanding of what drives discarding can help provide mitigation measures in the future. Altering management measures which result in high discard rates/ratios may prove beneficial not only to the economic viability of the fishery but also to the biological functioning of the ecosystem.

The present Ph.D. thesis, titled “Discarding Discards: Identification of influential factors and possible mitigation tools in demersal trawl fisheries”, investigates discarding practices in demersal trawl fisheries and identifies possibilities for reducing discards. In focus, the factors that determine discards, including environmental factors, fishing methods, management regulations, and biological factors have been analysed.

This includes an examination of the efficiency of technical regulations currently in force and retrospective analyses of the efficiency of such measures in the past. The thesis consists of a review and 4 papers. Paper I is an investigation of the factors that can potentially influence the discarding of commercial species in the Kattegat.

Previous studies that have investigated the factors that influence discarding have typically focused on the discarded portion as a whole, without considering that discards above and below minimum landing size (MLS) occur for different reasons. The study documented that the factors influential to discarding were different for the two subgroups (under and over MLS) and also for the different species.

Paper II focuses on discarding in the Baltic Sea cod (Gadus morhua) trawl fishery. Over the past 15 years extensive work has been conducted to improve the selectivity of the gears and subsequently reduce discards. This study investigated: i) the effects that technical measures, namely gear selectivity and minimum landing size (MLS), had on discards and; ii) a wide range of factors that can influence discards and may blur a potential effect of improved selectivity.

The results showed that when gear regulations are implemented correctly they are an effective management measure. However, their effectiveness is influenced by a diverse range of factors that if unaccounted for may distort a potential effect of improved/hampered selectivity. Paper III compiles discard data from 11370 fishing events collected across seven European Union (EU) Member States for the North Sea over the period 2003-2010.

Knowledge about the spatio-temporal nature of discards is imperative to researchers and regulators but is often lacking. Here we analysed the spatial and temporal distribution of cod discards throughout the entire North Sea together with the main driving factors behind its occurrence. We discuss how such information can be used to improve future fishing activities and their subsequent catch compositions under a discard ban.

Paper IV describes the discarding of plaice (Pleuronectes platessa) in the North Sea. Plaice play an important role in the North Sea benthic ecosystem, being one of the most abundant flatfish species and one of the most important species for the fishery. Nevertheless, the plaice fishery in the North Sea is characterised by a high discard ratio, where approximately 50% (by weight) of plaice are discarded.

Here we describe the general patterns in these data with particular focus on factors that could be important for management strategies in the future