Identifying simple and cost-effective gear solutions for an effective implementation of the new EU Common Fisheries Policy (CFP)

With the new Common Fisheries Policy (CFP) the European Union (EU) aimed at eliminating the discard of commercial species, introducing the obligation to land all catches (“landing obligation”). This catch-based approach, where all sizes of regulated species have to be landed and counted against quota, is designed to encourage fishermen to minimize unwanted catches.

Therefore, fishermen are now in need of fishing gear options to cope with the variability in unwanted catches and maximize their profit within the allowed catch limits. Flexible and specialized gear solutions, which can be used on a haul-by-haul basis to adjust size and species selectivity, can lead to an effective implementation of the CFP while maintaining the economic viability of the fishery.

These solutions are particularly urgent in mixed trawl fisheries, where “choke” species can limit the exploitation of more productive stocks. Therefore, this study focused on the Danish Nephrops (Nephrops norvegicus) directed mixed trawl fishery, one of the economically most important fisheries in Europe.

This multispecies fishery has one of the highest rates of unwanted catches, and is expected to be strongly affected by a fully implemented and controlled landing obligation. We investigated flexible gear modifications that could support alternative harvest strategies, such as the reduction of both undersized and commercial sized fish bycatch or the retention of only the most valuable bycatch species and sizes.

The thesis consists of a review and four papers. Paper I contains the development and test of the first counter-herding device for Nephrops-directed trawl fisheries. This flexible anterior modification, easily mountable and de-mountable on the gear at a haul-by-haul level, was designed to lead fish out of the trawl path.

Its efficiency varied across species and sizes, but was consistent regardless of diel period. The results showed a major reduction of catches of potentially unwanted fish species, in particular haddock (Melanogrammus aeglefinus) and whiting (Merlangius merlangus), with no effect on Nephrops catches.

Paper II focuses on a horizontally divided trawl codend, which could lead to a flexible separation of the catch in different compartments of the trawl. We attempted to use visual stimulation to improve species separation. Using Light Emitting Diodes (LED), we investigated if either positive or negative phototaxis could be used to improve fish vertical separation from Nephrops.

The results showed significant changes in vertical separation but no clear species-specific phototactic response. Moreover, overall LED lights increased the proportion of individuals entering the lower compartment together with Nephrops. Paper III continues the research on the horizontally divided trawl codend, but applying other types of behavioural stimulators.

We investigated if and to which extent it is possible to improve the vertical separation of fish from Nephrops by adding active stimulators designed to exploit fish avoidance behaviour. We tested two types of behaviour stimulators: a chain curtain at the entrance of the lower compartment and a set of rising float-lines ahead of the point of separation.

The results showed that species separation can be partially improved by the stimulators, but the effect may not be sufficient to justify the additional complexity in design respect to the baseline. Paper IV describes a meta-analytical approach to predict the size-selectivity of a gear with a combination of Bycatch Reduction Devices (BRDs) and to compare their performance under different catch scenarios.

We applied this theoretical approach to the Nephrops-directed trawl fishery, to identify the most pertinent BRDs combinations and the alternative harvest strategies that they could support. By including the results obtained in the previous papers, as well as relevant BRDs available in literature, we predicted the selectivity of up to 100 possible combinations.

Their performance was investigated for the target species, Nephrops, and two bycatch species, cod (Gadus morhua) and haddock. This meta-analytical approach can accelerate the process of identifying optimal uses of flexible gear solutions, broadening fishermen’s options when coping with the EU landing obligation.