Abstract

This report summarizes the outcome of the project “Improved knowledge for management of flatfish stocks in Danish waters”, funded by the European Maritime and Fisheries Fund and the Danish Fisheries Agency (“Fiskeri, natur og miljø - Marin biodiversitet”). The project focused on improving the biological data to support advice and sustainable management of three flatfish species in Denmark: turbot (Scophthalmus maximus), common sole (Solea solea) and European plaice (Pleuronectes platessa). These species are important to Danish fisheries, and they currently suffer from a lack of information on a number of questions of importance for proper stock assessment and management. While there were differences between knowledge gaps of species, there were also common themes, such as the identification of biological populations, their geographical distributions and migration between areas which are currently challenging a sustainable management, in particular in the Skagerrak/Kattegat, which was a focus area in all species in the current study.

In turbot, we reviewed how available evidence for population structure matches current stock assessment and management units for the species. Importantly, Kattegat and Skagerrak are currently merged into one management unit. However, merging Kattegat and Skagerrak did not seem to match available biological evidence, suggesting the presence of two different biological populations within the management unit, with interaction between North Sea/Atlantic and Baltic Sea populations mainly occurring in the Kattegat. This is based on substantial data, including genetic evidence and long-term data series. However, more information on the distribution of these populations in and outside spawning periods will be needed to refine current stock assessment and management practices to better match underlying biological variation (e.g. geographical distribution of populations across life stages). In addition, we addressed issues related to data quality for the stock assessment and confirmed the validity of catch data series and survey design in relation to the distribution of fishing efforts.

In sole, we used genetic data in combination with otolith microchemistry to identify populations and their migrations in the North Sea-Baltic Sea transition zone, with a particular focus on Kattegat/Skagerrak. In this area, previous work had indicated evidence for population mixing not well aligned with current assessment units, where Skagerrak and Kattegat are merged with the rest of the Baltic Sea transition zone. We found evidence for the presence of two biological populations in our data, with a strong degree of mixing in particular in the Skagerrak, with indications of the presence of spawning individuals from both populations. In addition, we found stronger mixing outside spawning season and also found mixing in other areas (e.g. northern part of the North Sea). Migration data and larval origin supported a hypothesis of extended distribution into the Skagerrak of a Baltic Sea population, and hence recruits to the western Baltic Sea may have been spawned as far north as the Skagerrak, where our current study indicates that both Baltic Sea and North Sea/Atlantic populations may reproduce. However, data also indicated that common sole, once recruited to a specific geographical area within the distribution of the two populations, are fairly stationary and do not migrate to a notable extent. Collectively, the results have provided an improved understanding of biological variability and population dynamics in the management areas, supporting a change in management procedures towards a population based assessment rather than the present area based assessment. However, while such population based stock assessment and management have been implemented in other species, further work, in particular related to more fine scale and quantitative estimation of population presence and migration will be needed to implement this change for sole.

In plaice, we focused on issues related to an apparent mis-match between stock assessment and quotas and the perception of the stock status by fishermen. In particular, in the Skagerrak where fishermen since 2017 are far from utilizing the quota that is based on the advice. In the project, we found distribution shifts possibly resulting in the availability of fewer plaice in the Skagerrak, matching the perception from the fishery. While the underlying reasons for these changes are unknown at present, we discuss potential sources of this variability, in particular in relation to potential local depletion and environmental changes that may have driven distribution changes in the Skagerrak.

Collectively, the project has improved our knowledge on stock dynamics and biological parameters to support the quality of stock assessment and management in all three species significantly. While there are still open questions, the results have also shown interesting common patterns across species, such as the identification of specific geographical areas with high level of interaction between biological populations in several species. As such, and in addition to improving single species biological advice, our project could also have broader impacts, for example in relation to future ecosystem based approaches to management, where an integrated understanding across species will be needed to secure an implementation on a sustainable basis. In addition, climate change will also challenge current management procedures and the balance of quota allocation between countries through changes in distributions and migrations of exploited populations. As such, an improved understanding and quantification of these changes will provide crucial decision support for future management.
Original languageEnglish
Place of PublicationKgs. Lyngby, Denmark
PublisherDTU Aqua
Number of pages74
ISBN (Electronic)978-87-7481-416-0
DOIs
Publication statusPublished - 2024
SeriesDTU Aqua-rapport
Number473-2024
ISSN1395-8216

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