Improving species and size selectivity in the Baltic cod trawl fishery with two simple codend designs
Introduction
In 1995 the Baltic Sea was one of the first EU (European Union) regions where selective sorting devices were adopted into legislation in the cod (Gadus morhua) trawl fishery (Madsen, 2007; Valentinsson et al., 2019). Technical regulations aimed at improving size selectivity of cod have been a key management strategy since. Fisheries regulations have changed on several occasions over the years to improve selectivity and reduce discards of cod (Suuronen et al., 2007; Feekings et al., 2013; Valentinsson et al., 2019), with limited attention on other commercial important species caught in cod directed fisheries.
A major change in the new EU common fishery policy was the introduction of a landing obligation (Sardà et al., 2016; Valentinsson et al., 2019). This was introduced in 2015 for cod and in 2017 for plaice (Pleuronectes platessa) in the Baltic Sea. The landing obligation implies a transfer from landing quotas to catch quotas. A minimum conservation reference size (MCRS) was introduced and fish below the MCRS cannot be used for human consumption (cod 35 cm; plaice 25 cm). Hence, these fish have a low economic value for fishermen but are still subtracted from the quota. This likely explains the relatively high discard rates of cod below MCRS since the introduction of the landing obligation (Valentinsson et al., 2019). The discard rate of cod was 8% in the western Baltic Sea (subdivision 22–24; ICES, 2019a) and 16 % in the eastern Baltic Sea (subdivision 25–32; ICES, 2019b) in 2018. Low landings of cod below the MCRS indicates the same pattern (ICES, 2019a, b).
The spawning stock biomass of the eastern cod stock in the Baltic Sea is low and declining (ICES, 2019a). The International Council for the Exploration of the Sea (ICES) advises that there should be zero catch for this species in 2020. The recruitment of the western cod stock (ICES subdivisions 22–24) has been low since 1999, with the 2018 and 2019 year classes being the lowest observed in more than 30 years (ICES, 2019b).
High discard rates for other commercially important species in the cod trawl fishery have also been observed. The discard rate of flounder (Platichthys flesus) in the cod trawl fishery was 33 % in 2018 in the waters west of Bornholm and in the southwestern central Baltic Sea (subdivision 24–32; ICES, 2019c), while the discard rate of plaice in the Baltic Sea (subdivision 24–32; ICES, 2019d) was 30 %. Flounder are often discarded due to their low economic value. Furthermore, fishers complain that flounder block the codend meshes, reducing the selectivity of the codend.
According to EU legislation, two alternative codend designs are allowed in the Baltic Sea (Valentinsson et al., 2019). The Bacoma codend with a square mesh panel inserted in the top panel of a conventional diamond-mesh codend and the T90 codend with conventional diamond meshes orientated at 90 degrees. Experiments conducted with a research vessel indicated the potential for improving selectivity for cod and plaice by changing mesh size and form in the conventional Bacoma codend (Wienbeck et al., 2014). Studies from other areas suggest that the shape of diamond meshes fit the morphology of plaice better than square meshes, opposed to cod (Frandsen et al., 2010; Madsen et al., 2006).
In this study, the performance of two codend designs were tested, using existing mesh types in new configurations. We used the conventional Bacoma codend (henceforth “Reference Bacoma”) as a reference codend for development and comparison of the two new codend designs. The goal was to develop a simple solution that can be easily adapted to improve size and species selection in demersal cod trawl fisheries.
The aim of the study was to reduce the catch of unwanted small cod, flounder and plaice which have a low market value. We paid attention on two different solutions: 1) improving size selectivity of cod in fisheries where the catch mainly consists of cod; and 2) a solution for a cod fishery where flounder and plaice are regularly caught as commercial bycatch. To ensure commercial application, the codends were tested from a commercial fishing vessel.
Section snippets
Codend designs
The new codend designs were based on the Reference Bacoma. This codend was chosen as a starting point as it is widely used and has two different mesh types (opposed to the alternative T90 codend), providing potential for optimization.
The Reference Bacoma has a stretched length of 6.3 m with 50 open diamond meshes crosswise in the lower panel, with a nominal inside mesh size of 105 mm (Fig. 1). We inserted a square-mesh panel with a nominal mesh size of 120 mm and a stretched length of about 5.6
Results
A total of 21 hauls with the twin trawl rig were conducted for the Square Bacoma and the New Bacoma, respectively. The average (± standard deviation) towing depth for the New Bacoma was 75 ± 2.5 m and 73 ± 3.2 m for the Square Bacoma.
Cod, flounder and plaice were caught in all 21 hauls in both experiments (Table 1), while catches of other species were limited (<5 kg) in all hauls. The average catch of cod in the reference codend was higher in Experiment 1 than in Experiment 2, however, the
Discussion
The New Bacoma codend significantly reduced catches of cod below 45 cm compared to the conventional Bacoma codend. Consequently, when using this codend there will be an immediate loss of marketable cod with the present MCRS of 35 cm. If the MCRS was reduced to 30 cm, similar to that in Kattegat and Skagerrak, allowing 30–35 cm fish to be sold for human consumption, loss of marketable cod would be reduced. However, promoting a more directed fishery for smaller cod may pose a risk to cod stocks.
CRediT authorship contribution statement
Niels Madsen: Conceptualization, Writing - review & editing, Data curation, Formal analysis, Funding acquisition, Investigation, Methodology, Project administration, Visualization, Writing - original draft. Ólafur A. Ingólfsson: Writing - review & editing, Formal analysis, Visualization, Writing - original draft. Hans Nilsson: Conceptualization, Methodology, Writing - original draft, Writing - review & editing. Petri Suuronen: Writing - review & editing, Writing - original draft.
Declaration of Competing Interest
The authors report no declarations of interest.
Acnowledegments
Thanks to the net makers at “Nexø Vodbinderi”, the skipper and crew of “Lis-Hansa” and DTU Aqua technicians for help with this study. We also thank our partners in the study, collegues and 3 anonymous referees for very usefull comments. The European Commission(Contract MARE/2010/11, LOT 1 (SI2.601456)) funded the study. The opinion expressed in this work does not reflect the views of the European Commission.
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2022, Ocean and Coastal ManagementCitation Excerpt :Considering the large catch losses of flatfish species in the topless trawl observed in the NW Atlantic trials (Chosid et al., 2008; Pol et al., 2003), applying topless trawls in the Baltic Sea might also reduce the catch efficiency on targeted flatfish species to unacceptable levels. Recent studies have demonstrated the potential of using codend selectivity to reduce the bycatch of Baltic cod beyond species minimum conservation reference size, without affecting the catchability of flatfish species (Madsen et al., 2021; Wienbeck et al., 2014). In particular, commercial sea trials using a square-mesh codend (so-called New Bacoma) of ∼125 mm mesh size significantly reduced cod catches up to 50 cm (Madsen et al., 2021).
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Present address: Section of Biology and Environmental Science, Department of Chemistry and Bioscience, Aalborg University, Fredrik Bajers Vej 7H, 9220 Aalborg, Denmark.