Caught and released: an overview of fishes’ sensitivity to being discarded as a tool to aid pursuing ecosystem-based management (COPE) (39416)

Project Details


The new European Common Fisheries Policy (CFP) has introduced substantial change to fisheries management, including a phased introduction of an obligation to land all catches taken from regulated stocks. However, the article 15 paragraph 4b of the CFP regulation (EU) No 1380/2013, allows for the possibility of returning at sea species for which ‘scientific evidence demonstrates high survival rates’. Such exemptions aims at reducing the risk under the European landing obligation of bringing onshore individuals that may otherwise survive the capture-and-discard process.

Central to any proposal for an exemption is the requirement for clear and defensible scientific evidence on discard survival rates. The ICES Workshop on Methods for Estimating Discard Survival (WKMEDS) has defined an appropriate method for assessing discard survival rates. In the evaluation of the scientific documentation, the specificity of each different species and fishery is emphasized as the fish is exposed to an array of stressors and potentially injurious events that will affect its survival potential during the capture and discard processes.

The two main aims of the project were to:
1) obtain survival rates for European plaice (Pleuronectes platessa) for otter trawl and Danish seine in Skagerrak. Additionally for the bottom trawl, effects of season, target species and catch separation were investigated. These studies followed the guidelines of ICES WKMEDS.
2) provide a catalogue where published survival rates of different flatfish species subject to the landing obligation under the Common Fisheries Policy (CFP) are put in relation to the discard ratios from Danish landings of different fisheries (métiers) in the North Sea, Skagerrak and Kattegat.

The project involved the following sub-aims:
3) to identify species for which sufficient data was available for conducting a meta-analysis. The objective of doing a meta-analysis was to provide managers with a reliable estimate of discards survival with uncertainty estimates and the effects of explanatory factors (e.g. gear, season, air exposure) by means of a systematic synthesis and statistical analysis of a collection of previous studies.
4) investigate if measures of impairment in fish condition could be used as indicators for discard survival and evaluate its usefulness to collect discard survival data from a range of fishing conditions without the costs and logistics of captive observation.
5) assess the seasonal differences in the magnitude of physiological distress from capture and handling, the temporal development in stress responses following retrieval of the fishing gear, and the effect of gear type.

DTU Aqua, National Institute of Aquatic Resources (coordinator)
Danish Agrifish Agency, Ministry of Environment and Food of Denmark
Danish Fishermen PO, Denmark

The project was funded by the European Maritime and Fisheries Fund (EMFF) and the Ministry of Environment and Food of Denmark.

Key findings

Discard survival for the bottom trawler vs the Danish seiner in Skagerrak:
Generally, fish caught by the trawler showed lower survival (44%, 95%-confidence interval: 37–52%) than those caught by the seiner (78%, 95%-confidence interval: 67–87%). Air exposure was found to be the most important factor in determining survivability of fish discarded from the trawler. For the seiner, bottom temperature was identified as an additional important determinant.

Discard survival of the bottom trawler, the effect of:
1) seasons:
In winter, survival was significantly higher (75%, Confidence Interval (CI): 61–78%) than in summer (44%, CI: 34–61%).
2) target species:
When targeting plaice, survival was significantly higher (73%, CI: 63–83%) than when targeting Nephrops (40%, CI: 14–59%) in winter.
3) standard vs divided codend:
Using the divided codend, an improvement in discard survival of undersized plaice was possible when targeting Nephrops, but without a significant difference from the 90mm diamond mesh codend with 270 mm diamond mesh SELTRA panel (37%, CI: 14–54%) when fish were caught in the 60mm square mesh lower compartment (61%, CI: 48–73%). In the 120mm square mesh upper compartment, survival was significantly higher (94% with CI: 81–100%), but few undersized individuals were caught. When targeting plaice, survival rates did not differ between codends.

Based on these discard survival rates, EU granted a year round high survival exemption for Danish seines in ICES division 3a (Skagerrak/Kattegat) and subarea 4 (North Sea), and a similar exemption for bottom trawls (OTB, PTB) of at least 120 mm mesh size when targeting fish during the winter months (1 November - 30 April) as described in Commission Delegated Regulation 2018/2035 and 2019/2238 (; In addition, a one-year exemption (until 31 December 2002) was given to bottom trawls (OTB, PTB) with a mesh size of at least 90-99 mm equipped with a SLETRA panel targeting fish in the winter months for ICES division 3a, and similarly for bottom trawls (OTB, PTB) with a mesh size of at least 80-99 mm in ICES subarea 4 as described in Commission Delegated Regulation 2019/2238.

The discard survival studies have provided input to ICES Working Group on Methods for Estimating Discard Survival (WGMEDS, 2017-2019) and ICES Workshop on the Inclusion of Discard Survival in Stock Assessment (WKSURVIVE, 2021). In a collaboration with WGMEDS, a modeling framework for meta-analysis of discard survival estimates was developed as methodological heterogeneity may be a challenge when conducting meta-analysis. A meta-analysis can potentially be done for Nephrops and sole in addition to plaice.

A catalogue of discards and known discard survival estimates were established for eight commercial flatfish species: brill (Scophthalmus rhombus), dab (Limanda limanda), flounder (Platichthys flesus), lemon sole (Microstomus kitt), plaice (Pleuronectes platessa), sole (Solea solea), turbot (Scophthalmus maximus) and witch flounder (Glyptocephalus cynoglossus). The most hazardous fisheries in terms of absolute and relative discard amounts were identified. Information on discards is relevant to managers when evaluating the extent of an high survival extension at the fleet level.

For the first time, lemon sole was assessed to identify a set of relevant assessment criteria, while such criteria were harmonized with previous studies for plaice. Unlike some previous studies, it was in this project not possible to obtain good predictions of plaice survival from scores of vitality, reflex impairment, visual damages or any derived indexes. This calls for further investigation of the selection of reflexes and damages, their physiological link to mortality and how their contribution to the mortality probability should be incorporated into the analysis.

The severity of the physiological distress caused by capture and handling will determine the ability of a fish to recover, as well as the time required to make a full recovery. The level of exhaustion based on the metabolic indicators was negatively influenced by increasing temperatures. Furthermore, the fish were negatively influenced by an increasing sorting time (air exposure) during which they rely on anaerobic metabolism, which gives a metabolite accumulation and associated distress. Overall, dopamine levels, plasma glucose, plasma pH, plasma pCO2, plasma bicarbonate, and muscle glycogen are all useful indicators of the magnitude of physiological distress experienced by the fish following capture and sorting. For most, these are not variables that can be employed by the fishing industry, but provide insight into the response variables to trawling.

Research area: Fisheries Technology
Research area: Aquaculture
Effective start/end date13/09/201631/12/2018


  • discard survival
  • European plaice (Pleuronectes platessa)
  • bottom trawl
  • Danish seine
  • Skagerrak
  • environmental factors
  • operational factors
  • high survival exemption