Effects of season, target species and codend design on the survival of undersized plaice (Pleuronectes platessa) discarded in the bottom otter trawl mixed fisheries in Skagerrak

Fisheries Research 219 (2019) 105311

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Effects of season, target species and codend design on the survival of undersized plaice (Pleuronectes platessa) discarded in the bottom otter trawl mixed fisheries in Skagerrak Esther Savinaa, a b

⁎,1

T

, Thomas Noackb,1, Junita D. Karlsena

Technical University of Denmark, National Institute of Aquatic Resources, North Sea Science Park, PO Box 101, DK-9850, Hirtshals, Denmark Thünen Institute of Baltic Sea Fisheries, Alter Hafen Süd 2, Rostock, 18069, Germany

A R T I C LE I N FO

A B S T R A C T

Handled by Bent Herrmann

Unaccounted fishing mortality is recognized as an important determinant in the management of bycatch, and discard survival studies have been conducted in commercial and recreational fisheries around the world. A range of environmental, operational and biological factors affect survival of discarded fish and should be considered when presenting survival estimates. The present study compared discard survival of plaice in the bottom otter trawl mixed fisheries in Skagerrak between (a) seasons, (b) target species and (c) codend designs. In the latter, a horizontally divided codend designed to reduce interactions between spiny Norway lobsters (Nephrops norvegicus) and fish during the fishing process was investigated for its capacity to reduce catch damages and improve fish survival. (a) In winter, survival was significantly higher (75%, Confidence Interval (CI): 61–78%) than in summer (44%, CI: 34–61%). (b) When targeting plaice, survival was significantly higher (73%, CI: 63–83%) than when targeting Nephrops (40%, CI: 14–59%) in winter. (c) Using the divided codend, an improvement in discard survival of undersized plaice was possible when targeting Nephrops, but without a significant difference from the 90 mm diamond mesh standard codend (37%, CI: 14–54%) when fish were caught in the 60 mm square mesh lower compartment (61%, CI: 48–73%). In the 120 mm 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.

Keywords: Captivity experiment Demersal fishery Discard survival analysis Landing obligation Norway lobster (Nephrops norvegicus) Horizontally divided codend

1. Introduction Unaccounted fishing mortality is recognized as an important determinant in the management of bycatch, and discard survival studies have been conducted in commercial and recreational fisheries around the world (Broadhurst et al., 2006; Davis, 2002; Uhlmann and Broadhurst, 2013). Recently, the European Common Fisheries Policy landing obligation has led to more discard survival studies. Indeed, fishermen can be exempted from landing all regulated species if, for a given species, the chance to survive the process of capture, handling and release as discard is, based on scientific evidence, considered high (Regulation (EU), 2013). The Scientific, Technical and Economic Committee for Fisheries (STECF) advises a case-by-case approach, i.e., estimates of survivability should be evaluated in the context of the fishery seeking an exemption, especially regarding the habitat, season and handling practices (Bailey et al., 2018; Ulrich and Doerner, 2017).

Therefore, discard survival studies have focused on specific species, gears and areas. European plaice (Pleuronectes platessa) is considered a resilient species (Morfin et al., 2017b) and represents an important target species in the demersal fishery of the North-East Atlantic, including the Skagerrak area (Noack et al., 2017). Previous studies have estimated the survival of plaice discarded from bottom otter trawlers in the English Channel (Mérillet et al., 2018), North Sea (Catchpole et al., 2015; van Beek et al., 1990), Skagerrak (Methling et al., 2017) and Baltic Sea (Kraak et al., 2018), beam trawlers in the North Sea (Berghahn et al., 1992; van Beek et al., 1990) and pulse trawlers in the North Sea (van der Reijden et al., 2017). These studies also investigated which environmental, operational and biological factors affected discard survival estimates. For example, increased survival rates have been found for plaice discarded in cold conditions (e.g. Kraak et al., 2018; Mérillet et al., 2018; Van Beek et al., 1990; van der Reijden et al.,

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Corresponding author. E-mail address: [email protected] (E. Savina). 1 Equal authorship. https://doi.org/10.1016/j.fishres.2019.105311 Received 21 February 2019; Received in revised form 7 June 2019; Accepted 15 June 2019 Available online 27 June 2019 0165-7836/ © 2019 Elsevier B.V. All rights reserved.

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twin rig was replaced by a horizontally divided codend made of Ultracross netting. It consisted of a 120 mm square mesh upper and a 60 mm square mesh lower compartment (Karlsen et al., 2015). The entrance of the upper and lower compartments were 60 and 30 cm high, respectively. Two frames (90 × 30 cm, 20 mm stainless steel pipes) to keep the lower compartment open. The vessel was chosen in collaboration with the Danish Fishermen Organisation to represent as much as possible the commercial practices of the fleet. Indeed, the size and length of the trawler were within the range of the fleet, i.e., 11.00–19.99 m and 67–365 kW respectively (2017, logbook database), and its gear (including ground rope and rigging) belongs to the standard gears used in the Danish fleet. The skipper and crew were asked to follow normal commercial practices during the trial, i.e., haul duration, fishing speed and fishing grounds.

2017). As the catch is hauled on board the vessel, the body temperature of fish may acutely change from a few degrees at the bottom to higher temperatures at the sea surface over a time scale of minutes. Temperature determines the metabolic rate and demands on blood circulation and cardiac function in ectothermic fish (Ekström et al., 2014; Mendonça and Gamperl, 2010; Vornanen, 2016). Recent studies have suggested that upper thermal limits in ectotherms are not determined by insufficient oxygen supply capacity (Ern et al., 2016), but by failure of electrical conduction systems (Vornanen, 2016). Individuals are especially subjected to acute temperature increase, facing challenges on cardiac performance and tissue oxygen delivery, in the warmer seasons in presence of a thermocline, which contributes to a reduced probability of surviving the capture and release process. In the North Sea (including Skagerrak) bottom otter trawl fishery, trawlers catch Norway lobster (Nephrops norvegicus) in addition to various species of flat- and roundfish. It is common practice to target plaice with a 120 mm mesh size codend, and Nephrops with 90 mm. Trawlers targeting plaice usually catch high proportions of plaice and small proportions of Nephrops, and vice versa. Catches of plaice cannot be circumvented entirely when targeting Nephrops. When mixed in the codend, fish can be injured by the spiny exoskeletons or claws of Nephrops (Karlsen et al., 2015). Differences in survival of discarded plaice between different catch compositions, and therefore target species such as plaice or Nephrops, are therefore likely. Separating fish from Nephrops using a horizontally divided codend can reduce catch-related damages to the fish (Karlsen et al., 2015). Thus, survival of fish discarded after being caught by such a codend might be higher. When using the divided codend, the catch is partly sorted during fishing and sorting time on board could potentially be reduced. This is a major advantage that can be used to reduce discard mortality, as air exposure is a key factor affecting survival (Morfin et al., 2017a; van der Reijden et al., 2017). This study investigated if discard survival of undersized plaice caught in the bottom otter trawl fishery in Skagerrak was affected (a) by fishing season, (b) by the species targeted by the fishery, or (c) if the standard codend was replaced by a horizontally divided codend. The study was partly designed to support the request for exemption from the landing obligation by the Danish authorities for the bottom otter trawlers in the North Sea (including Skagerrak). It provides managers and researchers with a survival probability for the fishery of interest, including the uncertainties from the haul selection. It also bring additional knowledge as to which biological, environmental and operational factors influence survival. Overall, it contributes to the further application and development of the International Council for the Exploration of the Sea (ICES) Working Group on Methods for Estimating Discard Survival (WKMEDS) guidelines.

2.2. Sampling and assessment on board the fishing vessel During each of the 2–3 hauls that were conducted per day, the operational variables date, start time, time when codend on surface, time when codend on board/in hopper (container in which the codend is emptied before being sorted on the conveyor belt), GPS position of the haul and environmental variables depth, air temperature, water temperatures at bottom and surface were recorded. Air temperature was measured in the hopper (“Handy Polaris 2”, Oxyguard, Denmark). Depth and water temperatures were recorded using two CTD loggers (Star-Oddi, Iceland) mounted on the footrope of each gear. The catch was treated as under commercial fishery and sorted by the fishermen. Catch handling differed slightly between seasons due to the divided codend used in winter. In summer, both codends were emptied simultaneously in a hopper (about 2 m fall), and sorted on a conveyor belt. Four to six plaice below the Minimum Conservation Reference Size (MCRS) of 27 cm were, instead of being discarded, sampled between four and ten sampling points from the beginning to the end of the sorting process giving a total of 30 fish sampled per haul. The time at which each sample was taken was recorded to document the average and range of sampling times relative to the total duration of the fish processing for the different hauls. Random selection of plaice was ensured by picking consecutive individuals on the sorting belt at a given point. For one of the haul, the condition of all plaice was assessed (vitality, following Benoît et al., 2010) so that the condition of the sampled plaice could be compared with the frequency distribution of all discarded plaice. The sampled fish from a sampling point were held in a 10 l bucket awaiting assessment. For each fish, starting time of the assessment and total length (cm below) were recorded. Eventually, each fish was tagged with a Passive Integrated Transponder (PIT) tag (“HPT 12”, Biomark, USA), implanted on the pigmented side just anterior of the head. The assessment lasted for less than one minute after which the fish was returned to a 10 l bucket filled with fresh seawater until all six individuals were assessed. Subsequently, the fish were randomly assigned a drawer of one of two survival units (SU) placed in the cooling room of the vessel. The SUs were custom-made for the experiments and consisted of eight drawers each with a volume of 24 l (40 cm x 60 cm x 11 cm) (Maaskant Shipyards Stellendam, Netherlands; Fig. 2A). Up to five individuals were stored in one drawer. Each SU was supplied by running surface sea water with a flow of at least 12 l/min. Sampling procedures in winter were as similar as possible to the summer trials. However, as the vessel offered only one fish hopper and sorting belt, and the catch from not only the two different codends, but also the two compartments of the divided codend had to be handled separately, each codend had to be handled one after the other. To exclude potential systematic effects of the second codend hanging in the water waiting to be retrieved, each codend was alternatively handled first. The first two of the three catch components (i.e., catch from the standard codend and upper compartment, or the lower and upper compartment) ran over the sorting belt one after the other and were

2. Material and methods 2.1. Study site, vessel and gear specifications Sea trials were conducted onboard the commercial stern trawler S84 “Ida-Katrine” (total length: 15.1 m, power: 221 kW) in Skagerrak, north of Hirtshals (Fig. 1). The summer trial from 17 August – 10 October 2017 consisted of three sub-cruises targeting plaice using standard commercial gear. Two identical nets were fished as twin rig. Their ground gears measured 54.9 m from wing-end to wing-end and were made of 40 mm rubber discs attached to a 12 mm steel wire. The 96 mm diamond mesh codends were made of 3 mm double polyethlyene (PE) twine knotted netting, were 8 m long and consisted of 100 open meshes in circumference. Their 270 mm escape windows (SELTRA) were made of 5 mm single PE twine. The winter trial from 8 to 26 March 2018 consisted of two sub-cruises where plaice was targeted during the first sub-cruise, and Nephrops during the second (Fig. 1). Plaice was caught on sandy bottoms close to the areas fished in summer, and Nephrops on muddy, slightly deeper grounds. One of the standard codends in the 2

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Fig. 1. Overview of the fishing operations in Skagerrak, north of Denmark. Dashed grey lines: hauls in summer 2017 targeting plaice (Pleuronectes platessa). Solid black lines: hauls in winter 2018 targeting plaice. Solid white lines: hauls in winter 2018 targeting Norway lobster (Nephrops norvegicus).

onboard the vessel depending on their holding time (i.e., if they originated from the first or second haul). As soon as the vessel approached the harbour, the water supply was disconnected from the SUs preventing the fish from experiencing harbour water of potentially reduced quality. The time from entering the harbour (closing water supply) until transferring the fish into the observation tanks in the land-based facilities varied considerably, but was usually about 15 min and never exceeded 45 min. During the 1.6 km of transportation, no additional oxygen suppliers were used, but oxygen saturation in the drawers was checked after the fish were transferred to the observation tanks.

stored separately in 90 l tubs. These tubs provided similar conditions for the catch as the fish hopper except for containing smaller catch weights. Fish were sampled from the tubs for further assessment. This allowed the third catch component (i.e. catch from the standard codend or lower compartment) to be sorted under fully commercial conditions on the sorting belt. The upper compartment was never handled last. Unlike the summer trials, four plaice below the MCRS of 27 cm were sampled at four evenly distributed sampling points from the beginning to the end of the sorting process, for a total of 16 fish per haul and compartment. As the upper compartment of the divided codend did not capture 16 undersized individuals per haul, additional individuals from the standard codend were sampled and stored in one of the four SUs used in winter. The water parameters oxygen saturation, salinity, water flow and water temperature in the SUs were monitored 2 to 4 times

Fig. 2. Transport and holding facilities for fish. A. Survival unit (118.3 x 78.3 x 65.7 cm) with 8 drawers (60 x 40 x 11 cm) used to hold fish onboard the vessel and transport them to the holding facility. B. Holding facility consisting of 4 rows of 4 tanks (1 m x 1 m) with semi-recirculated surface sea water. 3

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Fig. 3. Overview of the four types of Control Groups (CG). “acclim.”: acclimated controls transported to the fishing ground and treated similar to experimental fish, “trawl”: controls caught in 15 min hauls and treated similar to experimental fish, “land”: acclimated controls transferred directly to the observation tanks, “land+tag”: acclimated controls assessed and tagged before being transferred to the observation tanks.

experimental fish. These fish controlled for the sampling, the tagging, the transportation process (one way) and the holding facility, but might have additional mortality resulting from the capture process.

2.3. Controls Four types of controls were used (Fig. 3). Control Group (CG) “land” was caught prior to the experiment by the research vessel “Havfisken” (DTU-Aqua, Denmark) using a bottom otter trawl with 420 meshes (nominal mesh size: 120 mm) around the fishing circle with a 90 mm diamond mesh codend made of 4 mm double twine PE knotted netting. Haul duration was limited to 15 min and sampled control fish were stored inside the SUs. From the harbor, the SUs were transported 0.9 km to the laboratory where the fish were transferred into independent flow-through acclimation tanks (2 m x 2 m x 0.5 m) filled with 1400 l of seawater (water flow per tank: 10 l/min). After an acclimating period of at least 7 d during which the fish were fed shrimps (Pandalus borealis) ad libitum, 16 fish were transferred from the acclimation tanks directly into the observation tanks (one or two fish per tank), i.e., they were kept in the holding facility and were not brought back to the commercial vessel. Fish from CG “land” were moved from the acclimation tanks to the observation tanks as the same time as when the experimental fish were brought back from the commercial vessel to the observation tanks. Once in the observation tanks, CG “land” were treated as the experimental fish, i.e., checked for mortality (see 2.4), and therefore controlled for the holding conditions during the observation period, e.g., equal disturbance of checking fish for mortality. For the winter period, the number of individuals per CG was reduced to add CG “land + tag”. This CG consisted of 16 acclimated fish that were sampled similarly to CG “land”, i.e., by the research vessel prior to the experiment. Contrary to CG “land”, the CG “land + tag” were length measured and tagged in the facilities before being transferred into their respective observation tank (one fish per tank). CG “acclim.” consisted of ten acclimated fish, previously caught by the research vessel and left with no disturbance in the holding facility, that were brought to the commercial vessel distributed equally in the two SUs on each experimental day. Once on board the vessel, these control fish were treated like experimental fish and subsequently transferred back into the SUs. Fish of CG “acclim.” controlled for the sampling, tagging, transportation, and holding. However, they were exposed to one transportation process more than the experimental fish, i.e., from the facility to the fishing ground. Fish in CG “trawl” were caught by the commercial trawler at the start of each experimental day during short hauls of 15 min. After capture, ten fish were sampled on each experimental day, tagged, transported in a SU, and put into the observation tanks like the

2.4. Monitoring at the holding facility Both experimental and control fish were randomly transferred into one of the 16 observation tanks (1 m x 1 m x 0.5 m; Fig. 2B) filled with 300 l of surface seawater (max. 20 fish per tank). A sand layer of 2 cm covered the bottom of each tank. The observation tanks were part of a semi-recirculated system, with an hourly turnover rate of ˜200 l/min of which addition of water was ˜12 l/min. Each tank contained an air bubbler to keep oxygen levels above 80%. The dark-light ratio was kept constant with 8 h light (08:00 to 16:00) and 16 h dark. However, light needed to be switched on during assessments in the evening (˜1 h). All observation tanks were covered with two polystyrene plates (2.5 cm thick) to avoid direct illumination of the tanks and reduce stress levels. Two weeks were considered sufficient to observe all delayed mortalities resulting from capture and handling without adding additional stress. This was set out in Yochum et al. (2015) as a general principle, and verified in previous observations on plaice by Uhlmann et al. (2016) and van der Reijden et al. (2017). The first week, mortality assessment was done at arrival and thereafter three times a day (08:00, 14:00, 20:00). Non-moving fish with the blind side upward or showing a pronounced dark skin colour that contrasted the light coloured sand substrate underwent a detailed check above the water surface. If body, mouth and operculum movements were not present during a period of 30 s, the fish was declared dead and removed from the tank. The second week, monitoring was reduced to two assessments per day (08:00, 20:00) and fish were fed with shrimps (Pandulus borealis) ad libitum immediately before the lights were switched off (16:00). Excretions and food remains were removed during the evening assessment to prevent degraded organic matter from influencing the water quality. Salinity was recorded daily and ammonia levels were analysed weekly for each observation tanks (“VISOCOLOR ECO Ammonium 3”, Aquacultur, Germany). All experiments were in accordance with the European and Danish legislations on animal experimentation (Directive 2010/63/EU of the European Parliament and of the Council of 22 September 2010 on the protection of animals used for scientific purposes).

4

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Fig. 4. Overview of the sampling design, with the number of individuals sampled per subset (season, target species and catch component). The grey boxes indicate data used for the different comparisons of undersized plaice (Pleuronectes platessa) discard survival estimates.

by selecting m hauls with replacement from the pool of hauls of the specific subset of the specific case (e.g., data from the standard codend when targeting plaice in the winter, or data from the standard codend when targeting plaice in the summer) during each bootstrap repetition. The number of hauls selected from each subset was the same as the number of hauls for that subset (m). Within-haul uncertainty in the obtained survival probability was accounted for by randomly selecting fish with replacement from the selected haul. The number of fish selected from each haul was the same as the number of fish evaluated for that haul (nj). The resulting data for each bootstrap were then used to estimate the expected survival probability Sˆ based on Eq. 1 for each subset of each of the three cases.

2.5. Data analysis The study compared survival rates of discarded undersized plaice between (a) two different seasons, summer and winter, (b) two different target species, plaice and Nephrops, and (c) the three compartments of the two different codends (Fig. 4). We worked with subsets of the total dataset for each of the three cases. In summer, only the standard codend targeting plaice was used. This summer data was compared with data from the standard codend when targeting plaice in the winter for the seasonal comparison (Fig. 4a). In the same manner, the comparison between target species was done with data from the standard codend in winter (Fig. 4b). The comparison between the catches from standard codend, and the upper and lower compartments of the divided codend was done with data from the winter and run separately for each target species (Fig. 4c). In case confounding variables were identified within any of the subsets, observations not allowing for a balanced comparison were excluded. Only hauls for which the standard codend was sorted first were included in the seasonal comparison, i.e., 88 individuals were excluded from the winter data. When comparing survival rates between target species, individuals that experienced air exposure of more than 62 min were excluded from further analyses as maximum air exposure time observed for individuals from hauls targeting plaice was 62 min, i.e., 48 individuals were excluded from the data obtained when targeting Nephrops. When comparing survival rates between the three different catch components, 35 individuals from the standard codend with air exposure times exceeding 40 min were excluded from the analyses for hauls targeting plaice as 40 min was the maximum observed air exposure for individuals from the other two catch components. For the hauls targeting Nephrops, 12 and 18 individuals with air exposure times exceeding 120 min were excluded from the standard codend and lower compartment, respectively, as the maximum air exposure for individuals from the upper compartment was 120 min. For each subset of the three cases, i.e., seasons, target species and catch components, an initial data exploration was done by comparing Kaplan Meier survival curves (Kaplan and Meier, 1958) (Fig. 5 as an example, supplementary material S1–S4). Kaplan-Meier survival curves are a function of the observed data, i.e., representing the proportion of sampled individuals alive at each time point during captivity (Benoît et al., 2012). We used the Kaplan Meier survival curves to check that an asymptote was reached. For each of the three cases, we compared the observed survival probabilities of each subset after 14 d of observation averaged over hauls. A double bootstrap method was adapted to our data that is well established for evaluating fishing gear selectivity and catch efficiency (Herrmann et al., 2012; Wienbeck et al., 2014). The procedure accounted for between-haul variation in the obtained survival probability

m

Sˆ =

∑ j=1

n survivedj nj

m

(1)

With Sˆ the expected survival probability, m the total number of hauls for the specific subset, nj the total number of fish for haul j, and n survivedj the total number of fish that were alive after 14 days of observation for haul j. We performed 5000 bootstrap repetitions and calculated the Efron 95% percentile confidence limits (Efron, 1982) for the estimated survival probability for each subset for each of the three cases. Significant differences are observed when the confidence intervals do not overlap. 3. Results 3.1. Overview of hauls, catches and experimental conditions In total 22 experimental hauls were obtained. In summer, 12 hauls were conducted with the standard codend to target plaice. In winter, the standard and divided codends were used in 10 hauls of which Nephrops was targeted in four and plaice in the other six (Table 1). All operational factors were representative of commercial practices for the respective fishery, e.g., haul durations were longer and fishing areas deeper when targeting Nephrops than plaice. Differences in average air and bottom temperatures were the largest for hauls targeting plaice, with lower temperatures in winter (Table 1). Catches were the largest during the summer trials, and smallest when targeting plaice in winter. The sorting time depends on catch weight (and thus also vessel size) and composition, and the size of the crew onboard the vessel. Experience from DTU-Aqua observers at sea program suggests that in commercial conditions, sorting time is up to 1 h depending on catch weight when plaice is the main target species, and up to 2.5 h when Nephrops is the main target species. A proxy for sorting time is catch weight. The experimental catch weights were within the range of commercial hauls. The average catch weight per haul for commercial trawlers in the Skagerrak between 2015 and 2017 were 674 (53–2957) kg when using 5

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Fig. 5. Example of a Kaplan-Meier plot comparing survival over time for summer and winter (target: plaice, Pleuronectes platessa; catch component: standard codend).

mesh sizes ≥120 mm (Data Collection Framework database), i.e. mainly targeting plaice or round fish, and 559 (121–2236) kg when using mesh sizes < 120 mm (Data Collection Framework database), i.e., mainly targeting Nephrops (Table 1). The catch weights of the standard and divided codends towed in parallel were similar, but catches in the upper compartment were always lower (Table 1). This was also reflected by the number of individuals sampled from the respective catch components, with fewer individuals from the upper compartment. Due to low catches in the upper compartment, some of the fish were kept for longer periods than the real sorting time to provide with potential for comparison at longer air exposures with the other compartment (Table 1). It is therefore a “worst-case” scenario providing that most likely sorting duration for the upper compartment would remain limited. Average length of the sampled fish was similar for both seasons, target species and all catch components, with a tendency of larger fish in the upper compartment (Table 1). Average time fish spent in air was similar for both seasons when targeting plaice, but longer when targeting Nephrops (Table 1). Based on two consecutive hauls targeting plaice in the summer, the proportion of assessed individuals by vitality classes of the undersized plaice sampled for survival assessment seemed representative of all the undersized individuals caught (Fig. 6). Dissolved oxygen in the observation tanks were maintained above 80% saturation. Oxygen levels in the SUs were lower following transportation to the facilities, but never below 50% saturation. Salinity in the observation tanks ranged from 28 to 32 PSU, and ammonium levels never exceeded 20 μg/l. Survival of control fish was generally high with rates ranging from 87% to 100% (Table 2).

4. Discussion This study estimated survival rates for undersized plaice after commercial trawling in different seasons, when targeting different species and with two different codend designs. Characteristics of the hauls and catch handling varied between the different seasons and target species, but were within the range of commercial conditions and representative of the fisheries of interest. Comparison of mean survival estimates and their associated variabilities showed that all three considered factors can affect the survival probability of undersized plaice discarded in the Danish bottom otter trawl fishery. 4.1. Uncertainties in the discard survival estimates Fish were not released into their natural habitat, but into laboratory conditions, which can affect survival rates either positively or negatively. Additionally, fish were not exposed to predation, e.g. by sea birds (Garthe et al., 1996), whose quantitative effects remain unknown (Raby et al., 2014), but which likely means that survival estimates might be overestimated. Finally, handling of the fish during assessments and transportation likely increases stress and can negatively affect survival. This was therefore assessed by including several control groups in the experiment. The overall high survival of control groups (87–100%) indicated good experimental conditions. The 100% observed survivability of CG “land” indicated optimal holding conditions. Survival estimates of fish from CG “acclim.” and CG “trawl” were similar, and less than 100% in summer, indicating a negative effect of additional handling and transporting of the fish in summer. Fish sampling was minimally invasive, and previous studies concluded that the effect of tagging on survival is minimal (e.g. Huusko et al., 2016), suggesting that transportation was the factor affecting survival in summer. Similar survival rates of CG “acclim.” and CG “trawl” could also indicate that the mortality generated by an additional transportation process (CG “acclim.”) from the facilities to the fishing ground in summer is similar to being caught by a short trawl haul of 15 min (CG “trawl”) in summer. In order to properly disentangle the effects of transportation, catching process and fish assessment, the CG “land + tag” was added in the winter trials. CG “land + tag” experienced the assessment and tagging procedure, but not the transportation. However, in winter, none of the fish in CG “acclim.”, CG “trawl” or CG “land” died. This made it difficult to assign higher mortalities to either sampling or transportation. Furthermore, this likely means that the single dead fish in CG “land + tag” when targeting Nephrops was already impaired when entering the experiment. Fish got occasionally stuck in front of the second metal frame in the lower compartment of the divided codend, causing partial clogging. The reason for this was not known, and was not experienced in previous tests (Karlsen et al., 2015; Melli et al., 2019, 2018). It did not have an

3.2. Effects of season, target species and codend design on discard survival Survival of undersized plaice discarded in the summer, with 44% (CI: 34–61%; n = 333) of the fish being alive after 14 d, was significantly lower than the 75% (CI: 61–78%; n = 54) survival for fish discarded in the winter (Fig. 7a). Survival when targeting Nephrops was significantly lower (40%, CI: 14–59%; 53 individuals) than when targeting plaice (73%, CI: 63–83%; n = 142) (Fig. 7b). The comparison of the three different catch components demonstrated different patterns of survival rates depending on the target species (Fig. 7c). When targeting Nephrops, the average survival rate of fish from the upper compartment was significantly higher (94%, CI: 81–100%; n = 17) than from the standard codend (37%, CI: 14–54%; n = 89) and the lower compartment (61%, CI: 48–73%; n = 74). When targeting plaice, no significant differences in average survival rates of fish captured in the different compartments was found: lower compartment 77% (CI: 57–96%; n = 96); upper compartment: 92% (CI: 78–100%; n = 41); SC: 77% (CI: 64–89%; n = 107). 6

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Table 1 Haul characteristics for experimental hauls separated by target species and gear compartment. Values are given as mean (minimum – maximum), except for sorting time and air exposure as median (min-max). Catch components are indicated as ‘standard’ for the standard codend, ‘Upper’ for the upper and ‘Lower’ for the lower compartments of the divided codend. Season

Summer

Winter

Target species

Plaice

Nephrops

Plaice

Period of sea trials No. of hauls Haul duration (min) Depth (m) Air temperature (°C) Bottom temperature (°C) Catch weight (kg)

August-October 2017 12 141 (37–185) 34 (11–61) 15 (7–20) 14 (10–17) 450 (65–1509)

Sorting time (min)

40 (19-62)

Air exposure (min) sampling point 1

3 (1–6)

Air exposure (min) sampling point 2

9 (5-14)

Air exposure (min) sampling point 3

20 (8-30)

Air exposure (min) sampling point 4

25 (13-45)

Air exposure (min) sampling point 5

40 (19-62)

Air exposure (min) sampling point 6

NA

Air exposure (min) sampling point 7

NA

Air exposure (min) sampling point 8

NA

Air exposure (min) sampling point 9

NA

Air exposure (min) sampling point 10

NA

No. of individuals sampled

333

Fish length (cm)

23 (17–26)

March 2018 4 210 (180–239) 57 (37–69) 6 (4–7) 7 (7–7) 375 (200–500) Standard: 152 (53–198) Upper: 8 (5–10) Lower: 224 (150–296) Standard: 126 (117-135) Upper: 52 (39-106) Lower: 126 (108-149) Standard: 12 (1-21) Upper: 16 (4-25) Lower: 12 (1-27) Standard: 23 (12-34) Upper: 44 (16-56) Lower: 20 (17-39) Standard: 41 (21-46) Upper: 33 (33-33) Lower: 43 (30-58) Standard: 56 (29-61) Upper: 45 (45-45) Lower: 56 (45-74) Standard: 78 (59-117) Upper: 64 (64-64) Lower: 77 (60-121) Standard: 91 (75-94) Upper: 79 (79-79) Lower: 99 (75-135) Standard: 106 (90-111) Upper: 95 (95-95) Lower: 114 (90-149) Standard: 121 (105-122) Upper: 106 (106-106) Lower: 108 (105-136) Standard: 127 (119-135) Upper: NA Lower: 115 (115-115) Standard: 130 (130-130) Upper: NA Lower: NA Standard: 101 Upper: 17 Lower: 92 Standard: 23 (11–26) Upper: 24 (21–26) Lower: 21 (12–26)

March 2018 6 181 (177–185) 49 (16–59) −1 (−1–0) 7 (6–7) 150 (100–200) Standard: 54 (36–72) Upper: 11 (6–18) Lower: 81 (50–120) Standard: 36 (22-59) Upper: 22 (6-39) Lower: 30 (23-35) Standard: 2 (1-6) Upper: 6 (5-22) Lower: 7 (2-15) Standard: 12 (8-19) Upper: 19 (16-39) Lower: 16 (10-26) Standard: 19 (15-27) Upper: 26 (25-26) Lower: 23 (18-31) Standard: 26 (21-34) Upper: NA Lower: 30 (23-35) Standard: 40 (31-45) Upper: NA Lower: NA Standard: 47 (39-50) Upper: NA Lower: NA Standard: 54 (50-59) Upper: NA Lower: NA Standard: 53 (53-53) Upper: NA Lower: NA Standard: NA Upper: NA Lower: NA Standard: NA Upper: NA Lower: NA Standard: 142 Upper: 41 Lower: 96 Standard: 22 (13–26) Upper: 24 (17–26) Lower: 21 (14–26)

similar to 89% (CI: 83.7–92.9%) survival reported for the same season in Skagerrak (Methling et al., 2017). The higher survival estimate found by Methling et al. (2017) is likely due to a smaller vessel size (11.8 m) and larger fish (mean and standard deviation: 33.1 ± 3.3 cm). Indeed, an increase in survival probability with fish length has been identified for plaice caught in the North Sea (Uhlmann et al., 2016). When evaluating the request for a ‘high survival’ exemption from the landing obligation made by the Danish authorities as part of the joint recommendations for the North Sea, STECF (Bailey et al., 2018) advised for an exemption for the winter only.

effect on the vertical distribution of the catch, but may have induced additional handling time and therefore air exposure. Since air exposure was accounted for, these hauls were considered valid, but additional damages to the congested individuals might have contributed to a slight underestimation of the discard survival in the lower compartment. 4.2. Effect of season on discard survival Overall, the numbers of sampled individuals were well balanced between seasons and targets, even though there were more fish sampled in the summer than in the winter for the standard codend due to the sampling design. The survival estimate of plaice discarded in the summer (average: 44%, CI: 34–61%) was in line with the 42% survival estimated under similar conditions in the North Sea (Catchpole et al., 2015). The survival estimated of plaice discarded in the winter (75%, CI: 61–78%) was

4.3. Effect of target species on discard survival To our knowledge, this was the first study assessing the effect of target species of an otter trawl mixed fishery on plaice discard survival. Longer air exposures when targeting Nephrops were due to longer 7

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49 m with min-max: 16–59 m), although small, may have affected survival. Indeed, injuries related to barotrauma such as gas embolisms in organs or clotting of fine blood vessels cannot be excluded (Benoît et al., 2013; Methling et al., 2017; van der Reijden et al., 2017). When evaluating the request for a ‘high survival’ exemption from the landing obligation made by the Danish authorities as part of the joint recommendations for the North Sea, STECF (Bailey et al., 2018) advised for an exemption when fishing with mesh sizes ≥120 mm only, which is the codend mesh size usually used when targeting plaice. We expect significant differences in survival between target species in the summer as well, but the effect size may be different than that observed in the winter. The direct effect of codend mesh size on discard survival in plaice has not been investigated, but it has been proven without effect for Nephrops (Wileman et al., 1996). 4.4. Effect of catch separation on discard survival This is also, to our knowledge, the first study exploring the use of a horizontally divided codend to improve discard survival. When targeting Nephrops, the survival of undersized plaice was improved with the divided codend compared to the standard codend, but significant differences were observed for the upper compartment only. Similarly, Karlsen et al. (2015) showed that plaice in the upper compartment had significantly less scale loss than in the standard codend, but that this difference was not significant for the lower compartment. Nephrops were mainly caught in the lower compartment in both studies. By separating fish from Nephrops in the catch, harmful interactions between plaice and Nephrops were limited, therefore improving survival. As Nephrops are usually not found in the catch when targeting plaice, damages on the body surface were reduced and so the survival estimates were not significantly different between the gear compartments. The level of discolouration (blood spots/bruises) can significantly increase with increasing catch weight (Karlsen et al., 2015). The higher catch weights in the lower compartment compared with the standard codend might explain why we did not observe significant differences between these two gear compartments. This may come from differences in selectivity between the 90 mm meshes in the standard codend and the 60 mm meshes in the lower compartment of the divided codend. The standard codend was of a different netting material than the knotless Ultracross used in the divided codend, but results from Karlsen et al. (2015) suggested that the netting material had little effect on fish damage. We expect significant differences in survival between the compartments of the divided codend when targeting Nephrops in the summer as well, but the size of effect may be different to that observed in the winter. Fewer undersized individuals were sampled from the upper compartment compared to the others due to smaller catches in this compartment. Smaller catches in the upper compared to the lower compartment were also observed by Karlsen et al. (2015) and is likely explained by the larger, open square meshes in the upper compartment allowing for more, especially small fish to escape. However, we were unable to exclude the possibility that only a small proportion of individuals was entering the upper compartment. Plaice is closely associated with the sea bottom in their natural habitat and thus could be more prone to enter the lower compartment. Catch data from a blinded (40 mm square mesh) divided codend similar to the one used in our study and employed in the same fishery under similar commercial conditions, demonstrated that an average of 50% or more of the undersized plaice enter the upper compartment (Melli et al., 2019, 2018), suggesting that the few individuals caught in the upper compartment in our study was due to escapement through the large, open meshes. The present study did not consider survival of escapees, only that of individuals retained after the selection process in the codend. Considering that mortality rates as low as 15% have been observed for escapees of other flatfish species, e.g., winter flounder (Pseudopleuronectes americanus) (Dealteris and Reifsteck, 1993), one

Fig. 6. Proportion of assessed individuals by vitality classes (from 1 lively to 4 moribund, based on both body movements and damages following Benoît et al., 2010) for all individuals caught (“Catch”, n=301) and all individuals sampled for survival assessment (“Experimental”, n=30) during two consecutive hauls targeting plaice (Pleuronectes platessa) in the summer.

Table 2 Observed survival after 14 days for the experimental fish from the standard codend and the four control groups (CG). “acclim.”: acclimated controls transported to the fishing ground and treated similar to experimental fish, “trawl”: controls caught in 15 min hauls and treated similar to experimental fish, “land”: acclimated controls transferred directly to the observation tanks, “land + tag”: acclimated controls assessed and tagged before transferred to the observation tanks. Season

Target

Experimental or control

Experimental subset or control group

Number of individuals

Observed survival

Summer

Plaice

Experimental

Standard codend ”acclim.” ”trawl” ”land” Standard codend ”acclim.” ”trawl” ”land” “land + tag” Standard codend ”acclim.” ”trawl” “land” “land + tag”

333

0.42

60 60 50 142

0.87 0.92 1.00 0.74

10 10 16 16 53

1.00 1.00 1.00 1.00 0.40

10 10 16 16

1.00 1.00 1.00 0.94

Control

Winter

Plaice

Experimental Control

Nephrops

Experimental Control

sorting times either caused by larger catches, or to the fact that sorting Nephrops takes longer than sorting fish due to differences in body size and the presence of other benthic invertebrates in Nephrops-hauls. Plaice discarded when targeting Nephrops (40%, CI: 14–59%) showed reduced survival rates compared to when targeting plaice (73%, CI: 63–83%). Plaice interacts with the spiny bodies of Nephrops and their claws, both in the fishing gear and on board in the hopper, resulting in scale loss, skin damages and injuries (Karlsen et al., 2015). Such mechanical impairments to the skin may cause osmoregulatory challenges for the fish, and therefore additional stress (Benoît et al., 2013). Additionally, differences in fishing depths between hauls targeting Nephrops (mean: 57 m with min-max: 37–69 m) or plaice (mean: 8

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Fig. 7. Observed survival probability at asymptote after 14 d with 95% confidence interval of undersized plaice discard for the three different comparisons: a) season, i.e. summer vs. winter; b) target species, i.e. Norway lobster (Nephrops norvegicus) vs plaice (Pleuronectes platessa); and c) catch component for each target species, i.e. standard codend vs. upper and lower compartments of the divided codend.

might argue that the divided codend can be considered as a promising gear for the fishermen to meet the overall objectives of the landing obligation. Indeed, if the higher selectivity of the upper compartment of the divided codend for undersized plaice is further demonstrated, the use of such a modified gear could help the industry reduce the catch of undersized individuals.

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