Plaice Research Articles

The effect of electrical stimulation on the footrope and cod-end selection of a flatfish bottom trawl

The beam trawl fishery targeting sole is known for their substantial bycatch of flatfish below the minimum landing size. Pulse trawls were developed to improve the selectivity by replacing mechanical stimulation with electrical stimulation which immobilises fish in front of the footrope. Results are presented of an experiment on board of a commercial pulse trawler studying the effect of electrical stimulation on footrope and cod-end selectivity for three flatfish species - sole (Solea solea), plaice (Pleuronectes platessa) and dab (Limanda limanda) - in 29 paired hauls with electrical stimulation alternating between the starboard and portside gear. It was shown that electrical stimulation increased the footrope selection by a factor 2 in plaice and dab and a factor 7 in sole. The effect on sole is related to the specific response of sole which cramps into a U-shape. Footrope selection showed a small diurnal pattern with the highest selectivity of the pulse trawl during the day. Electrical stimulation and catch weight were shown to have a small effect on the cod-end selectivity retaining slightly more marketable sole. Cod-end mesh selection factors (SF) were estimated at 3.00 (se = 0.02), 2.11 (se = 0.02), and 2.3 (se = 0.1) for sole, plaice and dab, respectively. Selection ratio (SFA = selection range/mesh size) was estimated at 0.45 (se = 0.03), 0.23 (se = 0.02), and 0.41 (se = 0.04) for sole, plaice and dab, respectively. The SF and SFA of the pulse trawl is comparable to values reported for conventional beam or otter trawls.

Effect of High-Pressure Processing on Physico-Chemical, Microbiological and Sensory Traits in Fresh Fish Fillets (Salmo salar and Pleuronectes platessa).

High-pressure (HP) treatment could lead to several advantages when applied to fish and seafood since it would affect the extension of the shelf life of this highly perishable food. In this regard, this study aimed to evaluate the effect of high-pressure treatment (500 MPa for 2 min at a temperature of 4 °C) on changes in quality on two different kinds of fresh fish fillets (Salmo salar and Pleuronectes platessa). Specifically, physico-chemical (VOCs, untargeted metabolomics spectra, pH and color), microbiological (Enterobacteriaceae, Pseudomonas spp., mesophilic and psychrotrophic bacteria) and sensory traits were evaluated at different days of refrigerated storage. From the results obtained, it is possible to state that the high pressure significantly (p ≤ 0.05) reduced microbial growth for each investigated microorganism. Regarding the colorimetric coordinates, no remarkable effects on a* and b* indices were found, while a significant effect (p = 0.01) was observed on the colorimetric index L*, making the HP-treated samples lighter than their respective controls. The sensory analysis showed that for the odor attribute, the HP treatment seems to have had a stabilizing action during shelf-life. Moreover, the treated samples obtained a better score than the respective controls (p ≤ 0.05). With regards to texture and appearance attributes, the treatment seems to have had a significant (p ≤ 0.05) effect, making the treated samples more compact and opaque than controls, therefore resulting in the loss of the characteristics of raw fish for the treated samples. Moreover, from a chemical point of view, HP treatment prevents the development of volatile sulfides and delays the formation of histamine (p ≤ 0.05). Very interestingly, the metabolomic approach revealed novel dipeptide markers for the HP procedure.

Spatial variation in the gastrointestinal microbiome, diet, and nutritional condition of a juvenile flatfish among coastal habitats

Gut microbiota are important for the health, fitness and development of animal hosts, but little is known about these assemblages in wild populations of fish. Such knowledge is particularly important for juvenile life stages where nutritional intake critically determines early development, growth, and ultimately recruitment. We characterise the microbiome inhabiting the gut of young-of-the-year European plaice (‘YOY plaice’) on sandy beaches, their key juvenile habitat, and examine how these microbial communities vary spatially in relation to diet and nutritional condition of their plaice hosts. Body size, diet (stomach fullness and eukaryotic 18S ribosomal sequencing), nutritional condition (RNA:DNA) and gut microbiota (16S prokaryotic ribosomal sequencing) were compared in fish at two spatial scales: between beaches separated by 10s of kilometres and between sites at different depths on the same beach, separated by 10s of metres. The main microbial phyla in YOY plaice guts were Proteobacteria, Spirochaetes, Tenericutes and Verrucomicrobiae. Within the Proteobacteria there was an unusual dominance of Alphaproteobacteria. Differences in body size, diet and nutritional condition of YOY plaice between beaches were accompanied by differences in gut microbial assemblage structure. Notably, substantially reduced nutritional condition and size at one of the beaches was associated with lower stomach fullness, reduced consumption of annelids and differences in the abundance and presence of specific microbial taxa. Differences were also detected in microbial assemblages, body size, and diet between depths within the same nursery beach, although stomach fullness and nutritional condition did not vary significantly. The functional links between the environment, gut microbiota, and their hosts are potentially important mediators of the development of young fish through critical life stages. Our study indicates that these links need to be addressed at 10 km and even 10 m scales to capture the variability observed in wild populations of juvenile fish.

Comparison of the spatiotemporal distribution of three flatfish species in the Seine estuary nursery grounds

Nurseries are crucial habitats that play an important role for many marine fish species; which rely on them to complete their life cycle. Juvenile stages of dab (Limanda limanda), plaice (Pleuronectes platessa) and sole (Solea solea) present in the English Channel are common in the Seine estuary nursery grounds. To further explore the estuary's nursery function, we investigated the heterogeneity in spatial distribution patterns of these three flatfish at their juvenile stage, between 1996 and 2019. We used geostatistical indices and multivariate analyses to demonstrate species specific spatiotemporal dynamic. Sole favoured the most upstream part of the nursery, dab was found in the most marine areas, and plaice preferred the southern coast. We then performed clusters analysis based on spatial indices and spatial patterns extracted from a Minimum/Maximum Autocorrelation Factor (MAF). We showed that the average positions of the three flatfish species were stable across time. Each flatfish appeared to have its own spatial preference inside the nursery. No temporal variability in the spatial pattern nor trend was found that would correspond to the major stress imposed on the community by harbour development within the 2000–2005 period. We conclude that segregation of the juveniles of the three species within the nursery may reflect different ecological needs and underlying mechanisms to minimise interspecific competition.

Flatfish survivors have tales to tell: Cold seawater and reduced deployment duration contribute to the survival of European plaice (Pleuronectes platessa) discarded by Belgian beam trawlers

The ‘high survival’ exemption to the European Landing Obligation regulation intensified the relevance of quantifying the survival of commonly discarded European plaice (Pleuronectes platessa) within the discard-intensive Belgian beam-trawl fishery, together with the search for ways to maximize discard survival. In this study we worked together with vessel owners and the crews of three Belgian beam trawlers representing the small (≤221 kW) engine power, coastal (n = 1 vessel), ‘Eurocutter’ (1), and large (>221 kW) (1) fleet segments. During 12 commercial trips which included 99 beam-trawl deployments in 2014 and 2015, three scientific observers assessed the survival of 3849 plaice aboard the vessel (by checking for the presence of reflex responses and bleeding injury; on-board survival assessment) and counted how many survived among a sub-sample (n = 489 plaice) monitored for at least 9 days in captivity (post-release survival). These fish were monitored alongside 160 captive controls. Only three control fish died (<2%) during daily monitoring. Survival of plaice discarded from conventional trawls of Belgian beam-trawl vessels representing the three fleet segments was estimated to range between 41–58 %, 11–28 %, 2–4 % (95 % confidence interval; Kaplan-Meier models) for trips of the coastal (≤221 kW), Eurocutter (≤221 kW) and >221 kW vessel, respectively. More challenging operational fishing conditions of >221 kW compared to ≤221 kW vessels such as ∼150 min gear deployment and ∼37 min sorting durations meant that down to only 20 % of undersized plaice were still alive when landed on deck. Such immediate mortality was hardly observed among ≤221 kW vessels. These effects were cumulative to the effect of elevated seawater temperatures of >14 °C during summer, because despite warm waters, immediate mortality among ≤221 kW vessels was low compared to >221 kW vessel-trips. Out of those fish that were still alive after sorting at the point of discarding and monitored for 9 days, on average 23 % (0%–100 %, 95 % CI) survived. Post-release survival was associated with stress and injury sustained during the capture and sorting process (proxied by the reflex impairment and injury index) and seawater temperature. Survival rates can be improved by choosing when to fish, and restricting gear deployments to practicable minimum durations. Without further adaptations to current fishing practices, justifying a high survival exemption for discarded plaice in the Belgian beam-trawl fishery will remain a challenge in compliance with the Landing Obligation.

Commercial fish abundance estimation in the Belgian part of the North Sea through eDNA ddPCR analyses

Monitoring of fish assemblages in the Belgian part of the North Sea (BPNS) mainly happens through trawling. While effective, this method is invasive and destructive as it disturbs bottom communities, catches non-target species and removes organisms from the environment. A more sustainable alternative for monitoring marine diversity is the use of environmental DNA (eDNA) which comprises intra- and extracellular genetic material that comes from the shedding of organic material, like scales and mucus in the case of fish. When applying metabarcoding on eDNA, community composition can be inferred simply by analysing a small volume of water. Therefore the technique does not disturb the environment, and the high sensitivity of eDNA allows the detection of rare and transient species that are frequently missed by traditional sampling methods. Next to determining community composition, the amount of eDNA copies in the water could potentially be used to quantify target fish species in the marine environment. Here, we investigate whether eDNA concentrations from marine water samples correlate with local fish abundance estimates obtained via traditional beam trawling. Species specific Droplet Digital Polymerase Chain Reaction (ddPCR) assays were designed and tested for three economically important species: common sole (Solea solea), plaice (Pleuronectes platessa) and whiting (Merlangius merlangus). In march 2020, 12 sites in the BPNS were selected based on absence, low and high abundances of the three target species as observed in epibenthos monitoring data from previous years. In each site, 2L of seawater was collected with a niskin bottle from ca 1m above the sea floor. Subsequently, beam trawl transects of 1 km were conducted and all epibenthos species caught in the trawling net were morphologically identified, counted and weighted. Our results indicate promising correlations between eDNA concentrations obtained with the ddPCR assays and the number of specimens in the net for all three species. Some “false” positive results were obtained with the ddPCR, but these may actually be “true” positive detections because the fish might be present in the area but were not caught in the trawling transects. This warrants further investigation to see how far eDNA signals can be detected in the North Sea system. Next, 50 water samples were collected in Autumn 2020, involving more locations with or without the three fishes. This time samples were taken at the beginning, middle and end of the 1 km transects to investigate small scale horizontal variation in eDNA concentrations. The autumn samples are currently being processed. In March 2021, samples will be taken at different depths (surface, middle of the water column and ca 1m above the seafloor) to investigate whether there are any vertical patterns in eDNA distribution in a very well mixed system such as the BPNS. A DNA shedding experiment will be performed as well to estimate the rate at which the three fishes shed DNA. This will provide important information on how quickly fishes can be detected when they swim by. The information obtained with the field sampling and experimental setting will help to strengthen the correlation to a point that reliable abundance estimations of our target species become possible and will allow us to evaluate the potential of eDNA as a sustainable alternative/addition to traditional monitoring methods.

Determination of Pharmaceuticals, Heavy Metals, and Oxysterols in Fish Muscle.

The present study aimed to assess the levels of 98 multi-class pharmaceuticals including cardiovascular drugs, antidepressants, hypnotics, antibiotics, and sulfonamides occurring in the muscle tissue of fish caught in the Baltic Sea. The following fish species were collected: perch (Perca fluviatilis); flounder (Platichthys flesus); turbot (Scophthalmus maximus); plaice (Pleuronectes platessa); cod (Gadus morhua callarias); bream (Abramis brama); crucian (Carassius carassius). Additionally, in the examined fish muscle the levels of heavy metals and trace elements were determined (As; Ag; Au; Ba; Cd; Co; Cr; Cu; Hg; Li; Mo; Ni; Pb; Sb; Se; Sn; Tl; V) as well as the levels of cholesterol and its 5 derivatives (7-ketocholesterol; 7α-hydroxycholesterol; 7β-hydroxycholesterol; 5β,6β-epoxy-cholesterol; 5α,6α-epoxycholesterol). In the performed studies 11 out of 98 examined pharmaceuticals were detected in fish muscle. The levels of pharmaceuticals in fish muscle varied depending on the species. In the tissues of bream and crucian, no pharmaceuticals were found. Mercury, lead and arsenic were detected in the muscles of all examined fish. Based on the hazard factor for Hg, Pb, Cd, Ni (target hazard quotient, THQ < 1), it was found that the consumption of the studied fish does not constitute a health risk. However, the THQ for As remained >1 indicated possible risk from those metals. In the examined fish muscle the total cholesterol oxidation products (COPs) level of oxysterols were, respectively: 6.90 (cod) μg/g–4.18 μg/g (perch), which corresponded to 0.7–1.5% of cholesterol. The main COPs evaluated were 7-ketocholesterol (0.78 ± 0.14–1.79 ± 0.06 μg/g), 7β-hydroxycholesterol (0.50 ± 0.04–3.20 ± 2.95 μg/g) and 5β,6β-epoxycholesterol (0.66 ± 0.03–1.53 ± 0.66 μg/g). The assessment of health hazards due to contaminations is necessary, which may help to introduce national legislation and global standards aimed at reducing or even eliminating the exposure to contaminants.

Using a residency index to estimate the economic value of coastal habitat provisioning services for commercially important fish species

AbstractCoastal habitats worldwide face various threats, including sea level rise and land conversion. Coastal habitat loss has important economic consequences, as many of these habitats provide valuable ecosystem services including flood protection, carbon sequestration, and nursery areas for commercially fished species. Quantifying the economic value of these ecosystem services helps target policies for coastal habitat restoration. Here, we demonstrate how to quantify the contribution made by coastal habitats to the revenue (e.g., ex‐vessel values) of commercially fished species by estimating a residency index. This residency index weights the relative importance of a habitat along a species' lifecycle by explicitly incorporating the target species' life histories and the estimated proportion of time the species spends in that habitat at different life stages. We demonstrate how this method can be used to estimate the value of saltmarsh to UK commercial fisheries landings. This analysis suggests that UK saltmarsh contributes annually between 15% and 17.5% of total UK commercial landings for European seabass (Dicentrarchus labrax), European plaice (Pleuronectes platessa), and Common sole (Solea solea). Our findings support an economic argument for saltmarsh protection and restoration. Furthermore, our approach provides a general framework that integrates demographic methods and economic analyses to assess the value of saltmarsh and other coastal habitats for fisheries worldwide.

Evolution at two time frames: ancient structural variants involved in post-glacial divergence of the European plaice (Pleuronectes platessa).

Changing environmental conditions can lead to population diversification through differential selection on standing genetic variation. Structural variant (SV) polymorphisms provide examples of ancient alleles that in time become associated with novel environmental gradients. The European plaice (Pleuronectes platessa) is a marine flatfish showing large allele-frequency differences at two putative SVs associated with environmental variation. In this study, we explored the contribution of these SVs to population structure across the North East Atlantic. We compared genome-wide population structure using sets of RAD-sequencing SNPs with the spatial structure of the SVs. We found that in contrast to the rest of the genome, the SVs were only weakly associated with an isolation-by-distance pattern. Indeed, both SVs showed important variation in haplogroup frequencies, with the same haplogroup increasing both along the salinity gradient of the Baltic Sea, and found in high frequency in the northern-range margin of the Atlantic. Phylogenetic analyses suggested that the SV alleles are much older than the age of the Baltic Sea itself. These results suggest that the SVs are older than the age of the environmental gradients with which they currently co-vary. Altogether, our results suggest that the plaice SVs were shaped by evolutionary processes occurring at two time frames, firstly following their origin, ancient spread and maintenance in the ancestral populations, and secondly related to their current association with more recently formed environmental gradients such as those found in the North Sea-Baltic Sea transition zone.

Improving species and size selectivity in the Baltic cod trawl fishery with two simple codend designs

The eastern Baltic cod (Gadus morhua) stock is at a critically low level and recruitment of the western Baltic cod stock is at the lowest observed level in over 30 years. The EU (European Union) landing obligation was introduced in 2015 for cod and in 2017 for plaice (Pleuronectes platessa). Size selective codend designs have been used in the Baltic cod trawl fishery for 25 years; however, there are still relatively high discard rates for cod, plaice and European flounder (Platichthys flesus; henceforth flounder) in the trawl fishery and landings of individuals below the minimum conservation reference size (MCRS). These problems call for additional improvements of species and size selectivity in the cod trawl fishery. In this study, two trawl codend designs were developed and tested to improve species and size selectivity in the Baltic cod trawl fishery. The codends were tested onboard a commercial trawler and the test codends were compared to the conventional Bacoma codend, which was fished simultaneously in a twin trawl rig. New codend designs were based on the Bacoma design, where the first test codend had a bottom net panel made of larger diamond meshes (nominal 130 mm mesh size) to improve selectivity in a cod fishery with high catch rates of flatfish. This codend showed a reduction of plaice and flounder, and cod below 45 cm, in the catches. The other test codend was a full square mesh codend (nominal 120 mm mesh size) developed for a cod directed fishery with low flatfish catches, and showed a marked reduction in the catch of cod below 50 cm, with no differences for flounder and plaice catches. With cod stocks at critically low levels and relatively high discard rates of cod, plaice and flounder, the simple codend designs tested in this study could benefit the sustainability of the Baltic cod trawl fishery, but with some loss of commercially targeted sizes.

Near Infrared Reflectance Spectroscopy Coupled to Chemometrics as a Cost-Effective, Rapid, and Non-Destructive Tool for Fish Fraud Control: Monitoring Source, Condition, and Nutritional Value of Five Common Whitefish Species.

Fish fraud is a problematic issue for the industry. For it to be properly addressed will require the use of accurate, rapid, and cost-effective tools. In this work, near infrared reflectance spectroscopy (NIRS) was used to predict nutritional values (protein, lipids, and moisture) as well as to discriminate between sources (farmed vs. wild fish) and conditions (fresh or defrosted fish). Samples of five whitefish species-Alaskan pollock (Gadus chalcogrammu), Atlantic cod (G. morhua), European plaice (Pleuronectes platessa), common sole (Solea solea), and turbot (Psetta maxima)-including farmed, wild, fresh, and frozen ones, were scanned by a low-cost handheld near infrared reflectance spectrometer with a spectral range between 900 and 1700 nm. Several machine learning algorithms were explored for both regression and classification tasks, achieving precisions and coefficients of determination higher than 88% and 0.78, respectively. Principal component analysis (PCA) was used to cluster samples according to classes where good linear discriminations were denoted. Loadings from PCA revealed bands at 1150, 1200, and 1400 nm as the most discriminative spectral regions regarding classification of both source and condition, suggesting the absorbance of OH, CH, CH2, and CH3 groups as the most important ones. This study shows the use of NIRS and both linear and non-linear learners as a suitable strategy to address fish fraud and fish QC.

Legacy and Emerging Contaminants in Demersal Fish Species from Southern Norway and Implications for Food Safety.

The present study aimed at measuring the levels of legacy and emerging contaminants in fillet samples from four demersal fish caught in two fishing sites from Southern Norway, in order to assess possible implications for food safety. Levels of organochlorine compounds (OCs), organophosphate pesticides (OPs), polychlorinated biphenyls (PCBs), polybromodiphenyl ethers (PBDE), per- and polyfluoroalkyl substances (PFASs), and polycyclic aromatic hydrocarbons (PAHs) were measured in fillet from Atlantic cod (Gadus morhua), European plaice (Pleuronectes platessa), lemon sole (Microstomus kitt), and European flounder (Platichthys flesus) specimens. A negligible contamination by all the investigated chemicals was noted in both the fishing sites, as very low levels of OCs, PCBs, and PFASs were noted in a limited number of individuals for each species. Considering the levels of contaminants measured in fillets of the four demersal fish species, negligible risk for human health for Norwegian consumers can be supposed.

Projected impacts of warming seas on commercially fished species at a biogeographic boundary of the European continental shelf

Abstract Projecting the future effects of climate change on marine fished populations can help prepare the fishing industry and management systems for resulting ecological, social and economic changes. Generating projections using multiple climate scenarios can provide valuable insights for fisheries stakeholders regarding uncertainty arising from future climate data. Using a range of climate projections based on the Intergovernmental Panel on Climate Change A1B, RCP4.5 and RCP8.5 climate scenarios, we modelled abundance of eight commercially important bottom dwelling fish species across the Celtic Sea, English Channel and southern North Sea through the 21st century. This region spans a faunal boundary between cooler northern waters and warmer southern waters, where mean sea surface temperatures are projected to rise by 2 to 4°C by 2098. For each species, Generalized Additive Models were trained on spatially explicit abundance data from six surveys between 2001 and 2010. Annual and seasonal temperatures were key drivers of species abundance patterns. Models were used to project species abundance for each decade through to 2090. Projections suggest important future changes in the availability and catchability of fish species, with projected increases in abundance of red mullet Mullus surmuletus L., Dover sole Solea solea L., John dory Zeus faber L. and lemon sole Microstomus kitt L. and decreases in abundance of Atlantic cod Gadus morhua L., anglerfish Lophius piscatorius L. and megrim Lepidorhombus whiffiagonis L. European plaice Pleuronectes platessa L. appeared less affected by projected temperature changes. Most projected abundance responses were comparable among climate projections, but uncertainty in the rate and magnitude of changes often increased substantially beyond 2040. Synthesis and applications. These results indicate potential risks as well as some opportunities for demersal fisheries under climate change. These changes will challenge current management systems, with implications for decisions on target fishing mortality rates, fishing effort and allowable catches. Increasingly flexible and adaptive approaches that reduce climate impacts on species while also supporting industry adaptation are required.

A macroscopic and stereological imaging dataset of Pleuronectes platessa ovaries

The North Sea plaice, Pleuronectes platessa (Linnaeus, 1758), is a commonly studied commercial flatfish with poorly known ovarian histology. The following dataset is a collection of female plaice gonad images and their corresponding histological slides, collected during a complete season of the plaice’s reproduction cycle. Stereology was used to determine the percentage of different structures found throughout the ovaries. Inter-agent calibrations were accomplished in order to harmonize the stereological readings, and were based on a comprehensive reading protocol and histological lexicon that were specifically written for the plaice’s ovaries. The distribution and homogeneity of the different cell types found throughout the ovaries were also evaluated. This dataset can be used to automate the stereological reading process (through statistical learning methods for example) or to objectively determine the plaice’s maturity phase, and link that information to either macroscopic measurements or through image analysis of the full ovaries.

Dumped munitions: New insights into the metabolization of 2,4,6-trinitrotoluene in Baltic flatfish

Livers from dab (Limanda limanda), plaice (Pleuronectes platessa) and flounder (Platichthys flesus) sampled from the Baltic Sea were used to determine the interaction of flatfish CYP1A enzymes with 2,4,6-trinitrotoluene (TNT) in vitro. Competitive inhibition of 7-ethoxyresorufin-O-deethylase (EROD) and 7-methoxyresorufin-O-deethylase (MROD) could be demonstrated for all three flatfish species. The highest inhibition of CYP1A activities was measured in liver samples of flounder resulting in a half maximal inhibitory concentration (IC50) of 28.1 μM TNT. Due to their lower inhibition (EROD IC50 65.2 μM TNT, MROD IC50 40.3 μM TNT), dab liver samples were used to conduct in vitro metabolization experiments with TNT. The metabolization of TNT in fish was investigated with post-mitochondrial fractions (PMF) of dab liver as a model system after adding different cofactors. Rapid and time-dependent enzymatic degradation of TNT was observed. The concentrations of 4-amino-2,6-dinitrotoluene and 2-amino-4,6-dinitrotoluene increased in the samples over time. Additionally, 2,2,6,6-tetranitro-4,4-azoxytoluene was detected in one sample. The results of this study indicate that in vitro experiments are useful to investigate the xenobiotic metabolism of fish under controlled conditions prior to field studies. The metabolites found can serve as target compounds for marine monitoring of TNT contamination in munition dumpsites.

Genomic structural variants involved in local adaptation of the European plaice

A recommendation – based on reviews by Three anonymous reviewers – of the article: Le Moan, A. Bekkevold, D. and Hemmer-Hansen J. (2020). Evolution at two time-frames: ancient and common origin of two structural variants involved in local adaptation of the European plaice (Pleuronectes platessa). bioRxiv, 662577, ver. 5 peer-reviewed and recommended by PCI Evol Biol. doi: 10.1101/662577. doi: 10.1101/662577

Invasive Neogobius melanostomus in the Lithuanian Baltic Sea coast: Trophic role and impact on the diet of piscivorous fish

The partitioning of trophic niches is central for understanding the ecosystem processes associated with biological invasions. The recent successful establishment of Neogobius melanostomus in the Baltic Sea posed questions whether this invader found its own trophic niche, to what extent it competes with native benthivores, and if commercially important fish could be affected. In this study conducted along the Lithuanian Baltic coast, we used the stable isotope approach to identify potential trophic competitors of N. melanostomus. We also employed gut content analysis to quantify the incorporation of N. melanostomus into the diet of native piscivores. Furthermore, pre-invasion gut content data enabled us to check if there were any changes in the piscivorous fish diet after the invasion. We found N. melanostomus to be a subdominant benthivorous fish during the warm season of 2012–2013. It occupied a higher trophic position in spring (3.9) than in autumn (2.9), but spring specimens were also larger. The invader exploited a narrow (standard ellipse area 0.3–0.5‰2) and distinct core isotopic niche, but total niche area overlaps indicated potential trophic competition with Pleuronectes platessa, Platichthys flesus, Pomatoschistus minutus and Gasterosteus aculeatus in spring, and with P. minutus in autumn. The invader dominated the warm season (spring–autumn) diet of piscivorous fish (Gadus morhua, Myoxocephalus scorpius, Perca fluviatilis, Scophthalmus maximus) and its invasion was generally associated with decreased crustacean and increased fish-prey content. Therefore, it is plausible that N. melanostomus has created novel trophic links within the food web.

A meta-analysis of plaice size-selection data in otter trawl codends

The influence that mesh size, the number of open meshes around the circumference, twine thickness and catch size have on the size-selection of plaice (Pleuronectes platessa) in otter trawl codends is investigated by synthesising the data from 164 hauls using 25 gear configurations during 9 trips.All these variables have an effect on selection. The 50 % retention length (L50) increases by 1.9 cm for every 10 mm increase of mesh size and decreases by 0.24 cm for every doubling of catch. The selection range decreases by 3.8 % for each increase of 10 open meshes and increases by 12.3 % for each 1 mm increase of twine thickness. The mesh size and twine thickness effects have strong support from the data. The open meshes and catch size effects have less support and should be treated with some caution.There is good correspondence between the predictions of the analysis presented here and the L50 estimates of beam trawl codend selectivity trials, which is noteworthy given the many operational differences between the two fishing methods.

Modifying selectivity to reduce unwanted catches in an English trammel net and gill net common sole fishery

Modifying the design and selectivity of commercial fishing gears can reduce unwanted catches. Most gear selectivity research focusses on trawl gears, and there are relatively few examples of modified static nets, such as gill and trammel nets, which are used all over the world. Fishers operating in the southeast England’s common sole (Solea solea) static net fishery wanted to explore methods to try to reduce the unwanted catches of thornback ray (Raja clavata). In 2017, the fishery had common sole landings of 343 tonnes making it an important local fishery. The largest port, the beach launch port of Hastings, is the home port for 65 vessels. Here we describe the performance of a modified trammel net with reduced mesh size in the two outer walls, intended to reduce catches of thornback rays without reducing the landings of the target species of common sole. There no published studies on modifying the outer meshes of a trammel net. The fieldwork was undertaken onboard a vessel from Hastings, fishing over ten days. The catches from a standard gill net, a standard trammel net and a modified trammel net were compared. Six fleets of nets were fished, each made up of five different net sections. The three net designs were placed into the fleets in a random set design. All of the net designs had a minimum (inner) mesh size of 90 mm. There was no statistically significant difference in the number of thornback ray caught between the nets but the number of common sole was estimated at 87 % higher by value in the modified trammel net compared to the other nets. Therefore, when using the modified trammel net, considerably less fishing effort was required to catch the same quota of common sole, but with no increase in the rate of thornback ray catches. If this design were to be applied more widely in this fishery, it could reduce overall catches of thornback ray. The study shows the difference in selectivity towards common sole, plaice (Pleuronectes platessa), whiting (Merlangius merlangus) and thornback rays between a gill net and two trammel net designs.

Survival of undersized plaice (Pleuronectes platessa) discarded in the bottom otter trawl and Danish seine mixed fisheries in Skagerrak

The landing obligation of the new Common Fisheries Policy includes a “high survival”-exemption, which allows to continue discarding of fish with scientifically proven high chances to survive the process of capture, handling and release. This study presents survival rates of undersized plaice (Pleuronectes platessa) caught by two commercial fishing vessels targeting plaice in Skagerrak during the summer, a bottom otter trawler and a Danish anchor seiner. Instead of being discarded, the fish were assessed and monitored in on-land observation tanks for 14 days. For each gear, a Weibull mixture model was fitted to the data to estimate survival probabilities, including the uncertainties from the haul selection and conditions of the captivity experiment, and to determine which biological, environmental and operational factors influence survival. 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.