Beam Trawl Research Articles

Evaluating effects of partial fishing closures on the composition and structure of estuarine fish assemblages

Partial fishing closures are an integral component of contemporary aquatic resource conservation and fisheries management. This study examined whether assemblages of fishes differed between partially closed (PC) estuaries that permit recreational fishing compared to fully fished (FF) estuaries that permit commercial and recreational fisheries. Fish assemblages were quantitatively sampled in a standard and stratified manner using a multimesh gillnet and beam trawl that sampled different ichthyofaunal components in two PC and two FF estuaries across three years, ∼ six to eight years post commercial fishing closure and PC implementation. There was no global support for the hypothesis that assemblages, diversity and numbers of fishes would differ between PC and FF estuaries. Assemblages significantly and consistently differed among individual estuaries regardless of estuary management category. Differences between PC and FF estuaries in terms of numbers of species and individuals were inconsistent across years, with more species (gillnet) and individuals (trawl) occurring in PC estuaries in only one of three years. Only one species (Gerres subfasciatus) was more abundant (gillnet) in the PC category, most likely due to reduced fishery harvests. In contrast, juveniles of three harvested species (G. subfasciatus, Rhabdosargus sarba and Acanthopagrus spp.) occurred in greater numbers (trawl) in FF estuaries, potentially a result of strong recruitment and estuary-specific environmental conditions. This study demonstrated the complexity, and potential scale-dependent ecological and fishery-related constraints, in comparatively examining the effects of different fishery management arrangements on fish assemblages across estuary systems.

Evaluation of a novel research trap for surveys of blue swimmer crab populations

Surveying free-ranging crab populations is important for monitoring the health of exploited stocks and predicting future productivity. Here, we present a novel research trap design for use in fisheries-independent surveys of blue swimmer crab (Portunus armatus) populations, and evaluate the trap against some existing approaches within an independent survey framework. Specifically, the trap design aimed to improve efficiency for capturing smaller crabs, without adversely affecting catches of larger crabs. We quantify and report the abundance and selectivity of these traps, relative to co-located samples obtained using beam trawls and standard commercial round traps, to establish whether these small-mesh traps may offer any improvements over existing survey methods. Comparison and evaluation of these small-mesh traps against other existing survey gear, in different places and at different times, showed that the traps are more effective at catching smaller crabs when they are present, and equally or more effective at catching larger size classes of crabs. The beam trawl appeared to be effective at capturing a reasonable size range of crabs; however, the number of crabs caught (using a similar investment of time) was substantially lower than that caught in traps. This novel small-mesh research trap appears suitable for fisheries-independent surveys of portunid crab species.

Spatial and Temporal Distribution of Liocarcinus depurator (Crustacea: Decapod) Caught by Beam Trawl in the Southeastern Black Sea

This study was carried out to determine the temporal and spatial distribution of Liocarcinus depurator (Crustacea: Decapod) caught by beam trawl in the southeast Black Sea between December 2012 and November 2013. Sampling was executed at depths from shoreline to deeper 30 m by using a 2 m wide beam trawl from İyidere, Merkez and Çayeli stations. L. depurator was determined most intensely (1000 m2) at 0-5 m depth at Çayeli station and during the summer season. L. depurator started to migrate from shallow to deeper in autumn and left the coast completely in winter. It approached the shore again in the spring and spread at all depths in the summer season. In addition, while it was distributed in three different habitats according to CPUE values, the highest CPUE value was calculated at Çayeli station, which has a macroalgae structure.

Ecological Gradients of Epimegafaunal Distribution along The Sectors of Gulf of İzmir, Aegean Sea

During 2009-2010 epifaunal materials were collected seasonally with a beam trawl from seven fixed stations in the infralittoral of Gulf of Izmir to study spatiotemporal distribution of megabenthic fauna and their ecology. A total 153 megafaunal species were identified belonging to nine phyla, comprised mostly of 54 molluscans, 43 chordates (mostly fish), 20 arthropods, and 18 echinoderms. Nine alien species were recorded. Four species (two fish species, one gastropod and one Asteroidea species) were constant species and 16 species were common species in the study area. Lesueurigobius friesii, Buglossidium luteum, Turritellinella tricarinata and Astropecten irregularis were the most frequently occurred, and Lesueurigobius friesii, Varicorbula gibba, Anomia ephippium, Turritellinella tricarinata and Dentalium sp were the most abundantly occurred species. Excluding the evenness index, faunistic characteristics tended to increase as a factor of depth. Seasonal density (abundance and biomass) was minimal in April and maximal in February, followed by November having 2-fold higher abundance than that inJuly. Faunal assemblages were correlated with regions of the gulf and habitat type. Buglossidium luteum overspread the entire gulf excluding the inner gulf. Varicorbula gibba and Fulvia fragilis, a pollution indicator predominated the inner gulf. Hydrographical parameters and depth were associated to dictate faunal assemblages with difference among the sectors and habitats.

Recovery Signals of Rhodoliths Beds since Bottom Trawling Ban in the SCI Menorca Channel (Western Mediterranean)

One of the objectives of the LIFE IP INTEMARES project is to assess the impact of bottom trawling on the vulnerable benthic habitats of the circalittoral bottoms of the Menorca Channel (western Mediterranean), designated a Site of Community Importance (SCI) within the Natura 2000 network. The present study compares the epibenthic communities of four areas, subjected to different bottom trawl fishing intensity levels. The assignment of fishing effort levels was based on the fishing effort distribution in the area calculated from Vessel Monitoring System (VMS) data and the existence of two Fishing Protected Zones in the Menorca Channel. Biological samples were collected from 39 beam trawl stations, sampled during a scientific survey on April 2019. We compare the diversity, composition, and density of the epibenthic flora and fauna, together with the rhodoliths coverage and the morphology of the main species of rhodoliths of four areas subjected to different levels of bottom trawl fishing effort, including one that has never been impacted by trawling. Our results have shown negative impacts of bottom trawling on rhodoliths beds and the first signals of their recovery in areas recently closed to this fishery, which indicate that this is an effective measure for the conservation of this habitat of special interest and must be included in the management plan required to declare the Menorca Channel as a Special Area of Conservation.

Improving Scientific Knowledge of Mallorca Channel Seamounts (Western Mediterranean) within the Framework of Natura 2000 Network

The scientific exploration of Mallorca Channel seamounts (western Mediterranean) is improving the knowledge of the Ses Olives (SO), Ausias March (AM), and Emile Baudot (EB) seamounts for their inclusion in the Natura 2000 network. The aims are to map and characterize benthic species and habitats by means of a geological and biological multidisciplinary approach: high-resolution acoustics, sediment and rock dredges, beam trawl, bottom trawl, and underwater imagery. Among the seamounts, 15 different morphological features were differentiated, highlighting the presence of 4000 pockmarks, which are seafloor rounded depressions indicators of focused fluid flow escapes, usually gas and/or water, from beneath the seabed sediments. So far, a total of 547 species or taxa have been inventoried, with sponges, fishes, mollusks, and crustaceans the most diverse groups including new taxa and new geographical records. Up to 29 categories of benthic habitats have been found, highlighting those included in the Habitats Directive: maërl beds on the summits of AM and EB, pockmarks around the seamounts and coral reefs in their rocky escarpments as well as fields of Isidella elongata on sedimentary bathyal bottoms. Trawling is the main demersal fishery developed around SO and AM, which are targeted to deep water crustaceans: Parapenaeus longirostris, Nephrops norvegicus, and Aristeus antennatus. This study provides scientific information for the proposal of the Mallorca Channel seamounts as a Site of Community Importance and for its final declaration as a Special Area of Conservation.

Crustaceans (Malacostraca and Thecostraca) from the International Minho River, Iberian Peninsula

Crustaceans (Arthropoda) are a diverse and abundant group with chitinous exoskeleton, living on coastal/estuarine environments, at community invertebrate fauna. A survey on the species of some crustacean groups found on the Minho River estuary (international section) in the Iberian Peninsula, is presented with specimens collected through glass eel fishing bycatch, grab sampler, trammel net, beam trawl and fyke net sampling methods. A total of 98 specimens were examined belonging to 7 orders, 21 genera and 23 species (17 new records from Minho River, including one for Portugal). Brief diagnosis, ecological notes, species distributions and figures are provided intending to present taxonomic support on future projects in this region.

Effect of Fishery Closure on Discard Composition of Rapa Whelk, Rapana venosa (Valenciennes 1846) Beam Trawl Fisheries in Black Sea

Rapa whelk has become significant commercially, as there is a market that makes it export-oriented species. It has also gained special importance because of catching this species in all riparian countries in the Black Sea basin. The various exploitation methods of this species include beam trawl and diving. Here, the experimental beam trawl survey for Rapa whelk fishing was designed monthly in one year. Catch composition was examined spatially and temporally. Differences in estimated biomass of Rapa whelk and abundance of discard species were used to evaluate the effectiveness of seasonal fishing closure. Our results indicated that, discard composition of Rapa whelk fishery strongly reflected seasonal changes. Total estimated abundance and biomass values showed significant regional, seasonal and depth-related differences, also number of species and diversity index changes regionally and seasonally. Discard ratio was found very low in biomass. At the same time, seasonal fishery closure period was benefited for discard species in spring and summer period. The lowest distribution of Rapa whelk and discard species except blood cockle were found in winter season during fishing period. It becomes apparent that seasonal fishery closure is an important management measure both for stock sustainability, species diversity, and ecosystem health. Therefore, all Black Sea countries should consider similar management.

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.

Analysis of Demersal Fish Fauna off the Sea of Marmara, Turkey

Demersal fish fauna of the Sea of Marmara, Turkey was determined by bottom trawl surveys between March 2017-December 2018 at 34 stations with the monthly samplings. During the study, a total of 61 teleost and 12 cartilaginous fish species belonging to 42 families were sampled. The target, bycatch and discard rates of CPUE were determined as 13.40%, 69.64% and 16.95%, respectively. In total, 53.9% of the CPUE was stemmed from Trachurus trachurus. Mustelus mustelus, Raja clavata, Merluccius merluccius and Merlangius merlangius had the highest CPUE with a mean of 77.63, 71.86, 71.72 and 72.68 kg/km2, respectively. The highest biodiversity was observed in the southwestern part of the Marmara Sea. With increasing depth, the species number of the teleost fish decreased, whereas the species number of the cartilaginous fish increased. The mean CPUE values of the economical demersal fish species were lower in comparison to those reported from other regions in Turkey. Evidence suggests fish stocks with shallower distribution is under heavier threat against fishing pressure. Since commercial trawling is banned in the Sea of Marmara, beam trawl fishery can be considered as the major threat to demersal fish stocks in the region.

Reducing catch efficiency of rabbitfish (Siganus oramin) in a shrimp beam trawl fishery of the South China Sea

The bycatch of unwanted fish species is a main concern in shrimp beam trawl fishery of the South China Sea. This study evaluated the effect of using combined square-mesh and diamond-mesh codends (CSDM codends) on catch efficiency of a shrimp trawl fishery with the catch comparison and catch ratio approach. The trouser trawl method was applied using the commercial 22-mm diamond mesh codend (D22) as the baseline. The target species, greasyback shrimp (Metapenaeus ensis), and main fish bycatch rabbitfish (Siganus oramin) were chosen as the referred species. The results demonstrated that there was no significant change for the target species but a significant reduction on the unwanted fish bycatch when the CSDM codends were used. Compared with the D22 codend, the CSDM codends would reduce the catch efficiency of greasyback shrimp by less than 18% on average, and the reduction was not statistically significant; whereas catch efficiency of rabbitfish would be significantly reduced by more than 45% and the reduction was length-dependent. The results also showed that the catch efficiency of fish bycatch would reduce as the mesh sizes of the diamond-section of the tested codends increased. These promising results demonstrate that the CSDM codends have a potential to be applied to fishery management for mitigating fish bycatch in the shrimp fishery of the studied area.

Functional Pattern of Benthic Epifauna in the Chukchi Borderland, Arctic Deep Sea

Assessment of Arctic deep-sea ecosystem functioning is currently an urgent task considering that ongoing sea-ice reduction opens opportunities for resource exploitation of yet understudied deep-sea regions. We used Biological Trait Analysis to evaluate ecosystem functioning and test if common paradigms for deep-sea fauna apply to benthic epifauna of the deep-sea Arctic Chukchi Borderland (CBL). We also investigated the influence of environmental factors on the functional structure of the epifauna. The analysis was performed for 106 taxa collected with a beam trawl and a Remotely Operated Vehicle from 486 to 2610 m depth. The most common trait modalities were small-medium size, mobile, benthic direct and lecithotrophic larval development, and predatory feeding, which mostly supports the current view of epifauna in the global deep sea. Functional composition of epifauna differed between two depth strata (486–1059 m and 1882–2610 m), with depth and sediment carbon content explaining most of the functional variability. Proportional abundances of the modalities free-living, swimming, suspension feeders, opportunists/scavengers, internal fertilization and globulose were higher at deep stations. Functional redundancy (FR) was also higher there compared to the mid-depth stations, suggesting adaptation of fauna to the more homogeneous deep environment by fewer and shared traits. Mid-depth stations represented higher functional variability in terms of both trait modality composition and functional diversity, indicating more variable resource use in the more heterogeneous habitat. Food input correlated positively with the proportional abundance of the modalities tube-dwelling, sessile and deposit feeding. Areas with drop stones were associated with higher proportional abundance of the modalities attached, upright, and predators. Comparatively low FR may render the heterogeneous mid-depth area of the CBL vulnerable to disturbance through the risk of loss of functions. Across the study area, high occurrence of taxa with low dispersal ability among adult and larval life stages may prevent rapid adaptation to changes, reduce ability to recolonize and escape perturbation.

The compatibility of fishers and scientific surveys: increasing legitimacy without jeopardizing credibility

Abstract For a long time, fishery-independent surveys were only carried out by scientists. On-going criticism by Dutch fishers on the North Sea beam trawl survey prompted scientists to invite fishers on board. Since 2007, fishers have annually joined the survey as observers. Observing all steps in the survey like rigging the gear, the selection of fishing locations, catch sorting, and data registration enables fishers to provide feedback based on their experience instead of preconceptions. Where possible, their suggestions for improvements are incorporated. Since fishers joined, the nature of discussions about the survey has changed to being constructive. The fishing industry's trust in survey methods, results, and the scientific crew increased. Scientists gained a better understanding of the importance of the survey for fishers' livelihoods. The observers also inspired continuous scientific scrutiny of the survey, its setup, and objectives. We describe the process of incorporating fishers in a fishery-independent survey, its benefits, and challenges. We show how perceptions about the survey changed. Allowing stakeholders behind the scenes of a survey and taking their expertise into account contributes to a more reciprocal relation in the co-production of knowledge through collaborative research and increases legitimacy. We propose guidelines for involving stakeholders without compromising the survey's and professional credibility.

Impact of bottom trawling on sediment biogeochemistry: a modelling approach

Abstract. Bottom trawling in shelf seas can occur more than 10 times per year for a given location. This affects the benthic metabolism, through a mortality of the macrofauna, resuspension of organic matter from the sediment, and alterations of the physical sediment structure. However, the trawling impacts on organic carbon mineralization and associated processes are not well known. Using a modelling approach, the effects of increasing trawling frequencies on early diagenesis were studied in five different sedimentary environments, simulating the effects of a deeper-penetrating gear (e.g. a tickler chain beam trawl) versus a shallower, more variable penetrating gear (e.g. an electric pulse trawl). Trawling events strongly increased oxygen and nitrate concentrations in surface sediment layers and led to significantly lower amounts of ammonium (43 %–99 % reduction) and organic carbon in the top 10 cm of the sediment (62 %–96 % reduction). As a result, total mineralization rates in the sediment were decreased by up to 28 %. The effect on different mineralization processes differed both between sediment types and between trawling frequencies. The shallow-penetrating gear had a slightly smaller effect on benthic denitrification than the deeper-penetrating gear, but there were no statistically different results between gear types for all other parameters. Denitrification was reduced by 69 % in a fine sandy sediment, whereas nitrogen removal nearly doubled in a highly eutrophic mud. This suggests that even relatively low penetration depths from bottom fishing gears generate significant biogeochemical alterations. Physical organic carbon removal through trawl-induced resuspension of sediments, exacerbated by a removal of bioturbating macrofauna, was identified as the main cause of the changes in the mineralization process.

Quantifying abundance trends and environmental effects on a population of queen scallop Aequipecten opercularis targeted by artisanal fishers in a coastal upwelling area (Ría de Arousa, NW Spain) using a Bayesian spatial model

Queen scallop Aequipecten opercularis, is exploited by small-scale trawlers in separated aggregations along the east Atlantic coast and the Mediterranean Sea. However, population performance is poorly known. Here, we combine official information and on-board observers’ data over two decades to study the fishery ecology of an aggregation occurring in a coastal upwelling system (Ría de Arousa, NW Spain). Annual landings fluctuated around a mean of 170 tons while beam trawlers declined at a rate of 11 vessels per decade. A hurdle Bayesian spatial model fitted to observers’ catch and effort data showed that the probability of occurrence and abundance of the species increased with fishing effort and decreased in sandy bottoms. Moreover, abundance increased with upwelling intensity and decreased with continental runoff and along the fishing season. Furthermore, occurrence was higher in the inner part of the embayment while abundance increased in the central channel. The predicted index of standardized abundance correlated with the trend in landings, and year-to-year fluctuations in abundance were negatively and positively related to upwelling intensity and net primary production, respectively, during the spawning and settlement period.

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.

Sediment mobilization by bottom trawls: a model approach applied to the Dutch North Sea beam trawl fishery

Abstract Bottom trawls impact the seafloor and benthic ecosystem. One of the direct physical impacts is the mobilization of sediment in the wake of trawl gear components that are in contact with or are close to the seabed. The quantity of sediment mobilized is related to the hydrodynamic drag of the gear components and the type of sediment over which they are trawled. Here we present a methodology to estimate the sediment mobilization from hydrodynamic drag. The hydrodynamic drag of individual gear components is estimated using empirical measurements of similarly shaped objects, including cylinders, cubes, and nets. The method is applied to beam trawls used in the Dutch North Sea flatfish fishery and validated using measurements of beam trawl drag from the literature. Netting contributes most to the hydrodynamic drag of pulse trawls, while the tickler chains and chain mat comprise most of the hydrodynamic drag of conventional beam trawls. Taking account of the silt content of the areas trawled and the number of different beam trawl types used by the fleet, sediment mobilization is estimated as 9.2 and 5.3 kg m−2 for conventional 12 m beam and pulse trawls, respectively, and 4.2 and 4.3 kg m−2 for conventional 4.5 m beam and pulse trawls.

Length-Weight Relationship, Sex Ratio and Condition Factor of Merlangius merlangus (Linnaeus, 1758) From the Sea of Marmara, Turkey

In this study, some biological properties of Merlangius merlangus (Linnaeus, 1758) were examined. In this context, relationship of length-weight, distribution of length-frequency, sex ratios and condition factors were examined. A total of 303 individuals were taken by random sampling from fishing vessels (beam trawl) in the Sea of Marmara. Minimum length and weight values of all sample individuals was found 10.0 cm and 6.6 g, maximum length 41.2 cm and 535 g was found. The average length was found 17.15±0.49 cm and the average weight was 47.03±7.57 g. Length-weight relationship for all samples as W=0.0044TL3.1777 regression coefficient r2=0.98 was determined. Growth type, positive allometric was determined for all samples and both sexes (b˃3; P˂0.05). The sex ratio (F:M) was calculated as 1:1.27. The mean values of condition factor (K) were calculated as 0.729±0.089, 0.733±0.091 and 0.737±0.081 for male, female and combined sex respectively. There are no significant differences between male and female (P˃0.05).

Distribution and habitat of Scalpellidae (Cirripedia: Thoracica) in the Norwegian and Barents Seas

ABSTRACT We describe the distribution and habitats of Amigdoscalpellum hispidum, Weltnerium stroemii, Scalpellum scalpellum, Weltnerium cornutum, Catherinum striolatum, Weltnerium nymphocola and Hamatoscalpellum hamatum in the Norwegian and Barents Seas, and their morphology. These are seven of the eight scalpellid species recorded in the area. Expeditions from 2006 to 2018 of the MAREANO (Marine areal database for Norwegian waters) programme have provided a comprehensive collection of Scalpellidae that can contribute to our knowledge of their distribution. The collections were from 335 localities in 40–2300 m depth. Of the 1406 specimens collected, 1396 were collected by beam trawl and 37 were from grabs. Patchy distributions and the association of species with other organisms (e.g. hydroids and gorgonian corals) can explain the low abundance in grabs compared with beam trawls, the latter covering larger areas and mixed sediments better. Weltnerium stroemii was the most abundant species and had the largest depth and temperature range, followed by A. hispidum and W. nymphocola. Species were assigned into three geographic groups based on their depth distribution. Shelf species: W. stroemii, S. scalpellum and A. hispidum occurring mainly shallower than 400 m in water temperatures >4°C; Upper slope species: W. nymphocola, W. cornutum and H. hamatum, found mainly between 600–1000 m in temperatures from 4 to 0°C; Lower slope species: C. striolatum occurring below 1500 m in temperatures <0°C. The species C. striolatum and W. cornutum are new to the Norwegian fauna and future deep-sea exploration will probably discover further scalpellid species.

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.