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This study investigates particle dynamics around a towed circular cylinder near a wall. The flow field obtained from a numerical model based on the Reynolds-Average Navier–Stokes equations is utilized to estimate the particle trajectories computed using a Lagrangian approach. The simulated flow resembles a towed fishing gear and the particles which represent sea stars, muscles, and sediments are seeded at the seabed. The ejection of particles from the seabed is related to the upward flow induced by lee-wake vortex shedding. The effect of the gap between the cylinder and the seabed is investigated, where an increased distance leads to fewer ejected particles and distinct trajectories. Furthermore, for the same relative distance to the seabed, a larger diameter cylinder, and hence a different relative initial position and Reynolds number, leads to fewer ejected particles. The optimal position of the fishing net of a towed fishing gear relative to the cylinder is investigated based on the trajectories.

Sea stars (starfish) predate on various shellfish such as mussels and clams, and can have a negative impact on the blue-mussel production and aquaculture. This has encouraged the harvesting of sea stars to protect blue mussels and clams from predation, but they are also targeted commercially, as a valuable protein source used e.g. in animal feed. Towed fishing gears can be used to harvest sea stars in coastal regions, however, little is known scientifically about the hydrodynamics around these gears. To better understand the hydrodynamics around towed fishing gears, an experimental investigation has been carried out in a current flume within the DTU Civil and Mechanical Engineering Hydraulics Laboratory.

This study presents morphological data and estimates of the settling velocity and drag coefficient of sea stars (Asterias rubens) caught in the Limfjord, Denmark. A geometric model describing the sea star is presented and the thickness and arm width are determined as linear functions of arm length. The volume and mass of the sea stars is well predicted by the geometric model and is in agreement with the experimental measurements. The mean sea star density is determined to be 1095 kg/m3, the mean drag coefficient is estimated to be 2.3 and the settling velocity is shown to vary with the square root of the its size.

There is a need to improve fishing methods to select for certain sizes and species while excluding others. Experiments are conducted to quantify selectivity of fishing gears and how variables such as gear design (e.g. mesh size, mesh shape), environmental parameters (e.g. light, turbidity, substrate) or biological parameters (e.g. fish condition) alter selectivity; the resulting data need to be analyzed using specialized statistical methods in many cases. Here, we present a new tool for analyzing this type of data: an R package named selfisher. It allows estimating multiple fixed effects (e.g. fish length, total catch weight, environmental variables) and random effects (e.g. haul). A bootstrapping procedure is also provided. We demonstrate its use via four case studies including (A) covered codend analyses of four gears, (B) a paired gear study with numerous covariates, (C) a catch comparison study of unpaired hauls of gillnets and (D) a catch comparison study of paired hauls using polynomials and splines. This software will make it easier to model selectivity, teach statistical methods, and make analyses more repeatable.

Objectives, effort, and deliverables The primary project objectives and outcomes are organized within three overarching components, which have also formed the backbone of the work packages and associated tasks of the project. Fishing footprint: i) Development of a new methodology to estimate the exact spatial extent and gear footprint of Danish anchor seines. ii) The development of a hierarchical method that combines different sources of fisheries monitoring data (VMS, AIS, and Black Box data) to produce high-precision fishing pressure maps (grid cell resolution of 100x100m) of Danish demersal fisheries. Gear specific depletion rates: i) Development of a new methodology that theoretically derives gear-specific depletion rates, based on gear design and component-specific penetration depths, for ten different commercial fishing gears. ii) Quantification of the benthic impact (benthic fauna depletion) of the Danish Seine fishing gear based on in-project sea trials and experimental data. iii) Quantification of the impact on the benthic fauna community from a standard and a modified sandeel otter trawl based on experimental data from sea trials previously conducted by DTU Aqua. iv) Quantification of the impact on the benthic fauna community from a light-weight mussel dredge based on experimental data from sea trials previously conducted by DTU Aqua. Fishery specific assessment of benthic impact: i) Development of a new methodology to assess the amount of sediment mobilized by demersal fishing gears, based on the spatial distribution of both fishing activity and fine sediments, combined with gear-specific drag coefficients. ii) Integration of project results and external science-based methods and results into a framework, tailored to assess the benthic impact from the different Danish fisheries with Mobile Bottom Contacting Gears (MBCGs) on sandbanks in the North Sea, and applied to

Demersal fisheries trawling is widely acknowledged as one of the most intense forms of widespread benthic disturbance, resuspending extensive plumes of sediments and dissolved nutrients. However, difficulties associated with sampling within trawl plumes have hitherto limited our quantitative understanding of these widespread phenomena. This lack of knowledge hinders our ability to understand the broader consequences of demersal trawling and the development of new fishing gears to limit benthic disturbance. Here we present data from a series of novel in situ experiments using a specially designed trawl sled to quantitatively examine how trawl gear-induced drag and pressure influence the height and concentrations of resuspended sediments and nutrients within a trawl plume. Our data demonstrate that the composition of resuspended particles and sampled nutrients are both influenced by sampling height above the seafloor and the amount of drag exerted by the trawl gear (p < 0.001 in all cases), although the relative importance of these factors differed between the response variables examined. These differences likely reflect that sediment particles are more influenced by gravity than dissolved nutrients are. Our results demonstrate that trawl gear specification strongly influences the amount of dissolved and particulate material resuspended, suggesting that their design could be modified to reduce impacts on benthic ecosystems.

Fisheries management typically aims at controlling exploitation rate (e.g., Fbar) to ensure sustainable levels of stock size in accordance with established reference points (e.g., FMSY, BMSY). Population selectivity (“selectivity” hereafter), that is the distribution of fishing mortality over the different demographic components of an exploited fish stock, is also important because it affects both Maximum Sustainable Yield (MSY) and FMSY, as well as stock resilience to overfishing. The development of an appropriate metric could make selectivity operational as an additional lever for fisheries managers to achieve desirable outcomes. Additionally, such a selectiv-ity metric could inform managers on the uptake by fleets and effects on stocks of various technical measures. Here, we introduce three criteria for selectivity metrics: (a) sensitivity to selectivity changes, (b) robustness to recruitment variability and (c) robustness to changes in Fbar. Subsequently, we test a range of different selectivity metrics against these three criteria to identify the optimal metric. First, we simulate changes in selectivity, recruitment and Fbar on a virtual fish stock to study the met-rics under controlled conditions. We then apply two shortlisted selectivity metrics to six European fish stocks with a known history of technical measures to explore the metrics’ response in real-world situations. This process identified the ratio of F of the first recruited age–class to Fbar (Frec/Fbar) as an informative selectivity metric for fisheries management and advice.

There are numerous examples from across Europe where collaborative science-industry initiatives, which provide a bottom-up approach to trawl gear development, have been successful in developing selective gears. For fishers, the collaborative approach creates a sense of ownership and control over the gears developed and often a greater desire for them to achieve their objectives. Despite the many benefits of collaborative initiatives, a lack of clarity and transparency, incorrect incentives, unclear communication, and distrust or diverging perceptions can inhibit their performance. The present review examines the different collaborative approaches to fishing gear development that have been established within Europe, highlighting their salient features, comparing their objectives, and discussing the types and effectiveness of the incentives offered. Also examined is how the reform of the European Union's (EU) Common Fisheries Policy (CFP), together with the proposed reform of the technical measures framework, can potentially improve the flexibility in the technical regulations and allow for such initiatives to be a central part in improving the state of fish stocks throughout the EU. Finally, a framework is proposed on how initiatives pertaining to industry-led fishing gear development might look like under the reformed CFP.

Tickler-chain SumWing and electrode-fitted PulseWing trawls were compared to assess seabed impacts. Multi-beam echo sounder (MBES) bathymetry confirmed that the SumWing trawl tracks were consistently and uniformly deepened to 1.5 cm depth in contrast to 0.7 cm following PulseWing trawling. MBES backscatter strength analysis showed that SumWing trawls (3.11 dB) flattened seabed roughness significantly more than PulseWing trawls (2.37 dB). Sediment Profile Imagery (SPI) showed that SumWing trawls (mean, SD) homogenised the sediment deeper (3.4 cm, 0.9 cm) and removed more of the oxidised layer than PulseWing trawls (1 cm, 0.8 cm). The reduced PulseWing trawling impacts allowed a faster re-establishment of the oxidised layer and micro-topography. Particle size analysis suggested that SumWing trawls injected finer particles into the deeper sediment layers (∼4 cm depth), while PulseWing trawling only caused coarsening of the top layers (winnowing effect). Total penetration depth (mean, SD) of the SumWing trawls (4.1 cm, 0.9 cm) and PulseWing trawls (1.8 cm, 0.8 cm) was estimated by the depth of the disturbance layer and the layer of mobilized sediment (SumWing = 0.7 cm; PulseWing trawl = 0.8 cm). PulseWing trawls reduced most of the mechanical seabed impacts compared to SumWing trawls for this substrate and area characteristics

This study assesses the seabed pressure of towed fishing gears and models the physical impact (area and depth of seabed penetration) from trip-based information of vessel size, gear type, and catch. Traditionally fishing pressures are calculated top-down by making use of large-scale statistics such as logbook data. Here, we take a different approach starting from the gear itself (design and dimensions) to estimate the physical interactions with the seabed at the level of the individual fishing operation. We defined 14 distinct towed gear groups in European waters (eight otter trawl groups, three beam trawl groups, two demersal seine groups, and one dredge group), for which we established gear “footprints”. The footprint of a gear is defined as the relative contribution from individual larger gear components, such as trawl doors, sweeps, and groundgear, to the total area and severity of the gear's impact. An industry-based survey covering 13 countries provided the basis for estimating the relative impact-area contributions from individual gear components, whereas sediment penetration was estimated based on a literature review. For each gear group, a vessel size–gear size relationship was estimated to enable the prediction of gear footprint area and sediment penetration from vessel size. Application of these relationships with average vessel sizes and towing speeds provided hourly swept-area estimates by métier. Scottish seining has the largest overall gear footprint of ∼1.6 km2 h−1 of which 0.08 km2 has an impact at the subsurface level (sediment penetration ≥ 2 cm). Beam trawling for flatfish ranks low when comparing overall footprint size/hour but ranks substantially higher when comparing only impact at the subsurface level (0.19 km2h−1). These results have substantial implications for the definition, estimation, and monitoring of fishing pressure indicators, which are discussed in the context of an ecosystem approach to fisheries management.