Scotland's future catching policy: strategic environmental assessment report 2026

6. Assessment of Environmental Effects

The environmental baseline information (section 3) shows that the marine environment is subject to fishing-related pressures that contribute to the current state of our marine environment.

Section 6 assesses the environmental effects of the selectivity proposals in relation to the environmental issues screened into this SEA, and where applicable their associated UK MS descriptors (Table 1).

6.1 Overview of Potential Positive Environmental Effects of the selectivity proposals

6.1.1 Biodiversity, Flora, Fauna

The selectivity proposals seek to reduce unwanted catch of fish and bycatch of sensitive marine species to increase the sustainability of fishing activity in Scottish waters.

Large Mixed Demersal fleet segment

These vessels use demersal bottom trawls to catch whitefish (demersal) species and have a codend mesh size of 120mm or over.

The introduction of 100 mm square mesh codends is predicted to have significant positive effects on selectivity in Scottish waters. The more open square mesh geometry is expected to increase escapement of juvenile and small non-target fish, thereby reducing discards, enhances age and size structure, and supports more diverse and resilient demersal communities. Indirect benefits to sensitive species and ecosystem resilience are expected, particularly when the measure is applied consistently across relevant fleets and combined with complementary spatial and management measures. No significant adverse biodiversity effects are identified, provided overall effort is effectively managed and compliance with gear specifications is high.

Trials have demonstrated that square-mesh codends retain fewer discards of demersal species, greater escapement of undersized fish and improved sized selectivity in a mixed fishery context than conventional diamond-mesh codends of similar nominal mesh size, particularly for roundfish such as haddock and whiting.^[9] Other international trials have shown similar results.^[10] Although these particular studies were not carried out in Scottish waters and used square mesh smaller than 100mm for their target fishery, it is a useful proxy for the increased selectivity that square mesh offers. These findings support the conclusion that a 100mm square-mesh codend for the Scottish demersal fleet would provide improved size selectivity relative to the current 120 mm diamond-mesh codend baseline.

Mixed demersal fleet segment

The mixed demersal fleet use demersal bottom trawls to catch both whitefish, ground fish and Nephrops and utilise nets with a codend mesh size of both under 120mm and over 120mm depending on what they are targeting.

One Net Rule

UK fisheries legislation divides fisheries between over and under 120mm codend mesh size. Codend mesh size of 120mm and above is required for targeting whitefish to prevent excessive retention of undersized fish, while meshes below 120mm are permitted only for “directed” fisheries targeting species such as Nephrops, where smaller meshes are necessary but must be supplemented by additional selectivity measures. Historically, the “one net rule” allowed vessels to carry multiple nets provided they were either all above or all below 100mm. However, as legislation now uses 120mm as the regulatory threshold, this rule is no longer aligned with current requirements.

The proposed update would revise the one net rule to reflect the 120mm division, meaning vessels would be classified as either whitefish vessels (using only ≥120 mm nets) or directed fishery vessels (using only <120 mm nets), with only one category of net permitted on board at any time. Vessels operating under the directed fishery category would continue to be subject to mandatory additional selectivity measures, such as square mesh panels, along with supporting catch composition and spatial management controls such as haul by haul reporting.

Implementing a single category of net permitted on board at any time would deliver significant benefits for regulatory clarity, enforcement efficiency, stock protection and environmental sustainability, while also improving transparency, fairness and alignment with existing fisheries legislation.

Allowing only one mesh-size category on board prevents gear switching at sea, where vessels could alternate between whitefish and directed-fishery nets in ways that undermine selectivity rules and therefore reduce targeting whitefish with undersized meshes and circumventing catch-composition rules.

By ensuring that vessels targeting whitefish can only use ≥120mm codends, this reduces the capture of undersized fish. By stipulating that ‘directed fisheries’ must use <120mm codends, this ensures that directed fisheries remain subject to mandatory selectivity devices supporting reduction of undersized fish and reducing discards.

While there is limited peer-reviewed literature assessing the one-net rule as an isolated regulatory measure, indirect selectivity, compliance and enforcement evidence supports its underpinning rationale. Studies consistently demonstrate that misuse of small-mesh gears results in significantly higher retention and mortality of juvenile gadoids ^[11], regulatory complexity and variable gear configurations increase the risk of non-compliance and make enforcement harder^[12]. By restricting vessels to a single net category per trip, the one-net rule removes opportunities for mid-trip gear switching, strengthens regulatory clarity, and supports the consistent application of selectivity measures. On this basis, the measure is considered to be strongly evidence-led.

Catch Composition

The purpose of catch composition rules is to ensure that fishing activity remains consistent with the gear type, mesh size, and target fishery, while protecting undersized fish, supporting stock sustainability, and strengthening enforcement. The introduction of catch composition is predicted to have significant positive effects on selectivity in Scottish waters.

In practice, catch composition rules are designed to ensure that vessels using small-mesh gears remain genuinely focused on directed fisheries, while preventing the capture of excessive quantities of undersized and non-target whitefish reducing discards. It provides a quantitative test of whether the vessel is genuinely operating a directed fishery. If the proportion of undersized fish or whitefish in the catch exceeds the permitted thresholds, this indicates incorrect targeting. This prevents the ‘directed fishery’ from becoming a de facto whitefish fishery. Catch composition rules therefore convert gear intent into measurable compliance outcomes.

Haul-by-haul reporting

Haul-by-haul reporting is a fisheries monitoring and data recording system where a fishing vessel must record and report details of each individual haul rather than submitting aggregated daily trip level catch data. It supports catch composition and ‘move’ on rules by providing real-time, location-specific catch data that allows high bycatch or juvenile catch events to be detected immediately, spatially targeted move-on areas to be defined with precision, and compliance to be verified using vessel tracking and electronic monitoring systems. By linking catch composition directly to individual hauls, haul-by-haul reporting prevents repeated fishing in hotspots, strengthens enforcement certainty, and ensures that move-on rules deliver proportionate and effective protection for vulnerable fish stocks. The introduction of haul-by-haul reporting is predicted to have significant positive effects on selectivity in Scottish waters.

Dual Codend Separator Gear

Conventional single codend trawl configurations offer limited opportunity for real-time species separation in a mixed or directed fishery, resulting in discards and reduced stock recovery potential.

Dual codend separator (DCS) is predicted to have positive effects on selectivity in Scottish waters. The gear uses behavioural, mechanical or grid-based separation devices to direct different components of the catch (e.g. Nephrops and fish) into separate codends during towing. This allows target species to be retained while facilitating the escape or separate retention of non-target and undersized fish.

Evidence from at sea trials demonstrates that DCS fishing gear can improve species and size selectivity in mixed demersal fisheries by physically separating target and non-target components of the catch during the fishing process. Trials in Nephrops and whitefish fisheries have consistently shown that DCS gear reduce discard rates of unwanted fish, particularly undersized haddock, whiting and cod by directing different species into separate codends using behaviour-based or grid-based separation mechanisms while maintaining or improving retention of target species such as Nephrops supporting improved compliance with the landing obligation^[13]. The effectiveness of DCS gear is influenced by species behaviour, separator design, towing speed and gear rigging, with performance varying between fisheries and operational conditions. Overall, the evidence supports DCS gear as a proven technical measure for enhancing selectivity, reducing discards and supporting long term sustainability of stocks in mixed fisheries.

By enabling more precise targeting of commercial species while reducing incidental capture, DCS gear contributes to enhanced species selectivity and reduced ecosystem disturbance. These effects support long-term stock recovery and improved biodiversity resilience, particularly in heavily exploited mixed fisheries.

Small Mesh Demersal

These vessels use demersal bottom trawls to catch mainly Nephrops and some ground fish and utilise nets with a codend mesh size of between 80mm – 119mm.

Square mesh panels

The square mesh panel is one of the most commonly used selective devices. In this, the diamond mesh is turned through 45 degrees to the water flow thereby ensuring the meshes in the panel remain fully open throughout the fishing operation, allowing for the release of small fish. The current standard for square mesh panels in Scottish waters for any net between 70mm-119mm is that the square mesh panel is positioned so that the rearmost row of meshes of the square mesh panel are no more than 9-15m from the cod line depending on the fishery prosecuted.

The current standard in Scotland for square mesh panels in directed fisheries for the West of Scotland is 300mm square mesh as part of a 3m panel, fitted in the top side of the fishing net, no more than 12-15m from the cod-line.

There is an exemption to this standard however for lower powered, or lower length vessels (i.e. 200mm in a 2m panel instead of 300mm in a 3m panel, for vessels below 12 m in length and/or with engine power of 112 kW or less).

A study^[14] has outlined that removal of the low power exemptions for square mesh panels (i.e. 200mm instead of 300mm for vessels below 12 m in length over all and/or with engine power of 112 kW or less) should not cause any issues in terms of selectivity or retention of wanted catch – removing these power exemptions would ensure everyone is fishing the same standard of selectivity gear regardless of the size and power of the vessel. However, another study^[15] suggests that an increase to 300mm square mesh panel for these smaller, low powered vessels may result in a loss of medium sized marketable catches of Nephrops, as well as a cost of changing their gear.

While direct experimental comparisons between 200mm and 300mm square mesh panels are limited, the wider selectivity literature consistently demonstrates that increases in square mesh panel size result in progressively higher escapement of undersized fish and improved selectivity performance.

Demersal trawl selectivity trials demonstrates that larger square mesh panels consistently produce higher escapement rates and improved size selectivity for undersized gadoids compared with smaller panels, due to increased mesh opening area and longer effective contact time between fish and the selective surface ^[16]. Studies show that increasing square-mesh panel dimensions results in significant increases in L50^[17] and reductions in the retention of juvenile haddock and whiting, with performance strongly linked to panel size and position within the trawl ^[18]. On this basis, a 300mm square mesh panel is expected to deliver materially higher escapement of undersized gadoids and lower discard rates than a 200mm square mesh panel, particularly in mixed demersal and directed fisheries where smaller panels may become saturated under high catch loads. The evidence therefore supports the conclusion that 300 mm square mesh panels provide a more precautionary and effective selectivity standard than 200mm panels for juvenile fish protection.

Additionally, these trials consistently demonstrate that square mesh panels are most effective when positioned close to the codline, immediately ahead of the codend. These trials demonstrated that contribution of the panel to overall gear selectivity increases as it is moved close to the codline, due to greater fish contact and longer exposure time. When square mesh panels were located 3–6m from the codline they produced substantial increases in L50 for haddock and whiting, whereas panels positioned further forward (9–16m) delivered weaker improvements in selectivity. Subsequent reviews further conclude that square mesh panels positioned closest to the codline provide the greatest escapement of juvenile gadoids and lowest discard rates^[19]

Current legislation states that a square mesh panel, when installed must have no more than two open diamond meshes between the longitudinal side of the panel and the adjacent selvedge in the West of Scotland. In the North Sea licence conditions state there should be no more than five open diamond meshes between the longitudinal side of the panel and the adjacent selvedge. We are proposing to unify this measure across all of Scottish waters. Data suggests^[20] that we must ensure the rear most edge of the square mesh panel is not greater than 2 meshes from selvedge (stitched “seam” that joins the bottom and top half of the fishing net) as this prevents the panel being incorrectly inserted up the tapered section of the net with incorrect insertion having the potential to reduce selectivity.

A lifting strap is a piece of rope or wire loosely encircling the circumference of the codend or the strengthening bag, if any, and attached to it by means of loops or rings. Requiring lifting straps to be made of non-elastic material ensures that the straps are not restricting the meshes and reducing selectivity.

Flotation buoys can play a supporting role in maintaining the effective opening and vertical orientation of square mesh panels, particularly under high catch loads where gear deformation may otherwise reduce selectivity performance. By helping to stabilise panel geometry by counteracting net sagging and improve water flow through the mesh, appropriate buoyancy can improve the consistency and reliability of escapement opportunities. However, flotation does not directly enhance selectivity, and panel effectiveness remains primarily determined by mesh size, panel dimensions, placement close to the codline and fish behaviour.

These selectivity proposals for unwanted fish catch are expected to contribute towards the sustainability of targeted stocks, with possible indirect benefits for the wider environment. Improving data gaps and ensuring the availability of high-quality data for stock assessments will allow for more informed management decisions in the future that could result in improvements across a range of receptors and ultimately contribute to the sustainable management of targeted stocks.

As well as improving the status of the stocks themselves and contributing to improvements against UK MS commercial fish descriptor targets (D3), these actions may also benefit wider fish biodiversity and food webs, therefore contributing to improvements in UK MS targets under D1 and D4.

Sensitive Marine Species

Incidental bycatch or entanglement of sensitive marine species in fishing gear is a threat to the conservation and welfare of such species, including cetaceans, seals and seabirds. Such incidental events not only have a negative impact on the animal concerned and the recruitment of sensitive marine species, but also has significant impacts in terms of lost or damaged gear.

The UK administrations have a range of commitments through domestic, and International law to address the incidental catches of sensitive marine species. To meet these obligations, the Scottish Government has historically worked in partnership with the UK Government and the other devolved administrations, as well as independently. This includes taking forward research and monitoring to improve our understanding of bycatch in UK waters, such as the Marine Wildlife Bycatch Mitigation Initiative, and implementing action by working in partnership with the sector to reduce the numbers of sensitive marine species captured incidentally in fisheries.

Longlines

Longline fishing is the use of long, usually suspended, lines equipped with baited hooks which are deployed from a series of buoys. There are issues with incidental seabird mortality as they target the bait on the hooks as the gear is deployed or the catch as the lines are hauled. From data gathered through the UK Bycatch Monitoring Programme (UK BMP), bycatch of seabirds, which includes Northern fulmar, Northern gannet, Great skua and Manx shearwater is of greatest concern in this fleet segment ^[21].

In Scottish and wider North-East Atlantic longline fisheries, bird-scaring lines are recognised as one of the most effective and low-cost mitigation measures for reducing seabird bycatch, particularly of northern fulmar, gannet and large gulls. Operational trials in North Atlantic demersal and pelagic longline fisheries demonstrate that the use of bird scaring lines can reduce seabird bycatch by 70–100% compared with unmitigated gear, by creating a visual exclusion zone that deters birds from baited hooks during line setting^[22]. UK and Scottish seabird bycatch risk assessments identify longline fisheries as a key pressure on surface-feeding seabirds, particularly fulmar, and explicitly highlight bird-scaring lines as a primary best-practice mitigation measure ^[23].

The effectiveness of bird scaring lines is further enhanced when combined with offal management and deployment during periods of reduced seabird activity. Overall, the evidence base supports a high level of confidence that bird-scaring lines deliver positive biodiversity effects through substantial reductions in seabird bycatch without compromising fishing efficiency.

These noted operational fisheries trials also consistently demonstrate that night setting of longlines can substantially reduce seabird bycatch, often by 50–90% or more in some species, compared with daytime setting, by limiting visual detection of baited hooks by surface-feeding birds. They also recognise that discharging offal during longline setting substantially increases seabird attendance and attack rates on baited hooks, while retaining offal on board until hauling, or discharging it on the opposite side of the vessel from the setting line, significantly reduces seabird interactions and bycatch. UK and Scottish assessments of seabird bycatch identify night setting and offal management as core best-practice mitigation measures for longline fisheries, particularly for northern fulmar and large gulls, which remains one of the most frequently bycaught species in UK waters²¹²³.

Overall, the evidence supports these measures as highly effective, low-cost and operationally feasible measures for reducing seabird mortality.

Gillnets

Gillnets are bottom set nets typically deployed along the edge of the continental shelf, usually in pursuit of monkfish or hake, using monofilament nylon nets that are set on the seabed. The net is held to the seabed by a weighted foot-rope and held up by a floating head line.

The UK gillnet fleet is currently monitored under the UK Bycatch Monitoring Programme (UK BMP). Bycatch reported through the programme includes seals, harbour porpoise, common dolphin, and diving birds such as cormorants, shags and guillemots. To date, much of this monitoring effort has been directed to the south west of England where bycatch risk to these species is higher as nets are set predominantly in shallow inshore waters. Lower level monitoring has taken place in Scottish waters, and gillnetting activity in Scotland tends to take place in deeper waters. Therefore, we are seeking a call for evidence in order to improve our understanding regarding bycatch in the gillnet fishery in Scottish waters and any further selectivity measures that may be implemented within this fleet segment.

Monitoring is also underway in the tanglenet fishery targeting crawfish and which operates in waters adjacent to the Outer and Inner Hebrides. This will help improve understanding of potential bycatch risk in this seasonal fishery, which will run until September 2026.

Pots and Creels

The pots and creels fleet refers to fishing vessels that operate static, baited traps (pots or creels) deployed on the seabed to target species such as crab and lobster. These gears are passive fishing methods, meaning they do not tow gear through the water or across the seabed, but instead rely on animals entering traps voluntarily.

UK and Scottish reviews identify that entanglement in floating rope consistent with creel gear is a significant entanglement risk for cetaceans and basking sharks and explicitly recommend sinking groundlines as a core mitigation measure ^[24]. Operational trials demonstrate that replacing floating groundlines with sinking or neutrally buoyant groundlines substantially reduces the risk of entanglement of cetaceans by removing persistent rope loops from the water column^[25]. North Atlantic evidence shows that floating ropes are a primary driver of entanglement for large whales, particularly minke, humpback and critically endangered North Atlantic right whales, while sinking lines markedly lower encounter probability^[26].

Trials carried out by the Scottish Entanglement Alliance (SEA) on creel fleets using sinking groundline rather than floating rope, found that most fishers encountered very few problems with re-roped fleets, in some cases preferring them to gear made up with floating groundlines²⁷. These trials also showed the sinking rope lay lightly on the seabed with minimal movement.

Overall, the evidence supports sinking groundlines as a highly effective, low-impact and operationally feasible measure for reducing entanglement risk to cetaceans in static-gear fisheries.

The selectivity proposals outlined “support actions under the Marine Wildlife Bycatch Mitigation Initiative to reduce the risk, frequency and impact of fisheries on sensitive marine species” which is expected to deliver broader environmental benefits, for example for food webs and biodiversity. Implementing appropriate bycatch mitigation measures in these fisheries is expected to contribute to the population status of sensitive marine species.

6.1.2 Water

The selectivity proposals to reduce entanglement risk for cetaceans in the pots and creel fleet segment support water quality issues in regard to marine litter.

Reducing the loss of creel gear contributes to marine litter reduction by preventing plastic traps, ropes and marker buoys from entering the marine environment as persistent “ghost gear”, which can continue to fragment into secondary microplastics over long timescales^[27]. Lost creels accumulate on the seabed and in sensitive habitats such as reefs, kelp forests and soft sediments, contributing to long-term litter build-up, habitat smothering and secondary pollution^[28]. By preventing gear loss at source delivers a sustained reduction in marine litter inputs.

The Scottish Government also acknowledges the ongoing work with regard to the OSPAR convention to implement the second Regional Action Plan on Marine Litter. This includes action to tackle marine litter from land and sea-based sources, including fishing.

6.1.3 Climatic Factors

Increased selectivity indirectly contributes to the potential to lower vessel emissions by reducing the volume of unwanted catch and therefore the time a vessel must spend towing, hauling and sorting, which are the primary drivers of fuel use in commercial fisheries. Empirical analyses of fuel consumption show that trawling effort, tow duration and gear drag are strongly correlated with CO₂ emissions, meaning that achieving target catches with fewer hauls or shorter tow times may lead to lower fuel burn^[29]. Operational efficiency reviews similarly demonstrate that reducing non-marketable catch may reduce both main-engine fuel consumption and auxiliary-engine use during sorting and processing ^[30]. Selective gears reduce bulk catch volumes and bycatch mortality, thereby shortening fishing time required to achieve quota and improving energy efficiency per unit of retained catch³⁰. As a result, increased selectivity can be associated with a measurable decrease in greenhouse gas emissions through reductions in fishing effort, drag loads and time at sea.

6.2 Overview of Potential Negative Environmental Effects of the selectivity proposals /

6.2.1 Biodiversity, Flora, Fauna

Acknowledging that the selectivity proposals are at the beginning stages of their development, the assessment of likely negative effects identified a low risk of significant adverse effects on the environment from implementing individual measures. However, we do not yet know the potential environmental effects of implementing the combination of selectivity proposals.

Selectivity is highly sensitive to correct rigging, and improper installation of selective gears such as square mesh panels can significantly reduce their effectiveness. Inappropriately rigged gear alters mesh geometry, collapses escape openings, creates turbulence that prevents fish reaching selective zones, and allows fish to bypass panels altogether. Studies show that incorrect panel tensioning, wrong positioning or poor attachment can negate the selectivity benefits entirely, resulting in lower L50 values and increased retention of juvenile gadoids.¹⁸Correct rigging is therefore key to ensure to ensure selective devices function as intended and deliver discard-reduction objectives and therefore reduce negative environmental effects.

There are mitigation measures to deploy to ensure that selective gears perform as intended. These could focus on specifications supported by detailed technical guidance, industry training and gear inspections during routine compliance checks for gear compliance. Collectively, these actions will ensure that selective devices are rigged and maintained correctly, maximising their contribution to reducing unwanted catch and supporting stock sustainability objectives.

We recognise that management interventions brought in through the selectivity proposals may solve one issue, but unintended and unpredictable issues could arise because of the measures being implemented. For example, it is acknowledged that it is an unknown whether selectivity proposals may, through interventions intended to have a positive effect, lead to displacement of fishing activities to other locations or into fisheries due to reduced opportunities. This may result in negative environmental effects that fall outside the scope of the FCP.

The use of square mesh panels and square mesh codend presents a recognised risk to flatfish species because their laterally compressed body shape and benthic swimming behaviour make them more likely to pass through square mesh openings designed to release undersized roundfish such as haddock and whiting. Trials in Scottish demersal fisheries demonstrate that while square mesh panels improve L50 for haddock and whiting, they can simultaneously reduce retention of commercially valuable flatfish, leading to lower catch rates in mixed fisheries¹⁸. Reviews of selective gear performance confirm that this differential effect arises from both morphological differences and behavioural interactions with the panel, with flatfish making less contact with the escape zone and passing more readily through square meshes than roundfish ¹¹. As a result, square mesh panels must be carefully evaluated in fisheries where flatfish form an important part of the catch, to avoid unintended negative environmental consequences.

6.2.2 Effects identified by this assessment

The selectivity proposals are intended to reduce unwanted fish catch, and therefore reduce discarding, and bycatch of sensitive marine species. Therefore, the selectivity proposals are expected to reduce the risks to the future status of stocks in the long term thus giving positive benefit to the environment.

As well as impacting the commercial fish stocks themselves, the fisheries are likely to be impacting the wider environment.

Together, these actions will have the positive benefit of ensuring stock sustainability and contributing to improving the status of UK MS commercial fish stocks (D3) in the UK. In doing this there may also be improvements in overall fish biodiversity (D1) and the marine food webs (D4).

The assessment of the likely negative effects of the selectivity proposals in section 6 identified a low risk of significant adverse effects on the environment from implementing individual selectivity proposals. Therefore, no changes to the proposed selectivity measures are needed ahead of publishing the selectivity consultation. Where appropriate, the policies and actions will be developed and implemented to mitigate any potential negative effects identified by the current assessment.

The likely negative effects will also be considered when developing monitoring activities as part of the implementation process (see section 8), to ensure that any negative effects of the selectivity proposals individually or combined can be further reduced. Given the uncertainty as to the negative effects of implementing the selectivity proposals, monitoring changes to fishing activity resulting from the implementation of these will help identify any unintended consequences on the environment that could lead to significant negative environmental effects. Where likely unintended environmental consequences are identified, appropriate changes to management or mitigation can be implemented to reduce any negative environmental effects developing.