Nature Conservation Advice from NatureScot and JNCC: Demersal Fisheries Management Plans

4 Risks relating to PMFs in Scottish waters

Priority Marine Features (PMFs) in Scotland represent a selection of habitats and species identified for their conservation importance. These 81 features are acknowledged for their national significance and the role they play in supporting marine biodiversity. The list includes highly mobile marine mammals and fish, as well as habitats and benthic species of conservation interest. The purpose behind identifying PMFs is to focus conservation efforts, guide management actions, and ensure the protection and enhancement of marine biodiversity within Scottish waters.

Fisheries within the Demersal FMPs can potentially impact PMFs in three main ways:

(i) through the bycatch of mobile PMFs

(ii) the direct (targeted) and indirect (bycatch) removal of prey species on PMF species depend, and

(iii) physical impacts to benthic PMFs resulting in alterations to habitats or damage to low mobility/sessile PMFs.

4.1.1 Risks of bycatch to mobile PMFs from demersal fishing

4.1.1.1 Risk to marine mammals

The Scottish PMF list includes 16 marine mammals.

Demersal trawls and purse seines

The Scottish PMF list includes most of the UKs resident marine mammals. As mentioned in Section 3.1.1.1, marine mammal species face a limited risk of bycatch in demersal trawls and seines. However, these gears are not routinely monitored for marine mammal bycatch due to their perceived low risk. Continued or expanded monitoring, whether via observer coverage or remote electronic monitoring, would help to further refine risk assessments and lessen the need for a precautionary risk rating for marine mammals.

Static nets

Risks for seals and harbour porpoise have been outlined in Section 3.1.1.1 above. In 2020, bycatch in UK gillnet fisheries was estimated as 222 for common dolphin with an estimate of 20 occurring in Scottish waters (Kingston et al., 2023). No other marine mammal PMF species have been recorded as bycatch through the BMP over the 2017-2020 period, but all marine mammal PMF species are considered sensitive to bycatch in static nets.

Longlines

As stated in section 3.1.1.1., risk to marine mammals is expected to be low. In the 2017 to 2020 BMP bycatch recordings by gear type, longlines did not contain any marine mammal bycatch. Generally, the removal of non-target species, especially birds, is considered the highest risk to biodiversity by longline fisheries (Miles et al., 2020).

4.1.1.2 Risk to fish

The PMF list includes 31 fish species many of which are targeted commercially, including anglerfish, cod, saithe and whiting which are to be managed through the Demersal FMPs. Risks to commercial species should be somewhat mitigated through stock management.

Demersal trawls and purse seines

As discussed in Section 3.1.1.3 above, demersal trawls and seines present a potential bycatch risk to many fish species, including most of the fish species listed as PMFs. Many of these are also commercial species and so bycatch mortality should be monitored and mitigated through their own stock management processes. However, a number of PMF species have limited/zero total allowable catches and may be sensitive to bycatch in demersal trawls, particularly the longer-lived elasmobranch PMFs. While there is potential for the bycatch of PMF fish species in these gears, it is unclear what the scale of impact is likely to be and how this may affect the national status of fish PMFs. Continued monitoring and targeted assessment of bycatch records is recommended to improve understanding of these risks.

Static nets

In addition to the species listed in Section 3.1.1.3, there were 66 records of porbeagle shark bycatch and 25 records of sandy ray bycatch in gillnets recorded through the BMP 2017-2020. To date, this monitoring programme has focussed on recording bycatch events for a limited number of fish species – predominantly elasmobranchs and EMS fish species. Further evidence is required to downgrade a precautionary risk rating in the Scottish demersal static net fisheries.

Longlines

See discussion in section 3.1.1.3 above. No additional PMF fish species were recorded as bycatch in the UK longline fisheries between 2017-2020. However, as discussed above, not all PMF species are monitored within this programme and a number of deep-water fish species may be at risk of bycatch in longlines. Further monitoring of PMF fish bycatch in the Scottish hake longline fishery is recommended to better understand the risk of bycatch within this fishery and improve confidence in our assessment of risk.

4.1.1.3 Conclusions and recommendations relating to bycatch risk

In addition to the bycatch risks outlined for MPA designated species above, demersal trawls and seines covered by the Demersal FMPs present a further bycatch risk for all listed fish species, including several long-lived species with an increased vulnerability to bycatch mortality. For listed fish which are also commercial stocks, risks of bycatch may be mitigated through their own stock management processes, but significant gaps in the available evidence remain. Risks for PMF marine mammals are likely to echo the risks outlined above for MPA features. The risk rating for bycatch in demersal trawls and seines is assessed as moderate. Continued monitoring and enhanced data collection and research to address knowledge gaps may allow us to reassess this risk in future.

Risks relating to longlines are downgraded from the MPA feature assessment above as the PMF list does not include birds, which are the most vulnerable group in terms of bycatch in this gear. However, monitoring of commercial species bycatch, including sensitive fish and elasmobranch PMF species is limited in Scottish longline fisheries and further research is needed to understand the risk to these species. Therefore, a moderate risk rating is proposed for bycatch of PMFs in longlines.

The potential impact of static nets echo those proposed above for MPA features (bycatch of marine mammals and fish), as such the risk rating for bycatch of PMFs in static nets is assessed as high.

4.1.2 Risks of prey depletion to mobile PMFs from demersal fishing

Discussion of the impact of prey depletion on marine mammals listed as both MPA features and PMFs is given in section 3.1.2. above. A number of marine mammal and fish PMFs, in addition to species designated as MPA features, are likely to utilise demersal species at various life stages as prey resource. These species tend to have varied diets, making the direct risk from reducing the availability of prey species' numbers to PMF status relatively low. However, as stated in section 3.1.2 above, the dynamics of marine foodwebs are poorly understood. Given the breadth and scale of fisheries included in the Demersal FMPs, they are likely to impact spawning and recruitment, resulting in a reduction in the availability of juvenile gadoids and other prey species to marine predators listed as PMF.

4.1.2.1 Conclusions and recommendations relating to prey depletion risk

Recognising a potential impact pathway but lacking sufficient evidence to provide a confident conclusion, we advise that the risk of prey depletion to designated features of MPAs from demersal fisheries be considered moderate. Work done by the Demersal FMPs to ensure fisheries eventually meet the precautionary and sustainability objectives of the Fisheries Act will help to ensure stocks are fished sustainably, but additional consideration of predator needs is recommended.

4.1.3 Risks of physical impact on habitat PMFs from demersal fishing

Demersal trawls and seines

Demersal trawls and seines are known to have potentially significant impacts on benthic species and habitat PMFs.

The direct physical impacts of abrasion and penetration introduced by demersal trawling can affect local topography (Tuck et al., 1998; Fonteyne, 2000; Clements et al., 2018) and by removing and/or damaging infauna assemblages and sessile organisms, can reduce habitat complexity and alter community composition (Kefalas et al., 2003), which has the potential to significantly impact habitat functioning. Demersal seining is likely to have a lower impact with regard to this pressure; demersal seines have lighter ground gear than trawls and the footprint of a seining operation is typically smaller than the area affected by trawling (Buhl-Mortensen et al., 2013).

Abrasion impacts are documented to have lasting negative effects on the population density and diversity of benthic communities; The level of impact in a given area will depend on the intensity and frequency of the trawling or seining activity and the sensitivity of the species which make up the characterising community of the habitat (Ball et al., 2000; Hiddink et al., 2019; Whitton & Hiddink, 2023). Small, short-lived species show greater resilience and recovery capacity compared with the longer-lived and larger species (Bolam et al., 2017; Bastari et al., 2018; Gonzalez-Irusta et al., 2018; Hiddink et al., 2019), which can have significant implications for the benthic species and habitat PMFs which are characterised by fauna that are relatively large and long-lived (Greathead et al., 2007; OSPAR, 2010).

PMFs including flame shell beds, maerl beds, northern sea fan and sponge communities, deep sea sponge aggregations, seamount communities, cold-water coral reefs and coral gardens, are considered highly sensitive to physical disturbance including abrasion and penetration from demersal trawls and seines.

Static nets and longlines

The use of static gears is considered less damaging to benthic habitats than the use of mobile gears (Sewell and Hiscock, 2005), however, these gears can still cause physical impacts to the seabed and characteristic communities through abrasion, though the magnitude of these impacts is likely to be smaller than that of trawling (Eno et al., 2001; Lewis et al., 2009; Stephenson et al., 2017; Gall et al., 2020; Rees et al., 2021). Risk is greatest during setting and retrieval when elements of the gear may land heavily or be dragged across the seafloor (Grieve et al., 2014). In areas with stronger currents or during periods of bad weather, elements of static gear in contact with the seafloor may move, snagging on rocks and benthos and damaging fragile structures (Grieve et al., 2014).

4.1.3.1 Conclusions and recommendations relating to physical impacts

Demersal fishing, in particular trawling, has the capacity to impact the benthic habitats over which they occur, primarily through physical penetration, abrasion, and disturbance of the seabed.

Most benthic species and habitat PMFs are also represented across the Scottish MPA network, and in relevant sites these features will be offered some protection through the MPA management measures. Further work to identify and mitigate impacts on the 11 most sensitive benthic and habitat PMFs in inshore waters is also ongoing. Therefore, some of the physical impacts likely to affect the national status of PMFs will be mitigated, and the risk rating for physical impacts to benthic and habitat PMFs from demersal trawls and seines is assessed as moderate. The risk to these PMFs from demersal trawls and seines will vary depending on the intensity of the fishing pressure and the sensitivity of the individual PMF to that pressure. Physical impacts from static demersal gears are likely to represent a lower risk to benthos but there may be areas where high intensity static fishing can have a moderate impact on PMF habitats and benthic features through physical disturbance.

While some of the impacts from demersal fishing on PMFs may be mitigated through existing processes, it is worth noting that demersal fisheries are the main contributor to the UKs failure to meet GES for “D1, D6 seafloor integrity”. Recommendations for further work with respect to physical impacts of demersal fisheries are highlighted below under the assessment of risk for this descriptor.