Razor clam scientific trial – Fisheries Assessment

5. Assessment of impact of the razor clam scientific fishing trial on the national status of Priority Marine Features

This assessment examines the impact of razor clam fishing as authorised in the limited scientific trial on priority marine features outside Nature Conservation Marine Protected Areas, with reference to General Policy 9b of Scotland’s National Marine Plan.

General Policy 9b states that Development and use of the marine environment must not result in significant impact on the national status of Priority Marine Features. In this assessment, this is considered to signify significant adverse impacts.

Advice in relation to PMFs was sought from NatureScot.

5.1 Razor clam fishing activity

The razor clam scientific trial involves limited fishing in defined areas by a small number of authorised vessels. These vessels place an anchor on the seabed and then pull towards it at approximately 3 meters per minute, followed buy divers. The fishing gear in use on razor clam vessels is a lightweight electric current device towed at approximately 3 metres per minute, followed by divers (Blackadder, 2024). Individual fishing tows are limited by divers’ air supply, typically less than one hour. Electrodes leave a slight indent in the seabed, less than 1cm deep (Murray, Copland, Boulcott, Roberston, & Bailey, 2014).

5.2 Priority Marine Features in this assessment

NatureScot identifies 11 Priority Marine Features as most sensitive to bottom-contact mobile fishing gear. These are:

• Blue Mussel Beds
• Cold-water coral reefs
• Fan mussel aggregations
• Flame shell beds
• Horse mussel beds
• Maerl beds
• Maerl or coarse shell gravel with burrowing sea cucumbers
• Native oysters
• Northern sea fan and sponge communities
• Seagrass beds
• Serpulid aggregations

In the following sections the potential impact on the national status of each PMF is assessed against the specific advice produced by NatureScot and a conclusion on the probable impact of the activity is stated.

5.3 Blue mussel beds

Blue mussels are ubiquitous around Scotland’s coast on intertidal and subtidal rocky substrate. Subtidal beds on soft sediments are the form that have PMF status. Subtidal mussels are known to be present in the Sound of Sleat trial area, at the head of Loch Hourn. Otherwise, razor clam fishing trial areas are not known to contain blue mussel beds.

Razor clam fishing activity in the Sound of Sleat trial area is concentrated on grounds on the Arisaig coast (Blackadder L. H., 2024), a considerable distance from the Loch Hourn mussel beds.

Conclusion: Razor clam fishing activity does not have a significant adverse impact on the national status of blue mussel beds.

5.4 Cold-water coral reefs

No razor clam trial area is known to contain cold-water coral reefs.

Conclusion: Razor clam fishing activity does not have a significant adverse impact on the national status of cold-water coral reefs.

5.5 Fan mussel aggregations

Fan mussel aggregations mainly inhabit soft sediments in deep water. Fan mussels are known to be present in the Coll and Tiree razor clam trial area. Otherwise, razor clam fishing trial areas are not known to contain fan mussel aggregations.

The records in the Coll and Tiree trial area are in the deep water of the Passage of Tiree, which is beyond access by divers. Razor clam fishing activity in this trial area focuses on Gott Bay, which does not overlap with these deep water habitats.

Conclusion: Razor clam fishing activity does not have a significant adverse impact on the national status of fan mussel aggregations.

5.6 Flame shell beds

No razor clam trial area is known to contain flame shell beds.

Conclusion: Razor clam fishing activity does not have a significant adverse impact on the national status of flame shell beds.

5.7 Horse mussel beds

Horse mussel beds are known to be present in the Gigha razor clam trial area. Otherwise, razor clam trial areas are not known to contain horse mussel beds.

The beds in the Gigha trial area are to the north of Gigha, and do not overlap with the footprint of razor clam fishing activity (Blackadder L. H., 2024).

Horse mussels are sensitive to physical disturbance, surface and sub-surface abrasion, siltation changes and either targeted or incidental removal by fishing operations.

There is no risk associated with physical disturbance, surface or sub-surface abrasion or removals due to the physical separation between the authorised activity and known horse mussel beds.

Gathering razor clams does result in highly localised increase in turbidity as divers remove razor clams and swim close to the surface, however the risk this may pose to horse mussel beds is considered to be very low. This is because of the short duration of individual tows (typically around one hour) and the physical separation (approximately 4 km) between razor clam fishing activity and horse mussel beds.

Conclusion: Razor clam fishing activity does not have a significant adverse impact on the national status of horse mussel beds.

5.8 Maerl beds

Maerl beds are known to be present in the following razor clam trial areas:

• Gigha
• Sound of Sleat
• Sound of Harris

Otherwise, razor clam trial areas are not known to contain maerl beds.

5.8.1 Gigha

The maerl beds in the Gigha trial area are to the south of Gigha, and are within the associated shellfish harvesting area that authorises razor clam harvesting. To date, fishing activity in this trial area has focused on the soft sediment seabed in the Sound of Gigha, immediately to the north and south of Rhunahaorine point. These fishing areas are between 5 and 8km separate from known maerl beds.

Maerl beds are highly sensitive to physical disturbance, particularly in the form of abrasion and habitat removal / change, organic enrichment, siltation and changes in water flow.

Razor clam fishing activity is not considered to constitute a risk of physical disturbance, abrasion, habitat removal or change, organic enrichment or changes in water flow.

Gathering razor clams does result in highly localised increase in turbidity as divers remove razor clams and swim close to the surface, however the risk this may pose to maerl beds is considered to be very low. This is because of the short duration of individual tows (typically around one hour) and the physical separation between razor clam fishing activity and maerl beds.

5.8.2 Sound of Sleat

Known maerl beds in the Sound of Sleat trial area are in Loch Hourn, Loch Nevis and the Sound of Sleat itself in Bagh a’Mhuilinn. Fishing activity has occurred mainly in waters off the Arisaig coast, between 7 and 24km from known maerl beds.

Maerl beds are highly sensitive to physical disturbance, particularly in the form of abrasion and habitat removal / change, organic enrichment, siltation and changes in water flow.

Razor clam fishing activity is not considered to constitute a risk of physical disturbance, abrasion, habitat removal or change, organic enrichment or changes in water flow.

Gathering razor clams does result in highly localised increase in turbidity as divers remove razor clams and swim close to the surface, however the risk this may pose to maerl beds is considered to be very low. This is because of the short duration of individual tows (typically around one hour) and the physical separation between razor clam fishing activity and maerl beds.

5.8.3 Sound of Harris

In the Sound of Harris trial area, an area of maerl beds was excised from the trial area to the south of the island of Cheileagraigh at the commencement of the scientific trial in 2018. Further records are known to the south of the island of Sursaigh and at a sandbank in Loch Mhic Phail.

To date, no fishing activity has occurred in the trial at this area and there is no shellfish harvesting area in which fishing activity can proceed.

Therefore, there is no impact on maerl beds in this area.

5.8.4 Conclusion

Razor clam fishing activity does not have a significant adverse impact on the national status of maerl beds.

5.9 Maerl or coarse shell gravel with burrowing sea cucumbers

Maerl or coarse shell gravel with burrowing sea cucumbers is known to be present in the Benbecula razor trial area. Otherwise, razor clam trial areas are not known to contain this PMF.

To date, no fishing activity has occurred in the trial at this area and there is no shellfish harvesting area in which fishing activity can proceed.

Conclusion: Razor clam fishing activity does not have a significant adverse impact on the national status of maerl or coarse shell gravel with burrowing sea cucumbers.

5.10 Native oysters

Individual records of native oysters are located in the following razor clam trial areas:

• Colonsay
• Coll and Tiree
• Sound of Sleat
• Benbecula

There are further presence records, redacted to 20km British National Grid square resolution that overlap with the following areas:

• Firth of Clyde
• Gigha

The key locations to protect native oyster beds are the west coast of Mull and Ulva, and Loch Eishort. No razor clam fishing occurs in these areas.

Oysters have not been specifically tested, however analysis suggests non-target bivalves, if affected at all, are observed to immediately cease stimulated movement when an electric field is turned off (Murray, Copland, Boulcott, Roberston, & Bailey, 2014).

Towed bottom contacting fishing activity is not thought to be a high risk to oysters due to their distribution in shallow waters. Razor clam diving however can occur in shallow waters and the potential for adverse impact on the national status of native oysters is assessed for each area below.

5.10.1 Colonsay

The main razor clam ground in Colonsay is the Tarbert Bank. The northern portion of the bank, roughly 50% of the total, is open to razor clam fishing. Depths range from 10 to 20m. There are records of native oysters from the Tarbert Bank, so there is scope for overlap with razor clam fishing activity.

5.10.2 Coll and Tiree

Most razor clam fishing activity has occurred in Gott Bay. This is not known to be a location where native oysters are present. The presence records in the Coll and Tiree trial area do not overlap with the footprint of the fishing activity to date.

5.10.3 Sound of Sleat

There is a presence record for the Arisaig coast, which is in proximity to razor clam fishing activity, so there is scope for overlap with razor clam fishing activity.

5.10.4 Benbecula

To date, no fishing activity has occurred in the trial at this area and there is no shellfish harvesting area in which fishing activity can proceed.

5.10.5 Firth of Clyde

Records in the Firth of Clyde are redacted to 20km resolution so it is not possible to determine the extent to which razor clam fishing activity overlaps with native oyster records. The relevant grid squares overlap with the deeper portions of the razor clam trial area, which tend not to be fished compared to the shallower waters along the Ayrshire coast. No razor clam fishing occurs in Loch Ryan, where there is a managed native oyster fishery.

5.10.6 Gigha

Records near Gigha are redacted to 20km resolution so it is not possible to determine the extent to which razor clam fishing activity overlaps with native oyster records. The relevant grid square overlaps with the northern portion of the Gigha razor trial area, which does not overlap with the footprint of fishing activity.

5.10.7 Conclusion

There are several areas where native oysters are present close to, or overlapping with, razor clam fishing activity. In other areas, sensitive species redaction makes it impossible to determine the extent of potential overlap, though the possibility of overlap in these areas is considered to be low by nature of where the grid squares overlap relative to known fishing footprints in affected trial areas. Given best available knowledge of effects on non-target bivalve species, the suspected distribution of oysters in waters of the west coast of Scotland and the lack of overlap with key areas for protection, razor clam fishing is not considered to adversely impact the national status of native oysters.

5.11 Northern sea fan and sponge communities

Norther sea fan and sponge communities are known to occur in the following razor clam trial areas:

• Gigha
• Sound of Sleat
• Benbecula
• Sound of Harris

These communities occur on upward-facing and vertical bedrock, which will not be fished by razor clam vessels, which target soft sediments that razor clams inhabit.

Conclusion: Razor clam fishing activity does not have a significant adverse impact on the national status of northern sea fan and sponge communities.

5.12 Seagrass beds

Seagrass beds are known to occur in the following razor clam trial areas:

• Colonsay
• Sound of Sleat
• Benbecula
• Sound of Harris

Seagrass beds are sensitive to smothering, organic enrichment, nutrient enrichment, physical disturbance, changes in water flow and clarity, and INNS. Razor clam fishing is not considered to constitute a risk of smothering, organic enrichment, nutrient enrichment, change in water flow.

Like any vessel in the marine environment there is a risk of transfer of INNS but this is no higher in razor clam vessels than otherwise. A seasonal closure in spring incentivises vessels to be removed from the water for maintenance including removal of biofouling that may include INNS.

Gathering razor clams does result in highly localised increase in turbidity as divers remove razor clams and swim close to the surface, however the risk this may pose to seagrass beds is considered to be very low. This is because of the short duration of individual tows (typically around one hour) and the physical separation between razor clam fishing activity and maerl beds.

5.12.1 Colonsay

The records of seagrass beds in the waters around Colonsay are in two areas. One record to the west of Colonsay does not overlap with razor clam fishing activity. Two beds in Whistler’s Bay by Rubha Dubh on the east of Colonsay do overlap with where razor clam fishing activity has occurred.

5.12.2 Sound of Sleat

All records of seagrass beds are located in areas of the Sound of Sleat trial area that do not overlap with the footprint of razor clam fishing activity in the area. The closest known seagrass bed is approximately 4km from the northern extent of razor clam fishing activity in this trial area.

5.12.3 Benbecula

To date, no fishing activity has occurred in the trial at this area and there is no shellfish harvesting area in which fishing activity can proceed.

5.12.4 Sound of Harris

Harris is identified by NatureScot as a key location for protection of seagrass. To date, no fishing activity has occurred in the trial at this area and there is no shellfish harvesting area in which fishing activity can proceed.

5.12.5 Conclusion

Razor clam fishing activity does not have a significant adverse impact on the national status of seagrass beds. While razor clam fishing has minimal impact on the seabed, not enough is known about the effect of electric current or towed gear on seagrass. On a precautionary basis, the Colonsay razor trial area will be amended to prohibit fishing in the bay by Rubha Dubh on Colonsay to avoid overlap between razor clam fishing activity and known seagrass beds.

5.13 Serpulid aggregations

No razor clam trial areas contain serpulid aggregations.

Conclusion: Razor clam fishing activity does not have a significant adverse impact on the national status of serpulid aggregations.