Wild seaweed harvesting: strategic environmental assessment - environmental report

Investigates the sustainability and potential environmental impacts of wild seaweed and seagrass harvesting, maerl extraction and removal of beach-cast seaweed.

9. Risk Matrix and Mitigation Measures

9.1. Risk Matrix

9.1.1. The potential effects of wild harvesting that have been discussed in the preceding sections have been documented in a risk matrix ( Table 12). This presents information on the relative risks associated with harvesting each of the key seaweed and seagrass groups on each of the SEA topics. The evidence base underlying the risk matrix is provided in Appendix E .

9.1.2. The risk level that has been assigned represents the likely interaction of each harvesting method on each seaweed and seagrass group. The risk level was based on a consideration of the likely sensitivity of each group ( i.e. its resistance/tolerance) to different harvesting methods and the rate of (or time taken for) recovery (termed recoverability or resilience) once the pressure has been removed. The consideration of different harvesting methods allows the potential scale and duration of the loss associated with that method/activity type ( i.e. its magnitude/intensity, extent/coverage, frequency/duration and seasonality) to be taken into account.

9.1.3. The matrix provides a distinction between the regeneration of the target seaweed and/or seagrass resource itself ( i.e. the biotope) and the regeneration of its whole ecosystem which is reflected by other ecological functions ( i.e. ecological interactions, food web dynamics and production). A cumulative risk level is also included which indicates the highest risk across all SEA topics.

9.1.4. The assigned risk level has been based on the best available scientific evidence and impartial expert judgement. The evidence database that informed this risk matrix has been incorporated into the reference list provided in Section 13 . The level of confidence in the risk levels assigned to the matrix has been considered and is included as high (h), medium (m) and low (l) in Table 12. This takes account of the quality of the evidence or information, the degree to which evidence is applicable to the assessment and the degree of agreement between evidence types ( Table 13).

Table 12: Levels of risk associated with wild harvesting broad groups of seaweed and seagrass

Table 12: Levels of risk associated with wild harvesting broad groups of seaweed and seagrass

Table 12: Levels of risk associated with wild harvesting broad groups of seaweed and seagrass

Table 12: Levels of risk associated with wild harvesting broad groups of seaweed and seagrass

Table 13: Criteria used to assign confidence level to evidence base underpinning the risk matrix

Confidence Level


High (***)

Based on peer reviewed papers (observational or experimental) or grey literature reports by established agencies.

Assessment based on the same pressures acting on the same receptor in Scotland.

Studies agree on the direction and magnitude (of impact or recovery).

Medium (**)

Based on some peer reviewed papers but relies heavily on grey literature or expert judgement.

Assessment based on similar pressures on the receptor in other areas.

Studies agree on direction but not magnitude (of impact or recovery).

Low (*)

Based on expert judgement.

Assessment based on proxies for pressures e.g. natural disturbance events, grazing.

Studies do not agree on direction or magnitude (of impact or recovery).

9.1.5. Based on the risk matrix, harvesting for maerl should be prohibited in Scottish waters. The only likely method of harvesting possible for this group is dredging. Although there is limited direct evidence available of the effects of extracting maerl, expert judgement indicates that the risk of significant ecological and environmental effects would be high given the slow growth rate of these species. These species are also protected by MPA designations.

9.1.6. There is more evidence available on the effects of harvesting wracks, kelps and seagrasses by different harvesting methods. Based on the evidence, certain methods of wild harvesting are considered to be unsustainable and should be prohibited, namely mechanical cutting of living kelps and trawling/sledging/ dredging of living seagrasses. Other methods of harvesting kelps and seagrasses might require more detailed assessment and site specific management depending on the scale of the harvesting ( e.g. restrictions on harvesting methods, seasonal constraints etc.) because the ecological and/or environmental effects might potentially be significant. This would also be the case for harvesting of wracks and beach-cast material.

9.1.7. The ecological and environmental effects of wild harvesting of green and red seaweeds are likely to be small or negligible. However, there is very limited evidence of the effects of harvesting on these seaweeds and therefore the low risk assigned to these should be treated with caution.

9.2. Mitigation

9.2.1. Mitigation and enhancement measures will need to be considered where potentially significant effects may arise to ensure that wild harvesting activities are sustainable. With appropriate mitigation measures in place the residual effects may be reduced or minimised.

9.2.2. The specific mitigation that is appropriate will depend on the extent and scale of extraction which will only be known at the project level. In particular, it is important that any survey and monitoring requirements (particularly those used to develop sustainable harvesting strategies) reflect the scale, scope and complexity of the harvesting (Netalgae, 2012), as well as the level of risk (and confidence limits) of an ecological or environmental impact (see Table 12). At the same time, however, monitoring requirements should be proportionate to the scale of activity and the level of risk.

9.2.3. Potential mitigation measures that developers will need to consider at the project level where relevant and necessary are included in Table 14. These include recommended sustainable practices based on a review of current management practices in Europe (Netalgae, 2012).

Table 14: Generic mitigation measures to be considered at project level

Mitigation Measure

Example of Methods

Adoption of monitoring programmes

  • Prior to any harvesting, undertake an assessment of the status and biomass of the stock and also estimate percentage coverage of particular species of interest ( e.g. Blight et al., 2011);
  • Undertake pre- and post-harvesting survey work to record damage and regeneration of plants;
  • Record volumes, biomass and area of each species of seaweed harvested, along with date and location; and
  • Consider the use of nearby reference areas that are not harvested to help determine the scale of impact.

Alternative "less damaging" methods of harvesting and applying recommended harvesting techniques for specific species to encourage regeneration of harvested areas

  • Cutting heights should generally be as high as possible and well above the point of growth ( e.g. the meristem for kelps) and the holdfast left attached. The only case where this may not be feasible is in the case of Laminaria hyperborea where the stipes (which are below the meristem) are targeted for commercial use, and where the most sustainable mechanical methods involve removing the entire mature plant and leaving smaller immature plants to continue to grow.;
  • Where possible and relevant, less than one third ( i.e. 33%) of an individual plant should be harvested to allow for regrowth; and
  • Avoid the entire removal of any plants apart from the case of L. hyperborea as explained above.

Rotational fallowing and harvesting regimes i.e. providing fallow areas that are not harvested or harvested less frequently to ensure resource sustainability (re-growth and/or recruitment)

  • The total amount harvested should be set in accordance to the status and availability of the wild resource and the recovery rates of individual species. Only a small percentage of standing stock should be harvested where possible. For example, Comhairle Nan Eilean Siar (2013) advise that the annual harvest of Ascophyllum nodosum should be no more than 25% of the total accessible biomass;
  • Rotate harvesting areas to allow ample time for recovery. Harvested areas should be left for a period of time before harvesting again. The length of time will depend on the rates of recovery of the habitat and associated species, noting that growth rates of particular species can be site specific; and
  • Do not collect beach-cast seaweed from the entire length of strandlines. Leave larger proportions in place particularly during the months when overwintering birds may depend on it as a food source (October to March).

Harvest seaweeds during the active growth season to allow for quicker recovery

  • Harvest Laminaria hyperborea when growth of adults and juveniles is most rapid between winter and the end of summer (January to September) (Sjøtun et al., 1996); and
  • Harvest Ulva spp. during the rapid growth phase in spring and summer (May to August).

Harvest seaweeds outside of the reproductive season and ensure a substantial proportion of mature plants remain

  • Harvest Himanthalia elongata in summer (June to August) after the reproductive season if possible. If harvesting occurs during the reproductive season, then only one of the two main fronds should be harvested. Harvest Saccharina latissima, Alaria esculenta, F. vesiculosus, F. serratus during spring and summer (April to August), avoiding the autumn/winter reproductive season (October to February); and
  • Avoid harvesting A. nodosum during the spring (March to May) reproductive peak.

Avoidance of 'by catch' including brittlestars, stalked jellyfish, bryozoans, molluscs or their eggs

  • Where appropriate, rinse collected plants in situ to remove the majority of epifauna.

Limit and/or avoid harvesting in vulnerable areas

  • Limit/avoid harvesting in wave exposed and erosion prone coastal areas ( e.g. dunes) where kelps dissipate wave energy ( Figure 28)
  • Limit/avoid harvesting in designated sites where appropriate, i.e. where seaweeds ( e.g. kelps) are a qualifying or supporting feature of the site ( Figure 29 and Figure 30) and where seaweeds potentially support bird features ( Figure 20);
  • Follow the Scottish Marine Wildlife Watching Code [29] ;
  • Limit/avoid harvesting near charted archaeological features and HMPAs ( Figure 26 and Figure 27);
  • Limit/avoid harvesting beach-cast seaweed between October and April ( SNH, pers. comm.);
  • Avoid harvesting kelp in areas with high abundance of grazing sea urchins (Sjøtun et al., 2000); and
  • Consult crofters prior to large scale harvesting of beach-cast kelp to ensure that potential interactions are avoided or minimised.

Take extra care when harvesting to ensure that species or spores are not transferred to other areas.

  • Follow 'Check, Clean, Dry' biosecurity principles, checking, cleaning and drying all equipment and clothing when moving between sites to ensure that invasive species, pests and diseases are not spread to new areas [30] ; and
  • Develop a biosecurity plan as part of the monitoring strategy.

Figure 28: Distribution of kelp and areas of coastline that are wave exposed and soft

Figure 28: Distribution of kelp and areas of coastline that are wave exposed and soft

Figure 29: Distribution of kelp and SACs with interest features that could potentially support seaweed

Figure 29: Distribution of kelp and SACs with interest features that could potentially support seaweed

Figure 30: Distribution of kelp and relevant PMFs and MPAs that support this feature

Figure 30: Distribution of kelp and relevant PMFs and MPAs that support this feature


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