Atlantic salmon 2021: Scottish Marine and Freshwater Science Vol 14 No 4

3. Results

3.1 Scotland

• The pressure data coming from East coast rivers dominates the national picture (Figure 2) due to their greater river length and wetted area once standing waters are excluded.
• Exploitation of salmon by coastal and in-river nets is considered to be historic and of minor and declining importance, at least partly reflecting the legislative ban on the retention of salmon in coastal waters implemented in 2016.

• Predation by birds and seals, and upstream barriers are considered to be the most severe contemporary pressures acting on wild salmon.
• Of the emerging pressures, high river temperatures are thought to have the greatest current impact and this is expected to increase in coming years. Increasing high river flows under climate change, marine developments and the North American signal crayfish are also thought to be increasing pressures, although confidence in these assesments is lower, particularly in the case of high flows.

3.2 East

• The relative importance of different pressures on the East of Scotland (Figure 3) is generally similar to that across the country as a whole, although sea lice are not considered to have any substantial impact in this region.
• Coastal nets, acidification and to a lesser extent in-river nets are considered to be declining pressures.
• Bird predation and barriers to upstream migration are considered to be the greatest pressures on salmon at the current time.
• Pressures related to climate change are considered to be the greatest emerging threats, including increasing river temperatures and high flow extremes.
• Thermal discharges are considered to be of increasing concern, but are currently of very low severity. Marine developments and crayfish are also thought to be developing pressures, but their current severity is thought to be low and there is considerable uncertainty over impacts.

3.3 North

• In general it is considered that there are few pressures affecting salmon in Northern rivers (Figure 4).
• The greatest contemporary pressures are thought to be associated with predation of salmon by seals and birds, although there is substantial uncertainty over the impact of birds.
• Angling, fish disease, marine development and high river temperatures are considered to be increasing pressures.

3.4 West

• On the West of Scotland (Figure 5), coastal and in-river nets are considered to be historical pressures that are no longer having a major effect on salmon production.
• Sea lice from aquaculture is considered to be the greatest contemporary pressure, followed by predation from birds and seals.
• Climate change-related impacts of increasing temperature and high flows are highlighted as emerging pressures, but with very low current impacts. Thermal discharges are also highlighted as emerging pressures.

3.5 South West

• In the South West of Scotland (Figure 6) in-river netting is highlighted as a declining pressure, but with a greater remaining severity than in other regions. The loss of natural riparian vegetation was also considered to be a declining problem, potentially in response to improved forestry practices.
• Barriers to upstream migration were considered the greatest contemporary problem followed by bird and fish predation and illegal exploitation, although confidence in the latter pressure was low.
• High river temperature and escaped salmon from fish farms were highlighted as the dominant emerging pressures. The latter pressure was considered to be of greater impact at the present time, but with very low confidence.

3.6 Pressures with an unknown severity

• In some circumstances local fisheries biologists considered that a pressure may have an impact, but that the severity of the impact was too uncertain to estimate. In these circumstances, the severity of the pressure was recorded as "unknown", but the spatial extent of potential impacts was recorded. By calculating the percentage of river wetted area affected by this classification it is possible to highlight potentially important pressures for which there was a poor evidence base at the time of the pressure mapping exercise. The top ten most important pressures of unknown severity are shown in Table 1 below.
• In the case of predation and sea lice, these were also highlighted as regionally important pressures. In the case of marine development this highlights general concerns over a pressure for which there is not felt to be a strong evidence base on the likely severity of impacts.