Wild Salmon Strategy Science and Evidence Board advice to the Delivery Group: bird and seal predation summary

Bird and seal predation summary

Goosanders, mergansers and cormorants (fish-eating birds)

Based on extensive research, goosanders, mergansers and cormorants can take substantial numbers of salmon at the smolt and pre-smolt life stages, with daily intake per bird being higher in more northerly Scottish rivers where the diversity of prey fish species is lower.

There may be some potential to protect salmon at “pinch-points” where they are particularly vulnerable on their down-river migrations (eg near weirs and fish pass entrances/exits). However, from experience on the ground there is little evidence that such an approach is widely practical.

There is scepticism over the efficacy of scaring because of redistribution of birds to feed on other vulnerable salmon populations.

Estimating overall losses of salmon to fish-eating birds is complicated by uncertainties over the response of fish populations to harvesting by predators, particularly in wider rivers.

Ideally, from a scientific viewpoint, a next-step experiment to assess the level of impact would use a large scale controlled, replicated, bird culling approach- but this has not been deemed to be practical, acceptable or necessary.

Impacts of fish-eating birds on Scottish salmon populations are likely to be increasing substantially for two reasons. First, a perceived higher frequency of periods of low river flows during spring are probably making salmon smolts more vulnerable to predation on their down-river migration. Second, a reduction in egg deposition in depleted salmon populations is increasing the areas of river in which densities of juveniles are well below carrying capacity. Therefore, losses of salmon in the pre-smolt life-stages are predicted to have a direct effect on smolt production and hence population strength due to reduced scope for compensation. Losses of smolts due to fish-eating birds have long been expected to have a direct effect on salmon population strength. The incidence of smolts in bird diets has probably been underestimated in many cases due to methodological bias.

Understanding of population processes suggests that predators, including fish-eating birds, can impair the recovery of salmon populations even if other factors were predominant in the original decline.

Weak populations of salmon, which can be identified by local abundance of juvenile salmon, are now present in many Scottish rivers regardless of the overall river conservation grading, which is made at a whole-river scale. In particular, spring salmon, which enter rivers early in the year and generally spawn in upper tributaries, have time-limited special protection from harvesting across Scotland but are vulnerable to bird predation both “locally” as parr and throughout the entire lengths of rivers during the smolt run.

Fish-eating birds can be a significant pressure on salmon populations in poor conservation status along with many other factors.

Seals (harbour and grey)

Seals at sea eat a diverse range of prey; salmonids can be consumed but adult salmon are likely to be difficult for them to catch.

A small proportion of seals enter rivers on a regular basis and consume adult salmon among other fish species.

There can also be individual seals that specialise on eating salmon smolts.

Routine acoustic deterrent devices can have varying levels of success in reducing seal impacts in rivers, although the development of sonar-triggered acoustic startle devices to detect seals and deter them from entering rivers is a promising potential non-lethal solution.

Shooting river specialist seals is a potential approach to reduce their impacts on salmon populations where non-lethal options are found to be ineffective, whereas other options, such as trapping and relocation have not been deemed to be practical or effective.

In practice, there is an acute conflict between conserving locally declining harbour seals and endangered Atlantic salmon, reflected by a very low potential biological removal (PBR) value for harbour seals. In contrast, populations of grey seals are abundant and growing. Hence, there is a high PBR for grey seals and no conflict with conservation of salmon.

Although the overall conservation status of a river may be moderate or good, as measured by the assessment of adult salmon in the Salmon Conservation Regulations process, it still may contain weak populations of salmon, as identified by the juvenile salmon based National Electrofishing for Scotland process. Notably, populations of spring fish from upper tributaries have been very weak. Spring salmon, enter rivers early in the year, generally spawn in upper tributaries, and have time-limited special protection from retention by fisheries in spring (but not later in the year) across Scotland. They may be vulnerable to seal predation near the estuary during spring and in mid-upper river mainstem areas during spring and summer.

Seals can be a significant pressure on salmon populations in poor conservation status along with many other factors.