Scottish Animal Welfare Commission - proximity of seals to farmed fish: response to Marine Scotland

Evidence from other species

The effects of the presence of predators on their prey is well known to be stressful and profound, as it is fundamentally related to survival. In addition to lethal encounters with predators, animals that are preyed upon also show changes in foraging response and increased energetic expenditure from mounting a ‘fright’ or ‘flight’ response. This is typically expressed through activation of physiological pathways, leading to increased metabolism and other changes that promote effective escape (such as increased oxygenation of tissues), and behavioural responses. Stress, even for a relatively short period of time if severe, can lead to marked behavioural and physiological changes. For example, feeding responses were reduced for six days in two fish species towed in a net for 15 minutes (15) and for one species (walleye pollock, gadus chalcogrammus), the fish did not recover and the treatment resulted in 100% mortality. As a significant predator of salmon, seals are undoubtedly a stressor for salmon, but the extent and impact of this on fish welfare, health and behaviour needs to be assessed.

There are no published studies that assess the impact or extent of a stress response by salmon to the presence of seals. However, there are a few recently published papers that indicate that fish of other species show an acute physiological stress response in the presence of a predator. In a study of the response to practices such as handling and slaughter in European whitefish (Coregonus lavaretus), fish were implanted with cardiac monitors (16). During the experiment, an unintended event occurred where the holding cage of the group of test whitefish were held close to a cage of rainbow trout (Oncorhynchus mykiss), a natural predator of the whitefish. There was a sustained rise in heart rate that lasted at least 12 hours. The authors stated that response is suggestive of a major allostatic load on the fish that would have a negative impact on other physiological processes. In studies performed with qingbo (the Chinese barbed carp: Spinibarbus sinensis) and zebrafish (Danio rerio), the fish showed behavioural responses, increases in physiological stress responses (cortisol release), and increased metabolic rates in the presence of predator species (17, 18, 19). However, these papers only considered the short-term response of the fish and not a longer-term impact of the presence of a predator on behaviour and health.

There are studies of responses of prey species to the presence of predators in mammalian species that are of relevance here. Studies of predator-prey relationships on stress responses and longer-term consequences are sparse, but some intriguing studies relevant to the current issue are available, involving terrestrial and aquatic animals, including impacts of predation of seals themselves. Cape fur seals (Arctocephalus pusillus pusillus), which are preyed upon by great white sharks (Carcharodon carcharias), have been shown to have higher chronic physiological stress responses (faecal glucocorticoid metabolites) when living in areas of high shark predation, when compared to seals in areas of lower predation (20). Wild ungulate species are also vulnerable to predation, as are domesticated ungulates (sheep (Ovis aries)), cattle (Bos taurus), pigs (Sus domesticus)) and poultry species. In the presence of high predator density, wild sheep species escape to areas of their range that are perceived to be safer (something generally not available to domestic animals and farmed salmon which are confined) and then remain immobile until the threat has passed. With greater predation pressure animals maintain longer periods of reduced activity (21).

Greater flight distances, increased vigilance and reduced time spent in maintenance behaviours (such as feeding) are also seen in a range of other species when the predation risk increases (e.g. deer, moose (Alces alces), elk). Prolonged avoidance of pastures where predator attacks have occurred are also reported in ungulates where they have the opportunity to express these responses (22). Chronic or prolonged stress, such as may be experienced when animals are frequently exposed to predator attack or in high predator density areas, can cause reduced growth rates, impair reproductive function, reduce immune responses and increase disease susceptibility (23). In cattle farming in Australia, for example, predation by wild dogs (dingoes (Canis familiaris)) is thought to cause stress-related impacts including reduced weight gain and poor reproduction (lactation and delayed oestrus (24) even in the absence of direct attacks on individuals.