Table 2 Summary of evidence on intensity of unpleasantness of air exposure in trout, and consensus ratings on compatibility with definitions of intensity in Table 1 (N: No pain, A: Annoying, H: Hurtful, D: Disabling, E: Excruciating).
From: Quantifying the welfare impact of air asphyxia in rainbow trout slaughter for policy and practice
Summary of lines of evidence reviewed | Intensity categories | |||||
|---|---|---|---|---|---|---|
N | A | H | D | E | ||
I | Air exposure for as briefly as five seconds triggers the expression of neuromolecular states associated with negative emotions in fish25 | R | R | ? | + | + |
Conditioned place aversion occurs for air exposure associations40 | R | − | + | + | + | |
The greater and more immediate the survival threat, as in impaired oxygen intake, the more intense should be its unpleasantness | − | − | − | + | + | |
Intensive aversive reactions, vigorous movements of twisting and turning and escape attempts, upon removal from water37,38 | − | − | ? | + | + | |
Air exposure, even if for some seconds, is used as a reliable stress-inducing procedure in laboratory studies24,25,26 | − | − | ? | + | + | |
The acute stress associated with air exposure and oxygen deprivation leads to rapid increase in levels of stress hormones30,31,32,33 | − | − | ? | + | + | |
As little as 60 s elicit physiological stress responses greater than those triggered by longer-lasting stressors27,29 | − | − | ? | + | + | |
II, III | − | + | + | + | + | |
Aversion to CO2 is observed in its consistent use as a non-physical barrier for fish and a mechanism of self-transfer between tanks46,47 | R | − | + | + | + | |
Vigorous behavioural reactions, escape attempts and gasping for air are observed soon following exposure to high CO2 levels46,86 | − | − | ? | + | + | |
Several mechanisms in fish indicate that low pH and high CO2 lead to states of fear, anxiety and panic51,52,54,55,57,60 | − | − | − | + | + | |
The greater the survival threat, the more intense its unpleasantness | − | − | ? | + | + | |
In mammals, aversive responses to CO2 reduced by anxiolytics56 | R | − | + | + | + | |
III | Individuals are exhausted from depletion of energy reserves24,37,62 | − | − | + | + | ? |
Metabolic acidosis triggers pain receptors65 | R | ? | + | + | + | |
Lactate is a well known panicogen, leading to fear, anxiety, panic87 | − | − | − | + | + | |
Ischemic pain due to insufficient O2 and high levels of lactate may occur65,66. Ischemia causes release of inflammatory mediators67 | − | − | + | + | ? | |
IV | Prolonged hypercapnia and acidification of the cerebrospinal fluid depresses brain activity36,37,59,68,72 | + | + | + | − | − |
