How does percussive stunning work?

Percussive stunning works by shaking the brain relative to the skull. By hitting the skull at high speed, the skull moves and then the brain follows. This sudden jolt causes a disruption of the brain processes. If the jolt is big enough, the animal will be stunned. If the jolt is bigger still, the animal will be irrecoverably stunned, i.e. killed. This system follows the basic principles of physics, i.e. F = mv 2 . Therefore, a high velocity, low mass club, piston, or captive bolt, will significantly increase the efficacy of the blow.

What flesh quality problems arise from stressful harvest practices?

The quality defects in fish are quite complex. It is well recognised that by altering the diet we can change certain attributes, such as colour, flavour, and odour, but texture is thought to be in part associated with activity levels or physical processes exerted upon the fish, or fish flesh. There are also the direct physical effects of harvest stress leading to scale loss and bruising.

Textural problems in processing commonly represent at least half of all farmed salmon downgrades. This is mostly seen as separations between or within the muscle blocks, or gaping, which has massive flow-on consequences for filleting, portioning and slicing. Harvest and slaughter processes have the largest potential impact on textural quality, and differences are clear when farmers compare stressful practices with humane techniques. Bloodspotting and haemorrhaging in the flesh can also occur from blood vessel rupture due to poor bleed out of unstunned fish or strong muscular contractions resulting from vigorous escape attempts respectively. These defects are a global issue, and it has been highlighted by industry leaders as possibly the most important part of the salmon value chain today. Processors and retailers are very aware of the benefits of welfare at harvest issues, both in terms of quality and marketability.

Reducing fish activity during harvest also results in an increased pre-rigor time. Activity is a much bigger driver of rigor time than temperature, and by decreasing fish activity, pre-rigor times can be reduced from about 30 minutes for a particularly high activity carbon dioxide or live chilling harvest to over 24 hours for a particularly good percussive stun harvest (depending on how good the pre-stunning handling was). This opens up the possibility of pre-rigor filleting, which is known to reduce quality issues such as gaping. It can also lead to an increase in the shelf-life of the fish.

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