Harvesting is the first stage of fish processing and we now understand much better how vitally important this process is for final product quality. The investment in growing the fish up to harvest is considerable and the penalties for getting it wrong during harvest are significant. For this reason, professional advice and external audits are essential to ensure harvest operations are running to best practice. In addition, continual assessment of KPI’s for harvest performance and personnel training should be ongoing to assess the performance of harvest systems.
The key determinants of best practice in harvesting are operating efficiency and throughput rate (logistics and cost), welfare and product quality (inextricably linked), and occupational health and safety of personnel. There are a wide variety of slaughter techniques used for farmed salmon and many variations within each technique which may contribute a significant benefit or detriment to the overall harvest operation and resultant product quality.
The most common stunning and killing methods currently used for farmed salmon are:
· Percussive stunning;
· Electrical stunning;
· Carbon dioxide;
· Live chilling; and
· combinations of the above.
All methods are followed by exsanguination (bleeding).
Let’s just consider percussive stunning firstly but before we go too far, what do we mean by stunning?
Well, the Council Directive 93/119/EC of 22 December 1993 on the protection of animals at the time of slaughter or killing defines stunning as:
any process which, when applied to an animal, causes immediate loss of consciousness which lasts until death;
This was amended by COUNCIL REGULATION (EC) No 1099/2009 of 24 September 2009 on the protection of animals at the time of killing by:
‘stunning’ means any intentionally induced process which causes loss of consciousness and sensibility without pain, including any process resulting in instantaneous death;
So, the amendments start talking about pain and intent and instantaneous death.
Compare this with killing:
1993 - killing: any process which causes the death of an animal;
2009 - ‘killing’ means any intentionally induced process which causes the death of an animal;
Not much of a change there except for the intent to cause the death.
Therefore, a percussive blow (blow to the head), which causes instantaneous, irreversible insensibility fits both definitions of stunning and killing because it is instantaneous, there is intent in the process to cause death, and death results. This means what we refer to as percussive stunning also means killing and this is what is desired by percussive stunning because if we can achieve instantaneous death then we know that we have achieved good welfare and this will result in good quality.
Percussive Stunning – the science.
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 otherwise described as a traumatically induced derangement of the nervous system, resulting in an instantaneous diminution or loss of consciousness without gross anatomical changes in the brain. If the jolt is big enough, the animal will be stunned (with recovery possible). 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. Force (F) = mass (m) x acceleration (a) - (acceleration being change in velocity (v) over time (t)). Therefore, a high velocity, high mass club, piston, or captive bolt, will significantly increase the force of the blow. BUT it is a little more complex than that - in order to produce consistent stun (kill) efficacy and preserve carcass quality it is also important to reduce momentum.
The key issue here is high SPEED! or velocity with low mass to achieve force without momentum. Some percussive stunning equipment suppliers utilise a pneumatic cylinder (see pic below). This is not capable of high velocities and therefore would not meet many welfare standards and regulations for the slaughter of animals. However, the piston operated captive bolt utilises a higher mass bolt to achieve force which leads to high momentum, i.e.
F t = mv Since mv is momentum, we can see that the momentum conferred to an object by a force equals the force times the time the force is applied.
To illustrate this with an example - if the bolt strikes the fish’s skull with a momentum of 5 kg m/s, the force it applies to the skull depends on the time it takes for the skull to absorb the momentum. With a low mass high velocity bolt, the bolt exerts a high force on the fish’s skull, e.g. 100 N for 1/20th of a second. However, with a high mass, slow velocity bolt attached to a pneumatic piston the amount of time it takes to absorb the momentum is extended, and the bolt exerts a smaller force on the fish’s skull, e.g. 10 N for 1/2 a second BUT, it also causes more damage as the stroke length is fixed which causes increased penetration into the skull potentially leading to visible skull, eye, and jaw damage.
Figure 1. Pneumatic cylinders (Festo and Bosch 167)
The solution is to use a captive bolt fired at high velocity by a rapid discharge of stored air in the powerhead (Fig. 2). Some captive bolt guns use cartridges whilst others use a high pressure air supply. These are widely used in the red meat industry, i.e. for slaughtering sheep and cattle, as well as in the salmon industry. The pressure of supply is important in determining the force of the blow, however some captive bolts deliver considerably more force at 5 bar than others might do at 8 bar supply, so other factors such as powerhead air volume are also very important. The end result is that a low mass, high velocity captive bolt (which is free running within the cylinder such that stroke is partly dictated by resistance from the fish’s skull) causes a significant non-penetrating (low momentum) cerebral concussion. The impact can be further spread by the use of a hard plastic plate (commonly used by the salmon industry) that the bolt strikes which transfers the blow to a larger surface area on the fish’s skull.
Figure 2. Powerhead driven captive bolt showing head valve lifted by trigger mechanism (not shown) allowing rapid discharge of stored air in the powerhead to drive the captive bolt.
Percussive Stunning Efficacy Criteria
Percussive stunners work hard on a daily basis and need to be maintained. The criteria used to assess efficacy is below: (Score the first 50-100 fish where possible)
1. Excellent. - 99 to 100% instantly rendered insensible with one hit.
2. Acceptable. - 95 to 98% instantly rendered insensible with one hit.
3. Not Acceptable. - 90 to 94% instantly rendered insensible with one hit.
4. Serious Problem. - Less than 90% instantly rendered insensible with one hit. If one-hit efficacy falls below 95%, immediate action must be taken to improve the percentage. If the first hit fails to induce instantaneous insensibility, the fish must be immediately re-stunned with backup stunners.
Note: Any fish not effectively stunned (killed) should be immediately stunned (killed) by a backup stunner and bled (exsanguinated).
Why do we bleed then if the fish are rendered instaneously, irreversibly, insensible?
Exsanguination takes 4.5-6 minutes to cause death in salmon compared with 2-20 seconds in sheep and cattle. If fish have not been irreversibly stunned, and are also not bled, they can take >9.5 minutes to die from anoxia in a chilled ice slurry. As exsanguination takes so long to cause death, it is clearly unacceptable as a welfare-friendly slaughter method and it’s essential function should be viewed as only improving flesh quality by removing bloodspots (clotting) from the fish muscle.