Edible brown crab Cancer pagurus

Edible brown crab Cancer pagurus. For humane killing, electrical stunning is recommended to render the animals unconscious during boiling
Credit: Christophe Quintin.

Principles of humane slaughter

Humane slaughter of animals has been defined for fish by the Humane Slaughter Association as follows:

“As with mammals, a humane slaughter is one that results in an immediate loss of consciousness, or if slow acting, induces unconsciousness without discomfort or pain. This unconsciousness should persist until death intervenes.” 1

Methods that are not humane in themselves, such as boiling, should be preceded by humane stunning to render the animal unconscious (i.e. to anaesthetise) throughout the killing process.

This page describes methods of stunning/killing crustaceans according to how well they meet these criteria for humane slaughter, i.e. methods that:

  • meet the humane criteria (electrical stunning, AQUI-S)
  • do not but, being faster, are likely to be less inhumane (piercing of ganglia, splitting, high pressure killing)
  • are clearly inhumane (most methods including boiling, drowning, live carving/dismemberment)
  • are often claimed to humanely stun or kill but for which evidence is lacking or contrary (chilling, freezing).

For comparison, the methods for humane stunning/killing of fish are as follows:

  • percussive stunning (a blow to the head)2, 3
  • spiking the brain2
  • electrical stunning2, 3 and
  • use of food-grade anaesthetics (AQUI-S)3, 4.

The only method of stunning/killing crabs and lobsters that can produce an immediate loss of consciousness (within 1 second) is electrical stunning, enabling them to be killed without pain. In contrast to fish and other vertebrates, percussive stunning and spiking are not suitable methods for causing immediate loss of consciousness in crustaceans, due to the nervous system being less centralised.

The methods of spiking (crabs) and splitting (lobsters) are analogous to the spiking method of killing fish but take several seconds to perform. These methods are described below separately (along with high pressure killing of lobsters) since, although not immediate and not guaranteed to be pain-free, they are considerably faster than the other commercial methods except electrical stunning. Research suggests the humane killing of crustaceans may also be achieved using the fish anaesthetic AQUI-S which, though the process takes several minutes, appears not to cause distress.

Assessing welfare of stunning and killing methods

For any stunning/killing method to be considered humane, it is necessary to demonstrate that it either kills the animal, or that it causes unconsciousness that lasts until death. If the method does not cause an immediate loss of consciousness, then it is also necessary to demonstrate that it does not cause pain or distress before the animal becomes unconscious.

Pain and distress, or their absence, cannot be directly measured in animals and indirect evidence must be used, which includes the animal’s behavioural response and can also include the physiological response and electrical readings of brain or nerve activity. Behaviour is arguably the “best window” for inferring an animal’s emotional state 5 cited in 6 and is also the easiest to measure; however evidence based purely on behaviour is not always reliable since paralysis can occur without anaesthesia. Does an animal’s still behaviour indicate anaesthesia or is the animal simply paralysed?

Stress behaviour observed in crabs and lobsters includes trying to escape, thrashing and autotomy. Autotomy is a behavioural response in which limbs or other body parts are shed by the animal in response to damage or capture, or to stop the spread of potentially harmful stimuli to the rest of the body7 cited in 8. In a study of killing methods for Australian giant crab, treatment was considered to have caused pain when crabs dropped limbs, tore at their appendages or abdomens, became tensed and rigid or appeared to have muscle spasms.9

In his experiments on the killing of edible crabs, Baker10 stated that crabs only perform autotomy when they are vigorous and Baker stressed the need to study the effects of killing methods on vigorous animals.

Humane methods of killing crustaceans

Electrical stunning
A study in 201011 found electrical stunning to be the most efficient (i.e. humane) stunning method for edible crabs when compared with common commercial methods such as boiling, chilling, freezing, gassing (CO2) and immersion in salt solutions:

“Electrical stunning turned out the most efficient stunning method for edible crabs. With sufficient electrical current the animal could be rendered unconscious within 1 second”.

The Crustastun is a device, developed in the UK, for the humane electrical stunning and killing of crustaceans, including crabs, lobsters and crayfish. According to researchers at Glasgow University, the Crustastun causes no stress to the animal (as evidenced by blood lactate levels) above that caused by handling12 and that the result of the Crustastun is to silence the central nervous system immediately (within 1 second) without recovery13, 14. This conclusion is based on electrophysiological readings of exposed nerves in ‘Crustastunned’ crabs and lobsters and is supported by the finding that induced autotomy (shedding of legs) never occurred and has been found not to occur when the device is used commercially.

In a separate development, scientists in Norway have adapted the commercial dry stunner for fish (Stansas, from the equipment manufacturer Seaside) for the humane electrical stunning of edible crabs in bulk15. Crabs must be killed immediately after stunning (e.g. by boiling) to prevent recovery of consciousness.

Electrical stunning of crustaceans is discussed in more detail at: electrical stunning.

A study into the killing of Australian giant crab Pseudocarcinus gigas9 concluded they could be killed without apparent distress by use of the food-grade anaesthetic AQUI-S (or by the anaesthetic clove oil, which also has an anaesthetic effect in humans16 cited in 8, 31) added to the seawater tank in which they were held. Both these anaesthetics took 25 minutes or more to kill, during which the crabs did not show aversive behaviour and autotomy did not occur (i.e. they did not lose limbs). While recognising that absence of behavioural indicators of distress does not necessarily mean that killing is painless, since it is the case that an animal can be paralysed while still able to feel pain, the author concludes:

“…clove oil and AQUI-STM appear promising as treatments for the humane killing of crabs for human consumption”.

The fish anaesthetic AQUI-S is marketed as a water dispersible liquid anaesthetic for fin fish, crustaceans and shell fish that can be used for humane killing and also to remove stress from live transport and handling procedures17. AQUI-STM is approved in Australia, Chile, New Zealand, Korea, Costa Rica and Honduras with a zero withholding period and is reportedly the only aquatic anaesthetic that can be used for harvesting fish in those countries17. It was approved in Norway in 201317.

Less slow (and therefore likely to be less inhumane) methods of killing crustaceans

The following methods are reported to take several seconds (ranging from 6 to up to 15) to kill. Since 6 seconds is far from instant, they cannot be considered humane. However, being considerably faster that most commercial methods, they are likely to be less inhumane.

Piercing of ganglia (spiking)
A research article published in 1955 describes a study by Baker10, conducted under the auspices of the Universities Federation for Animal Welfare, to investigate methods of killing crabs. At a time prior to the development of electrical stunning for crustaceans, Baker concluded that piercing both of the crab’s two main nerve centres (the brain or posterior ganglion together with the ventral nerve mass or anterior ganglion) was the most humane method then available:

“The following methods intended to render crabs (Cancer pagurus) insensible before boiling were tried: immersion in strong salt solution, immersion in tap water, gradual increase of the temperature to boiling, and piercing of the brain and ventral nerve-mass with a steel-pointed awl. The responses of the crab to stimuli, especially the autotomy reaction on boiling, were carefully observed. The experiments showed that the best method is to pierce the brain and ventral-nerve mass. It is important to hold the awl at the proper angles when inserting it in the two positions”.

The nervous system of crustaceans consists of individual ganglia (well-defined clusters of nerve cells) connected along a main ventral nerve cord. In the crab, the nervous system is condensed into two main ganglia 8 (the posterior ganglion and the anterior ganglion). Crabs can be killed by piercing both these ganglia using an awl. Because both the two nerve centres must be destroyed, as compared to spiking the brain in fish, the process is not instant and therefore not humane. According to a study into the welfare impact of commercial killing methods for edible crab, the process must be performed by a skilled person and will take from 10 to 15 seconds.

Splitting (lobsters)
The nervous system of the lobster is less centralised than that of the crab and lobsters cannot be stunned by piercing the brain. In the lobster, a chain of ganglia (sometimes called ‘mini brains’18) with interconnecting nerves runs through the animals’ longitudinal midline (the length of the body)8. The splitting method seeks to destroy the central nervous system by cutting through all the ganglia with a sharp knife and involves cutting the animal into two identical longitudinal halves, starting at the front of the head and continuing through the thorax and straight down through the midline of the animal where the nervous system is located. Details on proper cutting methods, followed by rapid removal of visible nerve centres, is described in the Aquatic Animal Welfare Guidelines of the Australian National Aquatic Council19, which states that the process should take no longer than 10 seconds.

The method of splitting is demonstrated in the following clip from a promotional video for the Crustastun28:

High pressure killing
It has been reported in the media that some lobsters processed in the US are killed by high pressure in hydrostatic pressure processors20. The lobsters are crushed to death quickly in big batches, at the same time separating their meat from the shells without having to cook it. It is claimed that they are killed within 6 seconds21, though it is unclear if there is any evidence to support this.

A welfare advantage of this method is that, by enabling the killing of bacteria without cooking21, it could reduce the transport of live lobsters to restaurants and supermarkets. The method could presumably be made humane if the lobsters were electrically stunned before high pressure treatment.

Methods that are clearly inhumane

In its review of killing methods for animals used for research purposes22, a panel of scientists (the Animal Health and Welfare Scientific (AHAW) Panel) commissioned by the European Food Safely Authority, has concluded that the following methods of killing crustaceans are likely to cause pain and distress:

  • Any procedure involving the separation of the abdomen (tailpiece) from the thorax (tailing) or removal of tissue, flesh or limbs while the crustacean is still alive and fully conscious (including when in a chilled state).
  • Placing crustaceans in cold water and heating the water to boiling point.
  • Placing live crustaceans into hot or boiling water.
  • Placing live marine crustaceans in fresh water as they die from severe osmotic shock.
  • Unfocussed microwaves to the body as opposed to focal application to the head.

The first 4 of these are common commercial methods of killing crabs and lobsters used for food, including live carving, and are discussed below. Other commercial methods used that are also likely to cause distress include immersion in strong salt solution and immersion into water saturated with carbon dioxide, which are also discussed below.

Live carving and dismemberment
Another method for killing and processing crabs is carving11. As noted above, the AHAW panel of scientists has concluded that removing tissue, flesh or limbs from a conscious crustacean, even in a chilled state, is likely to cause pain. Roth and Øines report that edible crabs are often killed by carving without prior effective stunning, e.g. where the method intended to stun is chilling, which their experiments showed to be ineffective (see below). Conscious crabs are commonly placed into machines which rip of appendages before the body is split in two and the separate body parts are processed into food11. Some lobsters are killed by “tailing” whereby the body is severed, between the fore-half (cephalothorax) and abdomen, with a knife23. Lobsters are also killed by tearing off the limbs or head while the animal is still conscious, as are crayfish and languostine28.

Cutting into a conscious crustacean, other than by the correctly-performed methods of spiking and splitting discussed above, will clearly cause pain until the animal eventually dies because the central nervous system has not been destroyed.

Boiling and heating
As noted above, the AHAW panel of scientists has concluded that placing crustaceans into boiling water, or into cold water and heating the water to boiling point, are likely to cause pain and distress.

Placing conscious crabs and lobsters into boiling water causes them to shed their limbs (autotomy)23, 24 which is reportedly a common problem. A study by Baker10, published in 1955, found that crabs all crabs treated this way lost legs during boiling. In a later study25 cited in 26, Baker found that lobsters in full vigour, when they are plunged into boiling water, behave wildly, whipping their tails and trying to escape.

Roth and Øines11, predicted that edible crabs subjected to boiling will sense heat for a duration of 2.5 minutes, based on measurements of internal temperature. Chilling the crab to 2 °C prior to boiling will increase this duration to 3 minutes.

It has, in the past, been argued that placing crabs and lobsters in cold water and slowly heating the water to boiling point results in a loss of consciousness that is induced without pain before the animals is killed by boiling. However, in a study of this method for killing lobsters25 cited in 26, Baker observed aversive behaviour indicating distress. Baker researched bringing 14 lobsters slowly to the boil and the animals flipped violently as the temperature increased. The three lobsters heated in tap water regurgitated at least twice during the first ten minutes. Baker noted shaking, trembling and other uncoordinated movements; a general struggling, writhing or convulsive movement of the whole body without locomotion.

A later study by Gardner9 found that when Australian giant crabs were heated to 24 °C, in an attempt to anaesthetise them, the animals appeared uncomfortable and attempted to climb from the container as the temperature rose. Although apparently paralysed at 24 °C, their limbs constantly twitched.

Drowning (immersion of marine species into freshwater)
As noted above, the AHAW panel of scientists has concluded that placing live marine crustaceans into fresh water to die from osmotic shock is likely to cause pain and distress.

Killing marine crustaceans by immersion in freshwater is sometimes called “drowning” and will result in eventual death from loss of salts from the blood. Although low salinity conditions have the effect of aiding in the prevention of limb loss in lobsters, this treatment is likely to cause distress. Research has shown that immersing crabs in fresh water causes aversive behaviour9, 10. Gardner9 reported that Australian giant crabs immersed in freshwater became very active after 10 minutes, they tore at their abdomens and walking legs and autotomy occurred. The time taken for crabs to be killed by this method is long: 3 to 5 hours at 10 °C or at least 30 minutes in water heated to 38-49 °C, according to one author27.

Salt baths
Aaser reported that placing lobsters in common salt solution (35%) for a short period (1 minute or less) prior to boiling reduced the behaviour during boiling to a single flip of the abdomen29 cited in 10. Baker tried the same test on edible crabs, placing them in salt water for 10 minutes prior to boiling. The crabs displayed some abnormal behaviours (antenna immediately strongly retracted, followed by stillness followed by feeble walking) and, although spontaneous behaviour had ceased before 10 minutes had elapsed, autotomy occurred when the crabs were subsequently boiled10. Baker concluded that although placing these crabs in salt water will eventually result in death, there is no likelihood this slow death would be painless.

A study, by Roth and Øines11, into killing of edible crabs attempted to stun edible crabs by placing them into salt brine of 17% NaCl, 5% KCl or 20% KCl. The crabs exposed to 17% NaCl and 5% KCl displayed aversive behaviour, vigorously trying to escape, and were still conscious after 3 minutes (as indicated by behavioural responses to touching and handling). The crabs exposed to 20% KCl did not try to escape but took over 1 minute to lose all behavioural responses.

Carbon dioxide
Carbon dioxide has long been used to stun and kill aquatic animals, including edible crabs, according to a study by Roth and Øines11, which found carbon dioxide to be unsuitable for humane stunning. Crabs stunned in a water bath containing carbon dioxide showed aversive behaviour and some were still showing behavioural signs of consciousness when measured after 12 minutes. Gardner reported that Australian giant crabs exposed to this treatment thrashed and crushed their limbs, with some autotomy9.

Methods that are often claimed to humanely stun or kill but for which evidence is lacking or contrary

It is often considered that crustaceans can be stunned (i.e. rendered unconscious) without distress by chilling them in air or ice/ice slurry. However, the current author finds no research to support this view and chilling is elsewhere criticised as being slow and inconsistent and likely to cause discomfort by subjecting the animal to conditions that it would normally avoid24. On the contrary, separate studies have shown chilling on ice to be ineffective in stunning edible crab11 and Australian giant crab9 (temperate species).

According to the above mentioned AHAW review22, stunning of decapods can be achieved with a minimum of distress by chilling in air and they should then be killed by spiking or splitting. Crustaceans are cold-blooded animals and therefore chilling them with an air temperature of 4 °C or below induces a state of torpor, according to the AHAW review, and chilling in air at an appropriately low temperature for a minimum of 30 minutes will eventually kill them.

AHAW also considers that stunning of decapods can be achieved with a minimum of distress by chilling in ice/ice slurry for tropical species that are susceptible to cold temperatures (in which case they should be immersed in ice slurry for a minimum of 20 min depending on size and species, the ratio of ice to water should be 3:1 and the temperature should be maintained at or below minus 1 °C). AHAW does not recommend this for temperate species, which are likely to be adapted to cold temperatures.

The current author finds no clear evidence that chilling in air or ice/ice slurry results in anaesthesia, rather than just paralysis, nor that chilling is not itself aversive. The view that chilling effectively stuns appears to be based on the observations that crustaceans subjected to chilling do not show behavioural signs of distress, such as thrashing and autotomy, seen with of other methods of killing and there have not been studies on crustaceans that measure physiological indicators of distress during chilling and various other slaughter treatments8.

While AHAW recommends that chilled crabs and lobsters should be then killed by the spiking or splitting methods discussed above, it is especially concerning that elsewhere recommendations are made to chill crabs and lobsters prior to boiling. Roth and Øines11 found chilling to be an ineffective method of stunning edible crabs prior to boiling. Crabs that were subjected to chilling on ice at 0 °C were still showing behavioural responses to stimuli when tested after 100 minutes (at which point their internal temperature was 1.8 °C). The same researchers predicted that edible crabs will remain conscious during boiling for 2.5 minutes but even longer (3 minutes) if they have first been chilled to 2 °C.

Roth and Øines also showed that edible crabs placed in a freezer at -37 °C took 30-40 minutes to lose behavioural signs of consciousness. When placed in water at 12 °C following 60 minutes in the freezer, irreversible damage had been done and none recovered. All of the crabs lost two or more legs through autotomy, indicating stress.

Gardener9 found chilling to be an ineffective method of stunning Australian giant crabs. When Australian giant crabs were chilled for 14 hours to 2 °C and 5 °C, they remained active. When chilled for 2 hours at -1.5 °C, the animals retained movement (in response to tactile stimulation) of the antennae and limbs, though parts of the limbs became frozen.

To be a credible method of humane stunning, evidence of distress-free anaesthesia from chilling needs to be demonstrated on a species by species basis. Further, it should not be solely based on behavioural indicators. As a stunning method, chilling would need to be followed by a killing method that is sufficiently fast and reliable and that avoids warming the animal.

It seems likely that chilling crustaceans in air could also cause stress from exposure to air, which has been found to stressful to crabs and lobsters in a number of investigations of physiological and immune responses during live transport30.

The welfare affect of chilling on crustaceans is also important in transport, since most crustaceans destined for live markets are chilled prior to transport30.


From the literature available, the current author concludes that:

  • Electrical stunning, and in particular the Crustastun, is the most humane and most reliably humane method to stun/kill crabs, lobsters and other crustaceans.
  • Use of the fish anaesthetic AQUI-S may offer an alternative method of humane killing in countries where it has been approved
  • Most commercial methods of killing are likely to cause considerable suffering. Spiking, splitting and high pressure killing seem likely to be less inhumane since the killing process is much shorter, but since they are neither immediate nor likely to be distress-free, they cannot be considered humane.
  • To be credible, claims that chilling can render animals unconscious without distress need to be demonstrated on a species by species basis, and should not rely solely on behavioural indicators since it is possible that chilling causes paralysis without anaesthesia. Chilling should be followed by a sufficiently fast and reliable killing method that does not involve warming the animal.



A Mood. January 2014.


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