Flesh quality affected by Harvest practices
By Fernando Oyarzún
Background
Harvest of salmon involves several steps before live fish becomes human food. Before
slaughter fish are starved for few days to empty guts. Fish are crowded, pumped, killed,
bled and chilled at the farm, or transported alive by well boat to a slaughter facility where
the fish can be pumped into the slaughter house immediately from the boat or after being
held in waiting cages for a few days. Slaughter of fish involves stunning (most common
stunning methods are CO2 dissolved in water, percussive stunning, electric stunning and
chilling in cold water), bleeding (by severing gill arches or aorta), and cooling.
Starvation of fish, handling (transport and pumping) and stunning of fish implies stress.
While the handling and disturbances at the farm is limited, fish ready for harvest will
experience accumulative stress due to the harvest practices, such as repeated pumping
occations, transport, and starvation. This accumulative stress that the fish experience just
before slaughter prevent full recover and will ultimately impact the quality of the end
product.
This article split the harvest process and focuses on harvest practices that should be
under control.
Stress in fish
Farmed fish experience stress due to handling practices i.e. transport, crowding, pumping
and slaughter. As the primary and immediate response to stress, cortisol is excreted from
the interrenal cortex. Cortisol will then facilitate secondary stress responses such as
increased mobilization of glucose and the release of lactate under anaerobic situations.
Fish have developed these responses through evolution as a mechanism to danger and
the need for fast escape. However, as domestic animals experience repeated and chronic
stress, the mechanisms, that originally were necessary for survival, have become
maladaptive. In fish farming the stress responses have been associated with poor flesh
quality and early rigor development. Gaping increase, soft flesh and paler meat are
amongst other negative effects of vigorous physical activity pre mortem.
Post mortem processes
In live fish, muscle fibers contract continuously. The energy necessary for contraction is
supported by ATP, which is produced by cellular respiration. Respiration can be aerobic
(with oxygen), mainly in the red muscle or anaerobic (without oxygen), in the white muscle,
both alternate according to the nature of the exercise. During salmon slaughter bleeding of
the fish eventually stops oxygen supply. However respiration will still continue for some
time at the expenses of oxygen available in remaining blood and afterwards by anaerobic
glycolysis. When ATP is depleted, the muscle contracts and rigor starts. As a
consequence of anaerobic respiration, lactic acid is produced and accumulates in the
muscle with the subsequent decrease in muscle pH. The pH decrease in the muscle cells
causes protein denaturation and loss of water holding capacity. Loss of water will influence
meat texture.
Rigor mortis
Rigor mortis is characterized by muscle contraction, seen externally as progressive body
stiffness. Right after death, the muscle is soft and limp (called pre rigor), then it becomes
stiff and hard (in rigor), where after the muscle regain softness (post rigor). The longer the
rigor onset starts (time before stiffness) the better meat quality. The duration of the rigor
progression is mainly influenced by stress pre mortem (severe exercise, rough crowding,
long distance pumping, and improper stunning method) and high storage temperature.
Thus rigor in fish can be mastered if the mentioned elements here are understood and
controlled. Rigor mortis can be measured by changes in muscle contraction in whole fish
by tail bending, also called rigor index. The sag of the fish tail is measured when half of the
fish body is placed on a horizontal table and measurements are repeated several times
during fish storage. Another possibility is to follow rigor through fillet contraction. In farmed
fish stress and in particular temperature are technically easy to control.
Fish densities
Farmed salmon are normally kept under 25 kg/m3 and in farm sites located in optimal
water quality conditions. However during live transport and in waiting cages fish densities
arise to 120 and 50 kg/m3 respectively. These changes in environment will stress the fish,
but the degree of stress will depend on how thoroughly the water quality is controlled
during transport and on the water conditions where the waiting cages are placed.
Unfortunately many waiting cages are placed in shallow water, close to the slaughter
facility, and therefore exposed to bad water quality (increase in water temperature and low
oxygen level)
Crowding practices
Fish are crowded to increase the efficiency of pumping when the fish have to be moved.
Crowding of fish in waiting cages before slaughter often result in the increase in density
from 50 to 500 kg/m3 in less than one hour. Fish density and exhaustion among the fish
escalates with crowding time. The typical crowding starts with calm fish and then escalate
in intensity as the net is tightened. At the end of the crowding the activity often decreases,
most likely because the fish are exhausted. Air gasping and lateral side up are the typical
signs of the crowding. These behaviours are inevitable, but those signs should be seen at
the end of the crowding rather that at the beginning. It is the responsibility of the harvest
crew to handle the intensity of the crowding. Intense crowding (increased fish exercise)
accelerate rigor mortis onset.
Pumping
Pumping, that involves sucking and transportation of fish through pipelines is perhaps the
most stressful handling that the fish experience. Unpublished data shows that killing fish
after long distance pumping resulted in rigor development after 9 hours instead of 30 hours
in the case of killing beside the cage. In case pumping is inevitable it should be gentle and
continuous, without stops to ensure fish are not trapped inside the pipeline. Field
observations indicate that pumping is normally interrupted due to delays inside the factory
and fish might remain in the pipeline from few minutes up to 30 minutes. The longer it
takes, the higher the stress due the lack of oxygen and increased density. However, the
best results are obtained when fish are stunned before pumping.
Stunning methods
As red meat animals and poultry, fish should be render unconscious immediately or if it is
progressively, be without pain and fear. Conversely to farm land animals killing of salmon
are based on the batch concept. Industrial methods developed aimed to deal with big
numbers of animals at once. Scientist have indentified two types of methods: Slow
methods (that do not cause immediate unconsciousness) and fast methods (that cause
immediate unconsciousness). Slow methods expose fish to poor welfare and reduced
meat quality (Suffocation in air, in ice or cold water and CO2 among others) while well
applied fast methods (percussive stunning and electricity through the head stunning)
deliver good meat quality (less gaping, redder meat).
A slaughter method is considered humane when unconsciousness and insensitivity are
induced immediately by stunning of sufficient duration to ensure the animal does not
recover while bleeding to death
Fish death indicators
Are fish unconscious after stunning? Electroencephalogram (EEG), electrocardiogram
(ECD) and visually evoked response (VERs) are objective methods to indicate sensibility
and consciousness in animals; however behaviour can also be used. Behavioural
indicators like eye rolling and opercula movement has been well correlated to objective
methods and the lack of them after stunning should be used as behavioural indicators of
unconsciousness. Conscious fish (live fish) fish must not be bled or gutted for ethical
reasons and also for meat quality consequences (fast rigor development, increased gaping
and drip loss).To test the effectiveness of the stunning method, fish after stunning should
be placed in a container with clean water for 5 minutes and the indicators mentioned
above observed.
Live bleeding: The myth
Fish should be alive while bleeding to death. This assumption is based on a beating heart
to pump out the blood. But even if this happens why is blood spots still a problem? From
the animal welfare perspective fish must be unconscious or dead before bled out. Bleeding
of salmon is mainly done in water after severing gill arches, but since fish do live in water,
they have adapted to clot their wounds in water. Scientists investigated the different
bleeding procedures and concluded that a beating heart and number of gill arches cut are
irrelevant, this means cutting 2, 3, 4 or 8 gill deliver the same result. They also described a
methodology for blood spots counting using salted fillets superior to evaluations performed
in fresh and frozen condition. New evidence suggest that time before processing is by far
more important than the number of gills cut to improve blood spots in flesh. To remove
blood eliminates also the possibility of lipid oxidation while fish is stored in frozen
condition.
Chilling rates
Chilling of salmon immediately after death helps to delay rigor mortis and bacteria growth
extending the shelf life. Salmons should be chilled right after death, but temperatures
should not exceed -2.2oC since at this level 50% of the free water in the cell will freeze.
The salmon industry have developed good practices for chilling salmon with ice in bins but
there are new challenges in dead haul harvest due to increase in volume and long
distance journeys. RSW systems in replacement of ice offer a more reliable alternative for
long distance travels and easy pumping.
Recommendations
Harvesting of salmon after starvation takes few hours from crowding, pumping, killing,
bleeding and chilling. All of these elements are controllable by people in charge. The
operational procedures applied in this short period of time are crucial for animal welfare
and thus meat quality purposes, but also for the economic impact (weight loss, gaping,
meat colour, texture and shelf life). The control of the operational steps mentioned here
will enhance the benefits of salmon farming production.