Download our Protec Gill handbook

The fine art of gill protection
PROTEC GILL
Protec Gill - designed to support gill health and
recovery during disease, environmental and
treatment challenges.
GILLS – AN IMPORTANT MULTI-FUNCTIONING ORGAN
Fish gills are the most important site of contact between the fish and the water. They are not
only responsible for the exchange of the respiratory gases (oxygen and carbon dioxide) they
also play a part in maintaining salt balance and excreting waste. Skretting’s gill handbook has
been developed so you can learn more about this life sustaining organ, and how maintaining
good gill health will enable fish to perform to their full potential.
GILL STRUCTURE
Gills lie in what’s known as the branchial
cavity and are protected by a hard bony
Lamella
blood vessels
flap called an operculum. If you lift the
operculum, you will see four gill arches.
On each gill arch, there are lines of frills
called filaments. On the top and bottom
of the filaments, there is a series of
secondary folds called lamella. Inside the
lamella, you find tiny blood vessels. The
lamella walls consist of one cell layer,
and the entire lamella is about 10 microns
thick; a little more than half the thickness
of a strand of human hair. This branching
structure gives the gills its large surface
which is about 50 times larger than
the area of skin covering the body. Gill
surface area declines with increasing fish
Filament
weight which is why larger fish often are
the first to succumb to oxygen depletion.
Illustration showing how blood vessels coming from the heart
(in blue) and going to the body (in red) runs through the gill
arch. The water flows between the gill arches, through the gill
filaments, passing each gill lamella.
GILL FUNCTION
GAS EXCHANGE
When the fish opens its mouth, water flows in, past the gills, and out of the gill operculum in one
direction, while the blood in the gill flows in the opposite direction through the blood vessels in the
lamella. This process is called the counter-current principle. The combination of the large surface
area, the counter-current principle and the short distance between blood and water makes the gills
very efficient. Gills can extract up to 80% of the oxygen from the water passing through them.
CONTROLLING THE BODY’S WATER CONTENT
Chloride secretory cells located in the gill lamella play an important role in the transportation of
salts to and from the blood into the surrounding water; this process is known as osmoregulation.
In freshwater, water will continuously diffuse into the fish so it has to actively uptake salts from the
water to get enough ions in the body for it to function properly.
In seawater this process is reversed
and fish will have a lower concentration
of salts than that of the surrounding
Lamella
seawater, so it loses water and gains
salts. Fish need to get rid of this excess
salt to avoid dehydration.
ELIMINATION OF WASTE
The process of getting energy from
feed produces waste and a large
amount of this is in the form of
nitrogen. Nitrogen combines with other
molecules in the body to produce toxic
compounds such as ammonia. About
80 to 90% of a fishes nitrogenous
waste is excreted as ammonia by the
gills, with the remainder excreted by
the kidney as urea. Again, the short
distance between the blood and the
water the fish swims in is useful, as
The counter-current principle: A small segment of one of the gill
ammonia can easily be transported
filaments where the lamellae are located. Water and blood flow
in opposite directions past the lamellae making oxygen uptake
from the blood stream to the water via
very efficient.
the thin lamella wall.
WHAT HAPPENS WHEN GILLS ARE COMPROMISED?
There are many potential threats to gill integrity and function such as disease, poor water quality and
the side-effects of treatments. The very design that makes gills so efficient means that they are also
vulnerable to damage.
The most common response of the gill to any disorder are inflammatory responses in which the
distinguishing feature is either.
1) Hyperplasia - the enlargement of an organ caused by an increase in the reproduction rate of its cell
2) Hypertrophy – the enlargement of an organ or tissue from the increase in size of its cells.
Both hyperplasia and hypertrophy will lead to the lamellae becoming stuck or clubbed together.
Chronic irritation is also likely to cause a thickening of the mucous layer. These inflammatory symptoms
can significantly reduce the surface area available for gas and salt exchange as well as excretion. The
effects on farmed fish populations can be reduced appetite, a lower tolerance to stress and in severe
cases mortality. Today, gill disease is one of the most common causes of production loss in aquaculture.
AGD – AMOEBA CHALLENGES GILL HEALTH
An increasingly common challenge to
gill health is from the parasitic amoeba
Neoparamoeba perurans which causes
amoebic gill disease (AGD). Amoebas are
single celled organisms. In the human intestine
we have several naturally-occurring amoebas,
while other amoebas that end up in food or
drink can cause illness, for instance dysentery.
N. perurans only infects fish in sea water
and was first observed in Tasmania in 1985.
By 2015 nine of the twelve Atlantic salmon
producing countries worldwide had reported
confirmed cases of the disease. Even though
amoebas are small, they are large compared
to the gill filaments, with a length of up to
50μm. N. perurans can float well in water and
actively move over the surface of gills. They
reproduce quickly and easily by cell division.
When the amoeba attaches to the gills, its
presence causes a severe increase in the
number cells (hyperplasia) within the gill tissue
which in turn leads to the fusion of the gill
lamellar. These are the white spots visible
when you open the gill operculum and study
the gill filaments. If you look at the damage
under a microscope, you can see that the
lamellae are clumped together. Therefore,
the gills lose their surface area, and the gill
capacity is poorer. When the gills are severely
attacked, the fish can suffocate.
In Northern hemisphere salmon production
regions, AGD is most common in the autumn
when the temperature starts to drop from the
summer highs. The severity of the disease
reduces when the temperature falls to its
lowest winter levels.
It has been proven that N. perurans can
survive for at least 14 days in open sea water,
and further spread one kilometer from the
source. It also exists on other species living
in the sea, such as cleaner fish, shellfish and
hydroids. The risk of disease is likely to be
higher in farmed salmon, as the density of fish
and nutrients is higher.
A common scoring system has now been
adopted almost uniformly across the industry
to monitor the gills to see if the fish has been
infected by P. perurans. The damage is scored
in individual fish on a scale of 1 to 5. More
information about gill scoring can be found
on Skretting’s AGD poster and scorecard. A
score of 5 indicates the most serious damage,
where most of the gill tissue is altered and
there is a higher risk of mortality. It is therefore
common to treat the fish before the average
gill score of the unit has escalated. AGD is
treated with hydrogen peroxide or freshwater
baths. This requires time and resources, and
the treatment can be stressful for the fish.
Skretting research has shown that the
combination of several of our tried and tested
functional ingredients can modulate key
immune system parameters which are known
to support gill structure and function.
Amoebas may form runners used for movement and to catch food.
THE FUNCTIONAL FEED DEVELOPMENT PROCESS
At Skretting our rigorous process of functional feed development starts by screening
substances in the laboratory to establish the potential of the nutrient to either affect
pathogens directly or to modulate the immune system to achieve a positive effect. As these
innovative substances will be added to feed formulations the ingredients with potential are
then assessed for aspects such as stability, yields and bioavailability in finished feed products.
DEVELOPING PROTEC™ GILL
The role that functional feed ingredients play in supporting farmed fish health and in turn to
sustainable and economic production has been a major focus for Skretting Aquaculture Research
Centre over the last 25 years. In 1992, Skretting launched its first functional feed called Response,
whose ingredients were designed to strengthen the fish’s immune defense systems. Since then,
our researchers have discovered more functional ingredients that have been added to the feed that
work synergistically to improve fish health. With these upgrades, the name of the feed was changed
to Protec™. Today, the Protec™ solution is used for a number of species worldwide, and is effective
in preparing the fish for upcoming stress and challenges in farm conditions, such as handling,
vaccination and diseases.
With an extensive array of laboratory research, and repeated infection trials, a new feed that better
supports gill health and recovery during environmental, treatment and disease challenges such as
AGD has been developed. The feed is called Protec Gill and, as the name implies, it contains the full
Protec™ package, with added components that ensure better gill health. Protec Gill has produced
higher survival rates in three controlled AGD infection trials, and analysis of these results has shown
that the fish fed Protec Gill had significantly lower rates of mortality.
Protec ingredients inhibit the growth of amoeba
when exposed to mucus from fish fed different functional ingredients:
97
96
Growth Percentage
95
94
93
92
91
90
89
88
87
Neg. Control
Protec
Protec Gill
Pos. Control
Results show that when P. perurans amoeba are exposed to the mucus of Protec gill fed salmon
the growth of amoeba is inhibited.
Protec Gill Increases Survival
in fish challenged with P. Perurans
100
Percentage Survival
90
80
70
60
50
40
30
20
10
0
1
3
5
7
9
11
13
15
17
19
21
23
25
27
29
31
Days after infection
Control
Protec Gill
Challenge trials using Protec Gill against AGD infections were replicated three times to
confirm results. The fish that received Protec Gill had the lowest mortality rates every time,
and showed 27.1% 20.3% and 42.4% higher survival rates.
Protec Gill supports osmoregulation
in fish challenged with P. Perurans
174
Blood Sodium (mmol/L)
173
172
171
170
169
168
167
166
165
164
Control
Protec Gill
In seawater fish must replace the water which they constantly lose through osmosis.
However, drinking seawater brings with it a large quantity of salt into the blood and this has
to be removed by chloride secretory cells in the gills, which actively transport salts from the
blood back into the surrounding water.
Measuring blood sodium levels gives an indication of the integrity of gill structure and the
efficiency of gill function. Unhealthy or damaged gills will be less efficient in removing salts
from the blood. Research shows that Protec Gill fed fish are better able to maintain osmotic
regulation during an AGD challenge; the principle cause of death in AGD infected fish.
Protec Gill increases the expression of antibodies
Relative Expression
16
14
12
Control
Protec
10
8
6
4
2
0
IgM
IgT
An antibody, also known as an immunoglobulin, is a large, Y-shaped protein produced mainly
by plasma cells that is used by the immune system to identify and neutralise pathogens such
as bacteria and viruses.
Immunoglobulin M, or IgM for short, is a basic antibody that is produced by B cells. It is
the first antibody to appear in response to initial exposure to an antigen. IgT is the first
monomeric immunoglobulin to be shown in teleost fish. Recently discovered, IgT is produced
following exposure to toxins, pathogens or other foreign substance and is predominantly
found in the fish mucosal surfaces, suggesting a possible role of IgT in mucosal immunity.
Protec Gill increases anti-bacterial potential
1000
**
900
Iysozymes (U/ml)
800
700
*
600
500
400
300
200
100
0
Control
Protec
Protec Gill
Lysozymes are enzymes that increase the rate of destruction of the cell walls of certain
bacteria. Lysozyme is one of a number of immune related components occurring in fish
mucus and indicates the importance that mucus plays in providing the first line of defence
against a broad spectrum of pathogens. Research shows that the mucus produced by the
skin and gills in Protec Gill fed fish have higher levels of lysozyme and anti-bacterial potential.
SKRETTING’S RECOMMENDATIONS FOR MAINTAINING
GOOD GILL HEALTH
Proven functional feed solutions like Protec™ should be included as part of a structured approach to
performance and health management and alongside other strategies such as vaccination, disease
monitoring, and water quality management. When specific known gill stressors do develop, a
change to Protec Gill will focus the benefits of functional nutrients to where they will have the most
impact.
RECOMMENDATIONS FOR PROTEC GILL USE
Typical recommendations for Protec Gill use in Scottish seawater salmonid production:
Before and During Gill Health Challenges
Feeding Protec Gill 2 to 4 weeks ahead of known gill health risk periods and/or at the commencement
of gill disease outbreaks until its resolution, will provide additional mitigation against the effects of the
challenge.
Before and After Bath Treatments
Feeding Protec Gill for 2 weeks before and 2 weeks after Hydrogen Peroxide (H2O2) treatments will
support the regeneration of gill structure and function.
Skretting is the global leader in the production of innovative and sustainable nutritional solutions for aquaculture. We deliver
high quality feeds and services to farmers around the world, so that they can produce healthy and delicious food in a
sustainable manner. Skretting produces feed in 18 countries and has an annual production of 2 million tonnes of high quality
feed to a wide range of fish and shrimp species in all life stages.
www.skretting.co.uk