Water quality can be more
unstable in recirculating systems
than in large ponds or flow-
through systems. Water quality
fluctuations, such as temporary
increases in ammonia or nitrite, can, by themselves, result in
disease or significant losses. These
environmental fluctuations often
lead to suppressed immune
systems and greater susceptibility
to pathogens (i.e., disease-causing organisms, such as bacteria,
parasites, fungi, and viruses) and
disease outbreaks. Recirculating systems favor the
growth of many disease-causing
organisms and spread of disease.
There are a number of reasons for
this tendency, including higher
densities of fish when compared to other culture systems; build up of
biofilms and sediment and
subsequently pathogens in tanks,
sumps, or filtration components
(especially mechanical and
biological filters); and slower turn over of water. Over time, pathogens can become
concentrated (i.e., present in high
numbers). Most pathogens are
considered opportunistic, causing
disease only in fish with
suppressed immune systems. However, if pathogens become
sufficiently numerous they can also
cause disease in healthy fish. In
addition, the continuous flow of
water throughout a system can
spread pathogens rapidly, especially in a system lacking
adequate disinfection protocols or
components, such as ultraviolet
sterilization or ozone (see System
Disinfection below). Bacteria, parasites, fungi and
viruses can all become
concentrated in recirculating
systems. Bacteria that seem to
increase in number in recirculating
systems include Aeromonas spp., Vibrio spp., Mycobacterium spp.,
Streptococcus spp., and
Flavobacterium columnare
(Columnaris disease) (see UF/IFAS
Fact Sheets FA-14 Aeromonas Infections, FA-31 Vibrio Infections of Fish and VM-96 Mycobacteriosis in Fish; UF/IFAS Circular 57 Streptococcal Infections of Fish; and SRAC Publication No. 479b Columnaris Disease, respectively).
Parasites that tend to thrive and
spread relatively easily in
recirculating systems include
Ichthyophthirius,Cryptocaryon, Amyloodinium, Costia and
monogeneans (see UF/IFAS
Circulars 716 Introduction to Freshwater Fish Parasites and 920 Ichthyophthirius multifiliis (White
Spot) Infections in Fish; UF/IFAS Fact sheet Amyloodinium Infections in Fish VM-90;and UF/ IFAS Fact Sheet FA-28 Monogenean Parasites of Fish, respectively). Closed systems can also foster the
spread of fungi and viruses (see
UF/IFAS Fact Sheets VM-97 Fungal Diseases of Fish and FA-29 Introduction to Viral Diseases of
Fish, respectively). Adequate control of pathogens in a
system, and consequently
reduction of disease in these
systems, requires an
understanding of where pathogens
may be found, how they can be transmitted to fish, and how their
numbers may be reduced. In
addition, understanding the proper
use of chemicals to reduce or
eliminate pathogens is an essential
part of good management. Biosecurity Biosecurity has been mentioned in
Part 1 of this series (recommended
reading), but its importance
warrants further discussion. The
purpose of a biosecurity program
is to prevent entry of specific pathogens (disease-causing
organisms, i.e., bacteria, viruses,
parasites, or fungi) that may cause
significant disease and are not
present either in the environment
or on the fish in a given facility or system. In some cases, this is
achieved by extensive testing of
fish prior to receiving them from a
supplier, or during isolation and
quarantine, prior to placing them
in their intended system. For some pathogens, this may not
be an absolute elimination of risk
of entry, but primarily an overall
reduction of the number that do
enter the facility, so that fish
already on the property do not receive an overwhelming load. Biosecurity measures are
important not only when bringing
new fish into a facility; these
measures are also important for
reducing overall numbers of
potential pathogens in a given system, and to avoid transferring
pathogens from one system to
another. For this reason, it is
important to understand where
pathogens may be found
(reservoirs), and why quarantine, disinfection, and sanitation are
important to a good biosecurity
program. Pathogen Reservoirs There are many areas within an
aquaculture facility and
recirculating system that can act as
reservoirs for pathogens. The most
important reservoirs are the fish
themselves. Fish can act as asymptomatic carriers of disease.
In other words, they may be
immune to a specific pathogen but
still be able to shed the organism
into the water or transfer it to
other fish by contact. Sick and dead fish are often major reservoirs of
disease-causing organisms. For this
reason, sick, moribund (dying), and
dead fish should be removed as
soon as possible from a system and
disposed of according to county, state, or federal regulations. In
most instances, disposal can be as
simple as placing the dead fish in a
plastic bag and putting it in a trash
receptacle. Water can also act as a
reservoir. Water can spread pathogens to anything it touches. The ground (e.g., concrete slab)
can contain pockets of water that
contains pathogens. Equipment,
including nets, siphon hoses and
buckets, can also contain pockets of
disease-causing organisms. For this reason, disinfection of floors, and
use of footbaths (either small
containers or mats containing
disinfectants) placed at entrances
and exits to system rooms is
recommended, as is disinfection of all equipment when used with fish
in different tanks or vats or
systems. Nets should be kept off
the floor and placed in an
appropriate clean location to avoid
contamination. Quaternary ammonium
compounds are commonly used to
disinfect equipment but they must
be rinsed adequately prior to reuse
because these compounds are toxic
to fish (see UF/IFAS Fact Sheet VM-87, Sanitation Practices for Aquaculture Facilities). Chlorine can be used but will destroy nets and
must be neutralized or rinsed off
adequately to avoid killing fish.
Equipment disinfected with iodine-
containing compounds must also
be rinsed off prior to use because they can be toxic. Virkon Aquatic ®
is used by numerous aquaculture
facilities and has been shown to be
safe and effective against a wide
variety of aquatic pathogens when
used as directed. Contact a fish health or aquaculture specialist for
recommendations on disinfectants
for equipment, floors, and
footbaths. System hardware, including sumps
and filters, sediment, and tank
walls, are common sites for
pathogens. Sumps and tanks often
contain a fine film (biofilm) or
layer of sediment that may harbor pathogenic organisms. Sediment
on the bottom of sumps and tanks
should be vacuumed routinely.
Uneaten food lying on the bottom
of tanks can also provide areas for
pathogens to flourish. Filter beds, because of their
particulate nature, concentrate
microorganisms. Mechanical filters
should be backwashed, as
frequently as possible, to reduce
the loads of the undesirable (non- biofilter) bacteria, as well as other
potential pathogens. Pathogen Transmission Pathogens can be transmitted
several ways within a recirculating
system: in the water fish to fish by vectors and fomites in the food Introduction of water used to ship
fish can be a key source of
pathogens. Shipping water often
contains high numbers of bacteria
and may also contain parasites or
other pathogens. These organisms are easily transferred from tank to
tank in the recirculating water, or
by aerosolization (in mist or spray)
of water from one tank or system
to another. Within a single tank or vat,
pathogens can be spread directly
from fish to fish. Higher stocking
densities and increased fish-to-fish
contact (as seen in aggressive
species) can increase the rate of spread of pathogens. Vectors are organisms that can
transmit disease-causing
organisms from one animal to
another. For example, the
crustacean parasite Argulus (“fish
louse”) causes damage by itself, but it is also believed to transmit
bacteria and viruses between fish.
Leeches are another vector that
can transmit blood-borne parasites
and bacteria between fish.
Additionally, people can act as vectors by transmitting water and
pathogens from one tank to
another via their hands or arms. Fomites are inanimate objects that
can transmit diseases. Examples of
fomites in aquaculture systems
include equipment, such as nets
and siphon hoses, that are not
properly disinfected before being used in other tanks or vats. Food can also be a source of
disease. Frozen and live foods can
transmit bacteria, parasites,
viruses, and fungi. In addition,
feeds that have been improperly
stored can contain pathogenic bacteria or mycotoxins, dangerous
chemicals produced by the growth
of certain types of fungi in the feed
(see UF/IFAS Fact Sheet FA-95,
Molds in Fish Feeds and
Aflatoxicosis). Improperly stored feeds also have reduced nutritional
value, due to degradation of
micronutrients (e.g., reduction of
vitamin C levels) and
macronutrients (e.g., rancidity of
fats). System Disinfection or
Sterilization As described previously, water
may spread pathogens and also be
a potential reservoir for them.
Water from a tank containing sick
fish often carries numerous
disease-causing microorganisms. When this same water enters
another tank of fish, those fish are
then exposed to the
microorganisms and they will have
an increased risk of developing
disease. Disinfection helps to greatly reduce the spread of some
pathogens. Two techniques
commonly used to disinfect water
in aquaculture systems are
ultraviolet sterilization and
ozonation. Ultraviolet Sterilization Ultraviolet (UV) sterilizers typically
consist of UV-producing lamps
encased in a glass or quartz sleeve.
Water is passed over the lamps.
The lamps emit ultraviolet light (a
wavelength of approximately 254 nm is considered optimal) that
penetrates cells and damages
genetic material (DNA and RNA)
and proteins. For each type of microorganism, a
specific “zap dose,” measured in
microwatt seconds per square
centimeter, is required to
selectively sterilize the system (i.e.,
kill the unwanted organism). The zap dose is determined by the
intensity or wattage of the lamp,
contact time or flow rate of the
water, water clarity, and size and
biological characteristics of the
target organism. In general, larger organisms require a larger zap
dose (see Table 1 and Figure 1);
however, the specific structure of
certain viruses (which are
generally much smaller than
bacteria) makes some of them more difficult to “kill” than
other larger organisms. In general,
the zap dose required is lowest for
gram-negative bacteria, and it
increases progressively for gram-
positive bacteria, viruses, spore- forming bacteria, and protozoans. Table 1. Recommended zap doses for different organisms

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