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Balance is everything
– how environment changes
affect farm fish
Urszula Ejlak
Introduction
There are few organisms such susceptible to their environment as fish. They literally soak in their sur-rounding, no matter how nutritious or poisonous it may be. It is important for fish farmers and veterinarians to know how to control water parameters, so they can pro-vide their customers healthy fish products. For example, we all know about dangers of diluted pesticides flowing around, but we rarely think of hyperoxygenated water as a toxic factor. This article highlights some most com-mon environmental illnesses in farm fish in Poland, which we can quite easily avoid by paying attention to proper farm hygiene.
To properly understand the fish struggle, we need to focus on their immune system for a moment. In inten-sive breeding, immunocompetence might be broken by:
• high density, • poor nutrition, • low water quality, • contaminated facilities or • new fish coming to the stock.
It may be also influenced by genetic predispositions (both innate and acquired immunity), age and species of the fish, seasons and photoperiod. For example, cold water fish do not need to produce immunoglobulins, as bacterial growth in such temperatures is very slow. It may prolong the immune response up to 28 days as well. As in other animals, the vital role in fighting infec-tion is played by leukocytes. They wander around the body, differentiating cells and molecules between „self” and „non-self” and trigger immune response to destroy the „non-self” ones.
The first thing to realize is that fish have no bone marrow nor lymph nodes, so blood cells (including leukocytes) are produced in other tissues than in mam-mals. Some of them may surpise us, for example soft
tissue of orbit and skull (Holocephali fish) or intestinal spiral fold in young lamprey. In Elasmobranchii (e.g. sharks), there is a noticeable Leydig’s organ under the esophageal mucosa. The most important blood-produc-ing organs in bony fish are spleen and anterior kidney, where immunity and hematopoesis take place.
Those viscera are highly influenced by cortisol, „the stress hormone”. During manipulation (vaccination, transport etc.) we can detect involution of kidney and spleen, cortisol also blocks forming leukocytes and their adhesion to damaged tissue, activity of kidney phagocytes is lowered, in blood smear we can see leu-kopenia (especially lymphopenia T). This effects in de-creased immune response, so even weak pathogens can induce disease.
When we talk about fighting pathogens, we may not forget about the first barrier-skin covered with mucus. Mucous glands, present in the medium layer of epider-mis, produce substance full of lysozyme (which damag-es bacterial cell walls and is also prdamag-esent in our tears and saliva), complement compounds (essential for trigger-ing the non-specific and specific immune response) and natural antibodies and immunoglobulins. It prevents bacteria, fungi and parasites from adhesion to skin. It also separates the skin from irritating factors (i.e. am-monia, detergents in water), so we can notice an over-production of mucus in such cases.
Temperature
In Poland, freshwater temperature oscilates between 0 °C and 30 °C. In summer, heated water flows to the surface and the lowest temperature is found in the bot-tom. On the contrary, during winter there is a cold ice surface and warmer water below. Water in reservoirs deeper than 1 m is permanently separated into three layers. The bottom layer is called hypolimnion and pre-Summary:
Many fish diseases do not start with presence of highly infectious factors, but in small environmental imbalanc-es. They tend to handicap the immunological response, and, in consequence, sentisize to infections. On the other hand, severe pH, temperature and water-soluble com-pound level changes may be lethal itselves. This article presents some most common cases in fish farming in Po-land.
Key words: fish, environment, disease, hygiene, toxicity,
oxy-gen, ammonia
Urszula Ejlak: DVM-graduated from Warsaw University of
Life Sciences. Intrested in farm fish diseases, was an intern at University of Warmia and Mazury in Olsztyn, Inland Fisheries Institute, Norwegian School of Veterinary Science and worked with fish veterinarians in Haugesund (Norway). Presently working as a small animal internist in a veterinary clinic in Warsaw
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sents a stable temperature of about 4 °C. Water at this temperature has the highest density and can dissolve the most oxygen. Fish, which are familiar with this law of physics, can survive winter staying in hypolimnion.
While sudden temperature drop usually does not af-fect fish much, extraordinary rise may be lethal. Obvi-ously it depends on season, age of the fish (there is zero tolerance for temperature change during fry transpor-tation, for adult fish it is 4°C), their accommodation et caetera. For example for carpio 37 °C is fatal (28.1 °C – 40.6 °C according to Wolny, 1976), for brown bullhead it is 36 °C in summer and 29 °C in winter. For adult rainbow trout, there is singnificantly increased mortal-ity over 21 °C. To minimalize loss, it is necessary to stop feeding. Optimal water temperature (14–18 °C) dur-ing summer can be maintained by introducdur-ing a two-source water collecting system (recruintment from sur-face waters as well as from hypolimnion). Temperature rise causes respiratory stress, faster breathing (shown by excessive movement of operculum) and swimming near surface with open mouth (called piping). Faster metabolism requires more nourishment, but eventually fish stop feeding. In such circumstances leukocytosis and erythropaenia occur.
Dissociation and dissolution happen faster in warmer water (unfortunately, it does not apply to oxy-gen). Combined with increased fish metabolism in such cases, we may observe severe intoxications (especially with heavy metals and phenols) that would not happen if water was colder.
pH
Appropiate pH range for rainbow trout is 6.5–8.2; for carp it is 5.0–9.0. For fisheries, it is the best to keep neutral (7.0) or weak alkaline (7.0–8.5) water. Daily pH fluctuations in ponds are normal and fish tolerate it well. However, the basic rule is not to change it over 0.2–0.5 pH units per day. There are simple, portable pH meters avaliable and they are easy to use. Colorimetric tests are more expensive, but more reliable.
Calcium carbonate is a natural water buffer, pre-venting sudden pH changes. Similar buffers exist widely in nature, for example in blood, to maintain the stable environment for chemical reactions. While the calcium carbonate level is down, more calcium hydroxide is formed, which results in increased water basicity. This may induce fish alkalosis. At pH over 9.0 we can observe skin loss, frayed fins, dilated pupils and turbid lenses. Gill tissue is swollen, reddish and covered with mucus. This leads to lower oxygen intake and hypoxia.
When partial pressure of CO2 in water is lower than it is in fish blood, carbon dioxide is removed from tis-sues into water. This imbalance of gases in blood end up in suffocation despite the proper amount of oxygen (hypercarbia).
Carbon dioxide exists in water in molecular (90%) and conjugated form (as CaCO3). High CO2 levels are typical for water from underground sources. It may also occur during transport or cloudy days. In deep reser-voirs (e.g. lakes) we can observe stratification of carbon
dioxide concentration. Maximum level of free CO2 is 25 mg/dm-3 for carp and 20 mg/dm-3 for trout (Własow
and Guziur, 2008). Transgression causes fish acidosis, with such symptoms as tremors, hyperactivity, troubled transpiration, excessive mucus production, pale gills and skin (even skin loss) and death. Chronic exposure to low pH leads to fertility problems, slower growth, spinal deformities and accumulation of heavy metals in tissues.
Most commonly, too acidic pH is present after heavy rains. Humic acids are flushed from soil, especially in ponds near coniferous forests, moors, coal mines. Met-abolic activity of water flora and fauna are considered a secondary source of low water pH. Photosynthesis processed by water flora has a major influence on water pH. Assimilation of carbon dioxide decreases the level of CaCO3 in water. One may not forget about increased toxicity of metals (e.g. aluminium, chrome), ammonia and other factors in low pH.
Table 1. Average monthly growth of rainbow trout (fry to 2-year-old fish), depending on average monthly water temperature
Source: Goryczko, Grudniewska, 2015.
Fig. 1. Water carbonate buffer
Source: Noga, 2010.
Temperature [°C] 1 2 4 6 8 10 12 14 16 18
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Gas level imbalances
Oxygen balanceAnimal oxygen intake is variable and hard to pre-dict. As far as we know, the most important factors in-creasing oxygen demand are: young age, metabolic rate (much higher in warmer water), good nutritional status, activity and excitability.
Rainbow trout is capable of using 40% of oxygen dissolved in water. Level of oxygen adaptation, as well as oxygen solubility, depend on water temperature. 60% saturation was established as a full oxygen adaptation level. Amount of avaliable oxygen is described as a sub-traction between saturation level at inflow and adapta-tion threshold value (below this level fish are unable to uptake oxygen). According to Wieniawski (1969), it is 5 mg O2/l for rainbow trout.
Hypoxia
We can divide sources of hypoxia in two groups: too low oxygen delivery into water and too high use by fish and macrophytes, which are main oxygen consumers in water. First condition often happens in late winter, when ice cover is a barrier between water and atmosphere, and snow layer ceases underwater photosynthesis. It
can take place in summer as well, because gases are poorly soluble in warmer water. Cloudy weather may decrease aquatic flora metabolism. The second group has a lot to do with water plants. While it is desired to have a certain amount of algae in a pond to increase oxygen level, too much algae can cause wide DO (dil-luted oxygen) level fluctuations. They produce oxygen during the day, but use a lot of it at night, so we can observe very low DO level just before sunrise. This is a time of day when we see the most hypoxia symptoms. Overcrowding in ponds and sudden algae crush also de-crease DO, as it is absorbed by both live and decaying matter. This is one of the main reasons why it is so im-portant to clean water from dead fish, plants and other pollution.
Aerators are rarely used in Polish aquaculture. Mix-ing water with pure oxygen (liquid or from concentra-tors) is becoming more and more popular. Main source of water oxygen are photosynthesis, diffusion and water turbulence. Dissolved oxygen is one of the most impor-tant factors for all aquatic organisms. It is vital to keep balance between stock density and water macrophytes. In most fish, diluted oxygen level below 5 mg/l is high-ly stressful. Salmonids need as much as 8 mg/l of DO to maintain healthy and undisturbed growth (Prost, 1989). 6 mg/l is lethal for most of Salmonids, while it is
quite comfortable for carps (Własow and Guziur, 2008). If oxygen saturation changes slowly, carps can even sur-vive in 1.8 mgDO/l (Prost, 1989).
To diagnose environmental hypoxia, we need to measure oxygen level in situ, as there is no way to col-lect water samples without disturbing gas levels. In most cases, we must rely on clinical observations and histo-ry, however, TDS meters are increasing in popularity. Fish gathering near the inflow might be the first sign of hypoxia. They tend to jump out of water, pipe near the surface, stop feeding, show increased operculum move-ments and finally die with flared opercula and open mouth. Smaller specimens are more likely to survive than bigger ones. Hypoxia must be differentiated from gill parasitosis and nitrite poisoning.
Average yearly precipitation in Poland is 600 mm. It is not too much and leads to draughts during sum-mer and significantly lowers the water level in ponds, lakes and rivers. Combined with high temperatures and light exposure, it causes oxygen deprivation. The most common way to deal with the problem (besides aera-tion and oxygenaaera-tion) is to stop feeding for a few days to prevent additional oxygen loss from decayingfodder (as said before, fish will not eat anyway). Amount of oxy-gen, used by aerobic bacteria to oxidate organic com-pounds is described as Biochemical Oxygen Demand (BOD). This parameter is a widely-used surrogate of the degree of organic pollution in water. BOD5(measured for five days) for rainbow trout is <4 mg/l (Gory-czko and Grudniewska, 2015) and <15 mg/l for carp (Własow and Guziur, 2008).
Fasting slows down fish metabolism, lowersoxygen intake and helps purify the water.In acute cases, pad-dlewheels and air pumps must be used to prevent sud-den fish death. As DO levels vary throughout the day, it should be measured at dusk and two to three hours later. Farmers should monitor water turbidity caused by
Table 2. Oxygen demand according to water temperature (for fish over 25 g)
Source: Goryczko i Grudniewska, 2015.
Temp. [°C] Max. O2 saturation [mg/l] Adaptation treshold [mg/l] Oxygen demand for 1 kg of fish
[mg/min] 1 14.2 8.5 0.3 2 13.8 8.3 0.4 5 12.8 7.6 0.7 10 11.3 6.8 1.4 15 10.2 6.1 2.3 20 9.2 5.5 3.6 25 8.4 5.0 5.5
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macrophytes. Secchi disc or a stick should be visible at least 50 cm below the water surface.
Gas bubble disease
Everyone has heard of divers’ decompression sick-ness. When they emerge too fast, little gas bubbles form emboli in their blood vessels. Similar condition may appear in fish, but is caused by supersaturated water. When partial pressure of oxygen and nitrogen is higher than in atmosphere, those gases decompress. It is com-mon in well lightened, shallow ponds with lush macro-phythe growth, or in spring water sources. Sometimes, water is overly saturated during fish transport.
At first, for example when atmospheric pressure suddenly drops after a storm, microscopic bubbles form in small veins (in fins, around the eyes, in gills). Later, we may observe bigger bubbles between the skin and epidermis or in buccal mucosa. They may grow up to 7 milimeters. If they appear in lateral line, fish cannot detect signals from surrounding water, which prob-ably increases mortality during fish migration. In farm
fish the symptoms are: exophthalmos, corneal opacity (leading to blindness) and frayed fins. Fish staying in supersaturated water during the day may be hyperac-tive, disturbed, and later operculum movements are slowed down. In larvae, gases may accumulate in yolk sac, causing balance problems. Commonly, fish move fast, jump up or away from water and die with their mouths open, which may look like suffocation. In acute cases the disease may take 4 minutes from the first symptoms to death.
Treatment requires eliminating excess gas from the reservoir. Aerating the water allows equilibrating gas levels, but may not be practical in use. Farmers can build wide „steps” at the inflow to enlarge the surface of contact between water and air.
Ammonia
It is one of the parameters that should be measured regularly. We can use simple chemistry kits to check TAN (total ammonia nitrogen) in water. Ammonia is
a product of fish metabolism and increased concentra-tion is observed during intensive feeding, fish growth and high stock density. Some of ammonia comes from bacterial reduction of nitrates and nitrites, dead fish and uneaten food, industrial waste or fertilizers. Due to temperature and pH changes proportions between NH3 (unionized ammonia – UIA) and NH4+
(ammo-nium) vary. Water hardness and salinity also play a vi-tal role in this balance. The non-soluble form, NH3, shows high toxicity to fish. Lethal dose (50% of fish die within 48 hours) is 1,0–1,5 mg/dm-3 of NH
3 for carp and
0,5–0,8 mg/dm-3 for rainbow trout (Własow and
Gu-ziur, 2008).
However, much lower concentration (0,14–0,4 mg/ dm-3) may lead to serious illness (Własow and Guziur,
2008). Ammonia shows a complex effect: haemolytic, protoplasmic and neural. It can cause branchionecrosis (severe necrotic damage of gill tissue), ammonia poi-soning and autointoxication. The last symptom is ob-served, when ammonia is not excreted outside through the gills, but remains inside the body due to high tem-perature and water pH, even if ammonia concentration in water is low. The pH increase may be caused by over-growth ofphytoplankton and confervae.
Chronic exposure to ammonia slows fish growth, lowers immunocompetence against infections and worsens blood oxygen transport and distribution into tissues. Proper feeding and water cleaning can prevent autointoxication. To reduce ammonia buildup, liming and aeration is required. It rises pH and allows NH3 conversion to non-toxic NH4+ It is important to
remem-ber that formaldehyde pollution may give us false posi-tive results in some ammonia measuring tests.
Rarely, the same reasons may lead to nitrite poison-ing. Nitrite oxidizing bacteria (Nitrospira and Nitro-bacter) may not be sufficient enough to deal with high nitrite level after the ammonia was oxidized to nitrites.
Table 3. Percentage of toxic (non-soluble) ammonia in water, due to pH and temperature changes
Total ammonia level was not changed.
Source: Goryczko and Grudniewska, 2015. pH Temperature [°C] 10 12 14 16 18 20 7.0 0.19 0.21 0.25 0.29 0.34 0.40 7.1 0.23 0.27 0.32 0.37 0.42 0.50 7.2 0.29 0.34 0.40 0.46 0.53 0.63 7.3 0.37 0.43 0.51 0.58 0.67 0.79 7.4 0.47 0.54 0.64 0.73 0.84 0.99 7.5 0.59 0.68 0.80 0.92 1.06 1.24 7.6 0.74 0.85 1.00 1.16 1.33 1.56 7.7 0.92 1.07 1.26 1.45 1.67 1.96 7.8 1.16 1.35 1.58 1.82 2.09 2.45 7.9 1.46 1.69 1.98 2.29 2.62 3.06 8.0 1.83 2.12 2.48 2.86 3.28 3.83 8.1 2.29 2.65 3.11 3.58 4.09 4.77 8.2 2.86 3.32 3.88 4.46 5.10 5.94 8.3 3.58 4.14 4.84 5.55 6.33 7.36 8.4 4.46 5.15 6.01 6.89 7.84 9.09 8.5 5.55 6.40 7.45 8.52 9.68 11.20
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Lower temperatures may lead to nitrite accumulation as well, and it usually occurs in autumn. After the nitrite passes through gills into blood, it oxidizes hemoglobin to methemoglobin (MetHb). The reaction is similar to NO toxication in humans (suffocation during fires, for example). Gills may turn chocolate brown, but MetHb level must be as high as 40% for that symptom. The farmer may observe lethargy, hypoxia, dyspnea. Any additional stress can be lethal.
Kitchen salt (NaCl) is an effective remedy. Cl- ions
compete with nitrite in gill cell transmitters. After measuring the nitrite level, we should achieve at least 3 mg chloride to 1 mg nitrite ratio by adding salt to wa-ter. This level is usually safe for freshwater fish (up to 50 mg NaCl per liter). It is also a good idea to supplement ascorbates in fish food (it prevents MetHb forming).
Problem of the straggling water florais very com-mon in shallow, still water reservoirs, especially close to fertilized fields.
Iron toxicity
This macroelement is present in water as kations:Fe2+
(commonly as a soluble agent) and Fe3+ (mostly
non-soluble). In acid and poorly oxygenated water, iron is mostly diluted. There is no specific data on iron toxicity, as it highly depends on other factors and water param-eters. In general, maximum level of iron compounds for drinking water is 1.0 mg/l, for salmonids 0.5 mg/l (Goryczko and Grudniewska, 2015) and for carps 1.5–2 mg/l (Wojda, 2006).
In water polluted with ferrum, we observe severe breathing problems in fish. The iron cations form non-soluble deposit on gills, which deteriorates the gas ex-change. Expansion of ferrum – reducing bacteria makes it even worse. Gill tissue darkens and turns brownish.
Fish gather near the fresh water source and die with operculums wide open and mouths agape.
In hatcheries, we must avoid iron level over 0,35 mg/l. Organinc, non- organic suspensions and ferrum compounds easily precipitate on eggs, lowering gas ex-change and leading to increased mortality. For this rea-son, all hatcheries must be provided with efficient water filters. Copper pipes and galvanized components should be avoided, as fish embryos are highly sensitive to cop-per and zinc ions.
What goes around, comes around
Providing our fish well-balanced, clean water is es-sential for successful aquaculture production. However, we must not forget that outflow water affects the envi-ronment too. Low in oxygen, high in ammonia, excre-ments, food remains and decayed matter should be fil-tered thoroughly before discharging into natural water reservoirs. Careless behavior can stimulate algae over-growth, dying of delicate species as crayfish. We should do our best not to destroy beautiful wild ecosystems around the farm.
References
Noga EJ (2010). Fish disease- diagnosis and treatment. Second Edi-tion, Wiley-Blackwell.
Własow T, Guziur J (2008). Higiena ryb i środowiska hodowlanego
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Prost M (1989). Choroby ryb. Warszawa: Państwowe Wydawnictwo Rolnicze i Leśne.
Kolman R (2010). Jesiotry. Chów i hodowla. Olsztyn: Wydawnictwo IRŚ.
Wojda R (2006). Karp. Chów i hodowla. Poradnik hodowcy. Olsztyn: Wydawnictwo IRŚ.
Guziur J et al. (2003). Rybactwo stawowe. Warszawa: Oficyna Wy-dawnicza Hoża.
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tęczowe-go . Olsztyn: Wydawnictwo IRŚ.
Wolny P (1976). Karp. Państwowe Wydawnictwo Rolnicze i Leśne. Kilarski W (2012). Anatomia ryb. Poznań: Powszechne
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