Biotic and Abiotic stressors and its diagnostic methods in fishes

  • Vel Selvi R1, S. Dasgupta2, K S Vijay Amirtharaj1 and Bharathi S3
    1. Fisheries College and Research Institute, Thoothukudi - 628008
    2. 1ICAR – Central Institute Of Fisheries Education, Kolkata -700091
    3. Coastal Aquaculture Authority, Chennai - 600035
  • In print edition October,2020

Bio-monitoring is science for assessing the environmental condition which may be surrounding water or air by using biotic organism. Fish and its response are a good indicator of qualitative assessment and accumulation of chemicals in organism tissues as quantitative assessment. Most commonly used method is the Haematological technique to determine the sub-lethal effects of pollutants and it is a quick screening method as sudden changes in blood parameters due to environmental or physiological stressors. Other biochemical parameters also used to assess the stress condition in fish such as superoxide dismutase, catalase are the indicator for evaluating the environment. Diagnosis of disease in fish by using blood parameters like RBC (Red blood cell), Hb (Haemoglobin), PCV (Packed Cell Volume), TLC (Total Leucocyte Count), MCH (Mean Corpuscular haemoglobin). Freshwater fish on exposure to pollutant shows significant changes in haematology. ROS (Reactive Oxygen Species) such as hydrogen peroxide (H2O2), superoxide anion (O2.) and hydroxyl radical (OH.) are produced during stress condition. According to Seyle (1950), stress is a sum of all physiological responses by which animal tries to maintain or re-establish a normal metabolism in the face of physical or chemical force.

Fish stressors

Stressors are classified as acute or chronic depending on their duration and frequency. Acute stressors exist in a short period such as handling stress. Chronic fish stressors exist as the constant that causes a prolonged physiological response.

Physical stressor
- handling, capture, confinement and transport
Chemical stressor
- contaminants, low oxygen and acidification
Perceived stressor - Predators
Abiotic stressor:

Water quality parameter plays an important role for maintaining healthy animals and reduces the stress. Temperature, oxygen saturation, nitrogen compounds, carbon dioxide, pH and salinity are the main water quality parameters. Changes in one of these factors may cause fluctuation of the other parameters.

1. Temperature

Fishes are coldblooded animal. Johansen et al. (2006) reported that increased temperature may also increase metabolism, stress levels, physical activity, appetite, growth and other parameters that increase oxygen demand. In the brain, high ammonia levels cause high levels of extracellular glutamate by increasing glutamate release or/and decreasing glutamate synaptic reuptake. In the brain, high ammonia levels cause high levels of living thing salt by increasing salt to unleash or/and decreasing salt conjugation (Rao et al., 1992; Bosman et al., 1992; Schimdt et al., 1993). Otto (1974), Heath et al. (1993) and Bennett and Beitinger (1997) all found that fluctuating temperature regimes increase high temperature tolerance to fish. Hokanson et al. (1977) found that fluctuation of temperature within the preferred temperature range shows greater growth than constant temperatures with the same mean.

2. Nitrogenous compounds

Urea, faeces and feed are converted into ammonia and then into nitrite and nitrate in water. Ammonia toxicity expressed as total ammonia ([NH3] + [NH4+], mg N/L), clearly increases with water pH (Randall et al., 2002). Swimming fish have elevated internal ammonia levels when compared with resting fish. Mommsen and Hochachka (1988) reported that the ammonia level increased in the white muscle in rainbow trout due to a breakdown of adenylates to inosine monophosphate (IMP) and NH4+. Fed fish have plasma ammonia levels similar to those associated with death due to environmental ammonia exposure (Wicks and Randall, 2002a) and yet survive to feed. Wilson et al. (1998) also presumed that the rainbow trout, Oncorhynchus mykiss, decreased ammonia production when exposed to pH 10.

3. Carbon dioxide and pH

Fish has an average blood pH of 7.4. If the pH drops below 5 or rises above 10 would cause stress to fish (Wurts et al., 1992). Slow accumulation and dissolved oxygen above 5 mg/l may cause tolerate capacity of catfish to 20 to 30 mg/L CO2.

Abiotic factor causing stress
Table 1: Common symptoms and its remedies for abiotic stress in fish
S.No Factors Symptoms Remedies
1 Temperature Increase metabolism, stress levels, physical activity, appetite Locate tank away from direct sunlight
2 Oxygen Small bubbles under the skin, in fins and around the head & eyes Regular aeration, Locate tank away from direct sunlight, partial water exchange
3 Ammonia, Nitrite, Nitrate Inflammed gills and fin edges; black spots; loss of balance Make partial water exchange and apply aeration; use Zeolite activated carbon
4 pH Acidosis:fast swimming movement; gasping at the surface; loss of colour & appetite

Alkalosis: serious damage to gills; disintegration of fin edges; general opaqueness of skin
Avoid overstocking and understocking with plants, partial water exchange; add lime@5-10ppm.

Locate heavily planted tanks; away from prolonged sunlight; carryout immediate partial water exchange
4. Salinity

Disturbance of water balance in the fish (osmoregulation) is caused by changes in minerals metabolism. In this condition, freshwater fish can absorb water from the environment (hyperosmotic); saltwater fish lose water to the environment (hypo osmotic), this disruption increases energy requirements for osmoregulation.

5. Light

Fish physiology and its behaviour are affected by photoperiod. Manipulation of light is used in fish farming to manipulate maturation, increase appetite and stimulate weight gain. Blue light prevents an increase of stress-induced cortisol in the Nile tilapia. The response of the fish held in the blue environment has shown inhibition of cortisol (Volpato et al., 2001). Fish (tench) under black treatment exhibited a significantly lower blood cortisol concentration than those held under ‘white’ light (Owen et al., 2010).

Biotic factors
Infectious diseases

Fish occur in surface waters are prone to a wide variety of diseases. Fish are subject to infection by viruses, bacteria, and fungi and cause disease. Fish are also parasitized by tapeworms, trematodes (grubs), nematodes (roundworms), leeches, and licen. Disease occurs when fish are in poor condition, injured, starved, crowded, or rough handling, low oxygen level, high temperatures. Some early warning symptoms shows that fish under stress.


The skin used as indicators of health. Skin ulceration in fish due to acute stress. Reddening of the skin, fins, operculum, vent and caudal area of the tail. Dark colouration is the indicator of osmotic imbalance problems and build-up of mucous indicate that surface irritation.The mucus is the first line of defence against several infections, so detection of changes in the mucus may indicate the presence of pathogens.


The eyes should be observed for indications of disease. For example, eye enlargement and distension (“Popeye”).


Gills are very sensitive to any waterborne irritants and inflammation. Paleness and erosion of the gills are most readily observable change. Presence of red spots may be an indicator of haemorrhagic problems, which reduces the function of gills. Fouling, mucous build-up or parasites (ciliate protistans, monogeneans, copepods, fungi, etc.) may also reduce functional surface area and may be indicative of other health problems.


Any variation from the normal body shape of fish is a sign of a disease. Young fish are affected by “pinhead” as an indicator of developmental problems. Skeletal deformities such as lordosis and scoliosis are caused by improper nutrition. Dropsy is a swollen abdomen (“pot belly”)

Surface parasite

Surface parasites like copepods, ciliates or flatworms infection lead to stress. These parasites are attached to the surface or encysted larval stages seen in the fins, or skin. Lymphocystis or other environmental problems cause abnormal growths (tumorous diseases).

Non infectious disease
Nutrient deficiency Disease
Tryptophan, Magnesium, Phosphorus,Vitamin C, Essential fatty acids Scoliosis/Lordosis
Methionine, Tryptophan, Zinc, Magnesium, Copper Selenium, Manganese, Vitamin A, Riboflavin Cataract
Lysine, Tryptophan, Zinc, Riboflavin, Inositol, Niacin, Vitamin C Fin erosion
Choline, Essential fatty acids Fatty liver
Pantothenic acid, Niacin, Folic acid, Vitamin A, Vitamin E, Oxidized fish oil Exophthalmia
Pantothenic acid, Niacin, Thiamine, Inositol, Vitamin C Vitamin A, Vitamin K Fin/Skin haemorrhage
Diagnostic method:
Heart rate and respiration

Stress condition may lead to increases heart and respiration rates and it may be monitored by looking at operculum ventilation rates.

Blood samples

Stress has a major effect on blood parameters. Cortisol, glycemia, haemoglobin and hematocrit are the plasma indicators of stress (Hatting, 1976; Pickering, 1981; Kumschnabel & Lackner, 1993). The low blood pressure is the main reason for the wide variation in blood parameter. Anaesthesia has a major effect on the heart and maintains adequate blood pressure during blood sampling. Diversion of blood flows from the intestinal organs to the skeletal muscle to support the ‘fight or flight’ mechanisms during stress. If the stress level is high, very few blood cells with large amounts of plasma may present for the balance of ions and water. Blood glucose is a major indicator of stress.

Enzyme analysis

Stress enzymes are the indicator of fish under stress such as SOD, Catalase, GOT, GPT. When acclimating to an increased level of oxidative stress, SOD concentration typically increased with the degree of stress condition. Catalase is an indicator of environmental stress.

Molecular biological methods

Stress proteins in fish used as biomarkers of cellular. Increased expression of metallothionein in the liver, gills and the intestine due to thermal stress.


Stress compromises the fish’s natural defences against invading pathogen. When disease outbreaks occur, the underlying stress factors, as well as disease organism, should be identified. Disease treatment is an artificial way of slowing down an infection so that the fish’s immune system has time to respond. Any stress which adversely affects the fish cause death.


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