by: Prof. Dr. Safwat Kamal
Egypt

(A) Heavy Metals
Mercury – Cadmium – Lead – Copper – Zinc
Heavy metals are among the most significant pollutants leading to fish contamination, posing a high risk to both fish and human health. This is because they affect vital processes and reproduction in fish.

• Most of these elements have the property of accumulating in tissues.
• They cause various poisoning cases in fish and humans.
• It has been observed that when these elements exceed the globally permissible limits set by the World Health Organization (WHO), they impact vital processes in fish and weaken their immune systems, increasing their susceptibility to diseases. Fish are first attacked by parasites and then by other microbes such as bacteria and fungi.

Recent studies have also confirmed a strong relationship between parasitic infections and concentrations of heavy metals. For instance, fish with high larval stages of trematodes (metacercariae) contain significantly higher concentrations of heavy metals compared to fish with lower or no larval stages. Another example is the estimation of heavy metals like zinc and cadmium in fish infected with worms. These examples confirm that parasites have a high capacity to absorb heavy metals, leading to their accumulation in the tissues and flesh of these fish. Below is a clarification of these heavy metals and their health significance.

1. Mercury
   The primary source of this element found in water comes from industrial waste and agricultural runoff, where its inorganic compounds are used as pesticides, and organic compounds are used as fungicides. The toxicity of this element is due to its accumulation from the water's surface to the bottom, often through a process called methylation—where organic and inorganic mercury compounds combine in sediments and transform into a more toxic compound, methylmercury, aided by certain types of bacteria. Notably, the concentration of mercury in fish flesh has been estimated to exceed that in their diet by about ten times, leading to damage in vital tissues and internal organs, as well as adverse effects on reproduction and fry development. In humans, this element causes Minamata Disease due to the consumption of fish contaminated with methylmercury, manifesting in neurological symptoms and motor function impairment.

2. Cadmium
   The primary source of cadmium is plastic manufacturing waste and battery production. It causes poisoning even at very low concentrations over prolonged exposure. Toxic symptoms manifest as failure to hatch eggs and inability of larvae to grow and mature. It can also cause anemia, changes in red blood cells, and damage to internal organs and the central nervous system. In humans, cadmium exposure from contaminated fish can lead to Itai-itai disease, often observed in workers and fishermen, characterized by anemia and kidney failure.

3. Lead
   Air is the primary source of lead pollution, mainly from vehicle exhaust. The toxicity of lead in fish is influenced by its presence in water; for instance, its solubility can be reduced by increasing the alkalinity or calcium and magnesium concentrations in the water. Lead poisoning causes damage to gill tissues, leading to suffocation and death in fish. In chronic cases, it results in changes in blood composition, as well as damage to internal organs and the central nervous system. Lead poisoning in humans can lead to various illnesses, including anemia, especially in children and pregnant women, along with neurological impairments.

4. Copper
   This element is highly toxic to fish when dissolved in water, with its toxicity increasing in the presence of other metals such as zinc and cadmium. Its primary source in water comes from its use in fish farming to treat various fish diseases. Clinical symptoms in fish include severe respiratory distress, where fish often jump to the surface to take in air, and excessive mucus covering their gills and external surfaces. In humans, copper toxicity results from consuming large amounts of copper, leading to damage to blood vessels, liver, and kidney tissues, along with severe inflammation of the central nervous system.

5. Zinc
   Zinc is also toxic to fish when dissolved in water, with symptoms of poisoning resembling those of copper poisoning. The toxicity of zinc can increase significantly in the presence of cadmium and copper. Reducing zinc toxicity can be achieved by increasing calcium concentrations in the water and avoiding the use of zinc-coated pipes or tanks in fish farms. Symptoms of zinc poisoning include vomiting, dehydration, mineral imbalance, severe abdominal pain, and muscle weakness. In some cases, zinc poisoning can lead to acute kidney failure.

(B) Substances in Water That Are Toxic to Fish

1. Oxygen-demanding wastes: Household waste, animal food remnants, dairy waste and products.

2. Toxic elements: Aluminum, antimony, arsenic, mercury, zinc, copper, iron.

3. Ions: Naturally occurring ions in water include calcium, magnesium, sodium, potassium, carbonate, and sulfate ions.

4. Toxic gases: Ammonia, chlorine, carbon dioxide, ozone, hydrogen sulfide.

5. Organic substances: Alcohols, volatile substances, organic acids, cyanides, oils, phenolic compounds.

6. Pesticides: Insecticides.

7. Therapeutic compounds: Malachite green, nitrofurans, formaldehyde, antibiotics.

8. Poisoning from oxygen-demanding wastes
   Symptoms of oxygen deficiency are often evident in fish farms, such as repeated jumping of fish to the water's surface, lethargy, and reduced activity. This can be determined by measuring:

• A. Biochemical oxygen demand.
• B. Chemical oxygen demand.

2. Poisoning from toxic elements
   The presence of these toxic elements can cause mortality. They often accumulate in the liver, kidneys, bones, and gills, making early detection difficult. Fish growth and reproduction rates decline. Effective tests in this case include blood analysis and water component analysis along with pH measurement.

3. Poisoning from naturally occurring ions in water
   Symptoms of this type of poisoning are hard to notice without geographical knowledge of the area and the water source. High concentrations of these ions may affect kidney functions, leading to fish body edema, weight loss, and physiological dysfunctions, ultimately resulting in fish mortality. Required tests include blood and enzyme profiles, in addition to water analysis.

4. Poisoning from toxic gases
   Ammonia arises from industries, cleaning materials, and the decomposition of certain bacteria, dissolving in water. Fish poisoned by chlorine exhibit loss of balance and distress, often followed by death. Hydrogen sulfide gas is produced by bacterial reactions in lake and swamp bottoms, affecting egg hatching and killing young fry, thus reducing production and reproduction rates. To measure sulfur levels in water, samples should be taken with minimal aeration and movement, and analyzed immediately, as atmospheric oxygen can react with sulfur, reducing its concentration.

5. Poisoning from organic toxins
   Oils, petroleum, and volatile substances can accumulate in fish bodies, causing numerous pathological changes such as corneal ulcers, increased macrophage cells in the kidneys and spleen, and hemorrhagic spots in the liver. Phenolic compounds can cause damage to gill, liver, and kidney tissues, leading to edema, anemia, and mortality. Common toxic substances for fish include household and industrial cleaners found in wastewater.

6. Poisoning from pesticides
   Symptoms vary based on the type of pesticide and fish species, with possible signs of spasms leading to paralysis. Exposure can cause congenital deformities and reduced growth rates. Highly toxic pesticides for fish include rotenone, which causes suffocation, loss of balance, and high mortality rates.

7. Poisoning from therapeutic compounds
   Experiments have shown that many antibiotics, such as kanamycin and erythromycin, can have toxic effects on fish. Prolonged use may cause hemorrhagic spots in the liver and kidneys, along with congenital deformities in fry. Additionally, sulfonamide compounds accumulating in the kidneys can disrupt their functions. Malachite green can cause inflammation of the lining membranes of the digestive tract and gills, reducing calcium and cholesterol levels in the blood. Copper sulfate is often highly toxic when treatment duration or dosage increases, leading to decreased red blood cells and protein levels, and damaging liver and muscle cells, resulting in reduced cholesterol levels. Notably, all these changes do not revert to normal even after stopping copper sulfate treatment.