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The Aquatic Environment

VI. The Aquatic Environment (including water quality)

Knowledge of this provides a basis for the interpretation of disease associated with inhospitable environments. The subject of water quality was reviewed extensively by Ferguson and is extensively referred to in this section on water quality.
A. Temperature
Each species has its own preferred temperature range. Being at temperatures beyond this range represent a stress, with rapid changes being more stressful. The more rapid the temperature change, the more stressful to the fish. In addition to the metabolic alterations, the sequelae of temperature stress include opportunistic infections, secondary to stress.
B. Oxygen
The concentration of 02 in water is low as compared to air, (At 15°C, one liter of freshwater contains 7cc
02 compared to 21 volume % in air). The gills have evolved to be very effective in extracting 02 from the water to the blood. In warm or polluted waters, wherein 02 levels are low, the "cost" of extraction of 02 may be too high for normal maintenance of normal life processes. (In such a circumstance, the energy required for ventilation may exceed that released by the oxygen to the extent that the fish develops the respiratory distress syndrome.

Different species have different levels of oxygen requirement e.g. Salmonids having a high requirement while catfish have a lower requirement.
When a fish is removed from water the gill filaments collapse on contact with air thus greatly reducing the surface area and resulting in hypoxia.
Clinical signs of low oxygen: Fish gather at inlets or gasp at the surface and decrease their activity. If the decline in oxygen in the water is slow, fish can become acclimated to some extent by such means as elevating their hematocrit.

C. Supersaturation of water with gas (nitrogen, oxygen)

If air is forced under pressure into water such as at the bottom of waterfalls or in an aquarium with excessive aerator-pump activity, the water can become supersaturated with atmospheric gases. Supersaturation can also occur if water temperature rises, (since the oxygen-holding capacity declines as temperature rises, (since the oxygen-holding capacity declines as temperature rises). If this change occurs rapidly, gas dissolved in the blood (from water which has entered the body of a fish via the gills) comes out of solution forming gas emboli in blood vessels.

D. Suspended Solids

These can be of various types. Non-inert clays may adsorb substances (such as heavy metals, pesticides, NH3). When these laden particles come in contact and adhere to the gill surface, they can act as carriers of these absorbed materials to the gills thereby enhancing toxicity.

E. Ammonia

Ammonia is produced as a waste product of nitrogen metabolism and in fish is excreted via the gills. If present in water at low levels, it is of no significance. However, in cases such as overcrowding, or wherein materials such as organics are present in the water, fish (especially the young) develop toxicity.

Free ammonia (NH3) is highly toxic; whereas bound ammonia is much less so. In acidic water, most ammonia is in the bound form while in alkaline water free ammonia may be more of a problem.

Lesion — high ammonia levels cause epithelial hyperplasia of the gills thereby interfering with oxygen diffusion from the water to the gills, as well as elimination of wastes.

F Nitrite

Nitrite levels may be high from agricultural run-off. Sewage is another important source of nitrogen. Nitrite is actively absorbed through the gills (and blood levels may be 10 x that of surrounding water. Nitrite oxidizes the iron in hemoglobin forming methemoglobin and this pigment lacks the ability to bind reversibly to oxygen.

Lesion — High levels of methemoglobin result in a brown color of the blood (so-called "chocolate blood disease" of channel catfish farms).

G. Nitrate

Nitrate is formed by the complete oxidation of ammonia. It is naturally present, sometimes in high concentration in surface waters of fish farms. It is less toxic than nitrites. It may promote algae bloom which can have serious sequelae to fish.

H. Carbon dioxide

CO2 is excreted by the gills. In water, it forms carbonic acid, which reduced the pH. Thus increasing levels of CO2 in the blood reduces the affinity of hemoglobin for oxygen (Bohr effect).

Lesions — High environmental levels of CO2 predispose to nephrocalcinosis. The lesions lead to precipitation of mineral within tubular renal interstitium and tubules. This initiates a severe granulomatosis responses. In addition to kidneys this mineralization can occur in the lamina propria of the stomach, as well as in musculature dorsal to the kidneys.

I. Chlorine

This is sometimes added to water to destroy pathogens or may be from industrial sources.

Raising the pH leads to dissociation of hypochlorous acid yielding oxygen (thus a strong oxidizing agent) which leads to gill necrosis.

Agents to remove chlorine include activated charcoal and natural clays (zeolites). Merely aerating the water vigorously will remove much of the chlorine.

J. "Hardness"

The buffering ability of water is due to carbonates, bicarbonates and hydroxides. Freshwater in limestone areas is well buffered. Water with good buffering capacity generally have a stable pH and thus able to resist some of the agricultural and domestic pollution.

K. Salinity

Salinity measures the total salts dissolved in water. Some species can adapt naturally from fresh water to seawater e.g.salmon. Most are not tolerant of such changes although a few species e.g. rainbow trout can largely adapt, but in some situations (e.g. when respiratory demand is great) may struggle to maintain their oxygen balance.

L. pH

In fish in aquaria, the pH can be maintained in the optimal range for the species. In nature, however, such environmental events as heavy rain flushing out peat bogs (humic acids) or such occurrences as acid rain can lead to acute toxicity (from acid pH) with lamellar epithelial edema and necrosis of gills. The effects can be acute mortality, reduced growth, or skeletal deformities, but especially reproductive failure. Some species of fish have disappeared from such ponds and rivers.

M. Light intensity

Associated problems range from hatchability percent to sunburn in some species.

N. Heavy Metals

Usually trace amounts, but high in region of deposits.

Cadmium and copper are extremely toxic in soft water (hard water tends to precipitate them out). Damage to gills is prominent, with lamellae fusion and edema. Zinc has similar effect, but additionally can cause scoliosis.

Mercury causes branchial and renal changes due to increased permeability of cell membrane and impaired mitochondrial ATP production.

O. Pesticides and Herbicides (organic toxins)

Pesticide and herbicide toxicity and should be kept in mind in case of large fish kills.