Английский язык: Сборник текстов и упражнений для студентов специальностей "Промышленное рыболовство". Дьякова Н.П. - 58 стр.

quirements of the species under culture and of the temperature at any given time will
not only provide the culturist with valuable information about how well the animals
are growing and how much to feed them, it will help to establish the disease resis-
tance status of the animals.
Dissolved Oxygen:
Oxygen enters water by dissolution from the atmosphere and through the release of
that element by plants during photosynthesis. Animals with gills respire by absorbing
oxygen that has been dissolved in water directly into the blood steam through diffu-
sion as a general rule, if the water contains 5 parts per millon (ppm or mgll) of oxy-
gen, it will support aquatic organisms. Some fish, such as tilapia, can survive at very
low concentration of oxygen, while others, such as trout, are stressed if the concentra-
tion falls below 5 ppm.
Daily changes in temperature are very small relative to the changes that can occur
seasonally, particularly in temperature climates. Daily changes in dissolved oxygen,
on the other hand, can be substantial. Dissolved oxygen begins to increase at about
dawn, when photosynthetic production of oxygen by the plant community begins. As
the sun rises, photosynthetic oxygen production increases with the increasing amount
of light energy available. While both plants and animals respire continuously, the rate
of oxygen production exceeds respiration and there is a net increase in the dissolved
oxygen level. At dusk, when there is insufficient light for photosynthesis, the oxygen
level begins to drop because of respiration demands, and the drop continues through
the night. As long as the lowest morning dissolved oxygen level is not below about 5
ppm, there should be no problem. However, the lowest level of dissolved oxygen can
change dramatically from one day to the next. Daily production of oxygen can be in-
fluenced by the weather (cloudy days don’t support as much photosynthetic activity
as clear days) and by the biomass of culture organisms present. As the fish or shell-
fish being raised grow, they extract more oxygen from the pond each day.
Ammonia:
Ammonia occurs in two forms, unionized (NH3) and ionized (NH4). The ratio be-
tween the two depends on temperature, pH and a few other factors. Ammonia is ra-
pidly converted to nitrate (N03) by plants and bacteria in aquatic systems. Thus, in
ponds where there are plenty of plants and bacteria present ammonia toxicity is not
usually a problem. In raceways and other water systems where animals are reared at
high densities, ammonia removal is often not as efficient as in a pond, and toxicity
can occur. Different species of aquaculture interest have different tolerances for am-
monia. Tilapia can tolerate high concentrations of total ammonia (several ppm), whe-
reas trout are highly susceptible to levels well below 1 ppm.

                                  Word supplement:

      poikilothermic-                          пойкилотермное животное (имеющее
                                               непостоянную температуру тела)
      dissolution                              растворение, ражжижение

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