Английский язык: Сборник текстов и упражнений. Дьякова Н.П. - 75 стр.

mals 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 insuf-
ficient light for photosynthesis, the oxygen level begins to drop because of respi-
ration 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 influenced 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 shellfish 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
between the two depends on temperature, pH and a few other factors. Ammonia is
rapidly converted to nitrate (NО3) 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 to-
lerances for ammonia. Tilapia can tolerate high concentrations of total ammonia
(several ppm), whereas trout are highly susceptible to levels well below 1 ppm.

                                 Word supplement

   poikilothermic                     пойкилотермное животное
                                      (имеющее непостоянную
                                      температуру тела)
   dissolution                        растворение, разжижение
   respire                            дышать
   bloodstream                        кровообращение, кровоток
   unionozed ammonia (NH3)            аммиак
   ionized ammonia (NH4)              ионизированный аммиак
   nitrate                            нитрат



                                 Text 27
                         NUTRITION AND FEEDING

    Under natural conditions in ponds, lakes, rivers and the ocean, fishes rely on
natural productivity for their nourishment. Some aquaculturists also use natural
food organisms to provide nourishment for the culture species. In China, for ex-
ample, ponds are stocked with various species of carp that feed on different parts
of the food chain. The fish ponds may be fertilized to help promote growth of
phytoplankton, rooplankton, rooted aquatic macrophytes and benthic organisms,
each of which is fed upon by a different type of carp. In Japan and a few other
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