Info

Cadmium, /¿g/L

<10

<10

Chromium, /¿g/L

<100

<100

Copper, /¿g/L

<25

<25

Lead, /¿g/L

<100

<100

Mercury, ¡x g/L

<0.1

<0.1

Zinc, /¿g/L

<100

<100

Source: Refs. 2, 8, and 9.

Temperature, °C Salinity, g/kg Dissolved oxygen, mg/L PH

Carbon dioxide, mg/L Hydrogen sulfide, /¿g/L Cadmium, /¿g/L Chromium, /¿g/L Copper, /¿g/L Mercury, /¿g/L Nickel, /¿g/L Lead, /¿g/L Zinc, /¿g/L

1—40 (depends on species) 1-40 (depends on species) >6 <7.9-8.2 <10 100 <10 <1 <3 <25 <3 <0.1 <5 <4 <25

tion when literature information is unavailable. Synergistic effects that occur among water quality variables can have an influence on the tolerance a species has under any given set of circumstances. Ammonia is a good example. Ionized ammonia (NH+) is not particularly lethal to aquatic animals, but unionized ammonia (NH3) can be toxic even when present at a fraction of a part per million (depending on species). The percentage of unionized ammonia in the water at any given total ammonia concentration changes in relation to such factors as temperature and pH. As either temperature or pH increases so does the percentage of unionized ammonia relative to the level of total ammonia. Thus, in warmwater and in seawater (which generally has a higher pH than freshwater), ammonia toxicity will occur at a lower total ammonia concentration than in cool or lower pH water.

Another example is dissolved oxygen (DO). The amount of DO that water can hold at saturation is affected by both temperature and salinity. The warmer and/or saline the water, the lower the saturation DO level. Atmospheric pressure also affects oxygen saturation. The saturation oxygen level decreases as elevation increases.

Biocides should not be present in water used for aquaculture. Sources of herbicides and pesticides include runoff from agricultural land, contamination of the water table, and spray drift from crop-dusting activity. Excessive levels of phosphorus and nitrogen may occur where runoff from fertilized land enters an aquaculture facility either from surface runoff or groundwater contamination. Trace metal levels should be low as indicated in Tables 4 and 5.

Most aquaculture facilities release water constantly or periodically into the environment without passing it through a municipal sewage treatment plant. The effects of aquaculture effluents on natural systems have become a subject of intense scrutiny in recent years and have, in some instances, resulted in opposition to further development of aquaculture facilities in some locales, particularly in public waters. There have even been demands that some existing operations should be shut down.

Regulation of aquaculture varies greatly both between and within nations. Some governmental agencies with jurisdiction over aquaculture have placed severe restrictions on the levels of such nutrients as phosphorus and nitrogen that can be released into receiving waters. Regulations on suspended solids levels in effluent water are also common. The installation of settling ponds or created wetlands, exposure of the water to filter feeding animals that will remove solids, and mechanical filtration have been used to treat effluents. Reduction or removal of dissolved nutrients through tertiary treatment is possible but is generally not economically feasible with current technology. Research is currently under way to develop feeds containing reduced levels of nutrients or to provide nutrients in forms that the culture animals can better utilize. The goal in both approaches is to reduce losses of nutrients to the environment through excretion.

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