Tilapia Aren’t to Blame For High Nitrogen Level At Kaua`i Shrimp Pond
I wish to respond to something reported in Teresa Dawson’s article in the June 2002 issue of Environment Hawai`i, “CEATECH Violates Discharge Permit, Blames Tilapia for High Nitrogen Levels.” I would like to offer a different possible explanation for the high levels of nitrogen than that offered by the Department of Health.
She reports that Mark Tomomitsu of the DOH’s Clean Water Branch agrees with CEATECH that tilapia are responsible for nitrogen levels in water discharged from the company’s shrimp ponds into Kawaiele channel that exceed permitted levels. Alhtough tilapia do excrete ammonia, I feel that the problem is much more complicated than that.
Aquaculture production methods initially facilitate aerobic, healthy conditions. These methods include fertilization, water exchange, aeration, feeding, liming, and periodic pond drying. Fertilization and animal waste increase the growth of algae that utilize nitrogen. Photosynthesis by algae in ponds is the main source of good dissolved oxygen levels. Water exchange improves water quality. Aeration maintains adequate dissolved oxygen levels, especially at night, when algae respire and utilize more oxygen. Liming helps buffer pH and facilitates aerobic conditions through the biogenesis of calcium. Periodic pond drying prevents anaerobic conditions to develop on pond bottoms. Such best-management practices allow for aquaculture of animals at much denser levels without the development of high dissolved nitrogen levels in the ponds and effluent water. The drawback is that these practices dramatically increase bound nitrogen levels (mostly protein) in the large biomass of algae and animals being aquacultured.
If the phytoplankton blooms are not monitored and controlled through water exchanges, algae blooms can become too dense and can inhibit light penetration. This increases daytime algae respiration and carbon dioxide levels, while depleting nutrients, especially phosphorous. Anaerobic conditions will develop on pond bottoms, along with reduced pH levels, and eutrophic wastewater. Inevitably, algae blooms crash.
Algae make up the largest biomass in ponds, amounting often to tons in a given pond. When algae crash, it creates low dissolved oxygen levels and reduced pH, the conditions in which anaerobic bacteria thrive. The resulting death of shrimp and fish leads to even heavier bacterial blooms, resulting in even lower dissolved oxygen levels and higher ammonia levels. Anaerobic conditions kill denitrifying bacteria. Ammonia and nitrite displace oxygen in the bodies of fish and shrimp. Mass mortality results. Animals that survive may be vulnerable to parasites and disease that infect not only the aquacultured animals but also those in the effluent system. Rotenone, used by CEATECH at one point to kill tilapia, does not lower ammonia and nitrite levels since dead fish only compound the imbalance in the system and create even higher ammonia levels!
Anaerobic conditions resulting from the rotting biomass of fish, shrimp, and algae destroys the aerobic cycle of nitrogen and calcium. Anaerobic bacteria produce hydrogen sulfide (H2SO4), which damages aquatic organisms in the surrounding environment. Dead fish and the spread of diseases into the wild can occur on a large scale. Trouble-shooting before problems evolve is necessary to avoid compounded disasters. Guessing by authorities only makes matters worse.
Tank culture of shrimp and fish is a completely different situation from earthen ponds. Unlike the ponds, in tanks, the cultivated animals represent the largest biomass. High dissolved nitrogen levels are common in effluent water from tanks. Still, tanks in full sunlight can develop algae blooms and must be monitored closely for this.
Ponds are analogous to pastures for cattle, where plant life biomass is much greater than animal biomass. Tanks are analogous to feedlots, where animal biomass is fully dominant and all nutrients are obtained from the feed.
By the way, the tilapia at CEATECH is the saltwater tilapia Seratherodon melanotheron, which dominates the Kekaha area and has displaced mossambicus in coastal areas. It was introduced to Kaua`i in 1980 and has spread beyond Hanalei, where it eats taro roots in the Hanalei Valley. Seratherodon melanotheron was introduced to O`ahu by the predecessor agency to the National Marine Fisheries Service in 1962. It was introduced to the Big Island in 1990.
Wayne Okamura
Okamura Fish Farms, Inc.
Kapa`au, Hawai`i
Volume 13, Number 1 July 2002
Leave a Reply