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Farmed fish are fed pelleted feed to provide a balanced diet for optimum growth rates. Feeds contain nutrients such as nitrogen (N) and phosphorus (P) as well as trace elements. Approximately 70% of the P and 15% of the N fed to fish may be lost through faeces. Since fish typically utilise only 30% of the ingested N and P, the remainder is voided. Most of the voided N is dissolved, whereas for P, the majority is associated with the solid material.
Fish sludge can be defined as the sediment that deposits at the bottom of fish tanks and a variable quantity of water, depending on the system used to separate the solid and liquid fractions. It comprises, uneaten fish pellets, faecal material, soluble metabolite products and also any soil particles that enter the tanks/races, e.g. in re-circulating river/lake fed systems and sediment that is dredged or pumped from the lake/sea floor with the faeces and feed. Sludge has to be removed from the tanks and races by mechanical filters and pumps in order to maintain high quality water to maintain high fish vitality and minimise risks of disease within the stock. It is also removed to reduce the risk of enriching the nutrient pool of receiving watercourses.
Under certain environmental conditions, P and N can be released from nutrient rich sludges and may stimulate algal growth. Fish farm effluents containing P and N have been reported to have caused eutrophication of receiving waters. Sludge must be also removed from below lake and sea cages to prevent the production of anoxic gases from poisoning the fish and to reduce the growth of micro organisms that can remove oxygen from the water (dissolved oxygen also is low under fish cages).
Fish sludge contains nutrients and organic matter, which can be returned to the land to fertilise crops and provide much needed organic material to certain soils. This represents a low cost 'disposal' option. Their use would result in a reduced requirement for 'bagged' fertiliser and could therefore save farmers money.
However, sludges could also contain harmful substances, such as heavy metals and pathogens which would limit their suitability for fertilising crops. Also, those sludges derived from salt water fish farms could also contain significant quantities of sodium (Na) which may impact on soil structure. In order to optimise the use of fish sludges on land and minimise their environmental impact, it is essential to know the nutrient content and availability as well as the content of heavy metals, Na and viable pathogens.
Prepared by Dr. Dave Chadwick1 and Dr. Francisco Salazar2
1Manures and Farm Resources Team, IGER, North Wyke Research Station, Okehampton, Devon EX20 2SB. UK. Email:david chadwick
2National Institute for Agricultural Research (INIA), Remehue Research Centre, Osorno, Chile.
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