Maintenance of appropriate water quality is essential in aquaculture farm management to ensure optimal growth and survival of aquaculture species. Poor water quality is linked with disease problems contributing to the collapse of several prawn farming industries in some South East Asian countries.
Unmanaged blue-green algal growth in aquaculture ponds cause poor water quality following algal degradation. When algae reach their maximum growth phase, they flourish for a period and then die.
This is known as an algal “crash”. After a crash or periodical collapse of algal populations, the decomposition of these dead algae utilizes a large amount of oxygen and can cause oxygen deficits and increased concentration of toxic ammonia.
Insufficient oxygen and high ammonia concentrations may, in turn: kill aquaculture species; promote disease; and/or temporarily reduce the feeding and growth rates of fish/prawns.
Blue-green algae produce toxins and cause stress to aquaculture species:
Blue-green algae produce toxins which may cause toxic and physical stress to fish/prawns. Exposure to harmful algal neurotoxins can alter swimming and social behavior in fish and alter neuronal activity. Microcystins (a toxin produced by blue-green algae) can accumulate in the tissues of fish (particularly in the viscera - liver, kidney, etc.), and in shellfish.
Blue-green algae produce taste and odour compounds and cause off-flavour problems:
The objectionable odours or tastes detected in aquaculture products are referred as ‘off-flavour’. Off-flavour is one of the most economically significant problems encountered in catfish and prawn aquaculture. Most common “earthy” and “musty” off-flavours are caused primarily to the absorption of odorous compounds (such as geosmin (GSM) and 2-methylisoborneol (2- MIB)) from the water through skin and gills. These compounds accumulate in the fatty tissue of the aquaculture species.
These odorous compounds are metabolites produced by certain species of blue-green algae. Off-flavour problems cause inconsistent product quality that may adversely affect consumer demand and ultimately, hinder industry development and decrease profits for producers and processors by increasing production costs. Up to 80% of harvestable catfish can have an off-flavour during any one year (Rimando & Schrader, 2003).
Figure 4. Geosmin (GSM) and 2-methylisoborneol (2-MIB) produced by certain species of blue-green algae and pass through skin and gills of aquaculture species and accumulate in the fatty tissues of fish or prawn heads (Rimando & Schrader, 2003).
“Off-flavour” is common in catfishes, but also occurs in penaeid shrimp. Off flavour compounds are soluble in lipid and tend to concentrate in shrimp heads. Geosmin also found to be the main compound associated with the seasonal occurrence of earthy-taints in UK farmed trout.
Using algicides to control blue-green algae in aquaculture ponds:
Currently there is no non-toxic, sustainable method to control blue-green algae in aquaculture ponds. However, using chemical algicides (copper sulfate and certain chelated or complexed copper products) may prevent blue-green algae.
But copper algicides are not suited to aquaculture. They are not selectively toxic to odour-producing blue-green algae. Copper also interacts strongly with other water quality variables, and one important consequence of those interactions is that copper products become more toxic to fish and algae as water hardness and alkalinity decrease.
Copper sulfate treatments cause poor water quality. Increased aeration is required in copper treated ponds than in untreated ponds.
Copper can accumulate in the sediments of treated ponds, with a relatively large fraction of the copper in the sediment initially present in a potentially bioavailable form. In addition, due to their potential adverse environmental impacts, there may be local regulations in place restricting the use of algicides.
Eradicating algal blooms may not improve the situation and could potentially make the problem worse in the near term. Treatment of ponds with algicide chemicals can result in beneficial algae also being removed.
Algicides or chlorination may cause the algal cells to burst and this can cause toxins to be released into the water. In addition, when algae decompose, the phosphorus stored in the algal cells is released into the water. The released phosphorus is available to fuel new algae growth.
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