Blue-green algae pose toxic risks in shrimp and fish farming


The presence of large numbers of blue-green algae is a sign that aquaculture systems are not being properly managed.

The widespread pollution of the oceans is a serious problem, and there is every indication that dead zones resulting directly from excess nutrients promoting the growth of algae which in turn use up all the oxygen present in an area are becoming more frequent. . By 2011, more than 500 such areas had been identified.

It is highly likely that many other areas are in decline due to pollution. The constant addition of nutrients from billions of tons of untreated human sewage, agricultural runoff and industrial waste is slowly strangling the oceans’ ability to cope. The intensification of aquaculture systems in Southeast Asia could also contribute.

Blue-green algae

Blue-green algae (cyanophytes or cyanobacteria) sit on the phylogenetic barrier between bacteria and algae, with an ongoing tussle among taxonomists as to which group they belong to. They are precisely characterized as photosynthetic bacteria.

Blue-green algae are widely considered indicators of pollution, and their presence in large numbers in aquaculture production systems is a sign that the systems are not being properly managed. When influent comes from areas that are in the preliminary dead zone stage, heavy nutrient loads and unwanted algal species can be pumped into production areas.

Toxins

Perhaps the biggest threat from these algae is the toxins they produce. Many toxins have been characterized, including neurotoxins, hepatotoxins, cytotoxins and dermatotoxins. Some are widely documented, such as micromicrocystins and nodularin which can damage the liver, geosmin responsible for bad flavor and beta-methylamino-L-alanine (BMAA), a powerful toxin produced by many species which has been associated several serious illnesses. brain diseases, including amyotrophic lateral sclerosis (ALS) and Alzheimer’s disease.

While many strains are benign, many strains are not. They pose a serious threat to animal health and potentially to consumers if present in livestock products. There are no regulations requiring testing for some of the most potent toxins. It is highly likely that they are present to some extent in some animals.

A number of algal species have been implicated in fish kills and, to a lesser extent, in shrimp kills. Knowing that they are ubiquitous and that shrimp are commonly cultured in waters containing blue-greens, it is likely that their presence impacts productivity. Animals that ingest these toxins may not die, but may be weakened, increasing their susceptibility to pathogens. The presence of algae can also impact growth rates and feed conversion rates.

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Proactive pond management can limit algae problems.

Impacts on shrimp farming

Recent reports from Southeast Asia suggest that a number of potential toxins at work in many different countries could kill shrimp within the first few days after stocking. The pathology is consistent with that of a hepatotoxin, and several studies have implicated a role of benthic materials in this process.

It is common practice to kill incoming algae, zooplankton and other planktonic organisms using chlorine. This is mainly done to reduce the risk of introducing viral vectors into the ponds. Many blue-greens release toxins when they die and settle to the bottom as they die. Postlarval shrimp feeding on the benthos are exposed to these toxins. A number of toxins are biodegraded by the action of natural bacteria in ponds and generally no longer cause biological impacts. Some, however, persist and have a direct and indirect impact on animals.

It is essential for short-term production and long-term culture success that the presence of these algae and their environmental burdens are minimized. Although low mortality and progressive attrition in the culture environment are likely the result of the presence of these toxins, they pale in comparison to the potential risks to consumers of cultured products containing toxic materials.

Management

There are a number of potential solutions to algae problems, and it is always in the interest of farmers to minimize the presence of blue-green algae in their production systems. However, killing them outright can be problematic. It makes much more sense to proactively manage their presence by managing production systems to ensure proper nutrient balance.

Disposal of waste from intensive cultivation systems is bad practice. Best practice dictates that this waste be collected and digested using the same technologies that are currently used to degrade wastewater.

Algae risks are real and must be managed proactively. Tools based on enzyme immunoassays make it possible to test suspect animal tissues for the presence of potentially harmful toxins. Many of these toxins are readily metabolized, although in some cases their presence, even at very low levels, can stress animals and lead to problems with production and potential consumption.

September 2012

Aquaculture is an increasingly important source of safe, nutritious and sustainable seafood for people around the world. Globally, aquaculture production must double by 2030 to keep pace with demand. These increases in demand for aquaculture products, food security considerations and job creation have generated an increased need for skilled workers.

Find out how you can be part of this growing industry.

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