AQUACULTURE

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by Stephen G Newman

Many aquaculturists, whether they arefarming fish or shrimp, typically inponds, understand that high levels of

blue green algae are not desirable. Bluegreen algae are not really algae; it isphotosynthetic bacteria called cyanobacteria.It is among the oldest living group of bacteriawith evidence of its existence dating backmore than 3 billion years. Blue green algaeare diverse group of organisms, occupyingmany different environments and are verysuccessful ecologically. Its preferred nichesare those where the nutrient loads are“unbalanced” or eutrophic, that gives them acompetitive edge over the myriad algaespecies, including green, red, brown algaeand others that typically occupy marine,estuarine and freshwater environments.

Pond based aquaculture systems that relyon relatively low water exchange rates are ata greater risk for problems with these bacteriathan high flow through rate systems whereexcess nutrients can be carried off into effluentsettling ponds or into discharge canals thatenter the same water source that issubsequently used for influents. The termeutrophic refers to systems that have high

Cyanobacteria present in the pond havebeen found to inhibit the growth and varietyof macro and microorganisms andnegatively affect profitability.

levels of nutrients such as phosphorous andnitrogen that allow these photosyntheticbacteria to readily proliferate at the expenseof other more desirable organisms.

Emergence ofcyanobacteria in thearea

As human populations increases therehas been a concomitant increase in thetypes and amounts of waste products thatenter into estuarine and near shoreecosystems. Vast amounts of nitrogen andphosphorous originating from many differentsources enter these environments. Sourcesinclude agricultural run-off from feed lots,from the production of row crops fertilised

with chemicalfertilisers, frominadequatelytreated sewagefrom cities and smaller municipalities, as wellas, to a much lesser extent, run off fromaquaculture operations that do not complywith Best Aquaculture or Best ManagementPractices. These pollutants have resulted inthe development of innumerable dead zonesin these ecosystems. Cyanobacteriaproliferate in these areas resulting indepleted oxygen levels, which subsequentlycause fish, and other marine animal kills.The number of reported dead zonesincreases each year with more than 500now reported annually. This is a seriousproblem that has broad ranging implicationsfor aquaculture, as these are not

Stephen G Newman

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AQUACULTURERisk of toxins from blue green algae to aquaculture

environmentally suitable areas to producefish or shrimp.

Toxins and its risk

Cyanobacteria present in the pond havebeen found to inhibit the growth and variety ofmacro and microorganisms and negativelyaffect profitability. They can ruin the suitabilityof a pond to produce shrimp or fish. Theyhave also been found to produce a veryimpressive array of toxins. There are manyreported cases of problems from theconsumption of water that has beencontaminated with cyanobacterial toxins.There are also many instances where toxinshave been deposited in animals produced byaquaculture and a leading cause of what isknown as off-flavour. This muddy taste candestroy the value of a crop and in extremecases result in total loss of the crop. Savvy

consumers know that seafood that tastes thisway is not fit for human consumption. Themore responsible producers, typically throughthe processing plants, taste fish and shrimpbefore processing and reject product that hasoff flavour.

Cyanobacteria produce literally dozens of,if not hundreds or more, toxins. These toxinsare divided into two broad groups, theodorous metabolites (toxins are usuallymetabolic by products not compoundsproduced to assure access to food such asantibiotics) and bioactive metabolites. Themost common examples of metabolites thatcause foul smelling and foul tasting seafood(odorous) are those that are classicallyassociated with off-flavour, 2-methylisoborneol (MIB) and geosmin. These toxinsare produced by a number of differentspecies and are only released when the cellsdie. Their toxins are quite common. Geosmin

is the odour that often is noted after rainshowers. They can be quite toxic in additionto ruining the value of a crop.

Those metabolites that are bioactiveinclude toxins known as hepatotoxins thatdamage the liver and related organs such asthe hepatopancreas (HP). The most commonof which are the microcystins and thenodularins. Nodularins can contaminatedrinking water and can accumulate inbivalves, fish and even prawns. Microcystinshave been shown to accumulate in fish andprawns as well. In fact levels have beenreported that are high enough to potentiallyhave an adverse effect on humansconsuming them.

Cyanobacteria also produce cytotoxins,neurotoxins, dermatotoxins. The threat posedto humans that eat shrimp and fish obviouslyvaries with the type of toxin and the quantitypresent in the animal at the time it is

Reader enquiry number 7

AQUACULTURE

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Risk of toxins from blue green algae to aquaculture

consumed. Low levels are typically notproblematic and World Health Organisation(WHO) has established acceptable thresholdlevels of these toxins. However, there areserious concerns that some of these toxinsmay pose short and long-term threats toconsumers that eat them. Food poisoning fromconsuming toxins such as nodularin andmicrocystin does occur; the more fish andshrimp are produced in the samecontaminated environment the risk is certain toincrease. Consumption of water containingvarious toxins has resulted in hospitalisationsand even in some cases human deaths.Animal deaths are common.

Also among the toxins produced is Beta-methylamino L alanine or BMAA. This toxin isproduced by virtually every group ofcyanobacteria. It is an amino acid that is notfound in normal protein and is a potent slowacting neurotoxin. The toxin was discoveredas a result of a syndrome that was affecting agroup of Pacific Islander natives that wereconsuming fox bats. These are large fruiteating bats that ate cycad seeds that havebeen contaminated with BMAA producingbacteria. They were found to have a uniqueneurodegenerative disease, coinedamyotrophic lateral sclerosis-parkinsonism/dementia complex that appeared at afrequency that was fifty to one hundred timesgreater than the global incidence of ALS.These neurological problems have beenlinked to the presence of the BMAA in theirdiets. More recently BMAA has been linked toother neurodegenerative diseases including

Alzheimer’s. While it is likely that thesediseases are multifactorial, the presence ofBMAA appears to be a potentially important ifnot critical component. There are many whobelieve that this may be only one example ofa myriad of environmental toxins that we areunwittingly being exposed to that impacthuman health.

Control and prevention

While there have not been any extensivestudies to date on the presence of BMAA toxinin the environment (aside from its likely

presence in every type of cyanobacteria), ithas been isolated from wild fish, shrimp andcrab at what would be characterised asdisturbingly high levels in the US. There havebeen no wide studies of farmed fish or shrimpas of yet although given the wide range ofalgal species that have been found toproduce this toxin, it is likely present in somefarmed product as it a common practice forshrimp and fish to be produced in pondscontaining heavy levels of blue green algae.

This toxin does not belong in the humanfood chain at any level. The evidence isstrong enough to warrant forced testing for itspresence at harvest much as current HACCPguidelines require testing for specific antibioticresidues that are deemed illegal. While somemay view this as extreme, the evidencecontinues to accumulate that these toxins areubiquitous and that it can have direconsequences from long-term consumption.

Management

Given the nature of these bacteria, it isvery unlikely that they can be eliminatedentirely from production systems. The goalshould however, to be to manage aroundthem and take proactive steps to reduce theirnumbers through whatever means there areavailable. Controlling the growth of these

Blue greens on paddle wheels.

Blue green algae in affected pond.

AQUACULTURE

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Risk of toxins from bluegreen algae to aquaculture

bacteria in ponds becomes of paramountimportance if one is to control the levels ofpotentially problematic toxins.

Rather than killing the algae outright thebest solutions are those that involve trying toshift the ecosystem away from them.Perhaps the best possible approach is toseed the ponds with bacteria (typically stableBacillus species) and allow them to competewith the cyanobacteria for nutrients. This cancause a slow displacement of the algae in theponds and a shift away from thecyanobacteria to more desirable algalspecies. Another approach is to modifyproduction systems so that they become moreheterotrophic based. Through manipulation ofthe carbon to nitrogen ratios, the pond can bepushed towards biofloc technology.

Conclusion

Aquaculturists have become too blaséabout the presence of large numbers ofcyanobacteria in their ponds. While theevidence is at this time largely one ofcircumstance as no broad reaching in-depthstudies have been published that show howwidely disseminated many of the toxins thatthese organisms produce are, it is very likelythat in some production systems, high levels ofsome of the more onerous toxins are going tobe found to be present. Given the nature ofsome of them such as BMAA, it makes a lot ofsense for aquaculturists to utilise strategies thatreduce the presence of these bacteria in theirproduction systems. It is critical for the truesustainability of aquaculture that these issuesbe addressed and incorporated as a part ofBest Management Practices. Screening forBMAA should be required for all lots of farmedshrimp and as well for the fish that are farmedin similar environments before they are soldfor export and it would be wise for those whoregulate the local sale to consider thisrequirement as well.

Reader enquiry number 10

Stephen G Newman PhD is thePresident and CEO of AquaIn Tech Inc

Reader enquiry number 9