Rand News

Breeding better oysters

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MUCH of the bounty of the ocean is, these days, far less plentiful than it used to be. Scarcity has made oysters expensive, turning this unattractive mollusc into a delicacy for the rich. That could change if researchers find a way to breed a faster growing and larger oyster.  As many gardeners and farmers know, crossbreeding two wimpy specimens sometimes produces strong offspring an effect known as hybrid vigour. Hybrid vigour is common in plants and is found in some animals though, some speculate, it may be lacking in European royalty.

Several years ago Dennis Hedgecock of the University of Southern California and his colleagues discovered that oysters can hybridise. If a tiny inbred strain called “oyster 6” is bred with the similarly puny “oyster 7”, the result is a large and fast-growing oyster “oyster 6x7” which is easy to open and produces tens of millions of eggs. The problem, though, is that when oyster 6x7 is bred with itself, the resulting offspring are puny again. The hybrid does not, in the jargon, breed true.  If new hybrids were easy to generate in quantity, that would not matter. But oysters 6 and 7 themselves produce only around a million eggs per adult, and their shells are hard to open. Oyster farms each need tens of billions of eggs to operate commercially. Constantly regenerating the hybrid is not a viable approach.

To get around this problem, Dr Hedgecock and his colleagues took some other puny inbreds and created a second hybrid line, oyster 8x9. This is also big, fast-growing and easy to open, and, like oyster 6x7, it produces tens of millions of eggs. The trick is that although it too does not breed true itself, when it is hybridised with 6x7 to produce a super-duper 6x7x8x9 crossbreed, the outcome is just as large, fast-growing and tasty. The result of this two-stage crossbreeding process is that, though none of the hybrids involved breeds true by itself, a marketable hybrid oyster can nevertheless be turned out in large quantities. That is the hope, although the proof will come next year, when the hybrids are grown on a commercial scale.

Of course, it would help if more were known about what creates hybrid vigour in the first place. To this end, Dr Hedgecock has been looking at how hybrid oysters express their genes. He has done so by collecting and analysing the animals' messenger RNA. This molecule, as its name suggests, carries genetic information from the DNA of a cell's nucleus to the places where proteins are made under genetic instruction. If a great deal of messenger RNA for a particular gene exists in an animal's cells, it may indicate that this gene is particularly active. So far, the work has revealed that 350 genes  are expressed differently in the hybrid oysters than in the parent strains. The next step is to sort out what these genes do and which are responsible for large size and rapid growth.

If hybridisation works out, oyster farming could follow the same path as salmon farming, and turn a delicacy for the wealthy into the food of the masses. Unlike salmon, moreover, oysters are filter feeders that clean up the water column, making oyster farms healthy parts of the ocean. Salmon farms are environmentally controversial. Oyster farms should please consumers and environmentalists alike.  

Antibiotic resistance from fish food

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THE mucky sediment below fish farms usually teems with antibiotic-resistant bacteria. The presence of such bacteria is a cause of increasing concern because resistance can limit the ability to fight diseases, but it is also not that surprising: pisciculturalists have a long history of dosing fish they are breeding and rearing with antibiotics. But some scientists suspect there is more to it than that. One group, led by Jing Wang of Dalian University of Technology in China, has found that the problem is also linked to what the fish are being fed.

Dr Wang knew from previous reports that fish farmers who had not used antibiotics for years, or had never used them at all, still had sediment in their marine farms carrying bacteria with many of the genes associated with drug resistance. The genes had to be getting into the bacteria somehow; one possible pathway was through antibiotic-resistance genes in fish food mingling in various ways with bacteria in the sediment.f colleagues, Dr Wang set up an experiment to find out if that was the case. As they report in Environmental Science and Technology, the researchers obtained five commonly used fishmeal products and subjected each one to a detailed genetic analysis. This revealed the presence of 132 drug-resistance genes, suggesting that heavy antibiotic use on the fish products which are themselves ground up into fishmeal formulations, was behind the transfer of genes.

But that, too, was not as straightforward as it seemed. Further analysis revealed that of the five products, the one with the highest concentration of residual antibiotics was a fishmeal from Russia. It contained 54 nanograms of antibiotics per gram of food, although it had only eight resistance genes present. In contrast, a fishmeal from Peru had just 16 nanograms of antibiotics per gram of food, but carried a disturbing 41 resistance genes.

The next step was to discover whether mixing resistance genes from fish food into bacteria-rich sediments would allow the resistance traits to transfer over. To test this out, the team set up microcosms of fish farms in flasks containing 300 millilitres of seawater and 200 grams of sediment. The microcosms were incubated and gently shaken periodically for 50 days and then had a small amount of the Peruvian fishmeal added to them, or were left untouched to function as controls. The researchers regularly collected bacterial samples from the sediments for a further 50 days and analysed them.

The results were clear. Although the control microcosms started with some resistance genes present the number did not increase. In contrast, the number of resistance genes present in the microcosms exposed to the Peruvian fishmeal increased tenfold.

The discovery of fish food as a source of resistance genes migrating into oceanic bacteria is worrying, and the researchers say more work is needed to determine if these resistance traits can find their way into the human food chain. But, says Dr Wang, the Russian fishmeal, which clearly came from fish that had been given a lot of antibiotics before being ground up yet did not contain much resistant genetic material, points to a solution. This is to concentrate on processing methods that destroy the DNA in fishmeal with heat and chemicals. That should rid fish feed of much of its cargo of resistance genes before the food is packed and shipped.

Sustainable aquaculture is possible

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Aquaculture is in the spotlight again, with an ABC investigation raising concerns over the sustainability of the expansion of Tasmania’s salmon-farming industry.  Controversies over fish farming are newsworthy and emotive, particularly when company profits and communities are at stake. Unfortunately, independent scientific evidence is often used selectively or even ignored in these debates.  Science is an essential tool for managers and regulators when planning industry expansion, and Australia’s aquaculture industry does have a strong research base.  Fish farming can be sustainable, but only if it takes proper account of scientific research – and only if that research moves fast enough to give an up-to-date picture of the risks.

Demand for sustainable aquaculture

The ever-growing demand for seafood, combined with the limited opportunity to increase catch from wild fisheries, means we need more aquaculture. Farming already produces roughly 50% of the global seafood supply, and farmed fish production now exceeds that of farmed beef.  Intensive aquaculture is relatively new, with supply rising tenfold since the mid-1980s. It is thus unique among food production sectors in that its initial expansion has taken place in an era of unprecedented scrutiny from government, environmentalists and the community.

This scrutiny is warranted, given that many fish farms are in coastal waters considered as a multi-use, common resource. In Australia, the industry is subject to high environmental standards and constantly evolving management.  Intensive aquaculture has several inherent advantages over other forms of agriculture. These include efficient food conversion relatively limited use of fresh water; and the absence of fertilisers.  However, there are also significant sustainability challenges, including limiting marine feed ingredients; waste management; the use of drugs, colourants and other chemicals; impacts on wild marine species; management of fish health and welfare; site selection; and societal attitudes.  The aquaculture research community is acutely aware of these challenges. At a World Aquaculture conference in Adelaide in 2014, the program was dominated by issues related to sustainable development.

Planning for the future

In the forseeable future, world aquaculture production is projected to grow at least at its current and long term rate of 6.5% a year. Australia’s industry, while representing less than 0.1% of world production, is growing even faster: more than 7% a year over the past decade.  Given cost constraints, this future expansion will be mostly inland or in coastal marine environments. Scientific input will be crucial if this expansion is to be managed in a sustainable way.  Understanding the spatial and temporal variation in these conditions is critical. It is not in the industry’s interest to risk growing fish in marginal conditions.  Conditions are also becoming more challenging as a result of climate change – the oceans off Australia’s southeast are among the fastest warming on the planet.

Enlightened aquaculture businesses are trying to anticipate these conditions by working with scientists including CSIRO and the Bureau of Meteorology to understand future environmental risks on a range of timescales.  Seven-day ocean forecasts and medium-term outlooks covering several months will help the industry make decisions about cage locations, stocking density, diet, disease management, and when to harvest.

 

Meanwhile, longer-term planning, on time scales of years and decades, will be informed by climate models. For example, the industry can aim to breed fish to cope with changing conditions such as warmer water.  Of course, forecasts are never 100% accurate, meaning that aquaculture businesses still need to account for risk and uncertainty.

Planning for now

Science is clearly crucial for effective future planning. But it is also important to ensure that current management is the best it can be, and that current risks are managed.  In the case of finfish aquaculture, the potential for localised impacts on the seabed around sea cages is well known, and monitoring and management strategies well established.  The potential for adverse effects on the water in and around cages is also important, and water column monitoring is increasingly a management requirement.

Broader ecosystem interactions such as changes in fauna and flora on reefs around cages are progressively being recognised as an issue for many aquaculture regulators and managers.  As scientists’ understanding of these risks increases, regulators and managers can implement strategies to protect a broader suite of environmental assets and values.  However, there is no “one size fits all” management approach for this rapidly growing industry, and strategies need to be considered in the local contex. Science can provide a better understanding of a particular scenario, but it is up to managers to use this information wisely – and to exercise caution where risks are not well understood.

Fast responses

Management may aspire to be “best practice”, but it is important to recognise that this does not mean that it will be static or finite. Management should respond to changes in the environment and should adjust as the science and understanding develops.  It is important to acknowledge the different but complementary roles that science and management play in aquaculture planning. Scientists seek to understand the situation and share that understanding impartially and objectively. Regulators and managers need to make decisions with a much broader mandate, and as such need to consider factors beyond the science alone. Good planning needs to recognise the value of both.  Aquaculture development and policy needs to be able to trust the science, which in turn, must be delivered in a timely manner, to ensure long-term sustainability of this industry.