Sterilising farmed fish offers the aquaculture industry a number of benefits. For farmers themselves, it can prevent fish from becoming sexually mature – desirable since maturation reduces flesh quality and makes fish more susceptible to diseases. Sterilisation can also restrict their environmental impact in preventing farmed genes, which are often suboptimal for a life in the wild, from being introduced into wild populations by escapees. The current sterilisation method of choice for aquaculture is to induce triploidy. 

The process itself is fairly straightforward. Fertilised eggs are exposed to either high pressure or high temperature, disrupting chromosome movement during meiosis (cell division). Treated eggs retain more chromosomes than they normally would do – three instead of just two – rendering the animal infertile. Triploidisation as a sterilisation method has a number of benefits. It is already used in terrestrial agriculture systems. Most bananas we eat and seedless watermelons, for example, are triploid. It can also sterilise fish en masse – an essential element for commercial application. Other methods, such as surgical sterilisation, are much more labour intensive – and really only suitable for sterilising a small number of fish at a time.

As with everything, triploid is not perfect.  Once they reach harvestable size, triploid fish can have lower body weight than their diploid counterparts.  They are also more likely to suffer from skeletal deformities, and smaller and sometimes deformed gills which can impact their fitness and performance.  Disease is also a concern, though research comparing susceptibility of triploids and diploids to disease has produced mixed results.  Equally concerning, sterilization is not guaranteed.  Various studies have indicated a sterilization success rate between 97% and 100% for salmonid species, and can be less effective for males than females.  These problems have pushed the search for alternative methods for sterilization suitable for commercial use.

Dr Ten-Tsao Wong (University of Maryland Baltimore County) has been focusing his research on primordial germ cells – a group of cells inside the embryo which eventually become the eggs or sperm in adult fish.  Before they can become eggs or sperm, these cells must migrate into the developing gonad.  By altering gene expression, this migration can be disrupted and sterilization induced.  Meanwhile in Norway, Dr Anna Wargelius (Institute of Marine Research, Norway) has been using CRISPR to create Atlantic salmon without any germ cells in the first place. 

Both Wong’s germ-cell migration disruption and Wargelius’ CRISPR technique involves genetically modifying fish which, as Wong notes, presents a number of challenges. “When you want to make a sterile fish you have to make a transgenic version of each fish”, he explains; “You want a sterile salmon, you have to make a transgenic salmon.  You want a sterile trout, you have to make a transgenic trout”.  Wong also highlights the varying regulatory requirements countries have in place as a significant hurdle.  Even if approval is gained, there is the second hurdle to overcome – consumer choice, which has so far shown significant resistance to genetically modified fish. 

Wong, whose work is supported by USDA Biotechnology Risk Assessment Grant and NOAA Sea Grant Aquaculture Research Program, also looked at disrupting primordial germ cell development with an immersion technique rather than genetic modification.  He developed a special bath consisting of a synthetic molecule designed to disrupt gene expression, and transporter to carry the molecule into the eggs.  The treatment prevents gonads developing, creating sterilised fish.  Over the next few years Wong hopes to assess the performance of fish treated with the immersion-method, and compare to triploid fish.  If all goes well, the process potentially offers a number of advantages over genetic modification.  It does not require a new breed of transgenic fish to be created, bathing can be easily incorporated into farming methods currently used, and can sterilize a large number of eggs in one go.

With triploidy now being required by some management bodies, until alternative solutions are fully developed and accepted by regulators and the public, understanding why triploids develop problems and finding ways to resolve them, seems to be the way forward.  This includes thinking about what triploids eat, and where they live.

Triploid Atlantic salmon have a higher demand for phosphorous than diploids – particularly when they are juveniles and experiencing rapid growth.  Without sufficient phosphorous, work by Dr Per Gunnar Fjelldal (Institute of Marine Research, Norway) revealed, deformities – and even mortality – is more likely.  This means that triploids can’t simply be given the same diet as their diploid counterparts as they commonly are.  Exactly how much phosphorous should be added to the diet for optimal performance is an area of investigation.

It has also become apparent that triploid salmon are more sensitive to temperature and hypoxia (low oxygen) than diploids.  This, Dr Florian Sambraus (Institute of Marine Research, Norway) says, can impact growth, feed intake, and mortality.  In his laboratory experiments triploids showed significantly higher feed intake than the diploids when temperatures were between 3ᵒC and 9ᵒC, with intake peaking at 12ᵒC.  Diploids feed intake, which peaked at 15ᵒC, only surpassed triploids once temperatures exceeded 12ᵒC.  When they threw hypoxic conditions into the mix, triploids really struggled.  Feed intake lowered and at temperatures of 18 °C, mortality became a substantial issue.  Diploids on the other hand were much more tolerant to hypoxia.  The study offers clear advice for salmon aquaculturalists; use triploid salmon in locations where hypoxia is rarely - if ever – encountered - and where water temperatures go no higher than 15ᵒC.  If conditions are more variable where your farm is located, diploid might be the safest bet.

This story appeared on The Fish Site.