Getting the most from trout stocking – Part 2

Steve continues the discussion.

All over Australia, stocking is going on all the time. If we want to stock, and to be able to continue to stock, we have to ask questions and at least to some extent, know the answers. What are our stocking objectives? What will be the impacts on the environment? What if the fish we stock escape? How do we manage genetic diversity? How many fish should we stock for a given area of water? How do we know if our program has been effective, and what do we do if it all turns to custard?

This is more than just risk assessment, it’s clarity of objectives and assessment of cost, benefit, opportunity, threat, and alternatives. This is not really done anywhere in a consistent and comprehensive way. Yet ideally, all that should happen before we even ask how we can improve our stocking success.

As humans, we seem to have short memories when it comes to why we need answers to these things. Although we rightly revere our trout acclimatisation societies in New South Wales, we shouldn’t forget that at some point in history someone once put in their resume, “I recently led the program that successfully introduced carp to Australia.” Rabbits, sparrows, foxes, deer, blackberries, bananas, sugar cane, pineapples, mangoes, llamas, donkeys, camels… there’s an endless list of successfully acclimatised species.

Not all stockings turn out well.

The first of the acclimatisation societies was set up in 1861, looking at all forms of plant and animal life with the potential for successful introduction into Australia. Although State-based, these were private societies and involved high level patronage including Governors, and Directors of Botanic Gardens and Museums. By 1862, every State had one; they were obviously quite the rage. In their early records is a request from the Victorian Acclimatisation Society to the Duke of Newcastle, sitting on his peerage in the old country as Secretary of State for the Colonies, asking for a loan of a man-o’-war to bring out some salmon fry. The Duke replied that he, “…did not like converting a man-o’-war into a herring-boat!”. Which was a little intolerant, but maybe the Duke was unwell because by 1864, aged 53, he had died.

Before leaving this topic, I wonder if Victorians know just how close they got to having monkeys competing with koalas “for the amusement of the wayfarer, whom their gambols would delight as he lay under some gum tree in the forest on a sultry day.” And let’s face it, koalas don’t do much in the way of the gambol, so maybe we missed out?

The truth is, we can justify almost anything to ourselves when it comes to fish stocking. It is often great politically, and it’s easy to claim we can reduce pressure on other fisheries and bring economic benefits. But rarely are we called upon to back up these claims with any rigour.

What you can’t do

These days, you can’t just do what you want, and for good reason. Every State and Territory has policies and laws about what you can stock and where, so always check. Please don’t just throw in a few fish, even in a dam on your own property, without checking if it’s legal. In terms of adverse outcomes, at best, you’ll waste your money – if you put the wrong fish in the wrong place, they’ll just die and there’s no real harm done. At worst, you could end up being like that person with carp on their CV. And not forgetting there are also serious penalties for doing the wrong thing.

There are many rules. For example, you can’t stock a dam in NSW unless you can stop the fish from escaping into a natural creek or waterway. And there are good reasons for all this. Hatchery-reared fish may have diseases or parasites not found in populations of the same fish in a small remote creek. Or your stocked fish could overwhelm the local population. Your good intention to create a dam fishery could damage a well-established creek fishery.

And of course it’s illegal to stock some species, like carp, tilapia and redfin, into farm dams. You could introduce an unwelcome species and even create an environmental catastrophe.


Genetics are important. “Why?”, I hear you say. Well it’s all about alleles. Alleles are pairs or series of genes on a chromosome that determine hereditary characteristics. Constantly mixing these up is an important force in evolution. Not mixing them up (inbreeding) results in an increased risk of undesirable genes (disease and deformity) through a reduction in genetic diversity. Your 100 little hatchery rainbows are all pretty much clones; it’s unlikely there are more than a few mums and dads involved. A mature hen rainbow can produce around 3,000 progeny and hatchery fish, bred from hatchery fish, bred from hatchery fish can turn out a few weird-looking offspring! Depending upon the source hatchery, they are often bred as table fish and don’t have to live very long. So there is a risk that if these fish escape, they may dilute the quality of established stocks.

It might seem unlikely that our trout stocks in Australia are very genetically diverse given the small numbers of fish originally introduced – or so you would think. Some good science around this (Farrington et al 2004) found “no appreciable difference in genetic diversity was apparent between hatchery and self‐sustaining wild populations of rainbow trout from mainland Australia”. They also found that levels of genetic diversity within Australia didn’t show a marked difference from hatchery and wild populations in North America (our Australian rainbows came to us from California via New Zealand). Having said that, they did find significant and consistent genetic differences between Victorian and New South Wales rainbow trout.

There is no doubt that genetics are important and when rainbow trout are isolated geographically for long periods, they can be distinguished by their genes. How material this is from the perspective of stocking is still unclear. There’s good science from British Columbia (Winant et al 2018) which showed rainbow trout tested from above long-standing natural impassable barriers, were from unique gene pools with reduced levels of genetic diversity. Rainbows from below the barrier were genetically distinct and significantly more diverse than those from above.

Brown trout are one of the most genetically diverse vertebrates in the world. So much so, there has been a long-standing debate about whether there is more than one species. In the 19th century, there were more than 50 species described; in the early 20th century, this was settled to be just one. However, as recently as 2017, the literature was discussing that there are potentially 40-plus species. Ask any really experienced trout fisher about this and they will tell you they can recognise strains of brown trout from certain rivers or lakes by their colour and markings, so there is something in all this. But the internet lights up when you try to work through it and get a clear answer. It is daunting when faced with reports like, ‘Fine-scale phylogeographic contact zone in Austrian brown trout Salmo trutta reveals multiple waves of post-glacial colonization and a pre-dominance of natural versus anthropogenic admixture’. Somebody needs a hobby and I know a really good one!

Same species?

The point of all this is that genetics are a big deal for stocking programs of all species and we just have to live with this reality.

Closer to home, a study was undertaken by Victorian fisheries scientists. Strains of fish from Snobs Creek hatchery in Victoria and Great Lake in Tasmania were stocked into Millbrook Lakes (a private, catch-and-release fishery) to try and answer the question, whose rainbows are best? Both strains survived in catchable numbers for the duration of the trial until they were three to four years old. The good science answer was that there was no real difference in growth, survival, or catchability – but of course depending on where you live you all know whose fish are really the best!

Across the ditch in New Zealand, a selective breeding program around the Rotorua Lakes, developed a strain of rainbows which would spawn later (older). Some rainbows spawn at two years old; most at three, a few older than three. Once they become sexually mature, around half of all their energy goes into breeding and a lot die from exhaustion, infection or predation. So delaying spawning means more big fish, surviving on average for longer. After more than twenty years, the selective breeding program decreased the number of fish spawning at younger than 3 years to five percent; and they have more big fish as a result.

Age at sexual maturity can have a major influence on the ultimate growth potential of stocked diploid (‘regular’) rainbows – though this isn’t easily manipulated.

Meanwhile, in a totally different environment to New Zealand, the Pemberton Freshwater Research Centre in Western Australia on Lefroy Brook produces a unique genetic line of temperature-tolerant rainbow trout for their south-west region’s freshwater fishery.

Historical stockings

In NSW, the streams and rivers where stocking with trout is permitted, are those which have been stocked historically. In a very simple sense, if trout have become a part of an environment over the last 100 years then they can continue to be stocked. For many fisheries, if stocking did stop, then that fishery would die out. An example is the network of Monaro rivers and streams in southern NSW. There is very little habitat suitable for trout to spawn, and once every few years the streams pretty much dry up and the fish all die. Only stocking can re-establish those fisheries. Some of our legendary trout fisheries like the Maclaughlin River and the Kybeyan River would not exist without stocking. Other famous trout fisheries like the western lakes of Victoria, the Rotorua Lakes in NZ, and Penstock Lagoon in Tasmania, only exist because of stocking programs. And there are many more.

As with most eastern Monaro waters, Maclaughlin River browns grow very quickly during the good times, but stocking is needed to regenerate trout populations, which almost disappear during droughts.

Trout Unlimited

Trout Unlimited is a North American organisation with the mission to ‘conserve, protect and restore North America’s coldwater fisheries and their watersheds’. They have 300,000 members and supporters and are politically active in their work to ‘protect, reconnect, restore and sustain trout and salmon habitat’. You can read about their incredible efforts, but the point I want to make here is that they very much prefer to work on restoring and reconnecting habitat, rather than relying on hatchery stocking.

There is an amazing amount of information on their website and a quick summary would describe their fishery management policy as: don’t jump straight to hatchery stocking for a solution; there’s plenty you should try before you go there. First, fix the habitat, then look at river continuity to support fish migration (such as past dams and weirs), then control harvest to levels that encourage stocks to recover (controls on catch, harvest methods, catch-and-release). At the bottom of their hierarchy is hatcheries (not to be substituted for proper management and only with rigorous biological risk management). These are their “Four H’s”, a hierarchy of Habitat, Hydro, Harvest, and Hatchery. Common sense really.

Triploids and hybrids

Occasionally, a debate pops up about stocking triploid rainbows, or hybrids like tiger or cheetah trout; usually in one of those, ‘this is going to fix everything’ conversations. Both these fish share some common characteristics. Firstly, both triploids and hybrids are practically sterile and can’t breed either amongst themselves or with wild fish. So if one goal of a stocking program is to minimise the risk of genetic impacts on wild stocks, or to ensure a wild population does not become established, then these might be good options. Second, some advocates suggest they may grow faster and bigger, fight better, and live longer. They may even help control populations of less desirable species. (One strong rumour had them fixing both the pandemic and world peace!)

So what are they? Triploid rainbows have three sets of chromosomes instead of the normal amount, two (diploid). This is achieved by ‘shocking’ fertile eggs. For rainbows, this shocking can be achieved with heat, cold, pressure, or chemicals.

If we’re going to get real-Frankenstein about this, you can start by turning all your females into functional males (females that produce sperm) by treating them with hormones. Then, you can breed those with regular females and produce all female triploid offspring by bathing the fertilised eggs in a nice warm bath for 20 minutes which shocks the eggs into not ditching their third set of chromosomes. A cool recipe to share with the grandkids.

I’ve stocked triploid rainbows into a farm dam. I got fat rugby ball-shaped fish for two years, and then they just disappeared. The regular fish were still there so I stopped stocking triploids. I’ve heard that triploids may be less tolerant to higher water temperatures than their diploid counterparts, possibly one adverse consequence of the heat treatment of the eggs.

Between 2006 and 2010, a trial was conducted in Victoria using triploid brown trout to control roach (another successfully acclimatised English coarse fish). The results were inconclusive, the hypothesis not proven.

Meanwhile, hybrids are a cross between two similar species (horse + donkey = mule). A favourite hybrid is the tiger trout: a cross between a brown trout and a brook trout. They can appear naturally in the wild but fertilised eggs have a low survival rate, estimated at 5%. Bring man into the equation, and that jumps to 85% with the intervention of a bit of heat shocking of eggs. (I mean, who thought of that in the first place?)

Tiger trout – attractive and exciting to catch, although their popularity seems to come and go. Being sterile, they pose no risk of interbreeding if there are wild fish nearby. (Pic. Cale Berry)

You read about tigers occasionally. Normally in a local newspaper spruiking a special stocking of such exotics in a local lake, and always with an awful headline: “Tigers back in Tasmania – it’s True”. Tasmania and Victoria have tigers. They are reported to be aggressive and in winter a bit easier to catch. Cheetah trout, another hybrid of rainbow and brook trout were stocked in Lake Purrumbete in Victoria in 2019 with results yet to be assessed and reported.

Stocking of hybrid and triploid fish is not a new idea. It seems to come in waves. It’s almost like they become trendy, and then without a great deal of fuss or explanation they seem to drop off the stocking menu. Tiger trout were stocked exclusively in the Great Lakes between the US and Canada and then apparently discontinued – 50 years ago! Both go into my basket of distractions and sideshows.


My final point is about diseases, parasites and other nasties. Put any group of animals in an unnaturally large concentration and you will get disease outbreaks. We are more aware of this today than ever before because of the pandemic. Social distancing and self-isolation might be a bit unnatural for many of us, but COVID-19 can’t do its thing if we follow those rules. Fish in nature are pretty good at social distancing – and avoiding stressful situations. But put them into a fish farm with limited space, where they get stressed because they can’t escape contaminated water or uncomfortable temperatures, and any small outbreak of anything will spread very quickly. Releasing these fish into a wild population can result in that ailment spreading.


Sometimes, I literally scream when I hear there’s a stocking panacea for every instance of poor fishing. But other times, stocking does indeed make a positive difference, perhaps even creating a decent fishery which wouldn’t otherwise exist at all. The thing is, stocking must be done cleverly to get the right result, and too often, it isn’t done cleverly.

For example, we’ve learnt that size at stocking can be really important, and it’s all about the water in question and the desired result. Easy-to-catch trout for kids in a small water? Stock catchable-size fish and don’t worry too much about their long-term prospects. Recovery of a recently dried up but now refilling lake or stream? Perhaps fry or fingerlings are ideal, and good genetics are likely to be handy too. Waters with redfin? The stocked fish need to be close to 200mm to avoid being mostly wiped out in weeks.

Species/ hybrids? Talking purely about trout, the potential arguments are endless but survival in the stocked environment is right up there. For example, we know stocking brook trout into many otherwise suitable Australian trout waters just doesn’t work – the water gets too warm, and they can’t compete with established browns and or rainbows.

Effective stocking programs need quality, not just quantity.

And for deciding what’s stocked and at what size? Support from at least some science-based monitoring of returns makes so much sense. A little bit of research is likely to provide a whole lot of guidance about how to get the best ‘bang for stocking buck’ – something that’s likely to keep fishers, fisheries managers and even politicians happy.

Then there’s the actual releasing of stocked fish. We’ve seen how that can make a huge difference to outcomes. Matching ideal conditions with a pre-planned truckload of thousands of young trout isn’t always possible. However, aiming for a cooler time of year, and spreading the release over a wide area, close to cover (maybe with the assistance of angler volunteers) is sensible and doable.

A final plea: can we, as anglers, get the message to managers and politicians that stocking numbers aren’t everything. It might be good for the quick sell, but we know that numbers mean nothing without quality – being the right sized stocked fish, with fitness for purpose.