Trout Stocking

Stocking rivers with farmed trout

Some angling clubs and fishing syndicates use farmed brown trout to boost the number and size of fish available to anglers and (some say) to allow the trout populations to withstand high levels of fishing pressure, especially if anglers wish to kill fish to eat. 

However, many fisheries now stock fewer fish or have stopped stocking altogether, investing the money saved in habitat improvement to support the population of wild trout, and moving to catch and release.
The general picture from these clubs (see the case studies below) is that anglers are catching more, and in some cases, bigger, wild trout when farmed fish are no longer stocked. Even in low productivity rivers, trout will eventually grow to larger sizes. Our Conservation Officer team are available to help any angling club or syndicate looking make changes to their fisheries management. 

The regulatory agencies in the UK and Ireland require all farmed brown trout stocked into rivers and some lakes to be infertile (triploid) and to be stocked under licence. Where there’s a history of their stocking, the regulatory agencies might also allow rainbows to be stocked out. Details are on the appropriate agency websites e.g. EA here; Scottish Government here.

Stocking Picture

How many fish can my river support?

The number and size of trout that a river can support is largely determined by the availability of habitat, food and sufficiently good quality water at temperatures that trout can tolerate (i.e. less than 20°C for much of the year).

Adding farmed trout to a river which has a wild trout population will increase competition for food and for space between the stocked trout and their wild counterparts. Establishing the carrying capacity’ of a river is complex, and changes over short distances and through time, so the consequences of adding farmed trout to a river with an existing wild trout population can be hard to predict. 

Improving habitat (or changing how the river is managed) so that there is space and food for all life stages of wild trout is likely to increase the wild trout population. For more information on how to do this, check out our Wild Trout Survival Guide or request an Advisory Visit from one of our Conservation Officers.

Practising catch and release, rather than killing fish allows the fittest fish to grow large and satisfies angler demand for catching larger specimen fish, as long as there is good adult habitat and food available. These are valuable fish — less vulnerable to predation and making a greater contribution to the spawning population.

Paul Procter with large wild trout
WTT Vice President Paul Procter with one of many large wild trout, caught somewhere 'up north'!

Where have all my farmed trout gone?

Many fishing clubs keep records of fish caught, whether wild or farmed. It’s not uncommon for the number of farmed trout caught by anglers to be less than 5% of the number stocked into the river. Where do all the farmed trout go? 

Hatchery reared trout are bred to grow quickly on a pellet diet and to tolerate living in close proximity to other trout. Compared to wild trout, a domesticated life on the farm means farmed trout are less fit for life in the river. They may well have higher mortality rates, be more susceptible to predation and less able to compete for food and space. Trout captured from the wild to breed in a hatchery environment will show these domesticated traits remarkably quickly – within one or two generations.

Farmed trout also have a greater tendency to disperse (generally downstream) than wild trout, especially so where large numbers of fish are stocked at the same time and where river conditions (e.g. high water) make for a hostile environment for fish that have just left the confines of a farm pond. Where practically possible, trickle’ stocking on a little and often basis might reduce the number of stocked fish that move downstream. 

Unintended consequences of stocking farmed trout

Farmed trout are more naïve and may well be more susceptible to predation; in turn, predators may be attracted in by large numbers of new, stocked and energy-rich arrivals! Cormorants, in particular, are smart birds and quick to exploit a food resource. There is a view among some anglers that stocking reduces predation pressure on wild fish, but it may well be that stocking only worsens the situation!

Cormorant melvin grey
Cormorant. Image: Melvin Grey

Increasing densities of trout above the natural carrying capacity of the waterbody can cause negative impacts on the fitness of the wild trout population. This may mean that the wild trout breed less, are smaller, are less able to deal with extremes of temperature, floods or droughts.

There is potentially an impact on the whole food web within the river, with increased predation by trout of other fish such as bullhead, stone loach, trout and salmon parr and increased predation of some invertebrate families. There is evidence of these impacts where trout are introduced to streams or lakes with no existing wild trout population. However, if there is an existing population of wild trout, the effect of the introduction of farmed trout on the food web is difficult to establish. Any such effects will fluctuate over time and space but are likely to be amplified by the number of fish stocked, longevity and frequency of stocking and where stocked fish are larger than their wild counterparts.

Wild trout population recovery

It is sometimes thought that rivers that have been stocked with farmed fish for many years no longer have a wild trout population, and that stopping stocking will result in no trout. However, to the contrary, there is evidence that wild trout populations do recover once stocking with farmed fish ceases. Remnant populations of wild trout will often occupy small tributaries and will return to occupy the space vacated by farmed fish with surprising rapidity. If the river is reasonably accessible from the sea, then sea trout will come in to spawn and boost the resident as well as the migratory trout population.

The video below is from the USA and concerns rainbow trout and non-native brown trout, but the same principles have been shown to apply in the UK.

WTT view of stocking rivers with farmed fish

We have produced a position statement with a list of references, and also a concise summary document to help people make an informed decision about stocking and the protection of wild brown trout. This document was created in May 2012 when there was much debate about triploid / diploid farmed fish. We will produce an updated version in due course. 

The document was prepared by Dr Paul Gaskell with assistance from other WTT staff and our volunteer Advisory Pane. 

Wild Trout Trust — View on Stocking and Rationale
Wild Trout Trust — View on Stocking — Summary

Case studies

A number of angling clubs the Wild Trout Trust have worked with have taken the decision to cease stocking or reduce the numbers of fish introduced to their rivers. We asked them to provide case studies of their experiences describing how and why they took the decision and what the results have been.  Click on the links below for each case study (pdf documents or videos). 

River Nidd, North Yorkshire - Nidderdale Angling Club 

Afon Clwyd, North Wales - Denbigh & Clwyd Angling Club

River Leven, North Yorkshire - Hutton Rudby Fly Fishing Club. 

River Monnow, Monmouthshire / Herefordshire - Tregate Angling Club

River Ribble, LancashireThree 5 minute YouTube videos of a presentation by Neil Handy at the 2015 WTT Annual Get Together covering the experience of reducing stocking on the River Ribble.  Section 1. Section 2. Section 3.

Salisbury and District Angling Club. A large club of 2,200 members with extensive chalk stream fishing in Wiltshire. This is a video of a talk given by Andreas Topintzis at the 2016 WTT Annual Get Together.  An interesting (and amusing) story of big changes to a large club. 

River Wharfe, North Yorkshire - Ilkley Angling Association

Haddon Estate, Derbyshire

Further reading

An academic paper which looked at the impact of stocking sea trout in Shetland. See this news item Shetland stocking does not make for more trout’ and the full paper.

The history, science and future of stocking by Kyle Young. This article was first published in 2021 in Wild Fish’ by Salmon and Trout Conservation.