Skip to main content
Research and analysis

Brown trout stocking in protected rivers: Recapture and dispersal of stocked brown trout in southern chalk streams: summary

Published 21 October 2026

Applies to England

© Crown copyright 2026

This publication is licensed under the terms of the Open Government Licence v3.0 except where otherwise stated. To view this licence, visit nationalarchives.gov.uk/doc/open-government-licence/version/3 or write to the Information Policy Team, The National Archives, Kew, London TW9 4DU, or email: psi@nationalarchives.gov.uk.

Where we have identified any third party copyright information you will need to obtain permission from the copyright holders concerned.

This publication is available at https://www.gov.uk/government/publications/brown-trout-stocking-in-protected-rivers/brown-trout-stocking-in-protected-rivers-recapture-and-dispersal-of-stocked-brown-trout-in-southern-chalk-streams-summary

1. Chief Scientist’s Group report summary

Large numbers of farm-reared sterile female triploid brown trout are stocked into southern chalk streams to increase angling opportunities and catch rates by providing additional catchable fish. This project investigated the proportion of stocked brown trout caught by anglers and the prevalence of catch-and-release fishing. Time between release and recapture was studied, as was displacement of fish from their release point. The study was conducted collaboratively with representative chalk stream fisheries.

Publication of this study will not introduce any immediate changes to trout stocking regulation, based on this evidence. The information from this study contributes to the evidence base the Environment Agency is using to inform future policy and regulatory decisions relating to the number of trout that can be released, their size, and the timing of release. Measures that could mitigate impacts that fall outside of areas that the Environment Agency can regulate may be included in best practice guidance.

1.1 Background

Stocking with triploid brown trout is common in many southern chalk streams but there is little existing research on its impacts in this ecosystem. Stocking is regulated by the Environment Agency and forms part of the fisheries management strategy for many angling clubs and private fisheries. In recent years the environmental impact of trout stocking may have altered due to factors such as the release of larger trout. Ecological impacts were assumed to be short term with stocked fish being removed for the table when caught by anglers. However, catch and release angling has become more popular which may mean fewer stocked trout being removed from the system, prolonging potential ecological impacts. Artificially increasing trout numbers may result in impacts on wild fish populations and the wider ecosystem. Potential impacts include increased competition for space or food, attraction of predators, or stocked trout directly predating wild fish.

1.2 Approach

Between May and September 2024, 2,707 Floy tagged farm-reared triploid brown trout were released into fisheries in the River Test and River Avon catchments. The trout formed part of each fishery’s planned stocking programme and did not exceed the number that would ordinarily have been stocked. Angler catch records were used to calculate the proportion of tagged fish recaptured, the relative proportions of catch-and-release and consumptive angling, and the time between release and recapture dates. Where release and recapture locations were recorded, displacement distance was measured, this being the distance along the main river channel between release and recapture sites.

1.3 Results

Fisheries recaptured between none and 7% of the marked fish they released. Time between release and recapture ranged from 2 days to 59 weeks. Long term survival of some individuals was recorded with recaptures occurring over winter and during the 2025 trout season. There was no indication of a peak in catches of tagged fish shortly after their release. When anglers recorded whether they kept or released their catch, 45% were kept and 55% released.

Fish displacement did not increase with increasing length of time in river. For 168 trout for which release and recapture locations were available, 81% were recaptured less than 5 km, and 98% less than 6.0 km from their release point.

1.4 Conclusions

The study demonstrated that recapture rates following stocking can be low and that most trout were recaptured within 5 km of their release point. This suggests that there is potential for triploid stocking to affect the environment local to the release site. Displacements greater than 5 km, and over-wintering, were recorded for a few individuals suggesting that triploid stocking may also affect the environment temporally and spatially distant from release time and location.

1.5 Publication details

This summary relates to information from project SC240018, reported in detail in the following output:

This project was delivered by the Environment Agency’s Chief Scientist’s Group, which provides scientific knowledge, tools and techniques to enable us to protect and manage the environment as effectively as possible.

Enquiries: research@environment-agency.gov.uk.

© Environment Agency

Back to top