Anglers are being asked to help conserve fish stocks by agreeing to release more of the salmon and sea trout they catch.
The request follows reports that the salmon and sea trout numbers in Welsh rivers declined last year.
As part of efforts to address this, Natural Resources Wales would like to see anglers release at least 90% of their catch on 'at risk' salmon rivers.
Releasing more of the salmon and sea trout they catch will enable more fish to migrate upstream to spawn and produce more juvenile fish to boost the population.
At present an average of 72% of salmon and 76% of sea trout are released after being caught.
Figures from the Wye and Taff, where mandatory 100% catch and release byelaws are in place, suggest that catch and release can have a positive effect on fish stocks.
Both the Wye and Taff have shown an improvement in salmon numbers in 2013 and are now predicted to move out of the 'at risk' category by 2018.
This is more than likely due to a combination of catch and release and improvements in habitat and water quality.
Fish stocks in a total of 23 salmon and 27 sea trout rivers are assessed annually in Wales and placed in risk categories.
The assessments based on 2013 figures show 20 salmon rivers either 'at risk' or 'probably at risk'.
Natural Resources Wales particularly wants to see more salmon released on the rivers Ogmore, Tywi, Taf, Eastern and Western Cleddau, Rheidol, Nevern, Dyfi and Seiont.
The catch and release rate on the River Dee was already at an encouraging 81% last year but it is recommended that this should reach at least 90% this year.
There is also cause for concern over sea trout stocks especially on the 'at risk' rivers Loughor, Tywi, Taf and Eastern and Western Cleddau, as well as the 'probably at risk' Tawe, Nevern, Rheidol, Ystwyth, Dwyryd, Seiont and Conwy.
Rob Evans, fisheries advisor, Natural Resources Wales, said:
"The number of fish migrating into many Welsh rivers was worryingly low last year so we are asking all anglers to help by releasing more of the fish they catch to boost the next generation.
"Most anglers are already voluntarily practising catch and release and have been doing so for many years. Achieving 90% catch and release rates on the 'at risk' salmon rivers and further increasing the number of sea trout released, particularly the larger fish, may help to avoid the need for more mandatory byelaws.
"There are many pressures on salmon and sea trout these days including loss of habitat, low marine survival and a changing climate but studies have shown the vast majority of released fish, if handled carefully, survive to spawn successfully."
Catch and release is one of several initiatives to improve Welsh fish stocks.
Over the last 10 years Natural Resources Wales, Welsh Government and EU funded Sustainable Fisheries Programme has, in partnership with Rivers Trusts and others, invested millions of pounds to improve stocks.
This included opening up over 1,500km of access to spawning grounds by building fish passes and improving over 500km of habitat by, for example, fencing to exclude sheep and cattle from river banks.
Netsmen have also seen their catch restricted in recent years but there may be a need to impose further restrictions. This will be assessed later this year as part of the process of reviewing our Net Limitation Orders.
To give fish the best chance of survival anglers should use barbless single hooks, play the fish quickly, keep it in the water at all times and support the fish facing into the current until it is strong enough to swim away.