Arctic charr released into Llyn Padarn, Llanberis
Around 5,500 rare Arctic charr fish have been released into a Snowdonia lake to help restore their numbers.
The Arctic charr population at Llyn Padarn, Llanberis, has been dropping over a number of years with some blaming poor water quality.
It is the fourth year that juvenile charr have been released into the lake.
In previous years the fish have also been restocked at Llyn Crafnant, near Trefriw, Conwy, developed as a back-up source of charr for Llyn Padarn.
However, new stock will not be introduced into Crafnant this year as Natural Resources Wales (NRW) officers are monitoring populations in the lake to see if previous restocking efforts have been successful.
"We're committed to ensuring the future of the iconic Arctic charr population in Llyn Padarn and we've put great effort into restocking thousands of fish over the years," said NRW spokesman Tim Jones.
"At the same time, we're continuing to work with Dwr Cymru Welsh Water and others to find ways to improve water quality in the lake and create a better habitat for the charr to survive."
A breeding programme has been set up at NRW's Mawddach hatchery in a bid to protect the rare fish, which can only be found in a few cold, deep lakes in north Wales.
In July a report confirmed that nutrients from sewage effluent had damaged the water quality at Llyn Padarn following an incident in 2009.
NRW said the nutrients also caused a toxic algal bloom which closed the lake for weeks at a time.
Stricter limits have since been imposed on a local sewage works.