Glaslyn is a dynamic and productive tidal estuary system in Gwynedd, North Wales, flowing into Tremadog Bay and the wider Irish Sea. It is a significant water body for migratory fish and supports a diverse inshore fishery. The system includes the main Afon Glaslyn river and its tributaries, such as the Afon Cwm-llefrith, transitioning from freshwater upstream to fully saline conditions at its mouth near Borth-y-Gest. The estuary is characterized by its mixing of river and tidal currents, creating rich feeding grounds. The area is managed for both conservation and recreational angling, with specific regulations in place to protect fish stocks.