Doubtful Sound / Patea is a vast and remote fiord in Fiordland National Park, Southland, New Zealand. Stretching 40 kilometres from its head to the Tasman Sea, it is the deepest of the South Island's fiords, reaching a maximum depth of 421 metres. The fiord's dramatic U-shaped profile is lined with dense native rainforest and steep mountainous cliffs, creating a breathtaking but challenging environment for anglers. Access is exclusively by boat, with journeys typically originating from the towns of Te Anau or Manapouri. The area receives exceptionally high annual rainfall, averaging between 3,000 and 6,000 millimetres, which influences water conditions and fishing opportunities. This is a pristine marine environment with specific regulations designed to protect its unique and fragile ecosystems.