Black Flounder – wanders up the murky freshwater rivers

wanders up the murky freshwater rivers

Size: Length: 30–40 cm, Weight: 0.5–1 kg
Lifespan: 8–10 years
Diet: Small crustaceans, worms and insect larvae. The only flatfish that regularly ventures into pure fresh water, pushing far up rivers and into coastal lakes. Lives in murky, tannin-stained water where rivers meet the sea.
Habitat: Murky, tannin-stained water where the river meets the sea. The only flatfish that regularly ventures into pure fresh water, pushing far up rivers and into coastal lakes. The freshwater wanderer.
Range: Lowland rivers, lakes and estuaries throughout the country. Most common in murky, tannin-stained water where rivers meet the sea. Once common in many catchments, now vanished from those with dams, culverts and pollution.
Endemism: Endemic
Main Threats: Habitat loss from dam construction, culverts and wetland drainage. Water pollution from agricultural and urban runoff. Sedimentation from land clearance. Climate change affecting river flows.
Population: At Risk - Declining. The habit of swimming up rivers puts them directly in the path of dams, culverts and pollution. Once common in lowland waterways, they have vanished from many catchments where they used to thrive.
Conservation Status: Not Threatened
Human Risk: harmless
Handling Note: native flounder, handle with wet hands to protect slime
Conservation Note: Endemic marine fish; not assessed by NZTCS as marine fish are outside the scope of current threat classifications.
Te Ao Māori: The Black Flounder was known to Māori as a river fish. It was often caught in the tidal reaches where the fresh water meets the salt. Its dark colour made it easy to distinguish from the paler, saltwater flounders. Today it is the flatfish of the forgotten river. It is the one that used to be common, before the dams went in and the cows came to the banks. Its decline mirrors the degradation of lowland waterways. The loss of this species signals a broader ecological failure. Mana whenua observe this change with concern. The health of the river is tied to the presence of such indicators.

Dark ghost of the river mouth. The black flounder looks like a common flounder dipped in charcoal. The upper side is deep, dark olive-brown to almost black. Faint irregular blotches mark the skin. The underside is clean, creamy white. This provides perfect camouflage against the dark, muddy bottoms of the tidal rivers it loves. It is a fish that disappears into the mud. Adventurers of the flatfish family. While other flounders stick to salty estuaries, the black flounder pushes far upstream. It sometimes travels kilometres into pure fresh water. It has a higher tolerance for low salinity than any other New Zealand flatfish. This allows it to colonise coastal lakes and the deep, slow stretches of lowland rivers. Other flatfish cannot follow. A fish that goes where others cannot. Black flounder have a muddy, earthy flavour. This reflects their dark, freshwater habitat. Some people love it. Others find it too strong. But if caught from clean, flowing water and purged overnight, the muddy taste fades. A firm, sweet fillet emerges. It is worth the effort. A fish that rewards patience. To catch a black flounder is to catch the explorer. It is the dark, river-going flatfish that pushes into fresh water. It looks for a quieter life away from the salt. Once common in lowland waterways, it is now vanished from many catchments. Dams, culverts and pollution have taken their toll. It is the flatfish of the forgotten river. The one that used to be common, before the dams went in and the cows came to the banks. The river is brown. The water is slow. The black flounder hides in the mud, dark against dark, invisible. The dam upstream blocks the passage. The flounder cannot go further. It does not know why. It just knows it cannot go home. It carries on.