tolerates the brackish estuary mud

Size
Length: 25-45 cm, Wt: 0.2-0.6 kg
Lifespan
3-5 years
Diet
Benthic predator taking crustaceans, molluscs and worms from soft sediment. Detects buried prey using touch-sensitive dorsal fin rays and blind-side taste buds for identification.
Habitat
Mud and sandflats in estuaries, harbours and sheltered bays to 50 metres. Tolerates brackish conditions. Juveniles use shallow inshore mudflats as nursery habitat for development.
Range
Endemic to New Zealand. Found throughout both main islands and Chatham Islands. Most abundant in northern harbours, estuaries and sheltered bays to 50 metres depth.
Endemism
Endemic
Main Threats
Subject to commercial and recreational fishing pressure in northern harbours. Managed under QMS. Estuary sedimentation reduces juvenile nursery habitat quality significantly across northern regions.
Population
Most abundant in Kaipara, Manukau and Firth of Thames. Managed under QMS. Population stable under current management. Not assessed by DOC for conservation status or threat classification.
Conservation Status
Not Threatened
Human Risk
harmless
Handling Note
native flounder, harmless to humans, leave undisturbed
Conservation Note
Endemic marine fish; not assessed by NZTCS as marine fish are outside the scope of current threat classifications.
Te Ao Māori
To Māori, the yellow-belly flounder is pātiki tōtara. The name distinguishes it from other pātiki species by the yellowish colouring of its underside. This is likened to the reddish-brown bark of the tōtara tree. Flatfish broadly, and pātiki tōtara specifically, were important seasonal food sources. Coastal and estuarine iwi in the northern North Island relied on them. The Kaipara and Manukau harbours and the Firth of Thames were significant fisheries. Customary fishing rights in these harbours are held by mana whenua. Pātiki remain part of contemporary kaimoana practices. They are included in fisheries consultation processes. The cultural value is practical. Not ceremonial. The connection is enduring.
The yellow-belly flounder gives itself away from below. Rhombosolea leporina has an upper surface that is dark olive-green with large dark-edged scales. These blend with estuarine sediment and pass without comment. Turn it over and the underside reveals its character. White in juveniles, it becomes pale yellow with dark spots in adults. This colouring names the species. Māori described it as pātiki tōtara, the totara-coloured flatfish. The comparison is visual. The identity is distinct. Rhombosolea leporina is a righteye flounder endemic to New Zealand. It is short-bodied and oval. Plumper and rounder than the sand flounder it shares many habitats with. It is easily distinguished by those larger scales and the characteristic belly colouring. Both eyes sit on the right side of the head. They are positioned further back from the pointed snout than in most flounder species. This is because the eyes on this fish are not primarily for finding food. Small crustaceans, molluscs and worms buried in soft sediment are located instead by touch. The anterior rays of the dorsal fin carry sensory cells. These detect movement in the substrate. Taste buds on the fin rays and on the blind side of the head allow the fish to identify edible material. It does this from a mouthful of mud before committing to swallowing. This is a sophisticated system for an animal that looks, at first glance, like something that has made peace with lying still. The stillness is strategic. The sensing is active. The life cycle is structured around a habitat sequence. Larvae drift inshore and settle on sun-warmed shallow mudflats. Juveniles remain there for up to two years. As they grow, they move progressively into deeper estuarine channels. Then offshore. Adults spawn in coastal waters at 30 to 50 metres depth during spring. Adults migrate back to inshore feeding grounds through summer and autumn. Females grow faster and larger than males. They reach maturity at around 29 centimetres. Males mature at 24 centimetres. The largest females produce upward of a million eggs in a single spawning event. The output is high. The survival rate is low. The strategy is quantity. The yellow-belly flounder is most abundant in the Kaipara and Manukau harbours and the Firth of Thames. Set-netting has been a consistent fishery there for generations. It is managed under the Quota Management System as part of the flatfish group. The estuarine habitats it depends on for juvenile development remain under pressure. Sedimentation and coastal development affect the northern coastline. A fish that needs healthy mudflats at the start of its life is exposed to everything that damages those mudflats. In the northern North Island, that is a long list. The vulnerability is structural. The dependency is total. Without the mud, the cycle breaks. The fish persists where the habitat remains. It waits in the silt. It senses the vibration. It feeds. It carries on.