Yellowbelly flounder

Species of fish

The yellowbelly flounder (Rhombosolea leporina) is a flatfish of the genus Rhombosolea, found around New Zealand. A different species from the genus Rhombosolea is found in Australia and also goes by the name yellow belly flounder. The Māori people have commonly fished for R. leporina, and many other species of flatfish, throughout New Zealand's coastal waters for hundreds of years. The Māori name for this species is pātiki tōtara.

Rhombosolea leporina is part of the flounder family Pleuronectidae, where all species have both eyes on the right side of their bodies. This ocular side is a dark olive green, with dark edged, smooth scales. The underside/blindside is usually white in juveniles, eventually becoming yellow with dark spots as the flounder matures. Adult flounders have long, oval shaped bodies, with small eyes that are set further back from the pointed snout than most flounder species. They also have 13 17 rakers in each gill, on both sides of the body, and approximately 75 86 scales along the lateral line. The average length of adults is between 25–40 cm, with some female reaching up to 45 cm. Females are on average longer and heavier than males.

R. leporina is only found in the South Pacific and is endemic to New Zealand's coastal waters. This species is widely distributed throughout New Zealand's inshore areas, but is more abundant around the North Island than the South Island, due to the flounder favouring a warmer climate.

R. leporina lives in the demersal zone of mud/sand flats, estuaries, bays, harbours and other moderately shallow areas along the coastal sea margin. The demersal zone includes the seafloor and bottom of the water column that lies above it. Like many species of flounder, they are benthic fish, meaning they live and feed on the seafloor. Their preferred habitat temperature ranges from 12 18˚C, with larvae and juveniles preferring a higher temperature and adults preferring a lower temperature, especially during their breeding season. Their life cycle begins during spring when spawning season occurs in coastal waters averaging 15 °C and 30 50m deep. After the eggs have been laid by the female flounder, larvae take four to five days to hatch when reared in controlled conditions, at 15 °C, in the laboratory. The ocular right side, now with both eyes present, deepens in colour to blend in with surrounding benthic environment, camouflaging the flounder and protecting them from predators. The flounder move into deeper waters as they grow, reaching adulthood after growing longer than 20 cm, at approximately two years of age. It is unsure whether these parasites have a detrimental effect on the host.

High temperatures, low oxygen levels, and low pH can cause physiological stress in the flounder, making them more vulnerable to disease and other heath complications. R. leporina is also vulnerable to illnesses and deformities caused by effluent, chemical and industrial pollution run off in estuarine and harbour areas. Weirs built into inland waterways are another threat to R. leporina. Their purpose is to alter the water flow to reduce flooding, slow down the water's velocity, and raise water levels upstream. Weirs can cause mortalities and intercept flounders migrating from inland waterways, to offshore areas for spawning. Consequently, adults are unable to spawn and less offspring are produced, potentially having a negative impact on population numbers.