New Zealand Sea Fishes

Black Flounder – Rhombosolea retiaria

Black Flounder – Rhombosolea retiaria

By Allan Burgess

A Freshwater Flounder

The black flounder is mostly caught in fresh and brackish water, especially around river mouths. According to Arthur W. Parrott in the Queer and Rare Fishes of New Zealand, in the past large quantities of this species have been taken in Lake Ellesmere, Canterbury, at the mouths of the rivers that enter the lake, mostly amongst weeds. They are also fairly common in Lake Wairarapa and Onoke. Surprisingly they can travel very long distances up rivers see below; Where do you find black flounder? 

Size of Black Flounder

Their average length is 200 to 250 mm. They can attain a length of 450 mm but there are few records of large black flounders (McDowall, 1990). According to Tony Ayling in the Collins Guide to the Sea Fishes of New Zealand, large black flounders up to 450 mm and weighing 2kg are occasionally caught in Lake Ellesmere, Canterbury. 

Colouration of Black Flounder

The black flounder species is the most colourful and attractive of the flatfishes. Colouration can vary quite a bit with no two fish looking exactly alike. According to David H Graham in A Treasury of New Zealand Fishes, “the colour is dark, dull green with red spots which vary from the size of a mere dot to spots as large as a shilling”. A shilling measured 23.6 millimetres across.  

The distribution of the brick red spots can vary with almost every specimen in size and distribution. Sometimes the red spots have a smaller pale cream-coloured spot in their centres. Some have spots around the eyes and nose. While some individuals may have spots on their tail and fin membranes, others, like the one pictured above in a watercolour by Frank Edward Clark, do not.  

The underside of the black flounder is a yellowish-olive colour with each scale being tipped with brown. The undersides of other specimens can often be a muddy grey. David H. Graham notes that he also saw black flounders in the fish markets that had red spots both large and small on their undersides. Some specimens had many red spots on their bellies, whilst others had just a few. 

Graham, who was at one time a biologist at the Marine Fisheries Investigation and Biological Station and Hatchery at Portobello in Dunedin, found that when live black flounders were released into the station’s holding tanks, they had less control over changing their colour to suit their habitat than either the sand-flounder or the greenback flounder.

Graham also mentions being surprised to see many black flounders in the Christchurch Fish Market that had been caught in lakes with parts of their fins, and in some cases with their whole tail missing. Graham was told this was caused by the ravages of eels.

The black flounder has an oval-shaped body. The dorsal fin has between 65 and 67 rays, The anal fin has from 43 to 45 rays.

What Do Black Flounders Eat?

Investigation of the stomach contents of black flounders has shown they had been feeding on weed, worms and often muddy ooze from the bottom. They eat almost anything found in the mud including crustaceans like shrimps and crabs, as well as worms, insects, snails, and almost any organic material. They also eat small fish and are known to feed heavily on whitebait in the spring.   

Where do you find Black Flounder?

They are found all around the New Zealand coast. It is more prevalent on both the east and west coasts of the South Island and lower North Island. They are also found in the Waikato River.

Black flounder is only found in New Zealand.

They are caught in bays along the coast, in estuaries and in harbours. They are catadromous fish. This means they are born in saltwater, then migrate into freshwater as juveniles where they grow into adults before migrating back into the ocean to spawn.

Black flounder are also known as river flounder. They can be caught inside or near river mouths and can also be caught in lowland lakes. Black flounders are well-known for travelling many kilometres upstream and living quite happily in freshwater.

Yellowbelly flounders are a similar New Zealand flatfish that also enters river estuaries and lowland rivers but unlike the black flounder, do not venture beyond the intertidal zone to live in freshwater.     

According to Ron M. McDowall in New Zealand Freshwater Fishes – A Natural History and Guide, “there are reports of black flounder at Ohura, 250 km up the Whanganui River, near the confluence of the Moawhanga and Rangitikei Rivers, 88 km from the sea; at the Manawatu Gorge, 100 km up the Manawatu River; at the Burke Hut, on the Landsborough, a tributary of the Haast (60 km upstream – Walton, 1935); about 60 km up the Clarence River, in North Canterbury; in the headwaters of the Grey River, close to Springs Junction; and in the Pomahaka and Tuapeka Rivers, in the Clutha.” 

In order to penetrate these long distances upstream the black flounder must navigate strong fast flowing water and rapids. 

According to McDowall, the black flounder is almost completely unstudied. Nothing is known about their breeding, but McDowall suspected that “either the adults lay their eggs in rivers and these are washed downstream to the sea, or more likely, the adults migrate downstream to the sea, to spawn.” This seems the most likely as the numbers of black flounder found in freshwater decline during winter. 

Black flounders are known to travel very long distances at sea. There are reports from the Freshwater Division of the old New Zealand Marine Department that black flounders tagged in Lake Ellesmere have later been recaptured hundreds of km down the coast to the south. In one case 176 days later off the Nuggets in South Otago, 370 km away, and another was recaught in the Shag River 270 km south of Lake Ellesmere 265 days later.

Biology

The black flounder probably lays thousands of very small eggs. 

The eggs, which float near the surface of the sea, hatch most probably a few days after spawning. For their first few weeks, they are found in the surface plankton. At this stage, they still have their eyes on each side of their heads and swim upright like normal fish.

When they reach 10 mm in length a metamorphosis takes place. The eye on the left side migrates over the snout to the right side. at this stage, the tiny flounder sinks to the bottom to begin life as a flatfish. The black flounder lays on its left side. Black flounders are not found in freshwater until after this metamorphosis has occurred.

Very small juvenile black flounders only 10-15 mm long are found moving into rivers during the spring.

It is unknown for sure whether the adults die soon after spawning or return to freshwater.      

How do you Catch Black Flounder?

The old-time Maori would catch black flounder by net or spear (Elsdon Best, 1929). Nowadays they are caught by set netting and sold commercially but only in small numbers.

Flounders spend most of their lives hiding at the bottom where they feed. With their eyes high on their heads they are able to scan their surroundings very effectively. They change their skin colour to match the substrate and will shimmy their dorsal and anal fins to partially bury themselves both to avoid being seen by predators and to ambush small fishes. When disturbed they are capable of surprising quick bursts of speed.  

Are Black Flounders Good Eating? 

Black flounder is considered to be excellent eating. However, they do not keep as well as other flounder species and so must be processed or eaten as soon as possible after capture.

As with all flatfish, the most popular way of cooking them is after gutting place the whole fish in a frying pan with butter. Cook one side then turn over and cook the other side. Then place it on a plate and eat each side with a fork leaving the bones in place. Add only pepper and lemon juice either during or after cooking.

In the old time, black or spotted flounder was known to Maori as mohoao. They were taken between May and August in Lake Ellesmere/Te Waihora. It is the fattest of the flatfish found there. They were once caught by Maori in considerable numbers and preserved for future use. When caught they were cleaned and the heads cut off. They would then be hung up by the tail on racks to dry. A rough roof would be built over them to keep the rain off. 

When required for food they would be pounded to soften them. They would then be roasted, broiled or steamed (Elsdon Best, 1929).  

The black flounder was never as numerous as the sand flounder or yellowbelly flounder at any time.   

Right Eye Flounder

A Right Eye Flounder: if you hold the dark upper side of a black flounder, known as the ocular side, facing you with the head to the right, the gills will be at the bottom.

The black flounder is one of a larger group of flatfishes found in New Zealand which includes: 

Black Flounder Rhombosolea retiaria 

Yellow-Belly flounder Rhombosolea leporina 

Sand Flounder Rhombosolea plebeia 

Greenback Flounder Rhombosolea tapirina 

Lemon Sole Pelotretis flavilatus 

New Zealand Sole (common sole) Peltorhamphus novaezeelandiae 

Spotted Flounder Azygopus pinnifasciatus 

Brill Colistium guntheri  

Turbot Colistium nudipinnis. 

References

Te Waihora Mahinga Kai: a compilation of data and summary of existing research on freshwater fishes in Te Waihora. Prepared for the Whakaora Te Waihora Partners by NIWA 

A Treasury of New Zealand Fishes By David H. Graham 

The Queer and the Rare Fishes of New Zealand by Arthur W. Parrott. 

Collins Guide To The Sea Fishes of New Zealand by Tony Ayling, illustrated by Geoffrey J. Cox. 

Handbook of New Zealand Marine Fishers by Larry Paul and John Moreland, Illustrated by Eric Heath. 

New Zealand Freshwater Fishes – A Natural History and Guide by R.M. McDowall. 

A Photographic Guide to Freshwater Fishes of New Zealand by Stella McQueen, with photographs by Rod Morris. 

Fishing Methods and Devices of the Maori by Elsdon Best. 

This post was last modified on 07/02/2023 2:06 am

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