Sand Flounder – Rhombosolea plebeia, How to Catch – Excellent to Eat

Sand Flounder - Rhombosolea plebeia. The dorsal and anal fins on the front half of the body are folded down. Photo Allan Burgess.
Sand Flounder - Rhombosolea plebeia. The dorsal and anal fins on the front half of the body are folded down. Photo Allan Burgess.

Sand Flounder – Rhombosolea plebeia – Considered by many to be the best eating fish in the sea

Rhombosolea plebeia (NZ) : Sand flounder, 1873, by Frank Edward Clarke. Purchased 1921. Te Papa (1992-0035-2278/26)
Rhombosolea plebeia (NZ): Sand flounder, 1873, by Frank Edward Clarke. Purchased 1921. Te Papa (1992-0035-2278/26)

The Sand flounder is one of the four main species of flounder, or flatfish, sold in New Zealand fish shops and supermarkets. The other species are the Greenback flounder, Black flounder and Yellow-belly flounder. Sand lounder are also called dab, three-corners, whites and sometimes diamond or square. 

All flounders, along with Soles, Turbot and Brill begin life like any other fish with one eye on each side.  When very young their bodies are round-shaped and the fish swims in a vertical position like other fish. Then something very strange begins to occur.

The Wonderful Early Life of Flounders

According to David H. Graham in a Treasury of New Zealand Fishes, Sand flounder lays anything from one million to three million eggs. The eggs are buoyant and float on the surface of the water. The eggs are microscopic in size. The time taken for the eggs to hatch depends on the temperature of the seawater. Within just a few days of fertilisation, the baby flounder can be seen inside the egg with the aid of a microscope. A few days later the little fish becomes transparent and the beating heart can be seen under the microscope.

After a few more days the small fish, with a yolk sac still attached to the underside of its body, struggles free of the eggshell. Now measuring less than a quarter of an inch (6mm) in length it feeds from the attached yolk sac until it can fend for itself. At this stage, it is swimming vertically like most other fish with an eye on each side of its body.

It begins feeding on spores of seaweed and diatoms (a type of plankton). As they grow they feed on tiny shrimp-like animals in the plankton. After about three weeks, and by now half an inch (12mm) in length, they are still swimming at the surface. Its body shape now begins to broaden out and becomes wider compared with its length.

Now a remarkable series of changes begin in earnest. The gristle in the front part of the head is absorbed. Still, at the baby stage, the skull is cartilaginous like any other fish lava. There is a bar of cartilage above each eye. The left eye now begins to move upwards or over, just a little and very gradually until the left eye has moved right over the edge of the head and both eyes are now close together on the right side of the head. The bar of cartilage above the left eye – which is the one on the more – is absorbed to make way for its passage.

If you look carefully at the mouth of an adult flounder you will notice that it has a peculiar shape. This results from the twisting of the skull and bones to one side.

Now three months of age, and about an inch (25mm) in length, it finally sinks down to the seafloor to begin the life of a flatfish.

Left Eye and Right Eye Flounders

We tend to think of flounders as having a top and a bottom, however more correctly they are actually either Left Eye Flounders: Bothidae, or Right Eye Flounders Pleuronectidae.

Left Eye Flounders: if you hold the flounder dark side facing you with the head to the right, the gills will be at the top. This is the smaller group. It comprises the Finless flounder Mancopsetta milfordi, Witch (Megrim) Arnoglossus scapha and the Crested flounder Lophonectes gallus.

Right Eye Flounders

Right Eye Flounders: if you hold the dark side facing you with the head to the right, the gills will be at the bottom. This larger group includes the 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 and Turbot Colistium nudipinnis.

The Sand Flounder – Rhombosolea plebeia is white on the belly.
The Sand Flounder – Rhombosolea plebeia is white on the belly. The gut is easily removed with the fingers after slitting the belly with a knife. Photo, Allan Burgess.

Colour

Usually, greenish-brown above, but sometimes greyish. They often have faint mottling on the upper side. The underside is almost white. The scales are very small and can be left on and eaten.

Juvenile sand flounders look very similar to yellowbelly flounders.

Flounders Can Rapidly Change Their Skin Colour to Match the Seafloor

They hide on the seafloor not only by covering themselves with sand but also by changing their skin colour to match the colour of the seabed. Graham conducted experiments with sand flounders at the Portobelo Research station to learn how well and how quickly they could make the necessary change of skin colour to match the bottom of a tank they were introduced to. He discovered that this was a learned behaviour. If the seafloor was one they recognised their colour change was almost instant.

Graham conducted a series of experiments whereby he placed seven watertight boxes with seawater running into them side by side. He placed on the bottom of these the following (1) clean grey sand, (2) a brownish growth of weed, (3) clay-coloured gravel, (4) blue and white broken cockle shells, (5) a black-tarred floor, (6) a large white enamel dish, (7) a sheet of linoleum with a square and triangular pattern. He then introduced three sand flounders measuring between four inches (100mm) and seven inches (177mm) into each one.

The three fish placed in the sand changed colour to match the sand in just a few seconds.

Those in the second box of brownish weed took ten minutes to change, first to a mottled brown, and then to the brown colouring of the floor.

Next, those flounders placed in the clay-coloured gravel also changed in stages. After eight minutes they had assumed the exact colouring of the clay gravel right down to matching the contrasting colours of small stones.

The three flounder placed on the blue and white cockle shells had changed within five minutes to perfectly match the bottom.

The three placed in the box with the black-tarred bottom changed first to a dark brown with blotches that were almost black, and then after about twenty minutes gradually changed to a shiny blue-black.

Those in the white enamel dish showed the most stunning of the colour changes. No sooner had the fish reached the bottom than an immediate change took place. In a few seconds their dark colour faded away and they each turned a sickly white!

Those placed over the linoleum were at a disadvantage as they would have been unlikely to have struck such a square and triangular pattern anywhere on the seafloor. It took them two days before they could fully match the unusual pattern of the linoleum. But match it they did. It was interesting to see one half of the top of the flounder with a triangular coloured pattern near the head and squares of another colour near the tail.

Graham also discovered that by moving the fish from box to box the speed of their colour changes was greatly accelerated. After a few weeks of being moved between the boxes, they became “experts” able to change their skin colour to match the new bottom instantly.

Graham was sure the reasons for the colour changes, to perfectly match the bottom, are for protection from enemies, but also to catch its living food. Not only are they almost impossible to see from above by predators, but their perfectly blending into the bottom also makes them excellent camouflaged ambush predators themselves.

Under the flounder’s skin are colour sacs or pigment cells. The fish is able to expand or contract these colour cells to create a vast range of different colours.

Size of Sand Founder

The average length is 25 to 35 cm but can reach 45 cm. There is considerable variation in size around different parts of New Zealand. According to David H. Graham, Sand flounders kept in captivity increased about an inch (25mm) in length each month when fed freely on a diet of crustacean larvae caught from the Portabelo Marine Station jetty.

Graham noted that the growth rate of both small and mature flounders was unaffected by the cold during winter. Graham thought the flounders were a much more hardy fish than Greenbone or Blue Cod which don’t like the cold.

What Do Sand Flounders Eat?

The Sand flounder enjoys a varied diet. The only species of fish found in the stomachs of Sand flounders by Graham was the Ahuru (which looks similar to a red cod).

He also found Sand flounders to have eaten seven species of shellfish, small squids, small octopuses, five species of crabs, whalefeed, amphipods (shrimp-like crustaceans) and isopods (crustaceans that look similar to woodlice). Marine worms are an important food source for Sand flounders with at least eight different species being eaten.

Graham found brittle stars in the stomachs of almost every sand flounder he examined. They also eat Eel-grass and Sea Lettuce.

As you might expect of a fish that feeds on the bottom almost every Sand flounder’s stomach was found to contain sand and ooze.

Fish Species Known to Eat Sand Flounder

On examining the stomach contents of the following species they were found to contain the remains of Sand flounders: Tope (school shark), Spiny dogfish, Flathead, Ling and Toadfish. These were probably caught while the flounders were swimming.

Eating Qualities of Sand Flounder

Many consider the Sand flounder to be the best and sweetest of all fish to eat. The flesh is firm, white and delicate with a very fine flavour. It is quick and easy to cook. The simplest, and many would say the best, method of cooking is to coat the whole fish in white flour and gently fry in butter in a stove-top pan.

Where are Sand Founders Found?

Firstly, Sand flounders are very much a nocturnal fish both in captivity or in their natural habitat. They prefer to remain hidden in the sand during the day and don’t become active until dusk. They are experts at quickly landing on the bottom and tipping sand over themselves until only their eyes are showing. They perform this disappearing act so quickly they can all but vanish in the blink of an eye.

Flounders swim by undulating their fins from head to tail in a waving manner. As anyone who has speared flounder at night will know they can dart away in a zig-zagging motion with a surprisingly rapid burst of speed.

This species is found all around New Zealand being more abundant around the South Island. There are some excellent populations in North Island Bays.

According to D H. Graham in A Treasury of New Zealand Fishes, “Sand flounder is caught inside and outside of Otago Harbour from the extreme high-water mark to isolated pools at low-water left by the sea on mud-flats, to at least 63 fathoms (115m) outside the Otago Heads”.

Sand flounders prefer more open water with a firmer bottom than do yellow bellies. They are usually found in harbours, shallow bays and estuaries. They mostly prefer water less than 50m deep but are occasionally found in depths of 100 metres. 

Graham also reported that Sand flounder is catholic in choosing the sea bottom. He found that they live over Cockle-banks, patches of Eelgrass, and mud-flats inside and outside the harbour, in deep water on sand, mud, pebble, gravel or clay. In fact any type of bottom except rock.

Their growth is rapid, reaching the legal size of 23cm within 2 years. The females grow to a larger size than the males. The maximum daily limit per fisher is 20 in the Central Region. 

Be sure to check the Ministry of Primary Industries website for the recreational fishing rules in your area before you head out on your fishing trip. Catch limits, net mesh size and fish length may vary in different parts of New Zealand.  

Sand flounders are preyed upon by a variety of fish and birds. I have often seen shags catching and devouring juvenile flounder near river mouths that would appear at first glance to be too large for the bird to swallow but somehow they manage to do so.

Graham reported that those caught inside Otago Harbour were much better in quality than those caught outside. Within Otago Harbour, they were caught at low tide with seine nets along shallow beaches.

They were also caught in Otago Harbour at high tide in set nets. The set nets were usually set at night when the flounders would be swimming about.

Te Waihora – Lake Ellesmere Flounder Fishery

This is one of the three main species of flounder caught commercially in Canterbury’s Lake Ellesmere. Sand flounder make up approximately 12 per cent of the flatfish caught there.

A plot of the Te Waihora – Lake Ellesmere commercial fishery returns for the period 1990-2013 by species caught, shows that black flounders provide the bulk of the catch (633 tonnes, 43%) over the past 23 years, followed by sand flounders (216 tonnes,15%) and yellowbelly flounders (181 tonnes, 12%). 

There is however considerable variation in the numbers of flounder caught in the lake from year to year. There can be as much as a 10 fold increase or decrease between adjacent years. The timing of the arrival of flatfish into Lake Ellesmere is not fully understood, as most of the information gathered was mainly based on a sampling from 1994 when the lake was open to the sea for an extended period. Reference below from NIWA.

Flounder do not spawn in Lake Ellesmere

The main species of flatfish in Te Waihora – Lake Ellesmere do not spawn in the lake. The abundance or otherwise of flatfish in the lake depends on the recruitment of juveniles from the sea when the lake is open. 

According to the summary of known information by NIWA, some adult fish may also move into the lake when it is open, but probably not in great numbers. Maturing adult flounders migrate out of the lake during lake openings in winter and spawn at sea (Jellyman 2012). 

Adult yellowbelly and sand flounders often have pronounced ovaries throughout long periods of the year, indicating prolonged breeding seasons, while black flounders appear to mature more rapidly, with nearly mature fish congregating at Taumutu in July and August. If the lake is not open to the sea the gonads are reabsorbed into the flounder until the following year.

Flatfish spawning in the sea occurs over many months and juvenile flatfish may recruit into the lake over 4-6 months, but only if the lake is open.

Timing of black (Rhombosolea retiaria), sand (R. plebia) and yellowbelly (R. leporina) flounder spawning and recruitment are recorded as either “Spawning” (occurrence of ripe fish or larvae) or “Recruitment” (arrival of juveniles into shallow marine areas of estuaries/lakes). From Jellyman (2012).

When Do Sand Flounder Spawn?

According to the following studies, there appears to be some variation in spawning times around New Zealand.  The time of year during which sand flounder spawn is determined either via the occurrence of ripe fish or larvae or by recruitment (which of the arrival of juveniles into shallow marine areas of estuaries or lakes).

Sand flounders Te Waihora, Sept – Dec. Jellyman 2011

Ahuriri Lagoon, July – Dec Kilner & Akroyd. 1978

Hauraki Gulf, Jun – Nov Colman. 1973

Otago Coast Jul – Feb June – Jan. Roper & Jillett. 1981

Kaikoura, Jan – Oct Hickford & Schiel. 2003

Canterbury Bight, Jun-Dec Mundy. 1968

New Zealand, Sep-Dec Ayling & Cox. 1982

New Zealand, Winter and spring Paul. 2000 

How to Catch Sand Flounder

Recreational anglers catch sand flounder by set netting, beach seining, spearing, and sometimes by line fishing in suitable areas with light tackle on very small 8-10 long shank hooks. Baited hooks should be fished right on the bottom. Try a rig with a sinker at the bottom and a second smaller sinker above the hooks to ensure all your hooks rest on the sand.  

The old-time Maori referred to the sand flounder as patiki. They speared them at night on the mudflats using slender wooden spears. They used torches made from resinous wood to see and spear them. 

The best baits for flounder are live yabbies which can be caught from sandy beaches with the aid of a bait pump. 

Sand flounders will also take soft plastic baits fished along or near the bottom. Allow the soft bait jig to touch the bottom now and again which sends up a puff of sand which the flounder finds irresistible and will investigate the source. 

The best place to cast your baited hooks is into narrow channels left as the tide recedes as this will be where the flounder will congregate.

As the tide rises and the water becomes deeper flounder become more active.

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 Geffrey J. Cox.

 

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