Species focus: Sand Flounder

The New Zealand sand flounder is a member of the right-eyed flounder family Pleuronectidae.

In New Zealand there are also left-eyed flounders (Bothidae) and soles (Soleidae) and these are the most commonly encountered flatfish in coastal New Zealand waters, familiar to snorkellers, divers and other fishermen alike.

They are most commonly found in shallow bays, estuaries and harbours where they comfortably live in water depths of less than a metre lying on the bottom partially covered in mud or sand where their camouflage protects them well from predators. When approached carefully by divers they usually remain still. At night they are much more active, ambushing mobile invertebrates such as crustaceans, worms and brittlestars, whilst inadvertently ingesting seaweed and detritus.

Like other flatfishes, the sand flounder undergoes a remarkable metamorphosis. It begins life as a normal looking fish larva, swimming upright (laterally compressed) and with one eye on each side of its body. As it progresses through the various larval stages, firstly feeding off its yolk-sac, then on small planktonic prey, it gradually moves one eye around to the right side.

During this metamorphosis, swimming as an odd-shaped larva can be awkward and exhausting, and is sure to contribute greatly to mortality. By the time it reaches settlement stage, the fish is dorso-ventrally flattened like the flatfish we know. It now moves by undulating its marginal fins, whilst capable of rapid bursts with its tail when spooked.

As an adult flounder, the colouration varies markedly, adopting the colour and pattern of the surrounding substrate via special pigments in the skin. The underside is a pearly white. The unusual, twisted shape of the mouth is due to the movement of the skull and bones as the left eye migrates to the right side.

Flounders have no swim bladder, and only leave the seafloor for courtship and spawning, which they do after a couple of years, moving to deeper waters up to 50 metres in depth. From then on in they migrate back and forth to shallow waters in spring-summer and spawning grounds in autumn-winter.

The sand flounder is also a popular eating fish. Recreational fishers catch them with beach seines, setnets or spears, in harbours and estuaries and even on surf beaches. They are also commercially important, and harvested commercially with trawls and setnets. Traditionally, Maori would use wooden spears, using light from burning torches made of pine to spot them at night on the shallow sand and mudflats.

8 facts to know about sand flounder

1. A right eyed flounder.
   
2. Endemic to New Zealand.
   
3. In Maori, known as patiki, although this refers to several flounder species in general.
   
4. Also known as New Zealand dab, diamond, tinplate, square flounder.
   
5. Begins life with eyes on each side of the body.
   
6. Are known to be omnivorous as larvae, eating algae and spores in addition to zooplankton.
   
7. Adults reach about 45 cm in length.
   
8. Short lived, living only 3–4 years..