Yellowbelly flounder facts for kids

Quick facts for kids Yellowbelly flounder
Drawing by Dr Tony Ayling
Scientific classification
Synonyms
Rhombosolea millari Waite, 1911

The yellowbelly flounder (Rhombosolea leporina) is a type of flatfish found near New Zealand. It belongs to the Rhombosolea family. Another fish in Australia is also called yellow-belly flounder, but it's a different species. The Māori have fished for this flounder and other flatfish for hundreds of years. They call this fish 'patiki totara'.

What Does the Yellowbelly Flounder Look Like?

The yellowbelly flounder is part of the Pleuronectidae family. All fish in this family have both eyes on the right side of their bodies. The top side of the flounder, where its eyes are, is a dark olive green. It has smooth scales with dark edges. The underside, or "blind side," is usually white when the fish is young. As it gets older, this side turns yellow with dark spots.

Adult flounders have long, oval-shaped bodies. Their small eyes are set further back from their pointed snout than in most other flounder species. They have 13 to 17 rakers in each gill. They also have about 75 to 86 scales along their lateral line. Adult flounders are usually between 25 and 40 centimeters long. Some females can grow up to 45 centimeters. On average, female flounders are longer and heavier than males.

Where Do Yellowbelly Flounders Live?

The yellowbelly flounder is only found in the South Pacific. It lives only in the coastal waters around New Zealand. This species is common in many inshore areas of New Zealand. However, you will find more of them around the North Island. This is because the flounder prefers warmer water.

Yellowbelly Flounder Habitat

The yellowbelly flounder lives on the seafloor in areas with mud or sand. These areas include estuaries, bays, harbours, and other shallow places along the coast. The seafloor and the water just above it are called the demersal zone. Like many flounder species, they are benthic fish. This means they live and find their food on the seafloor.

They like water temperatures between 12 and 18 degrees Celsius. Young flounders and larvae prefer warmer water. Adult flounders prefer cooler water, especially when they are breeding. As flounders grow, they move to different habitats. Young flounders, less than 5 centimeters long, like warm, shallow mudflats heated by the sun.

When young flounders are 5 to 20 centimeters long, they move out with the ebb tide. They then live in slightly deeper channels closer to shore. Once flounders become adults, they move even further out into deeper, colder coastal waters. During their spawning season in winter and spring, adults move offshore. They go to coastal waters about 30 to 50 meters deep to lay their eggs.

Yellowbelly Flounder Life Cycle

Yellowbelly flounders usually live for three to four years. Some can live up to five years if conditions are good. This means most flounders will only lay eggs once in their lives. However, those that live longer than four years might lay eggs twice. Their life cycle begins in spring during the spawning season. This happens in coastal waters that are about 15 degrees Celsius and 30 to 50 meters deep.

After the female flounder lays her eggs, the tiny fish, called larvae, hatch in about four to five days. This happens when they are kept at 15 degrees Celsius in a lab. The embryo has a small yolk that feeds the larva. The larva then drifts with the tide from the open ocean, called the pelagic environment. It moves towards the shallow, sheltered mudflats along the coastline. The yolk doesn't feed the larva for very long. So, adult flounders try to breed as close to the coast as possible. This helps more young fish reach the inshore waters safely.

When they first hatch, R. leporina larvae look like other fish larvae. They are symmetrical, meaning both sides are the same. As the larva changes from living in the open ocean to living on the seafloor, it goes through metamorphosis. During this change, the left eye moves over to the right side of the body. This creates one side with both eyes (the ocular side) and one side without eyes (the blind side). Young flounders gather in their new home near the shore. They start to lie on their sides on the seafloor. The right side, which now has both eyes, gets darker. This helps the flounder blend in with the seafloor. This camouflage protects them from animals that want to eat them. As flounders grow, they move into deeper waters. They become adults when they are longer than 20 centimeters, usually around two years old.

Reproduction of Yellowbelly Flounders

Yellowbelly flounders become ready to reproduce after they are two years old. At this age, female flounders are about 29 centimeters long, and males are about 24 centimeters. Female flounders grow faster and are generally longer and heavier than males.

Female R. leporina are known for laying many eggs. The number of eggs can change based on the female's size and health. Larger females lay more eggs than smaller ones. For example, a 45-centimeter flounder can release up to 1.25 million eggs. A smaller female, only 30 centimeters long, might release around 250,000 eggs.

In the winter before spawning season, the reproductive organs, called gonads, grow larger in both males and females. The ovaries in females also start to prepare certain eggs for fertilization. Adult flounders move offshore during winter to breed in deeper coastal areas. This happens over the winter and spring months. Even though the breeding season is long, lasting at least six months, flounders only lay eggs once during this time. After laying eggs, they start to move back to estuarine habitats for the summer.

What Do Yellowbelly Flounders Eat?

Flounders are benthic feeders. This means they find and eat their food from the seafloor. Young flounders, less than two years old, are not very picky eaters. They mostly eat small invertebrates and detritus (decaying plant and animal matter). They find this food at the bottom of shallow mudflats. Their diet mainly includes amphipods. But they also eat other small crustaceans, annelids, molluscs, and nematodes. They use their uneven jaw to scoop this food into their mouth. This way of eating also means they scoop up a lot of sand, small rocks, and mud.

Young adult flounders continue to eat like juveniles. But they become pickier as they grow into full adults. They start eating larger invertebrates and less dirt. Adults can feed in the same areas as young flounders. However, they usually only eat crustaceans that have been washed in by the high tide, like mud crabs. When adults move offshore in winter and spring, they have also been known to eat whitebait.

R. leporina has small eyes that are set back from its snout. This means they can't see very well around their mouth. To make up for this, they have a very good sense of touch. They use the front rays of their dorsal fins and their lateral lines along their body. The front rays help them find prey hidden under the sand or in choppy water. The lateral lines sense pressure waves and low-frequency sounds in the water. This species also has fin rays with taste buds. They also have a tongue and more taste buds on the blind side of their head. These help the flounder tell the difference between food and unwanted things as it eats.

Threats to Yellowbelly Flounders

Predators of Flounders

During the day, flounders are less active. They use camouflage to avoid being eaten. The flounder blends in with the seafloor and buries itself in the sand. Only its eyes stick out, watching for food or danger. Large fish, sharks, and eels will eat flounders if they spot them.

Parasites Affecting Flounders

The nematode parasite Cucullanus antipodeus is often found in the stomach and gut of R. leporina. It is more common in young flounders because they are not picky eaters. Myxosporian parasites are also often found in the kidneys of flounders living in harbors. Scientists are not sure if these parasites seriously harm the flounder.

Diseases in Flounders

High temperatures, low oxygen levels, and low pH can cause stress in flounders. This stress makes them more likely to get sick. R. leporina can also get sick or have deformities from pollution. This pollution comes from effluent, chemicals, and industrial waste that runs into estuaries and harbors.

Human Activities and Flounders

People fish for flounders for fun and for business all over New Zealand. They use scoop nets or flatfish trawling. Trawling allows many flounders to be caught at once. There is concern about how New Zealand's flatfish species are managed. There aren't enough good ways to manage fishing, enforce catch limits, or use safe fishing methods. Flatfish trawling, the most common way to catch flounders, harms the benthic habitats on the seafloor. It can also catch other animals that are not meant to be caught, like marine mammals, seabirds, and other fish.

Even though the number of R. leporina is not a big concern right now, it is one of eight flatfish species managed as one group. Not keeping a close eye on how many flounders there are, and using general management plans, has raised worries about whether this species can be fished sustainably in the future.

Weirs built in waterways are another threat to R. leporina. Weirs are structures that change water flow. They help reduce flooding, slow down water, and raise water levels upstream. Weirs can kill flounders or stop them from moving from inland waters to offshore areas to lay eggs. If adult flounders cannot lay eggs, fewer young fish are born. This could negatively affect the number of flounders in the future.