New Zealand smelt facts for kids

Quick facts for kids New Zealand smelt
Scientific classification

The New Zealand smelt (Retropinna retropinna) is a small, silvery fish found only in New Zealand. People also call it the New Zealand common smelt or the New Zealand cucumber fish. It belongs to the smelt family, called Retropinnidae. You can find these fish in shallow parts of estuaries (where rivers meet the sea) and rivers. They usually grow to be about 8 to 13 centimeters long.

What is a New Zealand Smelt?

New Zealand smelt are long, silver-green fish. They have clear fins and are a bit bigger than your hand. You can spot a smelt by its special adipose fin. This is a small, soft bump located between its back fin and its tail.

Smelt also have scales and a tail that looks like a fork. Another cool way to tell them apart is their unique smell. When you catch one, it smells a bit like a cucumber! There are two types of smelt in New Zealand: the common smelt and Stokell's smelt. They look very similar, but you can tell them apart by looking closely at the size and number of their scales.

Where Do Smelt Live in New Zealand?

Common smelt are found all over the North Island of New Zealand. However, you won't find them in the South Island; only Stokell's smelt live there.

These fish live in coastal seas and estuaries. They also swim far up into freshwater streams. Even though smelt are not very strong swimmers against the current, they have been found very far inland. Some have been seen up to 300 kilometers from the coast! They can even live at high altitudes, up to 2000 meters above sea level.

Smelt Homes and Preferences

Smelt are quite sensitive fish. This means they need very specific conditions to be happy and healthy. They prefer water temperatures between 15 and 17 degrees Celsius. However, they can sometimes handle warmer water, up to 27 degrees.

They also like water that is slightly alkaline, with a pH of 8-9. Smelt prefer clear water with very little mud or dirt floating in it.

Some smelt spend part of their lives in freshwater and part in saltwater. These are called diadromous fish. Others spend most of their lives in the sea and only come into freshwater to lay their eggs. These are called anadromous fish. Smelt often live in large groups called shoals in estuaries. They spend most of their lives in the sea but return to freshwater rivers in the summer as adults. You can also find smelt living in lakes and lowland rivers.

The Life Cycle of a Smelt

To understand the smelt's life, we can think of them in two groups. There are smelt that live in lakes or man-made reservoirs (called lacustrine or reservoir smelt). Then there are the diadromous smelt, which move between fresh and saltwater.

Diadromous smelt start their lives in the sea, estuaries, or the lower parts of rivers. When they grow up, usually in spring, adult smelt swim upstream into lakes through rivers. They spend the summer and early autumn eating. Then, in late autumn, they lay their eggs on sandbars.

Smelt that live in lakes or reservoirs do not travel. They stay in their lake homes to eat and reproduce. Smelt are a semelparous species. This means that adult smelt usually die after they lay their eggs, typically when they are about one year old. Scientists think that smelt might lay eggs several times in one breeding season. However, they only live for one season of breeding.

Smelt like to lay their eggs on clean sand. The water should be between 0.5 and 2.5 meters deep and slow-moving. The warmer the water, the faster the eggs hatch. The best temperatures for hatching are between 14 and 18 degrees Celsius. Eggs from lake smelt hatch faster (8-10 days) than those from smelt that travel between fresh and saltwater (10-18 days).

Once the tiny smelt larvae hatch, they spread out in the water. They can be found as deep as 40 meters. Larvae are very small, from 4 millimeters up to about 30 millimeters long. When they grow past 25mm, they become juveniles. At this stage, they start to swim near the surface instead of just floating. They begin to form schools, and their depth range shrinks to 20 meters. Smelt naturally swim in schools near the surface.

When a smelt reaches 30mm long, it can start to migrate. By 50mm long, it is considered an adult. Adult smelt can swim at much greater depths, anywhere from 20 to 70 meters deep.

What Do Smelt Eat?

When smelt are tiny larvae, they mostly eat very small water animals called Rotifers. As they grow into juveniles, their diet changes. They start eating small crustaceans called Copepods and tiny water fleas called Cladocerans.

Adult smelt eat zooplankton and small insect larvae. They catch these near the water's surface. Not much else is known about what they eat. However, they don't eat plants that grow underwater. Smelt are usually seen near the surface or in the middle of streams, not feeding on the bottom.

Who Eats Smelt?

Smelt eggs can be eaten by adult bullies (another type of fish) in lakes. But there are no known predators that eat smelt larvae.

Adult smelt in lakes are a main food source for trout in the North Island. Young smelt that are migrating upstream are often caught by people who fish for 'whitebait'. Many birds also hunt smelt. Black and Red Billed Gulls mainly eat young smelt. Adult smelt are hunted by Pied Shags and Little Black Shags. These shags sometimes work together, swimming and diving in groups to catch the smelt. Shags are considered the main bird predators of adult smelt.

Smelt can also get parasites and diseases. Some common ones include:

• Black spot: This is a small cyst that forms under the fish's skin.
• Glochidia: These are tiny, young freshwater mussels. They attach to smelt to travel to new homes.
• Saprolegnia: This is a fungal infection that can affect a smelt's fins, especially if the fish has been injured.

Other Cool Facts About Smelt

In the past, Māori people traditionally caught smelt using special scoop nets called Hinaki. The fish could be eaten fresh or dried in the sun to keep them for later. Māori knew smelt by the names pōrohe and paraki.

The smelt also has another name, the cucumber fish. This is because of its special smell, which is like a fresh cucumber, especially when it has just been caught!

Images for kids

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  A New Zealand Smelt