Blue cod facts for kids

Quick facts for kids Blue cod
Conservation status
Least Concern (IUCN 3.1)
Scientific classification
Synonyms
Enchelyopus colias Forster, 1801

The New Zealand blue cod (Parapercis colias) is a cool-water ocean fish. It belongs to the Pinguipedidae family. People also call it Boston blue cod, New Zealand cod, or sand perch. Its Māori names are rāwaru, pākirikiri, and patutuki.

This fish lives only in New Zealand. You can find it in shallow waters around rocky coasts, down to about 150 meters deep. It's much more common south of Cook Strait. Blue cod are usually bluish-green to blue-black on top and white on their belly. Larger fish are often greenish-blue. Smaller ones have brown blotches. An adult can grow up to 60 centimeters long and weigh between 1 to 3 kilograms. They mostly eat small fish and crabs. Blue cod are territorial, meaning they defend their living space. They lay their eggs in the southern spring. Interestingly, blue cod can also change sex from female to male!

The New Zealand blue cod is a popular fish for people who fish for fun. It's also caught by commercial fishers. New Zealand manages blue cod numbers to keep them healthy. However, in some small areas, there are fewer blue cod because of too much fishing. About 2,000 to 2,500 tonnes of blue cod are caught each year.

How to Identify a Blue Cod

Blue cod are special because they are protogynous hermaphrodites. This means they are born female and can change into males later in life. P. colias is a bottom-dwelling fish found in New Zealand's coastal waters. Their colors change with age and sex.

Young blue cod are usually pale. They have two long dark stripes on their sides. As they grow, these stripes turn brown and are harder to see. When they are over 25 cm long, both males and females have a bluish-grey back and a white belly. Females might have a bit of orange and can turn green when they get bigger. They also have brownish spots near their pectoral fins (the fins behind their gills). Adult males are a distinct blue-grey with greenish sides. They have a golden-brown line above each eye.

Most blue cod are about 30–40 cm long and weigh 0.8–1.5 kg. But they can grow up to 60 cm and weigh 3 kg! Males are usually bigger than females. They can live for up to 32 years.

Unique Features of Blue Cod

Blue cod have a large, rounded head with scales. Their body is plump and covered with firm scales. They have two large eyes above their cheeks that can move separately. This helps them see almost everywhere! Their mouth is at the front with thick lips. Their pelvic fins (underneath their body) are usually placed forward on their throat and have brown dots. They have a single horizontal stripe with 10 to 12 scales across their side.

Their first dorsal fin (on their back) is small and short, followed by fins with five short spines. Their second dorsal fin is long. Here are some key features that help tell P. colias apart from other similar fish:

• They have eight teeth in the outer row of their lower jaw.
• The spines on their back (dorsal) fins get longer towards the back.
• Their soft dorsal fins have 20 rays, and their anal fins have 17.
• They have 10 to 11 scales from their lateral line to the base of their first soft dorsal ray.
• They have 23 to 26 gill rakers (structures that filter food).
• Their tail fins are rounded but look like they have two lobes, with 15 branched rays.

Why Blue Cod Stay on the Seabed

Most bony fish have a swim bladder. This helps them float at different depths without using much energy. But blue cod do not have a swim bladder! This means they will sink to the sea floor if they stop swimming. That's why they are called bottom-dwelling or "sedentary bottom-hugging" fish. Their flattened belly also shows this.

Blue cod usually swim using their pectoral fins. They only use their body muscles and tail fin for quick bursts of speed. When they rest on the seabed, sand can get into their gills. To fix this, blue cod often hold their breath and open their mouths wide, like a yawn, to take in fresh water. They have small teeth, but their sharp, strong pharyngeal teeth (in their throat) are used to crush their prey.

Where Blue Cod Live

Global Range

Fish similar to blue cod (called sandperches) are found in the southern Atlantic and Indo-Pacific oceans. However, the P. colias blue cod is found only in New Zealand. This means it is endemic to New Zealand.

New Zealand Range

You can find P. colias all around New Zealand's coastline, from the shore to the edge of the continental shelf. But they are not found around the Kermadec Islands or the Snares Islands / Tini Heke. They are much more common south of Cook Strait and are a well-known fish in the South Island. They are most often seen around Southland and the Chatham Islands.

Blue Cod Habitats

Blue cod live in depths up to 150 meters. They prefer rocky areas on gravel or sandy seabeds. They especially like habitats with large seaweeds or sponges.

Young blue cod are often found in "sponge gardens" (deeper than 16 meters). These gardens have orange finger sponges, large black sponges, and small yellow sponge clumps. These sponges offer lots of places for young fish to hide and stay safe. Adult blue cod mostly live at the edges of reefs and in deeper areas.

Blue cod are generalists, meaning they eat many different kinds of food. So, they like habitats with a wide variety of species. For example, kelp forests, which grow on rocky seabeds, are good places for blue cod. They can also be found in areas with fewer species, like bare rock flats (4-12 meters deep) where sea urchins graze on algae.

Scientists have studied the water conditions where blue cod live. They found that blue cod can be found in water with temperatures between 7.786 and 18.158 degrees Celsius. They also live in water with specific levels of salt (salinity), oxygen, and other chemicals.

Life Cycle and Reproduction

Spawning and Hatching

Female blue cod lay their eggs multiple times during late winter and spring. They don't release all their eggs at once but continuously over time. One male blue cod can mate with different groups of females. The eggs can drift up to 74 km from where they were laid and hatch after about 116 hours. Young fish are found below 20 meters deep and then move to shallower water in the summer.

Age, Growth, and Sex Change

Blue cod growth is measured by how their age relates to their body length. As mentioned, blue cod are protogynous hermaphrodites. When a blue cod reaches about 410 mm in length, half of the females are in the process of changing sex. This sex change often happens when their color changes from whitish with a brown band or an orange tint to a bluish color with a blue-green band.

Scientists don't fully understand why or when this sex change happens, as it can occur at different ages and sizes. Some studies suggest that if there are already many males in the population, females might be less likely to change sex. This means sex change might be a way for the population to keep a healthy balance of males and females, rather than just depending on the fish's size.

Blue cod grow at different rates depending on how much food is available, water temperature, and their habitat. Males grow faster and get larger than females. Blue cod can grow up to 50 cm long and weigh 4 kg. They can live for about 10 to 15 years, but some have been known to live up to 23 years. The oldest known blue cod lived for 32 years.

Blue cod reach maturity at different sizes in different places. For example, males in Northland might be ready to reproduce at 10–19 cm long (around 2–3 years old). But in Southland, males need to be 26–28 cm long (around 4–6 years old) to be mature. Both male and female blue cod are generally thought to be mature when they reach 28 cm.

Blue Cod Behavior

Like other similar fish, male blue cod protect their territories. Their home range gets bigger as they grow. Studies show that males tend to have large territories. They might live in small groups with three to five females. Even though they have stable territories, blue cod do move around. One study found that blue cod can move an average of 2.09 km. This might be them shifting their home range. It's also thought that blue cod move from coastal waters to deeper offshore waters in May each year. This could be to get ready for mating and laying eggs in early winter.

Migration

Blue cod seem to move around at certain times of the year, but not much is known about their migration. Some studies say they stay in one place, but one fish was recorded traveling 156 km over 20 months! This suggests they can sometimes travel long distances.

What Blue Cod Eat

Blue cod are generalists, meaning they eat many different things. They are not limited to just one type of food. Scientists have found 52 different types of food in their diet! Adult blue cod are pickier eaters than young ones. As blue cod grow bigger, they eat a wider variety of prey. This includes worms, crabs and shrimp, snails and clams, other fish, algae, and even their own eggs! This means blue cod are omnivores, eating from more than one level of the food chain. Basically, they eat whatever is common and available where they live.

What blue cod eat also depends on the region and whether the area is fished. One study showed that areas where oysters are dredged (removed from the seabed) can reduce the variety of food for blue cod. This can also negatively affect the size of the fish. Blue cod usually sneak up on their prey, grab it, and swallow it whole. If they get caught, they often spit out what's in their stomach.

In the "Our Big Blue Backyard" documentary about New Zealand marine life, blue cod in the Chatham Islands are shown waiting to eat pāua. Pāua are native sea snails that stick to rocks. Blue cod attack them when they move or when a starfish grabs them. Blue cod are known to be very hungry eaters.

Predators and Parasites

Who Eats Blue Cod?

Humans (Homo sapiens) catch a lot of blue cod, about 2,000 to 2,500 tonnes every year. Natural predators of blue cod include:

• Great white sharks
• Bottom-feeding animals like Yellow-eyed penguins and dolphins
• Sea birds like mollymawks

Many ocean predators eat a variety of foods, so there are likely other animals that eat blue cod that are not listed. Blue cod are also known to eat younger blue cod.

Blue Cod Parasites

Scientists have found different parasites living on blue cod. Here are some main groups:

Blood-sucking sea lice can also live on blue cod.

Diseases and Injuries

No specific diseases have been found in blue cod. However, they can get injuries. If a fish is caught with a hook, it can cause tissue damage and possibly lead to death. Even though there's a higher risk of infections from parasites, bacteria, or fungi after a hook injury, death is often caused by blood loss. Some research suggests that small hooks cause more blood loss problems for blue cod than large hooks.

Because blue cod don't have a swim bladder, they don't suffer from barotrauma. This is an injury that happens to fish with swim bladders when they are brought up quickly from deep water to the surface. Their swim bladder expands or bursts due to the rapid change in water pressure.

Cultural Uses

The heads of rāwaru (blue cod) were often given back to the sea by Māori as offerings to the god Maru before they returned from fishing.

History of the Name

Even though it's called "blue cod," P. colias is not actually a cod fish. The name "blue cod" has been used since the time of Captain Cook. Some early writers even called them "coal-fish," which came from unrelated European fish. However, this name is so widely used in New Zealand that it's unlikely to change now.

Managing Blue Cod Populations

Blue cod are very important for both commercial fishing and recreational fishing. As mentioned, about 2,000 to 2,500 tonnes of P. colias are caught each year. It's also the third most popular fish for recreational fishing. People even use blue cod flesh as bait for rock lobsters.

To protect blue cod, New Zealand's Ministry for Primary Industries (New Zealand) sets rules. These rules include a minimum length for fish that can be caught and a maximum number of fish a person can catch per day. These limits change depending on the area, based on how many blue cod are there. For example, in the South-East area of the South Island (from Clarence Point to Southland and extending to the Chatham Islands), you cannot take any fish shorter than 30 cm. Each fisher can only catch up to 30 fish per day.

To have sustainable fishing, it's important to keep blue cod populations healthy. This means constantly checking their numbers and setting quotas. However, since the largest fish in the population are usually males, they are often caught. This might cause females to change sex earlier, which is a concern for all fish that can change sex. Surprisingly, the ratio of males to females in blue cod is about 5 males to 1 female, which doesn't seem natural. This suggests that fishing might have changed the natural balance of the blue cod population. More research is needed to understand the full impact of fishing on blue cod.

Also, because blue cod tend to stay in one place, too much fishing in one area could lead to fewer fish there. Studies have shown that blue cod populations on the New Zealand mainland are genetically different from those around the Chatham Islands. This means they don't mix much. While there are some small genetic differences within mainland populations, blue cod don't travel far enough to have a big effect on the overall population numbers.

Images for kids

• 

  Parapercis colias female showing brown color phase, details of the eye and lips.

See also

In Spanish: Bacalao azul de Nueva Zelanda para niños