Blue cod
The New Zealand blue cod is a temperate marine ray-finned fish of the family Pinguipedidae. It is also known as Boston blue cod, New Zealand cod, and sand perch, and by its Māori names, rāwaru, pākirikiri, and patutuki.
It is exclusively found in New Zealand, in shallow waters around rocky coasts to a depth of, though it is far more common south of Cook Strait. It is bluish green to blue-black above with white toward the belly. Large examples are usually greenish blue in colour, while smaller ones are blotched in varying shades of brown. An adult may grow to in length and weigh from. It feeds mainly on small fish and crabs. Blue cod is territorial. Spawning takes place in southern spring. Blue cod can also change sex from female to male.
It is an important recreational species in the South Island and is commercially harvested. Blue cod populations are managed under New Zealand's fisheries quota management system, although they are becoming scarce in some small areas due to fishing pressure. Annual catch range is between 2,000 and 2,500 tonnes.
Names
Parapercis colias is most commonly known as the blue cod. It is also known in English as the New Zealand cod or the sand perch. In the indigenous Māori language, the species has a variety of names: kopukopu, pākirikiri, pātutuki, rāwaru and taipua.Identification
Parapercis colias are protogynous hermaphrodites, which means they have both female and male reproductive organs at the beginning and some females change sex to males later in their life. P. colias is a marine bottom dwelling fish that is found in coastal New Zealand waters. Their colouring varies on age and sex. Juveniles start off being generally pale in colour and have two long dark stripes along the sides which will turn brown and barely distinguishable when they become middle-sized fish. As they mature with a body length over 25 cm, both sexes would have a blueish-grey dorsal with a white underside. Females have a tinge of orange and there is a trend for them to become green when they grow larger. Brownish pigments can be found at the base of pectoral fin. On the other hand, adult males have distinctive blue-grey colour coat with greenish sides and a golden brown line can be found above each eye. Body length of P. colias is about 30–40 cm in general, but can up to 60 cm and their weight is 0.8–1.5 kg in general, but can up to 3 kg. Males tend to be larger than females. They can live up to 32 years old.Heads of P. colias are prominent and rounded with scales. They have a plump shaped body covered with firm scales. Above their non expanded cheeks are two large lateral eyes which can rotate independently, this allows them to see almost everywhere around them. P. colias possess a terminal mouth with bulbous lips. Their pelvic fins are generally placed forward on the throat with brown dots appear underneath it. There is a single horizontal stripe where ten to twelve scales above across their side of the body. The anterior section of their dorsal fin is small and short, followed by fins with five short spines. In contrast their second dorsal is long. A key used in an article of Cantwell can distinguish P. colias from other parapercids. Key features are listed below:
- The outer row of lower jaw has eight teeth
- Dorsal spines longer to the posterior
- Soft dorsal fins have 20 rays and anal fins with 17
- 10 to 11 scales are present from lateral line to base of first soft dorsal ray
- 23 to 26 counts of gill rakers
- Caudal fins have 15 branched rays that looks rounded but bi-lobed.
Swimming bladder is usually present in osteichthyids, it helps the fish to stay at a certain water pressure level without spending more energy. Swimming bladder in P. colias are absent. They will automatically sink to the sea floor if they stop swimming; therefore, they are called bottom-dwelling or sedentary bottom-hugging species. Their body shape of having a flattened abdomen indicates this as well. Normally, P. colias use their pectoral fin to swim, their body muscles and tails fin are only involved in swimming when a sudden burst or speed is needed, this type of swimming character is called labriform. As a result, their pelvic fins, used as props when they are resting on the seabed, are reduced and thickened. Due to their nature of being bottom-dwelling species, sand can clog their gills when they are resting. To remedy this, P. colias often hold their breath and open their mouths for a long interval like having a yawn to take good gulps of air every now and then. Speaking of having a yawn, if you look closely you will notice that P. colias do not have palatine teeth and only have small teeth; however, you would not want to feel their sharp and well developed pharyngeal teeth near their throat like their prey.
There are some other detailed morphological characteristics inside the body of P. colias that determine their taxonomy, such as the presence of a lentiform body and others, are mentioned by Eastman.
Distribution
Natural global range
fishes are widely found in the southern Atlantic and Indo-Pacific regions. However, P. colias is endemic to New Zealand.New Zealand range
P. colias is found from the shore to the shelf edge around New Zealand's entire coastline, but there are no records in either the Kermadec Islands or the Snares Islands. They are more abundant south of Cook Strait and they are an iconic species for the South Island. They are most common around Southland and the Chatham Islands.Habitat preferences
P. colias can be found at 150 m in depth occupying bedrock outcrops on gravel or sandy seabed. These habitats with macro algae or Sponges are even more preferred. Furthermore, their abundance in each habitat varies between age. Juvenile are found more frequently in sponge gardens dominated by orange finger sponges, large black massive sponge and the small bright yellow clumps of Polymastia granulosa that provide more shelters and safer refuges, whereas adults are mainly found on reef margins and deeper areas.Since P. colias are generalists, which means that they prey on various species instead of depending on only one species, habitats that are rich in diversity of species are also favoured by them. An example of this is the kelp forest which is often established on rock beds where it is also desirable for P. colias. However, they can be also found in less species diverse habitats such as barren rock flats that are dominated by sea urchins grazing on algae.
As mentioned before, P. colias can be found at 150 m in depth and was reported to be caught at 350m below the surface, other important aspects such as temperature, salinity and oxygen content in the ocean are often influenced by water depth. 766 samples were taken to determine the water temperature and chemistry ranges of P. colias as listed below:
Life cycle/phenology
Spawning and Hatching
Multiple spawning events of a female P. colias occur throughout late winter and spring in the centre to the outer continental shelf. Instead of a distinct spawning episode, their spawning is a continuous event that the eggs are released over time. One male can be spawning with different groups of female. Eggs could possibly drift 74 km away from the spawning site and hatch potentially after 116 hours. Young fish are found below 20 metres then move to shallow water in summer.Age, Growth, Sex and Maturity
Growth of P. colias is measured by the relationship between age and their body length. P. colias are protogynous hermaphrodites, when the length of P. colias reaches 410mm, 50% of the females are transitional. Sex inversion occurs with the colour change from whitish with a brown band at both sides or an orange tinge to bluish colour with a blue green band. Though sex inversion found to occur across a wide range of age and size, the cause of activating the sex inversion has not been well understood. Studies have shown that the proportion of sex change decreases afterwards as the presence of males in the population would discourage sex change. Hence, it is regarded as a response to the demographic structure in the population instead of a response to their size. Another study indicated that blue cod is most fecund at the size which closes to the average size of the first maturity of males. It has been regarded as a sign of potential sex change by reducing egg production for saving the resource. Sex change is also not well described that the transitional gonads has different definitions.Their growth rate can differ due to food supply, water temperature and habitat as other fish do. Sex can also influence the growth rate. Male grow faster and larger than female. P. colias have the potential to grow up to 50 cm in length and weight 4 kg. Growth ring interpretation shows that they can live about 10 to 15 years, but a study showed that the known maximum age of blue cod is 23 years old.
Maturity values are derived from the length of the fish. Their growth rate varies between locations. For example, male can reach maturity when their length reached 10–19 cm in Northland, but in Southland the male needs to reach 26–28 cm. Besides, males generally have a faster growth rate compared to females. Both female and male P. colias are assumed to reach sexual maturity with the measuring of 28 cm. The maximum lifespan of P. colias is 32 years.
Behaviours
Similar to other Parapercis species, male P. colias hold territories and their home range increases as the individual grows. Mutch found that they tend to hold large territories rather than but loose territories with small social groups that contain three to five females. Though they hold stable territories, P. colias moves from time to time. In a study, P. colias can move with on average of 2.09±2.12 km, this could potentially be home range shifts. It is reported that P. colias emigrate from coastal to offshore waters in May of each year, this is thought to be a preparation for mating and spawning in early winter.Thunder is known to agitate the fish. Reports since the 1950s have documented thunder activating blue cod's flight mechanisms, causing the fish to leap out of aquariums.