The New Zealand blue cod (Parapercis colias) is a temperate marine ray-finned fish It is also known as Boston blue cod, New Zealand cod, and sand perch, and by its Māori names, , , and .
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. Blue cod is territorial. , , and .
Identification
thumb|left|1828 diagrams by [[Georges Cuvier]]
alt=Blue Cod Closeup|thumb|left|Parapercis colias female showing brown colour phase, details of the eye and lips.
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. On the other hand, adult males have distinctive blue-grey colour coat with greenish sides They have a plump shaped body covered with firm scales. P. colias possess a terminal mouth
- The outer row of lower jaw has eight teeth
- Dorsal spines longer to the posterior (rear end)
- 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 (tail) fins have 15 branched rays that looks rounded but bi-lobed.
Other characters of detailed skeleton structures (osteology) of Parapercis species can be found in an article written by Gosline.
Swimming bladder is usually present in osteichthyids, it helps the fish to stay at a certain water pressure level (depth) without spending more energy. Swimming bladder in P. colias are absent. 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.
Distribution
Natural global range
Pinguipedid fishes (Sandperches) 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
thumb|Blue cod in [[Milford Sound on a bedrock outcrop]]
P. colias can be found at 150 m in depth or sandy seabed. These habitats with macro algae or Sponges are even more preferred. and deeper areas. 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 (4-12m deep) that are dominated by sea urchins grazing on algae. 766 samples were taken to determine the water temperature and chemistry ranges of P. colias as listed below:
:{||-
! scope="col" width="width: 1000em;" |
! scope="col" width="width: 20em;" |
! scope="col" width="width: 20em;" |
|-
|Temperature:
| 7.786 - 18.158
| (°C)
|-
|Nitrate:
| 0.205 - 18.689
| (μmol/L)
|-
|Salinity:
| 34.283 - 35.544
| (PPS)
|-
|Oxygen:
| 5.121 - 6.587
| (mL/L)
|-
|Phosphate:
| 0.258 - 1.333
| (μmol/L)
|-
|Silicate:
| 1.911 - 7.690
| (μmol/L)
|-
|}
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. One male can be spawning with different groups of female. 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. Growth ring interpretation shows that they can live about 10 to 15 years,
Maturity values are derived from the length of the fish. Besides, males generally have a faster growth rate compared to females.
Diet and foraging
Many fish species are generalists; thus, they are not limited by the predator-prey cycle. P. colias is one of them. It makes sense that when P. colias grows bigger, the variety of species in their diet also increases, this includes polychaetes, crustaceans, molluscs, algae and even its own spawn! and whether the area is fished or not plays an important role as well. In the Chatham Islands episode, blue cod (P. colias) waits to feed on pāua, an endemic sea-snail that attaches itself on hard surfaces such as rocks, at its most vulnerable phases – when they move or are grasped up by a starfish using hundreds of tube feet. Natural predators include:
- Great white shark
- Dolphins
- Sea birds, such as mollymawks
As many marine predators are generalists summarised the parasites found on blue cod including species in five main groups:
{| class="wikitable" style="text-align: left;"
|-
! style="width:20em;" |Group
! style="width:20em;" |Species
! style="width:20em;" |Location on host
|-
|Protozoa
|
- Trypanosoma parapercis
|
- Blood
|-
|Digenea
|
- Pancreadium otagoensis
- Plagioporus(Caudotestis) pachysomus
- Steringotrema rotundum
- Lecitocladium excisum
- Gonocerca phyddis
|
- Intestine
- Intestine
- Gall bladder and stomach
- Stomach
- Stomach
|-
|Monogenea
|
- Microcotyle constricta
|
- Gills
|-
|Nematoda
|
- Anisakis sp. larva
- Contracaecum (Thynnascaris) sp. larva
|
- Viscera, mesenteries and under peritoneum
- Stomach, intestine and body cavity
|-
|Copepoda
|
- Aethon percis
- Caligus buechlerae
|
- Gills
- Skin
|}
Blood sucking sea lice are also known to parasitise P. colias. Despite having greater risk for parasitic, bacterial, or fungal infections, mortality after having a hook injury is often caused by blood loss rather than disease. An article suggests that small hooks cause more blood loss problems than big hooks to P. colias. this means that they do not suffer from barotrauma – an injury due expansion or eruption of the swimming bladder in fish, it is often caused by rapid water pressure change when rising from deep water to water surface.
In a human context
thumb|A display of smoked blue cod in [[Melbourne, Victoria (1907)]]
Rāwaru was a valued traditional food for Māori who lived in the South Island, where the fish is more abundant. Heads of rāwaru (P. colias, blue cod) were often given back to the sea as offerings to the god Maru before Māori return from fishing.
The fish was an important species to early European settlers in New Zealand as a food source, and by the 1910s became even more popular in Australia, leading to the growth of the blue cod fishing industry in Southland. It is estimated that the marginal willingness to pay for an additional P. colias is $1.61 per fish and the average willingness to pay is $24.46 per fish. It is estimated that in 1999, 1.2 million P. colias was harvested, of which 70% were from the South Island. However, as the largest fish in the population would be the male, males are often being caught and is thought to affect the females changing their sex earlier. Surprisingly, the sex ratio male to females of P. colias is about 5:1