Blue cod
Blue cod | |
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Scientific classification | |
Domain: | Eukaryota |
Kingdom: | Animalia |
Phylum: | Chordata |
Class: | Actinopterygii |
Order: | Trachiniformes |
Family: | Pinguipedidae |
Genus: | Parapercis |
Species: | P. colias
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Binomial name | |
Parapercis colias (Forster, 1801)
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Synonyms[2] | |
Enchelyopus colias Forster, 1801 |
The New Zealand blue cod (Parapercis colias) is a temperate
It is exclusively found in New Zealand, in shallow waters around rocky coasts to a depth of 150 m, though it is far more common south of Cook Strait.[6] It is bluish green to blue-black above with white toward the belly.[7] Large examples are usually greenish blue in colour, while smaller ones are blotched in varying shades of brown.[7] An adult may grow to 60 cm in length and weigh from 1.0 to 3.0 kg.[8][9] It feeds mainly on small fish and crabs.[10] Blue cod is territorial.[11] Spawning takes place in southern spring.[12][13] Blue cod can also change sex from female to male.[14]
It is an important recreational species in the South Island and is commercially harvested.[15][16] 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.[17] Annual catch range is between 2,000 and 2,500 tonnes.[18]
Identification
Parapercis colias are
Heads of P. colias are prominent and rounded with scales.[12][8][22][23] They have a plump shaped body covered with firm scales.[16] Above their non expanded cheeks are two large lateral eyes[22] which can rotate independently, this allows them to see almost everywhere around them.[24] P. colias possess a terminal mouth[7] with bulbous lips.[12][24] Their pelvic fins are generally placed forward on the throat with brown dots appear underneath it.[21] There is a single horizontal stripe where ten to twelve scales above across their side of the body.[7] The anterior section of their dorsal fin is small and short,[15][21] followed by fins with five short spines.[8] In contrast their second dorsal is long.[21] A key used in an article of Cantwell can distinguish P. colias from other parapercids.[25] Key features are listed below:[25]
- The outer row of lower jaw has eight teeth
- Dorsalspines longer to the posterior (rear end)
- Soft anal finswith 17
- 10 to 11 scales are present from lateral line to base of first soft ray
- 23 to 26 counts of gill rakers
- rays that looks rounded but bi-lobed.[26]
Other characters of detailed skeleton structures (osteology) of Parapercis species can be found in an article written by Gosline.[27]
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 (an ocular vascular structure) and others, are mentioned by Eastman.[32]
Distribution
Natural global range
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
Habitat preferences
P. colias can be found at 150 m in depth
Since P. colias are generalists,[9] 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.[39][40][37] 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.[30]
As mentioned before, P. colias can be found at 150 m in depth and was reported to be caught at 350m below the surface,[6] other important aspects such as temperature, salinity and oxygen content in the ocean are often influenced by water depth.[22] 766 samples were taken to determine the water temperature and chemistry ranges of P. colias[20] as listed below:
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[12][13] in the centre to the outer continental shelf.[16] Instead of a distinct spawning episode, their spawning is a continuous event that the eggs are released over time.[41] One male can be spawning with different groups of female.[8] Eggs could possibly drift 74 km away from the spawning site and hatch potentially after 116 hours.[42] Young fish are found below 20 metres[21] then move to shallow water in summer.[16]
Age, Growth, Sex and Maturity
Growth of P. colias is measured by the relationship between age and their body length.
Their growth rate can differ due to food supply, water temperature and habitat as other fish do.[16] Sex can also influence the growth rate. Male grow faster and larger than female.[3][16] P. colias have the potential to grow up to 50 cm in length and weight 4 kg.[44] Growth ring interpretation shows that they can live about 10 to 15 years,[16] but a study showed that the known maximum age of blue cod is 23 years old.[45]
Maturity values are derived from the length of the fish.[43] Their growth rate varies between locations.[11][46] For example, male can reach maturity when their length reached 10–19 cm (which is about 2–3 years old) in Northland, but in Southland the male needs to reach 26–28 cm (which is about 4–6 years old).[46] Besides, males generally have a faster growth rate compared to females.[3][11][16] Both female and male P. colias are assumed to reach sexual maturity with the measuring of 28 cm.[19] The maximum lifespan of P. colias is 32 years.[18]
Behaviours
Similar to other Parapercis species, male P. colias hold territories and their home range increases as the individual grows.[11] Mutch found that they tend to hold large territories rather than but loose territories with small social groups that contain three to five females.[47] Though they hold stable territories, P. colias moves from time to time. In a study,[6] P. colias can move with on average of 2.09±2.12 km, this could potentially be home range shifts.[45] It is reported[34] 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.[48]
Migration
P. colias seem to be migratory at certain times in a year,[16] but little is known about their migration. Other studies indicated that they are relatively sedentary,[49][50][45] however, long-term dispersal has been suggested due to a record that one individual travelled 156 km over 20 months.[51]
Diet and foraging
Many fish species are
"Our Big Blue Backyard" is a documentary of New Zealand marine and shoreline species.[57] 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.[57] They are also said to be voracious.[57][34]
Predators, parasites, and diseases
Predators
- Great white shark[57]
- Benthic feeders, such as:
- Yellow-eyed penguins[58][59][60]
- Dolphins[61]
- Sea birds, such as mollymawks[47]
As many marine predators are generalists[52] there are many other potential predators not listed. P. colias are known to predate upon juveniles of their own species.[54]
Parasites
Hewitt and Hine[62] summarised the parasites found on blue cod including species in five main groups:
Group | Species | Location on host |
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Protozoa |
|
|
Digenea |
|
|
Monogenea |
|
|
Nematoda |
|
|
Copepoda |
|
Blood sucking sea lice are also known to parasitise P. colias.[57]
Disease and Injuries
No specific disease is found on P. colias, but some injuries have been recorded. Hooking injuries could cause tissue damage and possibly lead to mortality.[63] 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.[64] An article suggests that small hooks cause more blood loss problems than big hooks to P. colias.[63]
Compared to other bony fish, swimming bladder in P. colias is absent,[29] 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.[65]
In a human context
Rāwaru was a valued traditional food for Māori who lived in the South Island, where the fish is more abundant.[48] 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.[66]
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.[48]
History of the name
Blue cod (P. colias) is not a cod. The use of blue cod can date back to the time of Captain Cook.[16][23] Some early writers listed them as coal-fish which derived from unrelated European fish. However, this name is so widely used in New Zealand that it is unlikely to be changed.[16]
Economic and population management issue
Blue cod is well known by the public due to its commercial and recreational importance.
To achieve sustainable fishing, it is crucial to persist healthy population structure with continuous monitoring and quotas set to date.[68] 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.[38] This is an emerging issue to all hermaphrodites.[69] Surprisingly, the sex ratio male to females of P. colias is about 5:1[19] which is biologically implausible. This suggests that sex changes might not purely depend on fish length, but we cannot ignore the fact that fishery has changed the natural population composition of "P. colias". To understand the direct impact on P. colias despite population decline[38] further research is required.
In addition, the fundamental unit of concern of its management of population is the genetic structure that can lead to the reduction of the evolutionary potential for responding to environmental change, the increase of inbreeding risk and the force of selective genetic change.[46] Since a number of evidence indicated that blue cod are relatively sedentary,[49][50][45][46] the fishing pressure may lead to potential local depletion.[51] Recent studies showed that the genetic differences were significant between mainland List of islands of New Zealand population and Chatham Island population.[70][46] On the other hand, the differences within mainlandList of islands of New Zealand populations were limited while the pattern of the isolation by distance was detected.[46] Further research using microsatellite DNA markers suggested that some significant genetic differences between mainland population exist which indicate the potential long-distance dispersal, but the dispersal rate is too low to have demographic effect in the population.[70]
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External links
- Blue Cod Fishing and a link to the Blue Cod Fishing Regulations. Ministry of Fisheries.