Forget the fishing stories – here is a quick review of the current scientific facts about the biology of blue cod.
By far the most popular recreational target species in the South Island, blue cod (Parapercis colias) is a bottom-dwelling fish commonly found near reefs down to a depth of 150 metres. Generally avoiding kelp forests and large rocky platforms, blue cod are opportunistic carnivores ranging mainly over open ground at reef edges.
Blue cod are not actually related to the true cods; in fact they are weevers (genera Parapercis), which are part of the wider sandperch family, with about 50 species internationally, occurring in both shallow water and depths to over 200 metres.
Blue cod are endemic and occur throughout most of New Zealand. While found from the shore to the edge of the continental shelf, they are primarily coastal and most abundant in temperate areas south of Cook Strait. Often attracted to divers, these naturally inquisitive fish are easily coaxed to hand-feed, even in remote areas where divers are rare.
After spawning, blue cod eggs float to near the surface and hatch after about six days. Larvae then remain floating for another three days or so before settling to the sea floor at around half a centimetre long. Generally not observed until at least 3cm, baby blue cod are mainly found among rubble or associated with living structures such as sponges, horse mussels etc (biogenic reefs). These very young fish require some habitat structure to avoid predators (often larger blue cod) and to find food such as tiny crustaceans.
Typically a slow-moving fish, blue cod swim mainly propelled by their pectoral fins in a distinctive sculling motion, but are capable of sudden bursts of speed using their tail when required.
Large-scale tagging programmes have been done in the Marlborough Sounds in 1940−’41 and again in 1973−’76. These studies recorded maximum distances moved of 48km and 42km respectively; however, only 9% and 25% of blue cod moved more than a mile during the 13 and 28 months of these two studies.
Further south, tagging studies in Foveaux Strait (1998−1999) and Dusky Sound (2001−02) recorded maximum distances moved of 156km and 30km respectively. However, most fish moved less than 800m over 20 months in Foveaux Strait and less than 600m over 17 months in Dusky Sound. In fact, last year in Dusky Sound I recovered another three blue cod that I had tagged seven years earlier, and none had moved more than 350m.
While a few blue cod do move some distance, they are the exception, and the small amount of movement shown by most fish in these studies explains why blue cod may be vulnerable to over-fishing and local depletion. Blue cod can therefore benefit from smaller scale localised management in areas of high fishing pressure.
While blue cod may aggregate when feeding, they do not school. As with other Parapercis species, large male blue cod probably defend rather loose territories. This has been observed directly in Northland, where small social groups have been recorded, with the territory of a large dominant male encompassing the home ranges of three to five females. The size of the territory appears to increase as the size of the fish increases.
With large eyes, a wide mouth and an inquisitive nature, blue cod are voracious and opportunistic feeders. The main prey items of blue cod are crustaceans, small fish, shellfish, worms and small octopus. However, blue cod will eat almost any available animal matter, and their diet probably reflects what is most abundant in the environment. In one area, 52 different prey species were identified in the diet.
Blue cod grow to over 55cm in length, over 4kg in weight, and can reach a maximum age of more than 30 years. This moderately-slow growing species varies in growth rates around New Zealand, with males generally growing faster and larger than females. It takes up to 11 years for males and 12 years for females to reach the minimum legal size of 33cm in Southland. Further north growth may be a little faster, but it can still take seven years for males and 11 years for females to reach the 30cm size limit in the Marlborough Sounds.
The colour of blue cod changes with size; juveniles (5−15cm long) have a whitish body with two brown stripes running the entire length. Maturing fish darken to a rusty brown and the stripes become barely distinguishable. Beyond 25cm both sexes change colour to a mottled grey, which lasts until about 30cm, when a further change to green or blue occurs. Larger fish then develop a green-blue head, wide stripes and a pale belly. Both sexes occur in these colour phases, so it is not possible to sex blue cod by colour.
Like growth, size at sexual maturity also varies between locations. Fish in the Marlborough Sounds mature sexually at 21−26cm, compared to those in Southland which mature at 26−28cm. Blue cod have a relatively long spawning period, potentially ranging from early spring to mid summer, and evidence suggests that spawning occurs locally for both inshore and offshore populations.
Like other weevers, blue cod are able to change sex from female to male, but the presence of some large, older females suggests that not all blue cod undergo sex change. Just what controls sex change is largely unknown, but it is likely to be a complex process that is influenced by size, age and social interactions. In particular, the aggressive territorial behaviour of large males may be an important factor that suppresses sex inversion in smaller females. This may explain why catches from heavily-fished areas, such as the Marlborough Sounds, tend to be dominated by males (because there are no large males to suppress female sex change), while catches from areas with less fishing pressure, such as Dusky Sound, usually have a more balanced sex ratio. Just what effect a sex ratio biased towards males has on the reproductive output of blue cod populations is unknown, but a maximum legal size may be appropriate for blue cod in areas of high fishing pressure to ensure that some large fish remain in the population.
Blue cod are one of only a few species of weevers that are large and abundant enough to support significant fisheries. While some might say blue cod taste better than snapper, they are slower growing and need to be managed more conservatively.
Recreational fishers should also be aware that when blue cod swallow a hook, damage is often done to the gut or gills during the process of unhooking, causing returned fish to bleed to death within a few hours of being released. To improve the survival of returned undersized blue cod, fishers should endeavor to lip- or mouth-hook fish. Larger hooks are recommended because they are not as easily swallowed and catch fewer undersized fish. Avoiding small hooks could significantly reduce the mortality of returned undersize blue cod.
When returning undersized blue cod, handling time should also be kept to a minimum, as cod cannot survive more than a couple minutes out of the water. Fishers should also try to help undersized blue cod avoid predators such as birds and barracouta when releasing them. Escape tubes may be a useful method for avoiding bird predation, but serial returns of undersized blue cod will create a build up of predators both above and below the water. So if you find you are returning lots of undersize blue cod, move on and try somewhere else – don’t stay there waiting for the big ones to come to you.
Use only one hook per line
Use larger-sized hooks (at least size 6/0)
Keep tension on the line and retrieve the fish immediately when hooked
Keep handling time to a minimum
Try to avoid predators when releasing fish
Move on from areas where few legal-sized fish are being caught.
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