The blue cod is a very popular recreational fish found from the lower part of the North Island through but more predominantly in the South Island. The blue cod enjoys clear rocky ground to depths of about 150 metres. Large examples of blue cod are usually greenish blue in colour, while the smaller ones are blotched in varying shades of brown. The blue cod belong to a widespread tropical family of fishes but are in no way related to true cods such as red cod.
Blue cod is a bottom-dwelling species endemic to New Zealand. Although distributed throughout New Zealand near foul ground to a depth of 150m, they are more abundant south of Cook Strait and around the Chatham Islands. Growth may be influenced by a range of factors, including sex, habitat quality and fishing pressure relative to location (Carbines, 2004a). Size at sexual maturity also varies according to location. In Northland, maturity is reached at 10–19cm total length at an age of 2 years, whilst in the Marlborough Sounds it is reached at 21–26cm at 3–6 years. In Southland, the fish become mature between 26–28cm, at an age of 4–5 years. Blue cod have also been shown to be protogynous hermaphrodites, with individuals over a large length range changing sex from female to male (Carbines, 1998). Blue cod males appear to grow faster and larger than females. The maximum recorded age for this species is 32 years (Carbines, 2004b).
Blue cod have an annual reproductive cycle with an extended spawning season during late winter and spring. Spawning aggregations have been reported within inshore and mid shelf waters. It is also likely that spawning occurs in outer shelf waters. Ripe blue cod are also found in all areas fished commercially by blue cod fishers during the spawning season. Eggs are pelagic for about five days after spawning, and the larvae are pelagic for about five more days before settling onto the seabed. Juveniles are not caught by commercial potting or lining, and therefore blue cod are not vulnerable to the main commercial fishing methods until they are mature. Recreational methods do catch juveniles but the survival of these fish is good if they are caught using large hooks (6/0) and returned to the sea quickly.
Tagging experiments carried out in the Marlborough Sounds in the 1940s and 1970s suggested that most blue cod remained in the same area for extended periods. A more recent tagging experiment carried out in Foveaux Strait (Carbines, 2001) showed that although some blue cod moved as far as 156km, 60% travelled less than 1km. A similar pattern was found in Dusky Sound where four fish moved over 20km but 65% had moved less than 1km (Carbines & McKenzie, 2004).