Abstract
There is uncertainty whether blue cod (Parapercis colias) should be regarded as single stocks within the ten quota management areas around New Zealand as movement of blue cod is not well understood (Warren et al., 1997; Carbines, 1998).
The dynamic pool assumption in fisheries management states that fish in each stock intermingle and breed, but in reality there may be several substocks within each area (Warren et al., 1997; Carbines, 1998). Catch Per Unit Effort (CPUE) data can be inaccurate for determining stock size of blue cod due to their patchy distribution, where serial depletion of substocks can mask indicators of overfishing. Fisheries independent data such as a multisample tag/recapture programme is likely to give more accurate estimates of fish populations than those from a single tagging event. The Jolly-Seber multiple tag/recapture method gave more accurate estimates for the blue cod population in Paterson Inlet than the Peterson method, with estimates becoming more precise with increased tag/recapture events.
Carbines (1999) suggested blue cod in enclosed areas like Paterson Inlet and Marlborough Sounds move far less on average than those tagged in the open sea. Blue cod are also believed to differ in movement with size, where small blue cod move greater distances than larger territorial fish (Mutch, 1983). Blue cod showed localised movement of less than one kilometre in this study where movement only occurred within separate habitat patches. There appeared to be no evidence for size specific differences in net movement for blue cod inside the inlet. Movement data could prove useful in management of the blue cod fishery as it may help in determining whether stocks will replenish areas of reef where blue cod stocks have been depleted due to overfishing.
Growth rates and length at age data are important for the fishery, as growth determines how long a fish will take to reach minimum legal size, or to an age when the fish is sexually mature. These results were gathered using growth increment data from tag recaptures. Growth rates of blue cod were 2.7 mm/month, reaching minimum legal size at 5 years of age.
Blue cod in Paterson Inlet probably belong to the same stock, as there is a tendency for most fish to stay in the same habitat patch for extended periods. A multisample tag/recapture study for determining growth, movement and abundance may support better fisheries management by providing an insight into these important issues.