Blue cod survey in Marlborough, Tasman & Golden Bay
NIWA fisheries scientists will survey blue cod in the Marlborough Sounds and Tasman and Golden Bays during September and October, starting on 5 September.
The survey, the fifth in the region, is being carried out for the Ministry of Fisheries to assess changes in the blue cod population. Blue cod are the most popular catch with recreational fishers in Marlborough and also support a small commercial fishery.
The scientists will count and measure the sizes of blue cod caught and keep samples to estimate the ratio of males to females. Part of the research will again involve catching, tagging, and releasing up to 1000 fish in Pelorus Sound and around D’Urville Island to investigate fish movements and growth rates.
"We obtained very good returns from our previous tagging survey, which indicated two things. First, that there is heavy fishing pressure within the Marlborough Sounds, and secondly that most fish were caught within two kilometres of where they were released up to three years ago. These fish do not to move very much, which has important consequences for the sustainable management of the fishery", says Ken Grange, Regional Manager of NIWA in Nelson.
Blue cod are tagged using a small bright yellow tag attached to the base of the pelvic fin, which appears to cause no harm to the fish. Each tag is labelled with a unique number, and the address of NIWA in Nelson. "anyone catches a tagged fish, we ask that the fisher records the length of the fish and location of capture, and returns the tag to NIWA for a small reward", says Dr Grange. "If the fish is undersized, then record the tag number and location, measure the length of the fish, and release it carefully back in the water. Simply send that information in to NIWA".
"Surveys over the past decade have indicated that the numbers of legal-size blue cod throughout the Marlborough Sounds have decreased, mainly in the outer Sounds and around D’Urville Island, although the recent reduction in bag limits and the increase in legal minimum size may have reduced the level of decline. Since 2004, we have had 127 tags returned, which means that at least one in five fish has been caught again since being released. Information from re-captured tagged fish also provides us with important growth data."