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| New Zealand East Coast Snapper: A Mark Recapture Biomass Estimation Program Using Coded Wire TagsSnapper (Pagrus auratus; Family Sparidae) supports one of the largest commercial inshore fisheries in New Zealand. The 7,457 tons of snapper landed in 1993 had an export value in excess of US $37 million. Also, surveys indicate snapper is New Zealand's most important recreational fish. New Zealand snapper are managed as four distinct stocks and commercial fishing is constrained by catch quota. The East Coast snapper is the largest of these stocks, having a current commercial quota allocation of 4,900 tons. In order to estimate the maximum sustainable yield and the yield currently available from snapper fisheries, information on stock size, growth, productivity, spawning success and mortality rates are required. After several years of intensive research, the Ministry of Agriculture & Fisheries (MAF) has developed a mathematical model of the snapper population that can predict the outcome of different levels of fishing pressure. The accuracy and precision of this model depends strongly on an up-to-date estimate of stock size. Tagging is currently accepted as the best method of determining the size of New Zealand snapper stocks. In November of 1993, MAF New Zealand began a large and ambitious tagging program designed to assess the biomass of the East Coast New Zealand snapper stock. The program differed from all previous snapper tagging programs by the use Coded Wire Tags (CWT) instead of the familiar dart tag. This was the first time in recent years that these tags have been used to derive a biomass estimate of a sizeable marine fish population. The CWT was selected for four main reasons. First, the tags are much less damaging to the fish. The dart tags must be deeply inserted into the fish and "locked in" behind a bone. This leaves an open wound in the fish that is a possible site for infection. CWTs do not leave an open wound, and the tag site heals completely within a few weeks. Second, CWTs are retained extremely well in the fish. With dart tags, some break free or come loose because of poor tagging or infections. A low loss rate of CWTs means that less have to be used for the same amount of certainty in the final estimate of the stock. Third, as tags are recovered by MAF staff instead of being returned voluntarily, it assures more certainty over the details of how and when the fish was caught, its size, sex, and well-being. These details are critical to a proper analysis of a tagging program. Finally, when relying on the voluntary return of tags, some details can be lost or blurred. MAF can never be sure that all tags caught are returned, and a correction factor always has to be built into the analysis. Voluntary tag returns can even lead to real or alleged subversion, with perceptions of such practice usually stemming from mistrust between user groups. Having invisible tags planted, detected and collected by MAF staff alleviates the whole range of potential problems associated with voluntary tag returns. It was determined that 30,000 tagged snapper would need to be released, and 1,200 tons of commercial catch subsequently examined, to obtain an estimate with a precision level of 0.1. Chartered commercial longline vessels with MAF staff on board released 80% of the tagged snapper. Longline was chosen in preference to other fishing methods because it is least selective in respect to fish size, and mortality of tagged snapper caught by longline is very low. The remaining snapper were liberated from the MAF research trawler. This vessel trawled snapper for tagging principally from deeper water areas where longlining is less efficient. Once the fish were captured, a CWT was injected into a non-edible part of each fish. This wire is magnetized and can only be detected when using a magnetic tag detector, thus giving cleaner data. The MAF scientific staff is currently screening 25% of commercial snapper landings from the east coast. Results from the tag recovery phase to date are very encouraging. Tag catch ratios are within the bounds of current model predictions of stock size and fishing mortalities. Catches are regularly "seeded" to ascertain under-detection levels. Thus far; detection rates are better than 95%. An experiment is scheduled for the end of 1994 to determine the probability of mortality due to tagging. The biomass estimate in conjunction with other information on stock productivity should enable a more precise derivation of yield for the east coast snapper fishery. This analysis will ultimately lead to improved management of this very valuable and 'socially important' fishery resource. Authors: Jeremy McKenzie and Martin Cryer Ministry of Agriculture and Fisheries North Floor 3, 23 Hargeaves Street College Hill Ponsbey PO Box 3437, Aukland, New Zealand Phone + (09) 3794700 Fax + (09) 3774245
or contact NMT Biological Services at biology@nmt.us. | ||
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Northwest Marine Technology, Inc. (360) 468 - 3375 | Last edited: 02 Dec 2008 | ||
