Marine reserve effects on snapper (Pagrus auratus: Sparidae) in northern New Zealand

Abstract

The effects of marine reserve protection on the density of the snapper (Pagrus auratus: Sparidae) were examined at three marine reserves in northern New Zealand. Research effort was concentrated on three main areas: (1) movement patterns of snapper and levels of site fidelity, (2) development of effective survey methods and designs to detect changes in relative density, and (3) application of the methods to determine the magnitude of response of snapper to reserve protection. A recently developed tagging method, subdermal injection of visible implant fluorescent elastomer (VIFE), was found to provide a good method of identifying snapper underwater without the necessity of fish recapture. Over a two-week field experiment, 93% of tag implants to snapper caudal fins were retained, and these had no discernable effect on the health of the tagged fish. The method was then used in two tagging experiments, which showed that some snapper remain in particular reef areas for more than three years. The likelihood of a fish exhibiting site fidelity did not appear to be related to fish size. Difficulties in obtaining unbiased relative density data for snapper were solved by development of a new sampling method, baited underwater video (BUV). BUV was validated by comparison to underwater visual census and angling surveys done concurrently within the Cape Rodney to Okakari Point Marine Reserve. BUV allowed the collection of both unbiased relative density data, and accurate estimates of fish size. BUV was used to conduct four surveys inside and outside three marine reserves over a two-year period, to determine responses of snapper to protection. The relative density of fishable snapper (greater than the recreational minimum legal size limit of 270 mm caudal fork length) was 14 times greater within the reserves than outside them, biomass was 4-12 times greater, and egg production within the reserves was estimated to be 7-23 times greater than comparable stretches of coastline. There were large and consistent seasonal fluctuations in inshore snapper density, with autumn (April/May) densities higher than spring (October/November) densities at all locations.