Ecology of reef fishes in northeastern New Zealand and the relative importance of natural and human influences

Abstract

This thesis examined the relative importance of natural and human influences to the population and assemblage dynamics of reef fishes in northeastern New Zealand. In particular, how different reef fishes responded to the implementation of no-take and partial marine reserve protection, and physical factors responsible for spatial differences in fish abundance. Included were data from prior to the establishment of a no-take marine reserve, multiple fished reference locations, biannual sampling and the use of two independent methods to provide quantitative estimates of fish abundance and size. This combination of factors is rare in studies of marine reserves was an important strategy leading to an improved understanding of the mechanisms structuring fish communities. Responses of the reef fish assemblage to changes in fishing mortality were examined at the Poor Knights Islands Marine Reserve. Full no-take marine reserve protection was implemented on the 1st Oct 1998 but for the prior 17 years, the Poor Knights Reserve comprised only two small no-take zones and allowed recreational fishing in the rest of the reserve. Following implementation of no-take marine reserve status the reef fish community changed rapidly; there were no obvious changes at either reference location. Species targeted by fishers, such as Pagrus auratus (snapper) and Caprodon longimanus (pink maomao), responded most strongly to protection. An increase in the density of some non-targeted species can probably be attributed to climatic effects, rather than a reduction in fishing pressure. A decline in the abundance of several species at the Poor Knights may have been a result of natural mortality, or competitive or predatory interactions with snapper. Along with human influences (fishing), physical variables are important in determining the distribution and abundance of reef fish. Four locations (two mainland and two island) were surveyed in northeastern New Zealand to determine spatial patterns for seven labrid species, one of the most abundant and widespread taxa of reef fish in New Zealand. The underlying mechanisms were then explored through an examination of the relationship between swimming ability (as examined through pectoral fin morphology), exposure and depth. Each of the four locations consistently displayed distinct labrid assemblages, likely due to the influence of the East Auckland Current. Regardless of location, there was a consistent depthrelated trend for most labrids and a trend for some species to be associated with certain levels of wave exposure. By analogy with tropical labrid assemblages, it was expected that there would be a clear relationship between pectoral fin aspect ratio and depth and/or exposure. However, this relationship was not strongly evident suggesting that wave exposure may not be as important for labrids on northeastern New Zealand reefs as hypothesised for tropical coral reef systems. The response of snapper, an important recreational and commercial finfish, was investigated following the cessation of all fishing at the Poor Knights. The rate of response of snapper to protection was rapid, in areas that had previously been partially protected as well as in those that had been fully protected from fishing, with the overall density of legal sized fish increasing by 7.4 times over 4 years, likely a result of recolonisation rather than recruitment. The 818% increase in snapper biomass has the potential to enhance areas outside or within the reserve through the export of biomass (eggs and/or larvae and adults) - the daily batch fecundity was 11 to 18 times higher at the Poor Knights compared to the reference locations. The effects of partial protection on reef fish were further examined at the Mimiwhangata Marine Park, an area where recreational fishing is permitted but all commercial fishing has been prohibited for 18 years. Snapper showed no difference in abundance or size between the Mimiwhangata Marine Park and adjacent control areas, with the density of snapper most similar to fished reference locations. The lack of any recovery by snapper within the Marine Park, despite the exclusion of commercial fishers and restrictions on recreational fishing, and results from the Poor Knights, indicates that partial fishing regulations are ineffective as conservation tools for protecting targeted species or for fish communities in general (i.e. through reduction in by-catch). Results from this study provide evidence that recreational fishing has significant impacts on reef fishes.