Investigating the ecological effects of Long Bay-Okura Marine Reserve

Abstract

The Long Bay-Okura Marine Reserve (established 1995) is a 980-hectare no-take reserve located on the North Shore of Auckland. This reserve is influenced by the urban sprawl of New Zealand’s largest city through high sediment loads from coastal erosion and development. It was established to “protect a typical slice of Hauraki Gulf”, but despite its long protection, few investigations have explored the ecological benefits of this urban marine reserve and how effective it is at protecting species within its boundaries. This thesis examined the diversity of habitats in the reserve as well as the effect of 25 years of protection on two taonga species. To understand the current conditions of the benthic habitats and create a reference point for future comparisons, subtidal habitats were mapped inside and adjacent to the reserve using satellite and aerial imagery with geo-referenced drop camera images of the seafloor to validate habitat categories. Subtidal reef made up 7% of the reserve and was largely covered by canopy-forming large brown algae (Ecklonia radiata, Carpophyllum spp., Sargassum spp.), whereas low relief reef was dominated by turfing algae (e.g. Corallina officinalis) and high sand cover. Habitat composition was comparable inside and adjacent to the reserve. Baited video surveys of reef fishes were used to evaluate reserve protection of carnivorous fishes. As seen in other reserves in northern New Zealand, snapper (Chrysophrys auratus) were substantially larger and more abundant inside, with no legal sized individuals recorded in adjacent fished sites. Sea urchin (kina, Evechinus chloroticus) density and size were evaluated to assess the potential for reserve effects, either increase (protection of fished species) or decrease (via trophic cascades). Kina were generally larger and found at higher densities within the reserve, suggesting benefits from protection in this area and harvesting may have reduced densities on surrounding reefs. This contrasts patterns seen in other reserves in the Hauraki Gulf. Despite the degree of anthropogenic influence, Long Bay-Okura Marine Reserve protects several commercially, culturally, and recreationally important species. This demonstrates that reserves in urbanized areas can have significant ecological effects despite degraded habitat due to sedimentation and stressors associated with urban areas.