The extent of kina barrens over time at Hauturu-o-Toi and the Noises Islands

Abstract

Kelp forest habitats are the foundation for many species found on temperate rocky reefs. With fishing pressures decreasing populations of key urchin predators, the issues of urchin (kina) barrens and deforestation of kelp beds have become more prevalent. Satellite and aerial imagery were used to map the extent of kina barrens surrounding Hauturu-o-Toi and the Noises Islands in the Hauraki Gulf, northern New Zealand. GIS mapping software was used to digitise images from 1953, 1979 and 2019 for Hauturu-o-Toi, and for 1978 and 2019 for the Noises. This allowed a large-scale assessment of the current extent of kina barrens in each location and long-term changes in the extent of kina barrens at key locations. Aerial imagery and historic descriptions indicate that kina barrens did not occur in either location in the 1950’s but by the 1970’s kina barrens were a major habitat on subtidal reefs: covering 11.6% of reef at Hauturu-o-Toi and 23.97% at the Noises. This progression is consistent with industrial scale removal of predators, such as the spiny rock lobster and snapper, in the middle of last century. The extent of barren found to cover the reef habitat at the Noises increased between 1979 (24.0%) and in 2019 (49.5%) demonstrating that kina barrens are now a major habitat on the inner gulf reefs. This is contrary to much of the previous literature which claimed that the inner gulf was not subject to the issue of kina barrens to the extent that outer gulf islands were. Urchin barren covers 32.73% of reef habitat at Hauturu, dominating a shallow band around much of the island, but most extensive on the more wave exposed northern and eastern coasts. There was a positive relationship between the extent of kina barrens and wave exposure, with overall barren coverage and the lower depth of barrens increasing with exposure. The accuracy of habitat maps produced ranged from 67.9% to 79.5%, falling within the range of accuracies found in studies using similar methods, scope and detail. These habitat maps provide a clear visualisation of both how barren extent has changed over time, and where barren is to be found at each site. With marine protected areas (MPA’s) proposed for both locations, these habitat maps and the methods used provide an efficient and effective tool for monitoring future change in the extent of kina barrens and kelp forests on shallow reefs.