The impact of shading by oyster farm structure on seagrass, Zostera capricorni, in the Kaipara Harbour

Abstract

As aquaculture represents the fastest growing animal food based industry in the world it is critical that potential environmental impacts are understood and effectively managed to ensure the sustainable development of this industry. The expansion of oyster farming in New Zealand further from the coast, using modern 'hanging basket' technology, has lead to new environmental considerations that are likely to be different to traditional 'rack and rail' cultivation methods. One such consideration is the impact of shading by oyster farm structure on subtidal seagrass (Zostera capricorni), which is considered to be one of the most valuable ecological components of estuarine and coastal ecosystems. This study investigated the impact of shading by 'hanging basket' oyster (Crassostrea gigas) farm structure on seagrass in the Kaipara Harbour. This was achieved by determining; the distribution and light requirements of seagrass within the study site, the impact of the oyster farm on underwater light availability and seagrass distribution and density, as well as the the impact of reductions in underwater light availability on seagrass. The key findings of the research presented in this thesis suggest that light is a limiting factor for seagrass, particularly during winter when light availability was low. Light availability directly beneath oyster farm structure was found to be significant reduced, associated with lower seagrass densities, however, reductions in light were not great enough to prevent seagrass occupying this area. Analyses of historical and recent aerial photographs of the oyster farm in the Kaipara Harbour suggested that the oyster farm was having no detectable impact on seagrass distribution beneath the farm when compared with areas outside the farm. However, seagrass directly below the farm structure which was obscured could not be included in the analyses. Further, reductions in light availability comparable to reductions below oyster farm structure, using in situ shade screens above seagrass, were not found to have a significant impact on seagrass biomass or photosynthetic characteristics.