Hanging basket oyster farming: Assessing effects on seagrass using aerial photography

Abstract

Hanging baskets are rapidly being adopted in many parts of the world to grow shellfish, especially oysters. The adoption of this method is opening up new areas for aquaculture development. However, expansion into areas that have previously been unusable has been controversial, due to concerns about farms being established in areas with particularly high ecological values and sensitivity. This study investigated the impact of establishing a hanging basket oyster farm in the Kaipara Harbour in northern New Zealand on seagrass. Aerial photographs were analysed with a geographical imaging software in combination with field sampling to detect potential impacts. The hanging basket oyster farm technology was found to have no significant overall impact on seagrass beneath the farm. However, field sampling found a narrow band directly beneath the oyster farm growout lines of less than 5% of the farm area that had lower seagrass densities and abundance compared to adjacent control zones sampled within the farm. This area was obscured in aerial photographs by farm structures. This highly localised impact is likely due to shading or scour caused by farm structures. Overall, the results indicate that hanging basket technology for shellfish aquaculture has minimal environmental impact on underlying seagrass, representing an improvement over traditional culture technologies. Furthermore, the results confirm that remote sensing methods are useful tools for examining aquaculture impacts on seagrass communities, but only when combined with field sampling. © Inter-Research 2012.