Population biology, expansion and harvestability of the invasive Undaria pinnatifida in southern New Zealand

Abstract

In the marine environment, 84% of eco-regions have been subject to biological invasions. These invasions can be ecologically costly through displacing native species, shifting community structure, altering food webs, and distorting ecological processes such as nutrient cycling. The Laminarian kelp Undaria pinnatifida’s wide environmental tolerances, high dispersal capabilities and rapid growth rates have enabled the species to increase its global distribution throughout many temperate coastlines and is still expanding. The research conducted within this thesis aimed to further investigate the seasonal population dynamics, update vertical and geographic range expansions and assess the prospect of commercially harvesting Undaria pinnatifida throughout the Otago coastline in southern New Zealand. The research will add to baseline information on the invasive nature of this species and identify strategies to effectively manage its expansion throughout Southern New Zealand. Results from this study provided information on the environmental factors affecting seasonal shifts in Undaria’s population biology, updated the distributional advancements of Undaria and evaluated current impacts (if any) on native communities. Lastly, a sustainable control programme suitable for managing the prolific spread of Undaria throughout southern New Zealand was explored. Evidence from this study suggests water motion and mixing plays a vital role sustaining perennial populations of Undaria in southern New Zealand. The seasonal population structure of Undaria colonizing the inner portions of Otago Harbour closely resembles an annual lifecycle, whereby the lack of water motion and connectivity to the open ocean restricting nutrient availability is likely a contributing factor. Perennial tendencies were observed for both Outer Harbour and Open Coast Undaria populations. Undaria throughout the Open Coast were subjected to density and recruit peaks during autumn and spring along with a consistent presence of sporophytes year round. Although the Outer Harbour had a less obvious seasonal pattern compared to the Open Coast, favourable conditions of sufficient nutrient availability along with reduced disturbance resulted in a more stable population structure. Furthermore, Undaria’s invasion throughout southern New Zealand is continuing, with geographic and vertical distribution expansions increasing steadily from a previous survey conducted in 2009. Undaria has expanded into the last three previously un-invaded patches of coastline (Brinns Point, Puketeraki and Butterfly Bay) between Warrington and Matainaka (12.5 km of coastline). The species has now successfully established across the entire coastline of the East Otago Taiāpure customary protected area, and furthermore increased its vertical depth distribution across two (Warrington and Matinaka) of the three previously established sites within the East Otago Taiāpure. There was no evidence for Undaria modifying native macroalgal communities over the previous nine years. Whether or not any adverse ecological affects will transpose may not be fully understood for sometime, so it is vital for long term periodic monitoring to be conducted to understand the impact of this invasive species. A proposed management model of harvesting Undaria along the Otago coastline will not only assist in controlling the prolific spread of the species and fund monitoring programmes, but also financially benefit local communities. Further exploration into site-specific stock assessments along with exploring the potential of harvesting twice a season in cooler southern sites is recommended in order to attain an overall financial analysis of the control initiative. Lastly, investigating the steps involved in fucoidan extraction from the Undaria sporophyll along with processing other products would be financially beneficial and diversify the products provided from harvest.