Growth, Longevity and Density of the Adventive Asian Kelp Undaria Pinnatifida from the Intertidal Zone in Wellington Harbour and South Coast

Abstract

The brown alga Undaria pinnatifida is native to Southeast Asia, but is adventive in New Zealand since its first recorded sighting in Wellington Harbour in 1987. Preliminary observations in winter 1997 indicated that the species had spread beyond the Harbour to Wellington's South Coast. Investigations were made into the growth, longevity and density of U. pinnatifida sporophytes from the intertidal region of four study sites, two within Wellington Harbour (Freyberg and Thorndon Wharf) and two on Wellington's South Coast (Bait Shed and Gantry Channel) between July 1999 and October 2000. Morphological features were measured on banded mature sporophytes. Midrib width (MW) proved to be the best morphological feature to measure sporophyte growth. Mean midrib width growth measurements per sampling event at each study site correlated closely to seawater temperatures. Maximum growth occurred in winter/spring corresponding to minimum seawater temperatures. Sporophyte growth was minimal during summer. Significant differences in MW were observed between sampling events at each study site and between study sites. Seasonal differences in MW were observed on the South Coast between spring 1999 and summer 2000, and between winter and spring 2000. A significant difference was also observed in Wellington Harbour during spring 1999. Sporophytes from Wellington Harbour were larger in size than those found on the South Coast. Seasonal differences in mean midrib width measurements showed a significant difference in spring 1999, with greater MW values in Wellington Harbour. A significant difference was also observed in winter 2000, with the South Coast having the greatest MW growth measurements. The environmental conditions under which a seaweed grows largely determines its growth rate, nutrient status and size. In this study available sunlight was a significant factor influencing sporophyte growth at Thorndon Wharf from May 2000 to September 2000. Sporophytes were relatively short lived with a maximum of 6 months from the time of first being banded to its last appearance in the field. The majority of sporophtyes survived for a much shorter period and their longevity was influenced by the time of year and season. Sporophyte densities were considerably lower on the South Coast than in Wellington Harbour. Peak densities of 4467 sporophytes per 100 m2 at Freyberg and 4073 sporophytes per 100 m2 at Thorndon Wharf were observed in Wellington Harbour. In comparison, the Bait Shed had a peak density of 145 sprophytes per 100 m2 while the Gantry Channel had a peak density of 190 sporophytes per 100 m2. Seasonal differences were also observed between Wellington Harbour and South Coast in winter 1999 and spring 1999. This may be due to the differences in the timing of juvenile recruitment to each location, which was greatest during spring 2000 on the South Coast, and spring and winter 1999, and spring 2000 at Freyberg. Recommendations are made as to the management of U. pinnatifida on Wellington's South Coast, an area proposed for a marine reserve. Eradication of U. pinnatifida on Wellington's South Coast is not advised due to the unavoidable presence of potential vectors in the area, which are able to further spread U. pinnatifida. Preventing further spread by introducing a local surveillance programme to contain any infestation would best be suited to the area.