Individual study: Manual removal of the macroalga Undaria pinnatifida in Tinderbox Marine Reserve, Tasmania, Australia
Hewitt C.L., Campbell M.L., McEnnulty F., Moore K.M, Murfet N.B, Robertson B. & Schaffelke B. (2005) Efficacy of physical removal of a marine pest: the introduced kelp Undaria pinnatifida in a Tasmanian Marine Reserve. Biological Invasions, 7, 251-263
Biological invasions are considered one of the biggest threats to native biodiversity. Worldwide, many marine species have been introduced as a result of human activities. One eaxample is the brown algae Undaria pinnatifida, originally from the coastal Northwest Pacific of China, Japan, Korea and Russia. Since the 1970's this seaweed has spread and established populations in Argentina, Australia, New Zealand, Belgium, England, France, Italy, Spain and most recently California. The main ecological impact of U.pinnatifida in introduced areas is a change in native macroalgae community composition with associated decreases and losses of native species.
In Australia it was recorded in 1988 on the east cost of Tasmania, spreading at a rate along the coastline of approximately 10 km per year. The spread was likely due to local translocation by fishing, boating and aquacultural activities. In 1997, a small patch of Undaria was discovered in a marine nature reserve and manual removal was attempted as a method of control.
Study site: Tinderbox Marine Nature Reserve (jointly managed by the Parks and Wildlife Service and Marine Resources) protects a great variety of seaweeds, fish and invertebrates. The reserve (located south of the Tasmanian capital, Hobart) extends 1.4 km north-east of Tinderbox Bay to Piersons Point, to about 700 m south-west of the bay, and covers an area of about 45 hectares.
Occurence and control of Undaria pinnatifida: In January 1997, a small patch of 351 U.pinnatifida sporophytes (macroscopic life stage of the algae) covering 10 m² was reported in the Reserve. Manual removal was used as a rapid response when it was first discovered. A long term experiment was then set-up to asses the effectiveness of regular manual removal to control its spread.
Four belt transects (50 x 4 m) adjacent to each other (within an experimental area of 50 x 16 m) were established, running perpendicular to the shore. The position of individual sporophytes was mapped before removal. Divers removed all the sporophytes, carefully packing them into plastic bags in situ to avoid spore dispersal. The following was recorded: whether they were attached, drifting or mobile (attached to a small rocks or wood); and growth rate determined by dividing the stipe (stem like structure in algae) width by the number of days since the last collection and then averaging across the site).
Removals were carried out on a monthly basis beginning in September 1997. In July 1998 four additional transect were added as controls (i.e. no U.pinnatifida removal) and evaluations were carried out until March 1999 with two subsequent additional removals in July and August 1999.
Removal of Undaria pinnatifida: During the two years of the experiment 4,848 sporophytes were removed within transects. In the first year, manual removal resulted in a reduction in abundance of the algae from an average of 211 sporophytes to zero. During the second year the abundance of sporophytes in the removal area was higher than that in the control area, but from November 1998 the sporophytes numbers in the removal transect were lower than in control transects. In the third removal season, the number of recruited sporophytes in July and August 1999 was considerable lower than in the control transects compared with those where removal was undertaken. Very low numbers of fertile sporophytes were found in the removal areas.
Growth rate: The average rate of growth was 0.4 mm/day in the control transects but only 0.2 mm/day in the removal transects. The maximum growth rates however, were similar in removal and control transects, with 141 cm and 123 cm, respectively recorded over a period of 30 days during the summer months. The size class distribution showed that even with the removal effort, there was an prevalence of recruits in both transect types, except for the smallest size class, which were more abundant in the removal transects.
Abundance of U.pinnatifida was also found to be higher in rock and cobble habitats (where it could anchor more easily) compared to seagrass and sand habitats.
Conclusions: Although the removal was considered partially successful, to maintain control of sporophyte spread requires a bigger effort. Importantly, a method to remove microscopic life stages would be desirable.
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