Assessment of population management options for Styela clava

Deliverable 1
searched. Both search methods confirmed the Styela population is now widespread in Lyttelton Port’s
inner harbour. At the site level there was no relationship between the number of Styela detected in
above-water searches and the number detected by divers. Kriging analysis indicates Styela is likely to
be present on suitable man-made habitats around almost the entire inner Port area, with the possible
exception of the cattle Wharf and dry dock facilities on the western side of the inner Port. Outside the
inner harbour, Styela was also detected in low numbers along Cashin Quay, although kriging analysis
suggests it is likely to be absent from the far eastern end of the quay. Kriging analyses predicted that
during November-December 2006 only 7.5% of the total perimeter of Lyttelton Port was free of Styela
fouling.
The widespread Styela population in Lyttelton Port was typically present at mean densities < 0.35
individuals per m 2 . However three areas of locally high Styela density and total abundance were
discovered, all in Lyttelton Port’s inner harbour. These local nodes of high Styela abundance
(maximum densities of between 11 and 100 individuals per m 2 ) were associated with piles on the
western side of # two Wharf, pontoons located between Z Wharf and Gladstone Pier and the A and B
pontoons on the northern side of the inner harbour. Maximum Styela densities encountered in the Port
are however orders of magnitude lower than some extreme cases reported overseas. Styela has been
demonstrated to achieve densities of 500-1500 individuals per m 2 , although these situations are
exceptional and, in many areas where it has been introduced, Styela has persisted at much lower
densities (Leutzen 1999, Osman and Whitlatch 1999, Lambert and Lambert 2003).
The diver search strategy in Lyttelton Port concentrated effort on shallow waters within 5m of the
surface where Styela was predicted to be most abundant. This approach focused on the zone where the
organism appears most detectable, and also allowed more sites to be searched by divers in the time
available. Previous rapid detection surveys for Styela in New Zealand have detected the majority of
Styela specimens in these shallow depths, although the species can occur as deep as 25 m (Cohen
2005). To examine the distribution of the species in this deep water Port and determine the proportion
of populations likely to persist in deeper water, we compared Styela abundance from diver searches
above and below the 5m depth interval. Divers detected Styela on pile habitats down to approximately
13m. However diver estimates of Styela abundance in depths of between 5 and 13m were
approximately three fold lower then in the upper 5m of water column. Available evidence
incorporating above-water search results suggests that approximately 15% of the population exists
below 5m in Lyttelton Port. Concentrating diver search effort on substrata within 5m of the water
surface was therefore shown to maximise the chances of detecting Styela at each site. The strategy
provided the most reliable estimates of maximum Styela density and also protected divers from
potential injury by minimising the amount of bounce diving required during surveys.
The Styela population in Lyttelton Port has successfully reproduced and appears to have undergone a
considerable increase in abundance and distribution from November 2005 to November 2006. The
November 2005 rapid delimitation survey was conducted to establish whether Styela was present in
the Port. At that time above-water searches were conducted across areas that have subsequently been
gridded into 48 sites, and diver searches were conducted at 9 of these sites. A total of 15 Styela were
collected from 9 of the 48 inspected sites (19%) and maximum recorded densities at that time did not
exceed 1-10 per m 2 . All individuals detected were larger than 9cm total length (Gust et al 2006a). A
year later, the current survey detected nearly 1000 Styela, albeit with an approximate tripling of survey
effort. The percentage of sites where Styela was present grew from 19% to 60% overall, and the
MAF Biosecurity New Zealand Assessment of population management options for Styela clava • 43