Assessment of population management options for Styela clava

Deliverable 1
perimeter ~50 m long. Inspections of floating pontoons concentrated this search effort along the
length of discreet floating finger wharves and inspected the underneath of these structures and any of
their surfaces not visible to above-water searchers. Wharf pile inspections concentrated search effort
within five meters of the surface and inspected a linear set of pilings (typically >10 pilings per site),
ropes and other structures on the outer face of the wharf. Inspections of breakwalls involved divers
searching ~50m of substratum from the surface down to a maximum depth of five meters. The sites
searched by divers were recorded in the field on laminated maps of survey locations. Additional data
were recorded on water clarity (Secchi disc depth in meters), sea state (using the Beaufort scale), the
substrata searched and the time taken to search each site. As with above-water searches, the position
of any Styela found was recorded on laminated maps, GPS coordinates of the location were taken, and
we recorded the substratum it was attached to. Maximum density estimates for Styela were recorded
from diver inspections at each site using the semi-quantitative log scale described in 3.2 above.
4.6. Phase 2 – Validation of the diver survey technique
The survey technique used in phase 1 of the diving assumes that:
• The density of Styela on structures in the surface waters (< five meters) is at least equal to, or
greater than the density present at greater depths at the same site, and
• The density of Styela on the outer piles of wharves is at least equal to, or greater than, that
present on piles underneath wharves at the same site.
Violation of these assumptions means that sites identified as being free of Styela from above-water
searches and/or phase one of the diving, may contain the target organism in unsampled, less accessible
habitats within the site. To test these assumptions, we chose six sample sites where Styela was found
in phase one diving. We distributed the replicate re-surveyed sites throughout the Lyttelton Port to get
representative coverage of predominant wooden and concrete pile habitats. At each site we sampled at
least 10 piles on the outer perimeter of the wharf. Over the course of two dives the divers surveyed
each piling from the surface to the seafloor. Separate estimates of density were made above and below
five meters depth on each piling, using the semi-quantitative log scale described above.
In Tutukaka Marina and Magazine Bay Marina the physical layout of Marina structures enabled us to
survey piles both on the perimeter and underneath jetties. However in Lyttelton Port health and safety
concerns arose for divers beneath wharves and piers because of poor water clarity, light attenuation at
depth and the chance of entanglement on discarded wiring, cables, steel rods and other unseen
obstacles that we encountered beneath the wharves. These risks prevented us from comparing the
distribution of Styela on piles searched along the relatively well-lit outer perimeter of wharves and
those in the gloom underneath. However above-water field observations and initial diver attempts at
searching beneath wharves provided no indication of a difference in Styela abundance between piles
along the perimeter and beneath wharves. We consider the probability of Styela occurrence to be
equivalent for piles irrespective of their location beneath a wharf for subsequent kriging analyses.
Generalised Linear Models (GLM) were used to determine relationships between the presence and
abundance of Styela in above-water searches and both shallow (5m) diver searches.
MAF Biosecurity New Zealand Assessment of population management options for Styela clava • 17