Response to a marine incursion of Styela clava
By Andrew McFadden and Thomas Rawdon, Incursion Investigators (Animals), Investigation and Diagnostic Centres, MAFBNZ; and Brendan Gould, Senior Adviser (Marine), Post Border, MAFBNZ
On 14 October 2005, MAF Biosecurity New Zealand (MAFBNZ) initiated a large-scale response to an incursion of Styela clava (the clubbed tunicate sea squirt). As part of the response, Styela clava was declared an unwanted organism under the Biosecurity Act 1993.
Initial response objectives were to delimit the extent of Styela clava infestation and determine whether eradication was feasible.
Styela clava was identified in the Viaduct Harbour, Auckland on 8 September 2005 and the Port of Lyttelton, Christchurch the following month. Until then, Styela clava was not known in New Zealand. The most likely pathway was adult specimens on the hull or internal recesses of international vessels visiting New Zealand.
The organism can survive a wide range of marine environmental conditions. Native to the coastal waters around North Korea, Japan, Northern China and Siberia, it has invaded many other parts of the world.
Initial response management was three tiered. The National Response Centre (NRC), Wellington provided ministerial advice, technical support, public and media communications, and consultation with affected stakeholders.
The Investigation and Diagnostic Centre (IDC, Wallaceville) managed field teams as directed by the NRC.
Surveillance teams delimited the extent of Styela clava infestations at the initial incursion sites, and at other key locations around New Zealand's coast.
Detailed site surveys
The National Institute of Water and Atmospheric Research (NIWA) was contracted to carry out delimiting surveys of the Viaduct Harbour and Freemans Bay, Auckland and the Port of Lyttelton, Christchurch during October 2005.
Moderate densities (1–10 individuals per m2) were found in the Auckland locations. Styela clava was also found in the Westhaven Marina, Auckland, and there was evidence of infestation in the wider Waitemata Harbour. Infestation was lower at Lyttelton, with only 23 specimens found throughout the port.
Nationwide surveys
Evidence from the Auckland survey and public reporting suggested that Styela clava was widespread through the Hauraki Gulf and that it had been there for several years. It was likely that the high volume boat traffic from the Hauraki Gulf had seeded other locations with Styela clava.
Given the evidence that Styela clava may have spread beyond the Hauraki Gulf, further surveys were carried out at 26 other marine locations around New Zealand between November and December 2005 and at five locations over June 2006. Styela clava was detected at two of these: Magazine Bay Marina, Lyttelton and the Tutukaka Marina in Northland. At Tutukaka only two specimens were found.
Results from the nationwide survey can be seen in Figure 1.
In conjunction with the surveys, NIWA conducted experiments to determine the sensitivity of detecting Styela clava at different levels of water clarity. Results indicated there was a low probability of detecting small numbers of Styela clava at most survey locations.
Investigation of field reports
MAFBNZ encouraged the general public and marine groups to report suspected sightings of Styela clava.
If Styela clava could not be excluded, specimens were requested for identification by NIWA. Once it became apparent that a wide area within the Hauraki Gulf was infested, specimens were usually only requested when they came from outside this region.
The operations group received two independent reports of historical infestations of Styela clava. Specimens collected had been mis-identified at the time of the research, then reclassified as Styela clava upon review. These reports confirmed that Styela clava was present in the Hauraki Gulf in August 2002 and Lyttelton Harbour in May 2002. Anecdotal reports from aquaculture personnel from Waiheke Island suggest the organism may have been in the Hauraki Gulf some years before August 2002.
There were 378 reports made of possible Styela clava sightings between 30 September 2005 and 30 October 2006, 61 percent of these from coastal Auckland.
Only three of the 25 positive report case specimens were from outside the Hauraki Gulf: (Northland, Picton and Nelson). All three were from the hulls of moored boats.
The Northland specimens were collected from a boat berthed at Opua in the Bay of Islands. The boat had returned to New Zealand after sailing overseas.
The Nelson specimen was found on a boat that had been moored long-term in Nelson Port. Searches located a single live adult and one dead adult on another boat moored nearby, but no specimens were found on port structures. The second boat had been relocated from Auckland Harbour to Nelson Port in the past two years and may have been infested when it arrived. The period that both of these Nelson boats had been moored suggests that a small population of Styela clava may exist, although none has been found.
The Picton report consisted of a single juvenile specimen found on a boat recently arrived in Waikawa marina from Auckland Viaduct. No further specimens were found in the marina. Three further individual specimens of Styela clava have been found on three separate vessels moored in a Wellington marina. These are thought to have come from a heavily fouled yacht that had come to the marina from Auckland.
At-risk boat inspections
Vessels fouled with reproductively mature Styela clava can convey the organism to unaffected locations. The operations group received a number of reports of potentially affected boats. Owners were contacted and risk profiled, based on previous anti-foul treatment and manual cleaning of the boat, frequency of boat usage, and the presence of macro fouling on the hull.
A surveillance team assessed 232 boats entered into the 2005 Coastal Classic yacht race using a risk profile based on these criteria and arranged for them to be inspected if required. Of the 37 inspected, two were affected by Styela clava.
Styela clava was found and removed at the time of inspection from a barge moored at Kawau Island (Hauraki Gulf), and an ocean-going ship laid up in the Port of Auckland, but destined for Northland.
In addition, over the 2005 summer boating season, MAFBNZ marine advisers visited marinas throughout New Zealand to educate the public and to encourage regular voluntary cleaning and anti-foul treatment of boats.
Impact assessment of affected aquaculture sites
Seven aquaculture farms were found to be affected by Styela clava. Six of these had low numbers of the organism. The seventh, a mussel farm, was heavily infested (Figure 2). On this farm, an additional labour unit was required to cope with large quantities of Styela clava removed from mussel lines during harvest.
Despite the high infestation, no effect on mussel growth was observed. MAFBNZ has identified the need for a longitudinal study to quantify the affect of Styela clava on the quantities, quality and size of mussels.
Discussion
MAFBNZ could not have responded to the Styela clava incursions without support from the public, or without the assistance of other agencies including NIWA, Kingett Mitchell, Cawthron, regional councils and AgriQuality Ltd.
While high densities of Styela clava were found in the Hauraki Gulf, in other locations it was either not detected, or found at very low levels of infestation. Given that Styela clava had been in the Hauraki Gulf for at least three years, its absence in many other locations implies its spread via 'conveyers' such as boats over large distances may be unusual. Most of the spread within the Hauraki Gulf is likely to have come from localised recruitment or spread over short distances on conveyers such as buoys, trees or wood substrata.
Long-distance spread may, however, become more important as the Styela clava biomass increases in the Hauraki Gulf. More boats will become fouled, and there are few or no controls on movement of fouled boats. As evidenced by Styela clava-infested boats in Northland, Nelson and Wellington, fouled boats provide a suitable pathway for transfer of Styela clava between marine locations.
On one of these fouled boats, recently returned from overseas, there was an incidental finding of three crustaceans not previously recorded in New Zealand. This demonstrates how easily new organisms can enter New Zealand's marine environment. MAFBNZ is currently researching boat hull fouling for the purposes of developing border control standards to mitigate risk of new marine introductions.
Most of the Styela cava surveillance was centred on ports and marinas; however, the organism was reported from a wide range of marine habitats, including soft sediments attached to buried shell fragments. Extensive habitats such as these are difficult to survey.
Where populations are low, detection is difficult. At locations where it is easier to detect, it is more likely that Styela clava is well established, making eradication almost impossible.
As evidence for the pathway for transfer of infestation between Auckland and Lyttelton is weak, the observed populations of Styela clava from these two locations may be due to multiple incursions. Indeed, preliminary genetic analysis of Styela clava specimens indicates that specimens may have come from more than one overseas source.
In the case of a marine incursion such as this, the high cost of response, low detection probabilities, likely organism impact and the lack of barriers to further introductions and internal spread need to be considered in decisions about responses to future incursions.
- This article is an edited version of a paper which first appeared in Surveillance magazine, Vol. 34, No. 1, March 2007. For the original paper, visit:
- www.biosecurity.govt.nz/publications/surveillance/2007
Figure 1: Spatial distribution of Nationwide surveillance and report cases confirmed as being infested with Styela clava. Positive boat report cases do not necessarily imply established populations in the locations where Styela clava fouled boats were detected.
Figure 2: A mussel line heavily infested with Styela clava (examples of Styela clava
identified by arrows).
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Page last updated: 30 April 2008