Port Davey Introduced Marine Pest Survey August 2023

This survey was conducted as a partial repeat of previous introduced marine pest port surveys undertaken by Aquenal in Port Davey/Bathurst Channel in 2011 and 2003, with a particular focus on the Pacific oyster Magallana gigas following the detection of a live specimen in Schooner Cove in February 2022 that triggered an initial search by Natural Resources and Environment Tasmania (NRE Tas) in March 2022. However, no individuals were found, suggesting there had been no obvious recruitment or expansion of Magallana gigas within the estuarine system. This is despite extensive searching via targeted shoreline surveys by NRE Tas in March 2022 and this project (shoreline and immediate subtidal zone) in April 2022 and the broader pests survey in March 2023 that included more comprehensive subtidal diver-based searches. Shoreline searches showed that Carcinus meanas (European shore crab) now appears to be widespread and common within this system, following a similar pattern observed in eastern Tasmanian estuaries over the same period. However, unlike the moderate number of records in the 2002 survey, only one specimen of the introduced Metacarcinus novaezelandiae (Piecrust crab) was found, suggesting this species had not expanded its foothold in the region over this time.

No other target macroalgal or macrofaunal introduced species were found, despite extensive searching of suitable habitats, including species such as Grateloupia turuturu, Undaria pinatiffida, Petrolisthes elongatus, and Maoricolpus roseus that are now common in south east Tasmanian waters. M. roseus was recorded at Beabey Bay in Bathurst Channel in 2002 and all specimens removed. Interestingly, the phytoplankton analyses revealed no target phytoplankton species, including the invasive Gymnodinium catenatum that had previously been reported from this system as residual spore fragments in sediments. It appears likely that the tannin waters of the Bathurst Channel region are not suitable conditions for stimulation of Gymnodinium blooms due to the mix of low light availability and low salinity in the highly stratified surface waters. Likewise, the light limitation on phytoplankton growth may also limit opportunities for filter-feeding invasive species such as Maoricolpeus and Magallana.  Magallana gigas, a species with preference to intertidal habitats, may be additionally restricted in occupying intertidal to immediate subtidal habitats by the low salinity surface waters that are typical of this system. Native oysters were seen as far west as Playtpus Point in Bathurst Channel suggesting some oyster species can survive under these low productivity conditions, although typically at 3-5 m, they were below the strongest influence of fresh surface waters that may otherwise limit Magallana gigas distribution.

Survey methods included the collection of benthic infauna cores, sediment cores for phytoplankton cysts, underwater video transects, diver visual searches, crab traps and shoreline searches. Benthic cores recorded two families with known cryptogenic or introduced species - Spionidae (Polychaeta), and Ischyroceridae (Amphipoda). However, detailed taxonomic analysis demonstrated that the species from these families were native. Video transects undertaken at seven locations revealed only one cryptogenic non-target introduced species, the bryozoan Membranipora membranacea. This bryozoan species was found in shallow sites encrusting on various kelp species and had been observed during previous surveys.  Additionally, the seastar Astrostole scabra was observed during diver searches.  This species has previously been found in Port Davey during marine park surveys and while originally thought to be introduced, it is now generally thought to be a native species. Crab trapping at three target locations did not record any positive identifications of introduced species.

In light of increasing visitation rates and the recent discovery of Pacific oysters in this system, we recommend continuation of similar invasive marine species port surveys in the Port Davey to Bathurst Harbour estuarine system at 5-yearly intervals to allow early detection of such issues and at a stage where management intervention may be effective. In addition, following the confirmed detection of a live Pacific oyster in 2022, , a precautionary approach would be for NRE Tas to undertake annual surveys of the intertidal zone on headlands and rocky coast between Eve Point and Schooner Cove at summer peak low tides to ensure early detection of any vagrant Pacific oysters that may become established.

Clearly the unique environment of this tannin water influenced estuarine system may be playing a key role in its resilience to invasive species success. However, low-rainfall events in recent summers have severely reduced the tannin layer at times and potentially removed much of the protection that it provides, hence while these are temporary events, they do suggest this system may become increasingly vulnerable to pest species if this pattern continues in the face of a changing climate.

Finally, while there is a visitor guide available for this system, it is now 15 years old and in need of both an update and a program to ensure all visiting boats are able to access the guide and are aware of the need to follow appropriate protocols to ensure pest species are not transported on vessel hulls, on anchor mud or indeed as part of food provisions (e.g. a restriction on bringing live oysters into the region as a food item). Consideration should also be given to establishment of a permit system to be established for all marine visitors to Port Davey east of Breaksea Island and north of Whalers Point (the estuarine systems) to ensure all visitors are aware of the biosecurity risks and requirements.