Publications: Research reports and publications

Seafood safety risks from paralytic shellfish poisoning dionflagellate blooms in New Zealand: 2012-2013

17 September, 2013
Cawthron Report 2346. Prepared for the Ministry for Primary Industries, Food Safety.

EXECUTIVE SUMMARY

An intense bloom of A. catenella developed in Opua Bay, Queen Charlotte Sound during the 2013 summer. The bloom began in January, reached a climax in mid-March, and rapidly declined in early April.

At the height of the bloom cell concentrations exceeding 1 x106 cells/L existed in the Bay, causing extensive areas of visible 'red tide'.

Monitoring the Opua Bay dinoflagellate community provided a good early warning (> 1 month) of the appearance of toxicity in shellfish in adjacent areas of Tory Channel. It is recommended that routine sampling is carried out at this site between January and April each year.

In contrast to the same period in 2012, February and March 2013 in Marlborough was exceptionally fine and sunny. It is believed that this created the conditions (high light and a stable stratified water column) that were conducive to the proliferation of the bloom. Importantly, the strong south-easterly gales which disrupted the development of the bloom in March 2012 did not occur in 2013.

Monthly sampling of water column properties along a transect from Tory Channel to the head of Opua Bay provided data on the conditions associated with the development of the bloom. The pre-existing cyst bed, the hydrodynamic characteristics of the Inlet, seasonal thermal stratification, and the supply of nutrients from the deeply mixed, naturally nutrient enriched environment of Tory Channel are the main factors which drive the annual A. catenella bloom. An in situ net population doubling time of ~8.3 days was estimated.

Large swarms of the lobster krill Munida gregaria existed over the bloom period in Tory Channel and the Onepua/Opua Inlet and were found to be accumulating paralytic shellfish poisoning-toxins (PSP-toxins). An experiment showed that M. gregaria can reduce A. catenella cell numbers in the water column and it is possible it had some impact of the progression of the bloom. M. gregaria provides a route for food chain transmission of PSP-toxins to higher trophic levels.

The PSP-toxicities of shellfish throughout the Sound in 2013 were substantially lower than in 2011 and the closure period was consequently shorter. Toxicity at sites affected by Tory Chanel water flows were probably contaminated due to the export of cells from the Onepua/Opua Inlet. However, in the case of East Bay, it is likely that the bloom there developed independently, originating from cyst beds in this area.

Low numbers of A. catenella were observed in Port Underwood and Port Gore in late March early April.

A new analysis of A. catenella resting distribution in SCUBA retrieved sediment cores from Opua Bay, found apparently viable cysts at a depth of at least 20 cm. This suggests that the dinoflagellate has been resident in the Bay for multiple decades. Isotope dating of sediment strata is in progress to assign dates to sedimentary layers.

In December 2012 the most serious PSP poisoning event documented in NZ to date occurred, due to people consuming contaminated Tuatua from ocean beaches in the vicinity of Tauranga. At least 29 people became ill, several seriously, despite the fact that a public health warning was in place.

Affected shellfish associated with cases of human poisoning had high levels of toxicity (31 mg STX equiv'/kg and 14 mg STX equiv'/kg on the Lawrence HPLC screen and confirmation tests respectively) with toxin profiles dominated by the most toxic PSP-toxinanalogues, STX and neoSTX.

The cause of the event was a bloom of Alexandrium minutum, a species which has been implicated in PSP-toxin contamination in this area a number of times previously. Phytoplankton monitoring at the Tauranga and Bowentown sites provided an early warning of the appearance of Alexandrium minutum.

Historically, A. catenella has been the main cause of PSP-toxin contamination in the Bay of Plenty (BOP) but the toxin profiles of A. catenella and A. minutum are qualitatively different. The former has a profile containing a large proportion of low toxicity N-sulfocarbamoyl analogues (C1,2, GTX5); the latter produces a substantial proportion of high toxicity STX and neoSTX analogues.

It is assumed that scepticism by the public to toxic shellfish warnings in the BOP has arisen because of their high frequency and because the consumption of shellfish contaminated by A. catenella is less likely to cause illness than that by A. minutumPublic health warnings need to be able to distinguish between these different levels of hazard.

In late winter and spring 2012 the toxic dinoflagellate Gymnodinium catenatum appeared in water samples, and low levels of PSP-toxins were detected in shellfish, at a number of public health monitoring sites on the North Island west coast. This species had not been observed in this area since it was last reported in 2007. The widespread and damaging bloom of this species that took place in 2000–2001 also developed in this region at thistime of year. This suggests that a similar event, originating from the same area, could occur again.