Improved risk management of paralytic shellfish toxins in Southern Rock Lobster

During 2012 an extensive bloom of the toxic dinoflagellate Alexandrium catenella occurred on the east coast of Tasmania causing paralytic shellfish toxins (PST) to accumulate in bivalve shellfish at 12 times the regulatory maximum level (ML; 0.8 mg STX equiv. kg−1). Southern Rock Lobster, accumulated PST to 3.9 mg STX.2HCl equiv. kg−1, resulting in the first closure of an Australian lobster fishery due to marine biotoxins. Recurrent blooms since 2012 have had an on-going impact on both the commercial fishery in Tasmania, valued at AUD 97 M, and the significant recreational fishing sector.

The present body of work aimed to address the following knowledge gaps exposed by this novel risk: (1) the toxicokinetics associated with PST uptake and depuration in Southern Rock Lobster from A. catenella blooms; (2) the impact of PST on lobster health; (3) the supply chain risk of PST accumulation in Southern Rock Lobster; and (4) to determine and validate (where appropriate) cost-effective methods to adequately monitor and manage PST accumulation in lobster in the field. In elucidating these questions, information was sought that could inform management of public health and market access risks and determine whether PST accumulation could adversely affect lobster performance, health, and catchability.

To examine toxicokinetics of PST in Southern Rock Lobster, an experimental study was undertaken in a biosecure aquaculture facility in South Australia. Adult male lobsters were fed highly toxic mussels (6 mg STX.2HCl equiv. kg−1) sourced from the Tasmanian east coast for 4 weeks, then allowed to depurate for a further 5 weeks. Control (fed non-toxic mussels) and exposed lobster were harvested at regular intervals, tissues dissected and analysed for PST. The lobsters rapidly accumulated PST in the hepatopancreas (exponential rate of 6% per day), exceeding the bivalve ML within one week, and reaching a maximum of 9.0 mg STX.2HCl equiv. kg−1. Once toxic feed was removed, the lobster depurated at a rate of 7% per day. Toxins were found in lobster antennal glands at concentrations two orders of magnitude lower than found in the hepatopancreas. This is the first report of PST in lobster antennal glands which, along with the gills, represent possible excretion routes for PST. However, PST were not detected at significant levels in the lobster haemolymph, which rules out the possibility of non-destructive sampling of lobsters for biotoxin analyses.

During the experiment, lobster health was assessed to determine the impact of PST accumulation in the hepatopancreas. A comprehensive range of behavioural (vitality score, righting ability and reflex impairment score), health (haemocyte count, bacteriology, gill necrosis and parasite load), nutritional (hepatopancreas index and haemolymph refractive index) and haemolymph biochemical (21 parameters including electrolytes, metabolites, and enzymes) parameters were examined. Accumulation of PST increased the apparent feed intake but did not result in mortality nor significant changes in the behavioural, health, or nutritional measures suggesting limited gross impact on lobster performance. Furthermore, most haemolymph biochemical parameters measured exhibited no significant difference between control and exposed animals. However, in the lobsters fed toxic mussels, the concentration of potassium in the haemolymph increased with PST, whilst the concentration of lactate and the sodium:potassium ratio decreased with PST. In addition, lobsters with elevated levels of PST in the hepatopancreas showed a hyperglycaemic response, indicative of stress. These findings suggest that PST accumulation results in some measurable indicators of stress for lobsters, but these changes appear to be within the adaptive range for Southern Rock Lobster and do not result in a significant impairment of gross performance.

To determine whether Southern Rock Lobster could accumulate PST in the supply chain, a controlled experiment was conducted where adult male lobsters were exposed to highly toxic cultures of A. catenella at field relevant concentrations (2 × 105 cells L−1) over a period of 21 days. In contrast to the feeding experiment, no PST accumulated in lobster from exposure to toxic algal cells. The same lobster health parameters were assessed, with no change seen in any of the behavioural, health, nutritional or haemolymph biochemical parameters examined.

To assess appropriate management strategies, regulatory monitoring results since September 2012 were combined with field studies to examine uptake and depuration of toxins in Southern Rock Lobster on the east coast of Tasmania during A. catenella blooms. Results from 499 lobster hepatopancreas PST samples were analysed. A high degree of variability was seen across years, months, sites and between individuals. The highest risk sites are on the central east coast, with exceedances of the bivalve ML occurring between July and January. Mussel sentinel lines were installed in each lobster management zone on the east coast of Tasmania and monitored fortnightly during high-risk periods. These lines proved effective in indicating a risk of elevated PST in lobster hepatopancreas. Field uptake and depuration rates of PST in lobster were similar to those seen during the experimental studies (maximum of 2% and 3% per day respectively), but always less than rates measured simultaneously in mussels. Statistical analysis of hepatopancreas PST levels during bloom development and decline occurred to determine the level of confidence in the regulatory sampling regime (5 individual lobster hepatopancreas samples are analysed from each site during an event). When PST in the hepatopancreas of all lobsters sampled is < 0.42 mg STX equiv. kg-1 there is 97.5% confidence that any lobster from that site would be below the bivalve ML.

The Neogen™ rapid test kit, a qualitative lateral flow immunoassay for the detection of PST, was previously shown to be the best commercially available rapid screening test of PST in bivalves affected by Alexandrium catenella in Tasmania. In the present study we confirmed its applicability to rock lobster hepatopancreas matrices. Based on the analysed toxin profiles, the test kit provides high confidence in detecting PST at levels above 0.4 mg STX.2HCl equiv. kg-1 in lobster hepatopancreas tissues. That means that during years with dense widespread blooms the Neogen rapid test could facilitate monitoring programs by making faster decisions when PST levels are less than half the regulatory limit. Negative Neogen results would not need to be followed up with more expensive LC-MS analysis. Based on our field validation, this could present an average annual cost savings of $5,138 - $6,154 ($41,100 - $49,228 across the eight biotoxin seasons monitored) and provide increased sample turn-around (1-1.5 days vs. 2-3 days for confirmatory LC-MS analysis), which in turn translates to shorter fishery closures (should no PST be detected).

Further work testing samples contaminated with Gymnodinium/A. minutum derived PST is required to ascertain how applicable the Neogen assay is to these blooms (positive Neogen results at lower PST concentrations may limit the cost-effectiveness of the assay during blooms of these species).

The combined field and experimental work has informed improvements to the biotoxin risk management program for Southern Rock Lobster in Tasmania. Implications for biotoxin risk monitoring are: (1) lobsters continue to feed during bloom periods and high concentrations of PST can occur; (2) animal toxin monitoring should be frequent at the start of a bloom in case of a rapid accumulation of PST; (3) mussel sentinel lines are a cost-effective early warning system for toxin accumulation; (4) it is adequate to sample 5 individuals per site as long as a reduced trigger level of closure of harvest is employed; (5) depuration is relatively quick so that sampling to confirm re-opening should occur soon after bloom collapse (as indicated by mussel PST levels); (6) non-lethal sampling is not possible as haemolymph PST levels do not reflect levels in the hepatopancreas. Importantly, (7) lobsters exposed to toxic algae during wet storage in long supply chains (on vessel, in sea-cages or at processing facilities) do not take up PST. Furthermore, (8) survival, quality, and safety of this high-value product are not impacted by accumulation of PST or by exposure to toxic cells in the water.

In conducting this research, it became apparent that international regulations for maximum levels of PST in seafood and research into PST accumulation vary in the units used for PST concentration. Some standards/research reports use mg STX.2HCl equiv. kg-1 , some mg STX equiv. kg-1 (effectively producing total PST results that are 24% lower), and some only state mg kg-1 . Similarly, the toxic equivalency factors (TEF) used to convert analogue concentrations to total PST in toxicity equivalents are varied, and often not stated. A call for uniform reporting units was published to highlight this issue, recommending countries and researchers adopt the Codex Alimentarius Commission protocols of reporting in mg STX.2HCl equiv. kg-1 , using FAO/WHO TEFs. To align with international standards, PST concentrations in this report are expressed as STX.2HCl equiv. kg-1 , except for Chapter 6. This chapter directly relates to current Tasmanian PST regulations which are aligned with the Food Standards Australia New Zealand (FSANZ) biotoxin standard and the Australian bivalve regulatory PST level. An application to harmonise the Australian biotoxin standard (PST expressed as STX equiv. kg-1 ) with the international CODEX standard (PST expressed as STX.2HCl equiv. kg-1 ) has been submit