ASTOX - Amnesic Shellfish toxins in Shellfish harvesting Sites in Shetland

In 2019 the Scottish shellfish industry produced 7,080 tonnes of shellfish for the table with a further 3,493 tonnes on-growing. The greatest proportion, approximately 80%, of farmed mussels in Scotland come from Shetland which, has 137 active farm sites, 93 of them producing shellfish and the remaining 44 in different stages of the growing cycle. These employ 89 people, 58 of them full time and the rest part time or casual, making it an important source of employment on the island.

The waters around Shetland are well suited to farming mussels as they abound with microscopic, single celled algae (phytoplankton), the major food source for shellfish.

Unfortunately, out of the thousands of species of phytoplankton that exist, a few produce toxins.

The three main genus of harmful algae affecting Shellfish aquaculture in Shetland; Dinophysis sp. (panel A), Pseudo-nitzschia sp. (panels B and D) and Alexandrium sp. (panel C).

To protect human health the government carries out weekly official control monitoring (OCM), to determine the concentration of these toxins in farmed shellfish, and temporarily closes any site where concentrations exceed given thresholds.

The three genera that are responsible for most of the site closures in Shetland, are

It’s worth noting that not all of the species, or different strains of the same species, in these genera produce toxins.

The majority of farm closures in Shetland are due to high concentrations of the toxins produced by Dinophysis leading to DSP.

Activities and results

If any of these phytoplankton are eaten by mussels and those mussels are harvested and consumed it can have severe implications for human well-being.

Unfortunately, the cost of OCM is high and to help keep costs down, analysis of all other toxins stops at any site that is closed because concentrations of a particular toxin has breached safe levels. Only the problematic toxin is monitored until the site is given permission to re-open. In addition, a monitoring schedule has been introduced based on several risk assessments. For example, while the presence of DSP is monitored weekly, ASP is usually only monitored once a month.

In 2013 a large bloom of toxic Dinophysis, advected from offshore, led to the closure of most farm sites on Shetland for a period of several weeks. Intoxicated shellfish also reached market and resulted in 70 people seeking medical help. This event had a deleterious effect on the economic viability of the shellfish industry in Shetland. In an effort to mitigate the risks of a future toxic event, in 2014, the grower’s organisation, Seafood Shetland, approached SAMS and asked us to provide a risk assessment and forecast of possible harmful events. Since then, SAMS has provided a weekly bulletin and risk assessment to the industry. While this has proved beneficial to the farmers we are continuously trying to improve the accuracy of our forecasts.

As already mentioned, analysis of toxins partly ceases when a site is closed. This means that there are inevitable gaps in our understanding of the prevalence of toxins in farmed shellfish around Shetland. This project re-analysed archived material that was collected during routine monitoring in 2020 but not tested for the presence of ASP toxins. Most site closures in Shetland are in response to the toxins causing DSP. However, High numbers of Pseudo-nitzschia sp., the cells responsible for ASP, are often detected in the waters at the same time as Dinophysis sp., leading us to suspect that these toxins would also be present in the shellfish.

The study did indeed find these toxins present in most of the samples analysed. Fortunately, levels were below the official thresholds in all but one case. The site in question, experienced a large bloom of Pseudo-nitzschia and within two days the levels of toxins had breached official thresholds. The worrying aspect of this event was that the site was open but not scheduled for routine toxin monitoring. The bloom was noted by the phytoplankton monitoring programme at SAMS and an alert was issued but testing for toxins could not take place until the following week which in this case was too late. Fortunately, the farmers in Shetland also carry out their own testing before harvesting so this event did not result in any health incidents. It did, however, point out a potential gap in the monitoring system. Genetic analysis of the species present revealed that the bloom was composed mainly of Pseudo-nitzschia australis with a small quantity of P. multiseries and P. seriata, all of these species are known to be highly toxic.

It should be pointed out that, from a taxonomic point of view, there are two different groups of Pseudo-nitzschia, recognised and differentiated by their size. The Pseudo-nitzschia delicatissima group have a cell width that is 3 µm or less and a needle like shape while the Pseudo-nitzschia seriata are larger with expanded cell margins. The two groups are also more prevalent at different times of the year with the P. delicatissima group preferring late spring and early summer and the P. seriata group preferring late summer and autumn. This study found that the greatest risk of a toxic event was caused by species from the P. seriata group. Indeed, all of the species detected during the toxic event were part of the latter group.

This information has already been incorporated into our bulletin, early warning website and is used in our risk assessment based on expert evaluation. The testing schedule for ASPs has also been brought into question and this will form part of future discussions between CEFAS, SAMS and the regulatory bodies; FSS and the National Reference Laboratory.

For more information contact: Callum Whyte