How do algal blooms affect the fish you eat?

By Jackie Myers

Cyanobacterial aggregation. Wikimedia Commons.

The occurrence of cyanobacterial blooms - more commonly known as “blue-green” algal blooms - has been rising world-wide at an exponential rate in recent decades.  Why are we concerned about these blooms?

Blooms are almost always toxic due to the production of the hepatotoxin known as nodularin.

Many cyanobacteria produce toxins that represent human health hazards if sufficient levels are ingested in water or food, are inhaled, or come into direct dermal contact. Nodularia spumigena is a species of cyanobacteria that forms extensive blooms in estuarine and coastal systems world-wide. Blooms are almost always toxic due to the production of the hepatotoxin known as nodularin. These blooms often result in the accumulation of nodularin in seafood such as prawns, mussels and fish. When occurring in commercial and recreational fishing areas, the  N. spumigena blooms pose a serious food safety risk for a large number of people.

There has been a substantial increase in the frequency of blooms and the subsequent accumulation of nodularin into local seafood species.

This has been the case in the Gippsland Lakes, one of Australia’s largest lake systems situated in south-east Victoria. Over the last few decades there has been a substantial increase in the frequency of N. spumigena blooms and the subsequent accumulation of nodularin into local seafood species. Restrictions around the commercial and recreational harvest for prawns, mussels and finfish have been implemented in order to protect consumers. However, these restrictions have had a significant economic impact on the commercial fishing industry, as well as tourism operators in the region. Monitoring and managing seafood safety during toxic blooms has also placed a considerable economic burden on the government agencies involved.

In order to understand the seafood risks related to fish in the Gippsland Lakes during toxic N. spumigena blooms, the Fisheries Research and Development Corporation funded CAPIM to assess nodularin accumulation, tissue distribution and elimination in relevant fish species (1). This project also aimed to inform monitoring and management practices for seafood safety during cyanobacterial blooms.

In collaboration with scientists from Cawthron Institute, New Zealand, CAPIM scientists exposed two commercially and recreationally relevant fish species for the Gippsland Lakes - black bream (Acanthopagrus butcheri) and sand flathead (Platycephalus bassensis) - to N. spumigena cells in food at concentrations similar to those found in the environment.  They then determined nodularin toxin concentrations in fish tissues (liver, muscle and gut) to assess accumulation, tissue distribution and elimination.

Of the two species, sand flathead were found to pose a higher seafood risk.

Of the two species, sand flathead were found to pose a higher seafood risk, with uptake of nodularin toxin being three times greater than that observed for black bream. While the concentrations of nodularin detected in individual fish exceeded the Victorian health alert guidelines, the toxins were concentrated in the liver and not in muscle tissue. These results support measures enforced in the Gippsland Lakes during N. spumigena blooms, whereby fish may be safe to eat if once harvested they are gutted and gilled.

The project also showed that black bream accumulated lower concentrations of nodularin into tissues than those found in sand flathead and according to different patterns. These results challenge the suitability of the current use of black bream as an early-warning indicator to inform public health decision-making relating to seafood safety in the Gippsland Lakes during toxic blooms. More research is needed on species of commercial and recreational importance in the Gippsland lakes to find a sentinel species that  provides an early indication of risk and of the time when it is safe to consume fish following the decline of a bloom.

For more information on this project please contact Jackie Myers at jhmyers@unimelb.edu.au.

(1) This project was funded by the Fisheries Research and Development Corporation on behalf of the Australian Government.