Algal Blooms of Heterosigma akashiwo and Mugilidae Gill Alterations

Harmful algal bloom events (HABs) of the raphidophyte, Heterosigma akashiwo, have increased in frequency and severity in some South African estuaries and have been observed in conjunction with substantial fish kills. The impact of H. akashiwo on wild fish populations is still unclear, despite being responsible for massive economic losses in the aquaculture industry globally. Thus, histopathological techniques were used to assess the impacts of H. akashiwo HABs on three species of Mugilidae (Chelon richardsonii, Mugil cephalus and Chelon dumerili) during an in situ cage study (stocking density of < 0.75 kg m(-3)) in the agriculturally influenced Sundays Estuary, which has predictable spring/summer blooming periods of H. akashiwo. It was hypothesised that fishes kept in mesocosms situated in the bloom area of the estuary would have higher blood plasma cortisol and glucose levels and have more deleterious gill alterations than fishes kept adjacent to the bloom area. Blood plasma cortisol, glucose and gill histological and morphological indices were similar between the HAB and control treatments but differed between bloom phases. The proportion of secondary lamellae available for gas exchange (PAGE index) was lower during trials that were in a hypereutrophic bloom phase (> 60 Chl-a mu g.L-1) compared to the accumulation and decay phases. Gill histological alterations (HIGill index) were also more pronounced during the same trials. This study emphasises the need for further research including a toxicology study on the H. akashiwo strain, now common in some South African estuaries, to gain knowledge on the mechanisms by which H. akashiwo causes fish kills in order to protect the ecosystem services that estuaries provide.

Automated summary

This study assessed the impacts of harmful algal bloom events of Heterosigma akashiwo on fish populations in South African estuaries. It found that the blooms had effects on blood and gill morphology in fish, but these effects differed between bloom phases. The study highlights the need for further research on the toxicity of H. akashiwo to protect estuarine ecosystem services.