MITIGATION OF HARMFUL ALGAL BLOOMS SPECIES THAT AFFECT SCOTTISH AND NORWEGIAN ATLANTIC SALMON FISH FARMS USING MODIFIED CLAYS

Introduction

Understanding how to control microalgal species that negatively impact Atlantic salmon fish farms in fjord systems is a priority for the industry. Harmful Algal Blooms (HABs) in the waters of Scotland and Norway threaten both local food security and global aquaculture production, and they cause significant fish mortality and economic losses (Davidson et al., 2020). Several algal species can damage gill tissues or release toxic metabolites, compromising fish health and welfare in aquaculture systems (Hallegraeff et al., 2023; Planellas et al., 2026). The frequency, duration, and geographic extent of HAB events across Northern Europe are expected to increase with the changing climate (Murray et al., 2025). This increases the urgency for efficient and cost-effective mitigation strategies. At the moment, there are no widely adopted techniques in the salmon industry that are effective at mitigating the adverse effects of HAB (Anderson et al., 2025).

A promising technology is the application of modified clays (MC), which can adhere to algal cells, resulting in the flocculation and sinking of harmful species (Yu et al.,2017). These clays are modified with poly-aluminium chloride, which confers a positive surface charge on the materials, thereby altering their structure and increasing the attraction between the materials and the cells (Yu et al., 2017; Song et al., 2021). Furthermore, it is suggested that these MCs have the capacity to degrade algal toxins. In addition, MCs have been used in large-scale HABs across China, Malaysia and Turkey including open waters for fish aquaculture and enclosed culture ponds.

This study is the first in Europe to investigate the efficiency of commercially available MC (from the IOCAS) in removing seven selected HAB species from the water column under laboratory conditions at fjord-relevant temperatures and salinities. Furthermore, it will evaluate the potential impact of MCs on fish gill cell lines (RTgill-W1) as a proxy for fish toxicity. In addition, using this bioassays to assess unknown toxins and HPLC-MS/MS to detect known toxins. The study will also examine whether these materials can mitigate the toxic effects of harmful algae by adsorbing toxins.

Methodology

We performed a floc-and-sink experiment, in which cultures of algae were mixed with two different clays (MC-1 and MC-2) at a final concentration of 0.2 g L^-1, and cell removal was measured after 3 and 24 hours. These experiments targeted HAB species that cause physical damage to farmed fish: Chaetoceros curvisetus (CCAP 1010/12), and toxic species for fish: Alexandrium catenella (CCAP 1119/17), Alexandrium minutum (CCAP 1119/49), Karenia mikimotoi (RCC10703), Karlodinium veneficum (NORCCA K-1634), Chrysochromulina leadbeateri (NORCCA UIO394), and Prymnesium parvum (CCAP 946/7). At the same time, use the RTgill-W1 cell line as a bioassay to determine whether these clays are cytotoxic (and potentially toxic to fish) and whether they can adsorb the toxic compounds produced by the algae.This experiment was conducted on the cell line exposed to MC-1 and MC-2 alone at different concentrations and then assessed whether the MCs could mitigate the effects of the three species, A. catenella, K. mikimotoi and C. leadbeateri.

Results and Discussion

The results showed that, on a laboratory scale, both MCs reduced cell density in the water column by at least 50% for all taxa after 24 hours. For thecate dinoflagellates and diatoms, the removal efficiency was more than 75% in both types of clay at 3 hours (Figure 1A). In addition, athecate dinoflagellates show increased removal efficiency after 24 hours. Removal efficiency varied significantly among phytoplankton species and over time (Yu et al., 2017). Additionally, the RTgill-W1 bioassay MC-1 didn't show any reduction in cell viability in any of the concentrations tested. Furthermore, the cell line bioassay results suggest that MCs could adsorb toxic compounds produced by Alexandrium catenella, increasing the cell viability at higher concentrations of MCs(Figure 1B). This indicates sorption capacity of MCs for harmful algal compounds, as previously suggested by Song et al. (2021).

Figure 1: Effect of MCs on Alexandrium catenella. A) Removal efficiency over time of two type of MCs. B) Cell viability of RT-gill W1 on exposure of Alexandrium catenella supernatant with MCs

Overall, modified clays demonstrate considerable potential as a mitigation strategy against several HABs that affect fish. Concomitantly, it can adsorb toxic compounds, thereby increasing gill cell viability. Nevertheless, further research is required to evaluate their environmental safety and potential impacts on non-target organisms in European aquaculture, such as farmed fish.

Acknowledgments

We thank our collaborators, institutions, UHI Aquaculture Hub student travel funds, Salmon Scotland Student fund and SUPER DTP (NERC-UKRI) for supporting this research.

References

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