WWW.WAS.ORG • WORLD AQUACULTURE • DECEMBER 2024 25 Decreasing mussel activity levels by decreasing metabolism One promising approach is to utilise nature’s resilience toolbox, specifically metabolic depression. When entering a metabolically suppressed state, organisms can become less sensitive to, or even decoupled from, external stimuli. This metabolic response allows them to tolerate unfavourable environmental changes (Hand and Hardewig 1996, Guppy and Withers, 1999). Conditions that lower the metabolism as pre-treatments to make mussels “sleep” before live transport, therefore, have great potential for commercial application to reduce mussel loss along supply chains (Figure 2). Scoping the conditions that can lower mussel metabolism Following a literature review, three common inducers (low temperatures, low oxygen levels and anaesthetics) were identified to suppress the metabolism of mussels. To rapidly screen more efficacious conditions to lower metabolism, heart rate was used as a proxy for overall metabolism. Heart rate was measured with a noninvasive pulse sensor (Figure 3) in mussels exposed to various levels of inducers (Cheng et al. 2024). Comparison of the effectiveness of selected conditions for reducing mussel metabolism Our study revealed that lower temperatures and increasing concentrations of the food-safe anaesthetic magnesium chloride (MgCl2) were more effective at suppressing mussel metabolism than low oxygen conditions (Figure 4). Heart rate, used as a biomarker, revealed that 4 °C was the most effective temperature for reducing the metabolism of P. canaliculus, as heart rates nearly stopped at this temperature. The decrease in environmental temperature lowers body temperature, which in turn reduces enzyme activity and suppresses metabolism. Similarly, 40 g L-¹ MgCl 2 decreased mussel metabolism by 97% within two hours, making it a promising metabolic suppressant. Mg2+ ions are likely to relax heart muscle by blocking calcium ions (Ca2+), which are required for muscle contraction Aquaculture has become a crucial food source and is expected to play a key role in meeting the rising global food demand. Among the various forms of aquaculture, bivalve farming stands out due to its high yields and low environmental impact, making it one of the most sustainable ways to produce animal protein (Shumway et al. 2003, van der Schatte Olivier et al. 2020). As a key contributor to global aquaculture production, New Zealand produced 106,000 tonnes of aquaculture products in 2022—a 23% increase since 2000 (FAO 2024). One species in particular, the New Zealand endemic, green-lipped mussel (Perna canaliculus, Figure 1), has seen increasing demand due to its high quality and nutritional value. This species now has an annual production valued at $394 million NZD (Aquaculture New Zealand 2023). These mussels, which are cultivated exclusively in New Zealand’s waters due to their narrow geographical range, are in high demand globally. As consumers prefer fresher, higher-quality mussels, live transport of green-lipped mussels has been explored as a way to meet this demand as well as increase export revenue. Live transport, however, poses its own challenges. Mussels are often subjected to temperature fluctuations, prolonged periods of aerial exposure or closed seawater systems. Additionally, the mechanical processes involved in harvesting and transporting live mussels can cause significant stress. Combined, these stressors increase mussel energy demands, leading to a build-up of metabolic waste, depletion of energy reserves, and ultimately to reduced quality, survival rates, and shelf life, resulting in product downgrading and unnecessary food waste. A need for methods to reduce mussel mortality during live transport Mussel mortality rates in the supply chain can typically range from 2% to 7% (Barrento and Powell, 2016), translating to a potential loss of NZ$7.8 – 27.3 million. Given these challenges, it is crucial to develop new methods to enhance the resilience of mussels during live transport. Minimising stress during these processes is key to improving survival rates and extending shelf life, emphasising the need for innovative solutions to ensure the sustainability of New Zealand’s green-lipped mussel industry. Optimising Live Mussel Transport: Reducing Metabolism to Minimise Food Waste in the Supply Chain Martin C.F. Cheng, Leonardo N. Zamora, Norman L.C. Ragg, Brendon J. Dunphy FIGURE 1. New Zealand green-lipped mussel, Perna canaliculus. Photo by Cheng MCF. (CONTINUED ON PAGE 26)
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