“Recently, increasing evidence suggests seaweeds are more valuable as functional food or feed additives, with several bioactive properties that can be exploited to improve productivity in aquaculture. Numerous published reports and decades of successful use have highlighted the potential of seaweeds as rich sources of bioactive compounds with potential applications in animal nutrition. This brief review is focused on the benefits of macroalgal polysaccharides in aquaculture diets.”
Head of Nutrition
Ocean Harvest Technology
Despite improvements in recent years, acquaculture remains dependent on marine ingredients such as fishmeal and fish oil. A recent review estimated that the share of global fishmeal and fish oil used by the aquaculture sector versus livestock and non-food uses was 69% and 75%, respectively (Naylor et al., 2021). To improve sustainability, the aquaculture industry has increased the use of terrestrial plant-based ingredients. However, the use of plant-based ingredients such as grains and oilseeds present challenges in fish. In particular, carnivorous fish have difficulty digesting starch and other carbohydrates, including fibre, in these ingredients. Fish are also sensitive to antinutrients and toxins in plant-based ingredients. Replacing fishmeal and fish oil in aquatic feeds with terrestrial plant-sourced ingredients can impact the health of piscivorous species by altering the gastrointestinal microbiome, changing gut morphology and modifying immune function (Naylor et al., 2021).
Marine macroalgae or seaweeds consist of three diverse Phyla, the Phaeophyta (brown) Chlorophyta (green) and Rodophyta (red) seaweeds. Globally, appreciation for the potential of seaweeds to offer improved nutrition and ecosystem services is increasing (Naylor et al., 2021). In recent decades, research focused on the potential of seaweeds as substitutes for terrestrial plant and animal protein and energy has not yielded much success. However more recently, increasing evidence suggests seaweeds are more valuable as functional food or feed additives, with several bioactive properties that can be exploited to improve productivity in aquaculture. Numerous published reports and decades of successful use have highlighted the potential of seaweeds as rich sources of bioactive compounds with potential applications in animal nutrition (Jesus Raposo et al., 2016). This brief review is focused on the benefits of seaweed polysaccharides in aquaculture diets.
The non-nutritive effects of some seaweed polysaccharides (fibre) can be attributed to their colloidal properties. Some fibres can act as emulsifiers and colloids that, when incorporated into mixed feed, can increase the stability of the feed when exposed to water. The high temperatures associated with the extrusion process often employed in aquatic feed processing activate changes in seaweed fibre that lead to changes in feed texture (Wan et al., 2018). For example, a study by Hashim and Saat (1992) demonstrated that when Ulva seaweed or extracted carrageenan supplemented diets were immersed in water, these diets lost less mass compared to a similar diet without added seaweed. More importantly, seaweed fibres also possess prebotic properties in aquatic and terrestrial animals (Jesus Raposo et al, 2016). In addition, sulphated seaweed fibre can display bacteriostatic and immune modulating properties (Wan et al., 2018). For example, Cheng et al (2008) tested the bacteriostatic properties and reported increased survival rates in fish challenged with Vibrio alginolyticus when fed seaweed fibres. In another study, seaweed fibre extracts from the brown seaweed Laminaria sp. were reported to have immune stimulatory affects in Atlantic salmon (Dalmo and Seljelid, 1995). Peixoto et al. (2019) challenged European sea bass with Photobacterium damselae (PD) and reported that an extract from the red seaweed Gracilaria sp. added to diets modulated resistance to the PD infection. A similar study investigating the effects of a green seaweed Ulva ohnoi in diets for sea bass and sea bream concluded that the seaweed had an immunostimulatory effect and positively influenced oxidative metabolism (Martinez-Antequera et al., 2021). However, given the issues surrounding the use of antimicrobials and replacement of fishmeal in aquaculture, the potential of seaweed fibre is even greater as a prebiotic. Recently Liu et al. (2019) evaluated the prebiotic effects of seaweeds in Litopenaeus vannamei shrimp and reported enhanced hepatopancreas immunity and enhanced antioxidative status in shrimp consuming the diets containing seaweed. They also reported modifications in intestinal microbiota, where seaweed inclusion increased the relative abundance of beneficial bacteria such as Firmicutes and Bacillaceae, while decreasing pathogenic bacteria such as Gammaproteobacteria and Vibrionaceae. Niu et al., (2019) also conducted a white spot syndrome virus challenge trial, where mortality was reportedly lower in shrimp consuming seaweed-containing diets.
Given the bioactive properties of several seaweed components, which offer opportunities to improve aquatic feed properties and other functional benefits including immune stimulatory and antioxidative effects, seaweed represents a very attractive value-enhancing ingredient for aquaculture. The key question is whether these effects lead to improvements in performance. A recent study demonstrated that including seaweed meal in diets for Atlantic salmon improved protein efficiency ratio and feed conversion ratio (Kamunde et al., 2019). In another study with shrimp, the inclusion of Ulva lactuca seaweed meal in the diet resulted in improvements in growth and FCR as well as enhanced lipid and carotenoid content of the shrimp (Elizondo-Gonzalez et al., 2018). Additionally, sulphated polysaccharides (SP) extracted from Ulva intestinalis and Gracilaria persica were fed to rainbow trout for 8 weeks. The fish receiving the SP had significantly improved growth rates compared to control fish (Safavi et al., 2019), demonstrating that using the extracted SP provided similar benefits as seen with whole seaweed meal in other studies. Taken together, the results of the studies cited provide substantial evidence that seaweed fibre delivers prebiotic benefits in aquaculture through improved gut health, leading to higher productivity.
About Dr Jason Sands
Jason is Head of Nutrition at Ocean Harvest Technology Ltd. In this role Jason is responsible for leading research to develop technical documentation, strengthen the OceanFeed brand and support OHT’s sales and distribution teams by providing technical guidance for the OceanFeed range of products. Prior to Joining OHT, Jason worked various research and technical roles with public, academic and private companies. Jason holds a Bachelor and Master of science degrees in Animal Science from Tuskegee University and The University of Tennessee, respectively, and a PhD in Animal Nutrition from Purdue University.
