Introduction
Despite their ecological impact, invasive seaweeds may offer a promising opportunity for sustainable aquafeed development. Along the North Sea coastline, the non-endemic green and brown seaweeds such as Ulva australis (southern sea lettuce) and Sargassum muticum (Japanese wireweed) are now widespread. Their inclusion in shrimp and fish diets could reduce reliance on costly and less sustainable ingredients like fish meal and fish oil. Additionally, many studies demonstrated the antimicrobial properties of seaweed extracts and components. This is particularly relevant in Pacific white-leg shrimp (Litopenaeus vannamei) hatcheries, where Vibrio parahaemolyticus infections cause acute hepatopancreatic necrosis disease (AHPND) and result in high mortality and economic losses. Vaccination is ineffective due to the shrimp’s lack of adaptive immunity, and antibiotic use is strictly regulated to prevent the development of antibiotic resistance in the pathogen. Incorporating bioactive-rich seaweed extracts into pelletized feed may provide a functional immunostimulant solution that supports growth, disease resistance, and environmental sustainability. However, key questions remain about the specific bioactive compounds involved, optimal inclusion rates, and the cost-benefit analysis of the practical application in shrimp farming. This study aims to explore the potential of U. australis and S. muticum as alternative shrimp feed ingredients by evaluating their impact on shrimp growth performance and resistance to AHPND.
Materials and Methods
Seaweed extracts from two U. australis strains and one S. muticum strain collected along the North Sea coastline were prepared using 70% ethanol and lyophilized. Bioactive compounds were analyzed via high-performance liquid chromatography (HPLC) and total phenolic content (TPC) assays. The minimum inhibitory concentration (MIC) against V. parahaemolyticus (AHPND strain M0904) was determined using microbroth dilution. A four-week feeding trial tested 1% and 3% inclusion rates of one of the seaweed extracts in shrimp post-larvae (PL) diets, assessing survival rate (SR), weight gain (WG), feed conversion ratio (FCR), and specific growth rate (SGR). The control group was made of shrimp fed the standard diet without seaweed extracts. A pathogenic challenge evaluated the anti-Vibrio effects of these seaweed-supplemented feeds.
Results
The MIC of both U. australis strains was 12.5 mg/mL, and that of S. muticum was 25 mg/mL. Classified as a phenolic acid and a flavonoid, respectively, gallic acid and quercetin were identified in all extracts and have been extensively studied previously as antimicrobial and antioxidant agents. The estimated TPC in U. australis strain NL, U. australis strain FR, and S. muticum extract was 269 ± 7.86, 69 ± 6.50, and 89 ± 8.65 µg gallic acid equivalent (GAE) per gram dried extract (Table 1).
Preliminary results of the TPC and MIC values suggest that even low inclusion levels of seaweed extract in shrimp diets may confer protective effects against V. parahaemolyticus. A lower MIC combined with a higher TPC indicates stronger antimicrobial activity. Notably, experimental diets containing 3% of the U. australis strain NL showed promising trends in SR, WG, and SGR, with values of 97.78 ± 3.85%, 97.77 ± 26.16%, and 9.65 ± 1.97%/day, respectively, compared to 93.33 ± 5.77%, 92.70 ± 3.48%, and 9.37 ± 0.26%/day in the control group by week one. The FCR was considered relatively high concerning the digestive capacity in the adaptation phase in the first week of the experiment. Based on the SGR comparison, the treated PLs underwent efficient nutrient absorption from feed intake into their body mass. Key water qualities remained favorable and within the limits. These findings imply that seaweed supplementation has not had adverse effects and may contribute to enhanced immune function and growth performance in shrimp.
Conclusion
Selecting seaweed extracts with prospective antimicrobial and antioxidant potentials would reduce reliance on antibiotics or synthetic additives and aligning with sustainable aquaculture goals. Additionally, seaweed extracts may function as growth promoters to improve nutrient utilization. This study provides valuable insights for countries facing widespread macroalgae invasions and intensive aquaculture activities, where the challenges of managing foreign seaweed biomass, ensuring feed sustainability, and improving disease control must be simultaneously addressed. Understanding the relationship between the antimicrobial properties of invasive seaweeds as a dietary supplement and their therapeutic advantage on shrimp PL could be an ecologically and economically friendly prophylactic strategy to support stewardship in global aquaculture practices.
Keywords: Ulva australis, Sargassum muticum, AHPND, Litopenaeus vannamei, aquaculture sustainability, seaweed extracts