Seaweed, also known as macroalgae, represents a vast resource that can be categorized into three taxonomic groups: Rhodophyta (red), Chlorophyta (green), and Phaeophyceae (brown). They are a good source of essential nutrients such as proteins, minerals, vitamins, and omega-3 fatty acids. Seaweed also contains a wide range of functional metabolites, including polyphenols, polysaccharides, and pigments. This study comprehensively discusses seaweed and seaweed-derived metabolites and their potential as a functional feed ingredient in aquafeed for aquaculture production. Past research has discussed the nutritional role of seaweed in promoting the growth performance of fish, but their effects on immune response and gut health in fish have received considerably less attention in the published literature. Existing research, however, has demonstrated that dietary seaweed and seaweed-based metabolite supplementation positively impact the antioxidant status, disease resistance, and stress response in fish. Additionally, seaweed supplementation can promote the growth of beneficial bacteria and inhibit the proliferation of harmful bacteria, thereby improving gut health and nutrient absorption in fish. Nevertheless, an important balance remains between dietary seaweed inclusion level and the resultant metabolic alteration in fish. This review highlights the current state of knowledge and the associated importance of continued research endeavors regarding seaweed and seaweed-based functional metabolites as potential modulators of growth, immune and antioxidant response, and gut microbiota composition in fish.

Determining the macronutrient requirements for commercially valuable aquaculture species remains crucial for maximising production efficiency. Yet, such information is lacking for Australian hybrid abalone (Haliotis rubra × Haliotis laevigata), particularly with respect to life stage and water temperatures. The present study aimed to evaluate the effect of dietary protein inclusion level on the growth performance, nutrient utilisation and nutritional quality of juvenile (3·3 g) Australian hybrid abalone reared at three different temperatures representative of winter (12°C), average annual (17°C) and summer (22°C) grow-out periods and fed five diets containing graded dietary protein levels of 35, 38, 41, 44 and 47 %. Abalone growth increased with increasing water temperature with weight gains of approximately 100, 280 and 380 % of their initial weight at 12, 17 and 22°C, respectively. Furthermore, the present study clearly demonstrated that higher dietary protein inclusion levels (41 %) than those currently used commercially (35 %) would significantly improve the growth performance when water temperatures are ≥17°C without any adverse impacts on nutrient utilisation, nutrient deposition or nutritional quality of the abalone soft tissue. For example, at 22°C abalone fed a diet containing 41 % protein obtained a significantly higher weight gain percentage (421 %) compared with those fed a diet containing 35 % protein (356 %). Lastly, it is suggested that maintaining a dietary protein inclusion level of 35 % or implementing a ‘least cost’ feeding approach during cooler seasons, or where water temperatures are ∼12°C, may be beneficial, considering only marginal growth improvements were observed during these periods of slow growth.

Australian abalone aquaculture is characterised by a prolonged culture period and slow and variable growth, and abalone is cultured in fluctuating water temperatures ranging between 10 and 25°C with distinct seasons. Temperature is a crucial environmental factor influencing abalones physiology and energetics, leading to a change in nutritional requirements. However, feeds are generally formulated to match the nutritional requirements at their optimal temperature. Hence, there is a need to optimise dietary protein levels to match temperature-specific requirements during extreme conditions (winter and summer). Given this, a growth trial evaluating five experimental feeds consisting of graded protein inclusion levels (320, 350, 380, 410, and 440g·kg⁻¹) was conducted on subadult hybrid abalone (Haliotis rubra×H. laevigata) at three different temperatures reflecting winter (12°C), summer (22°C), and the annual average water temperature (17°C) for 143days. At lower water temperature (12°C), there was a marginal improvement in growth performance as dietary protein levels increased from 320 to 440g·kg⁻¹. However, at higher water temperatures (when the culture water temperature is above 17°C), there was a significant improvement in growth performance as dietary protein levels increased from 320 to 440g·kg⁻¹ as evidenced by an improved weight gain, specific growth rate, and feed conversion ratio. Furthermore, increasing dietary protein levels did not compromise the nutritional quality of the abalone tissue at all three tested temperatures. Therefore, during periods of higher water temperatures, feeding Australian hybrid abalone with a relatively high dietary protein level (410g·kg⁻¹) is expected to result in improved growth, shorter culture duration, and profit maximisation.