J Sci Food Agric 2019; 99:13–24

Margareth Øverland,* Liv T Mydland and Anders Skrede

Macroalgae or extracts have received increasing attention as safe alternatives to prophylactic and therapeutic agents in diets for farmed fish to prevent economic losses related to infectious dis-eases. All three groups of macroalgae, red, green, and brown, have been shown to exhibit antimicrobial properties116 and inhibitory effects against fish pathogens in vitro. Limited information exists on the effect of dietary macroalgal supplementation on the health of farmed fish in vivo, however, although there appears to be increasing interest in the use of macroalgae as a bioactive component in functional feeds for fish. Health-promoting effects include improved immunological responses, such as effects on lysozyme activity and increased complement pathway activity, increased antioxidant activity, and improved stress responses. Peixoto et al.,97 for instance, reported that supplementing diets for European sea bass with 75 g kg−1 of the red alga, Gracilaria, or a mixture of 75 g kg−1 of Gracilaria spp., brown Fucus spp., and green Ulva spp., may alter the metabolic rate, modulate the innate immune response, and cause antioxidant responses without compromising growth performance. The Gracilaria diet also resulted in increased glutathione S-transferase, an enzyme responsible for removing reactive oxygen species (ROS), suggesting that macroalgal supplementation may protect fish from ROS. The immunostimulatory properties of macroalgae may depend on the inclusion rate: Peixoto et al.97 reported a decrease in the hemolytic capacity of the alternative pathway complement system with the inclusion of 75 g kg−1 of Gracilaria or 75 g kg−1 of a macroalgal mixture, while Araujo et al.95 reported an increase in the plasma alternative complement when supplementing diets for rainbow trout with 50 g kg−1 of G. vermiculophylla,whereas a decrease in the immune response occurred at a higher inclu-sion level of 100 g kg−1. In Nile tilapia, inclusion of 100 g kg−1 of meal from U. rigida and U. lactuca increased the alternative complement, while inclusion at 50 g kg−1 had no effect.127 In the same species, inclusion of 50 g kg−1 of U. lactuca and Pterocla-dia capillacea improved growth performance and nutrient reten-tion as well as the stress response and survival rate after air exposure.128 In grouper (Epinephelus coioides), feeding diets con-taining 5 g kg−1 and 10 g kg−1 of laminarin improved growth rate and feed conversion ratio.129 In Atlantic salmon, diets containing 50 and 150 g kg−1 of P. palmata decreased serum activity of alanine transaminase, a biological indicator of liver health status, while there was no effect at 100 g kg−1.90 The research suggests that sev-eral macroalgal species, especially when used at low levels, provide health benefits when fed to fish and therefore have potential as ingredients of functional fish feed.
In general, these studies suggest that several macroalgal species and their extracts have beneficial health effects and potential as sources of bioactive compounds in feed for monogastric aqua-cultural and terrestrial livestock. However, reports of the effects on gut health of intact macroalgae and macroalgal extracts are inconsistent. This could be due to inhibitors in the intact macroalgae or the extracts, differences in bioactivity of compounds like laminarin or fucoidan from different macroalgal species, or differences in experimental design. In future work, attention needs to be paid to developing standard methods for extraction, isolation, and characterization of bioactive components in macroalgae as well as standardized methods to evaluate the impact of these on animal health in in vivo experiments.