Macroalgae, commonly known as seaweed, offer a novel and added-value dietary ingredient in formulated diets for fish. Production of biomass can be achieved without reliance on  expensive arable land, as seaweed may be collected from coastal regions or farmed. There are three taxonomic groups represented by the term ‘macroalgae’: Rhodophyta (red), Chlorophyta (green) and Phaeophyta (brown). Like terrestrial plants, nutritional content in macroalgae can vary greatly amongst species, genera, divisions, seasons and locations. Aside from their basic nutritional value, seaweeds contain a number of pigments, defensive and storage compounds, and secondary metabolites that could have beneficial effects on farmed fish. This review appraises the beneficial qualities of these macroalgae compounds and their potential for exploitation in commercial feed fish.

 Commercially formulated diets represent the largest proportion of the production costs in finfish aquaculture. Furthermore, a continuing stagnation in the production of fish meal (FM) and fish oil (FO) from fisheries when coupled with a rising demand for finfish diets has led to an overall increase in feed costs. Consequently, both research institutions and feed manufacturers are seeking novel economically and environmentally sustainable sources of feed materials as replacements for FM and FO. Ultimately, for these feed ingredient replacement alternatives to be accepted, they must maintain growth, and overall  health/survival and fillet quality of farmed fish, while meeting retailer and consumer expectations.

Replacement feed ingredients have a number of factors that affect their potential use. One potential feed ingredient, animal by-products, contains high levels of protein, lipid, vitamins and minerals. However, materials produced from waste streams of rendered terrestrial animals (e.g. offal and bone meal from bovine sources) are highly restricted in both the European Union (EC No 956/2008)  and the United States (US FDA CPG Sec. 675.400). One recent exception is legislation that permits the inclusion of poultry by-product and feather meals (category 3 sources) into aquafeeds. Food source constraints arose after bovine spongiform encephalopathy (BSE) disease outbreaks in the European countries during the 1990s. Restrictions on diets based on farmed animal tissue, issues of source availability and low-cost production have seen plants as the principal alternative to FM and FO use in aquafeeds (Gatlin et al. 2007; Naylor et al. 2009). More recently, processing plantbased meals, such as soybean, lupin and various pulses, through applying exogenous enzymes, chemicals and physical treatments, have allowed manufacturers to overcome the effects of antinutritional factors (ANF) and digestibility issues, which are commonly present in plant-derived ingredients. This has subsequently enabled elevated inclusion levels of plant meals in diets for less tolerant carnivorous finfish species, for example rainbow trout and Atlantic salmon. In recent years, the world has seen a wave of governmental subsidies and venture capital investments into the renewable energy sector. Aimed at reducing carbon dioxide emissions, farmers have taken advantage of these new incentives and moved from food to biofuel crop production. Furthermore, the non renewable  phosphate rich minerals used for fertilising plant crops are  gradually becoming scarce. These factors, combined with an increasing global demand for food, will probably see the price of plant meals and oils increase in the future to make it less attractive as an alternative to FM and FO in fish feeds. Consequently, questions have been raised by consumers on whether the use of animal and plant by-products really is safe and sustainable for finfish aquaculture.

An alternative to plant meal is marine macroalgae, produced without the need for arable land, freshwater and the expensive fertilisers associated with terrestrial crop production. Macroalgae, or seaweed, encompass algae that are multicellular, macroscopic and typically have a benthic lifestyle. They are a diverse assemblage of algal species due to an early divergence in their divisions, which include Division Rhodophyta (red), Division Chlorophyta (green) and the Division Ochrophyta, Class Phaeophyta (brown). Marine macroalgae are found in the intertidal, subtidal coastal zones and estuarine habitats. Life in these habitats can be a challenge, and the organisms living there can experience a variety of physical stressors: temperature, light, salinity, desiccation and wave action, alongside biological pressures: predation, competition, parasitism and allelopathy).

Many of the secondary metabolites produced by the algae are seen as adaptive responses to these selective pressures. These metabolites include functional proteins, peptides , mycosporine-like amino acids, carotenoids, phenolics, fatty acids, vitamins, functional carbohydrates  and other secondary metabolites . Many of the compounds found in macroalgae are not yet fully understood. In addition to aiding macroalgal survival, some of these compounds have been found to have a beneficial effect on animals (e.g. pigs, broiler chicks, ruminants and finfish) when they are administered into feeds.

Indeed, a comprehensive review of seaweeds as a viable protein source for monogastric livestock was undertaken by Angell et al. (2016) covering land animals and selected fish species, although these authors focussed principally on amino acid composition, protein concentration and development to optimise these nutrients in various seaweed products with further processing. However, the other important nutritional benefits of seaweeds were not highlighted in the latter review and important aspects not appreciated in the wider context of animal production and health.

With such promising attributes, seaweeds may fill more than just a nutritional role in aquafeeds: they may also promote fish health and fish welfare. This review will comprehensively examine the potential of both nutritional and functional components available in seaweeds that could benefit farmed fish. This review will focus on the current knowledge of the effects of seaweeds on fish growth performance, health and muscle quality. An economic assessment and examination of the current legislative feed safety regulations are used to evaluate whether seaweed is currently viable for commercial fish diets.

Source : https://onlinelibrary.wiley.com/doi/abs/10.1111/raq.12241

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