Solutions to fishmeal inclusion in aquafeeds are multi-faceted. Apart from inclusion of plant protein sources (from more plant species) and animal by-products, other initiatives included:
- pre-processing techniques of plant material to reduce the effects of anti-nutritional factors in order to enhance nutritional value;
- breeding of plants with a better amino acid profile and less antinutritional factors;
- selecting fish species with lower marine protein requirements (e.g. herbivores);
- converting low grade land animal by-products into high-value protein, and
- most recently, the use of new innovative protein sources.
The excessive reliance of aquaculture on fishmeal and fish oil has already led to dedicated research such as RAFOA (Research on Alternatives to Fish Oil in Aquaculture, coordinated by the University of Stirling, Scotland) and PEPPA (Perspectives of Plant Protein Use in Aquaculture, coordinated by INRA, France) under the fifth framework of the EU. The targeted reduction of dependency on fishmeal and oil by this research is given in Table 32.
It has been established that blends of vegetable oils can replace fish oil for the major part of the growth period in several farmed fish (Atlantic salmon, rainbow trout, European seabass and gilthead seabream) (Aquamax, 2008). This has been achieved by blending vegetable oil to mimic the levels of total saturated, total monounsaturated and total polyunsaturated fatty acids in fish oils, and their high levels of omega-3 polyunsaturates, except that of the C18 linolenic acid (18:3w3).
Biological enhancement through micro-organisms such as yeast and bacterial and fungal fermentations have been investigated to determine their capacity to reduce the effects of anti-nutrients in plant materials and current results demonstrate great potential of this method for removing anti-nutrients and adding essential nutrients such as protein and amino acids (Mukhopadhyay and Ray, 1999; Bairagi et al., 2004).
The overall goal of these processes is to increase protein concentration and decrease the levels of anti-nutrients (Gatlin et al., 2007). The knowledge on genetic manipulations to achieve better traits such as protein and oil content of plants has been extended to lower the anti-nutritional factors. Genetic manipulations have been investigated to achieve low levels of phytic acid and thereby enhance available phosphorous (Guttieri et al., 2004), increase essential amino acids such as lysine (Gibbon and Larkins 2005; Stepansky et al., 2004), increase the levels of oil (Laurie et al., 2004) and increase micronutrients such as antioxidatives (Capell and Christou, 2004).
Innovative new protein sources are mainly focused on microbial and algal species. However, cost of production will be an issue with most of the manufacturers of microbial proteins (Aquaculture Innovation, 2008). Locating the microbial protein manufacturing facilities very close to the major feed manufacturing locations may be a measure to keep the cost down by minimizing transport costs, which tend to increase over time. A company in Asia is developing microbial product at present with considerable interest from Asia’s largest integrated feed company, Charoen Pokphand Group Thailand (Aquaculture Innovation, 2008). It will be interesting to see the uptake of this type of product in the aquafeed sector when commercialized in the near future.
There are several benefits of microbial and plankton products. Single-cell products include products from bacteria, microalgae, protists and yeasts comprised of protein and omega-3 oils. Plankton, including copepods, euphausiids, amphipods and krill, which feed in low trophic levels, contain bioactive compounds like omega-3, bound phospholipids and axastanthin and have the potential to serve as a source of protein, oil, attractants and pigments (Hardy, 2004). However, exploitation of plankton should strike a balance to avoid negative ecological consequences to organisms in higher trophic levels.
Converting low-grade land-animal by-products into high-value aquafeed protein, with the appropriate amino acid balance, may be an innovative and a low-cost method to achieve the high levels of fishmeal replacement necessary (Aquaculture Innovation, 2008). In addition to plankton, other invertebrates used as protein sources to replace fishmeal are polychaete worms and terrestrial insects. Polychaetes include both marine worms (e.g. Nereis virens) and the earthworms (e.g. Eisenia foetida and Endrilus eugineae). On dry matter basis, the earthworm has 60–70 percent protein with high essential amino acid content, especially lysine and methionine. Other nutrients include 6–11 percent fat, 5–21 percent carbohydrate, 2–3 percent minerals and a range of vitamins, particularly niacin and vitamin B12. Marine worms, particularly known to induce sexual maturity, are used in broodstock and maturation feeds. The main limitation is their high moisture content (60 percent) and availability. These worms are a potentially valuable source of protein if they can be produced and processed economically. Among terrestrial insects, silkworm pupae, which contain a high content of free fatty acids, are being used. The de-oiled pupae are found to be most appropriate because of their high protein content and well-balanced amino acids.
In order to overcome poor growth and reduced immunity due to replacement of fishmeal, several initiatives have been taken in the feed industry. Some of the common initiatives are listed below.
Antibiotics: Drugs of natural or synthetic origin that have the capacity to kill or to inhibit the growth of micro-organisms are administered through feed. Prophylactic antibiotics are mostly used in intensive aquaculture to bolster the immune system, which tends to be weakened by stress caused by manipulations and high stocking densities or by replacement of fishmeal. Antibiotics are also used as growth promoters because of their positive effects on weight gain, feed utilization and mortality reduction. However, antibiotics administered through feed can find their way to the wider aquatic environment through unconsumed feed and faeces. Consequently, residual antibiotics exert selective pressure, which alters composition of the indigenous micro- flora and increases their resistance to antibiotics. Residual antibiotics have also been detected in fish and shellfish products destined for human consumption with similar consequences. This has led to stern measures against use of antibiotics ranging from total ban to severe restrictions of their use in aquaculture.
Nutrient supplementation: Poor performance in terms of immune-competence and disease resistance is partly due to deficiencies of nutrients, particularly amino acids/proteins, vitamins and minerals. These nutrients include the amino acids, arginine, glutamine and L–tryptophan. While arginine plays a key role in the microbial killing mechanism, glutamine serves as a source of energy for the immune system and as a precursor for nucleotide synthesis and facilitates proliferation of immunocytes during infection. L–tryptophan suppresses aggression in juvenile cod and cannibalism in juvenile grouper and prevents cortisol responses to stress in rainbow trout.
Probiotics, prebiotics and synbiotics: Probiotics are applied through feed or culture medium to prevent against pathogenic bacteria by minimizing the numbers of potentially pathogenic microbes by competitive exclusion, thereby modifying the composition of the microbial community in the organism as well as the culture medium in favour of harmless/beneficial microbes. Probiotics are natural viable micro- organisms such as Bacillus spp bacteria that have a beneficial effect on the health of the host upon ingestion by improving properties of its indigenous microflora. In general, the gastrointestinal microbiota of fishes, including those produced in aquaculture, have been poorly characterized, especially the anaerobic microbiota and, therefore, more detailed studies of the microbial community of cultured fish are needed to potentially enhance the effectiveness of probiotic supplementation (Gatlin et al., 2007).
Prebiotics are non-digestible food ingredients, which have beneficial effects on the host by selectively stimulating growth and/or activating a limited number of health promoting bacteria in the intestinal tract, thus improving the host’s intestinal balance and consequently decreasing the incidence of infection (Gibson and Roberfroid, 1995). Mostly oligosaccharides, such as mannan-oligosaccharides, fructo-oligosaccharides, transgalacto-oligosaccharide and inulin act as prebiotics (Vulevic, Rastall and Gibson, 2004).
Synbiotics are mixtures of probiotics and prebiotics, which are beneficial to the host by improving the survival and implantation of live microbial dietary supplements in the gastrointestinal tract by selectively stimulating the growth and/or activating the metabolism of one or a limited number of heat-promoting bacteria and thus improving host welfare. This is a new concept in aquaculture.
Nucleotides: These are low molecular weight biological compounds, which are building blocks of DNA and RNA and play vital roles in various physiological and biochemical functions of the body, often regarded as conditionally essential nutrients particularly during periods of rapid growth and physiological stress (Uauy, 1994). Dietary nucleotides are more preferential because de novo synthesis and salvage of nucleotides are metabolically costly processes that account for 5–10 percent of the energy used in the synthesis of tissue protein (Carver, 1994; Grimble, 1994). Dietary nucleotides in aquaculture have shown a number of beneficial effects such as:
- enhanced feed intake observed in largemouth bass (Kubitza, Lovshin and Lovel, 1997);
- improvement in growth observed in tilapia (Ramadan and Atef, 1991) and salmonids (Adamek et al., 1996; Burrells et al., 2001);
- increased resistance to pathogens observed in salmonids (Burrells et al., 2001; Leonardi, Sandino and Klempau, 2003) and hybrid striped bass (Li, Lewis and Gatlin, 2004); and
- increased resistance to stress in salmonids (Burrells et al., 2001; Leonardi, Sandino and Klempau, 2003).
Acidifiers: Acidifiers are potential alternatives to antibiotics and include organic acids (formic, acetic, propionic, lactic and citric) and organic salts (calcium formate, sodium formate, potassium diformate, calcium propionate and calcium lactate). The modes of positive influence of acidifiers are:
- reducing pH of feeds, thus inhibiting growth of microbes, some of which are potentially pathogenic;
- reducing pH in the stomach and small intestines, thus improving pepsin activity particularly during periods where levels of free hydrochloric acid are reduced, e.g. during high feed intake in young animals or when animals are fed diets with high protein content; and
- supplying energy for metabolism, as organic acids contain substantial amounts of energy, e.g. propionic acid contains one to five times more energy than wheat (Diebold and Eidelsburger, 2006).
Acidifiers have been reported to positively improve performance in arctic charr (Ringø, 1991), rainbow trout (de Wet, 2005) and tilapia (Ramli, Heindl and Sunanto, 2005).
Enzymes: Feed enzyme supplements have been predominantly used in pig and poultry diets. Use of enzymes in aquaculture has been relatively low perhaps due to reliance on fishmeal as a major source of protein in aquafeeds. Fishmeal is highly digestible and, therefore, little could be gained by adding enzymes. However, the application of enzymes in aquafeed deserves adequate consideration with the increasing utilization of plant products as partial or complete replacement of fishmeal. Plant products usually contain large amounts of fibre and a number of anti-nutritional factors that limit their nutrient availability, and feed enzymes have often been used to increase nutrient availability by both releasing bound nutrients and breaking down compounds. Feed enzymes work best when they complement endogenous enzymes in breaking down compounds to a size, that can be easily utilized by the animal. In this regard, phytase has proven consistently to improve availability of P, while protease and carbohydrase enzymes have given variable responses. Gatlin et al. (2007) reviewed the following researchable issues and approaches to increased use of plant products in aquafeeds.
Enhancing utilization by genetic selection of fish: Of great interest is the determination of whether carnivorous fish that have a natural capacity to utilize protein as their main energy source can be genetically selected for effecting improved utilization of plant material.
Optimizing bioactive compounds: Several plant feedstuffs contain bioactive compounds that may have positive or negative effects on aquatic animals and, thus, investigations to adjust their concentrations accordingly in aquafeeds are needed.
Monitoring effects of plant feedstuffs on fish product quality and consumer health: Given the physiological, nutritional, environmental and compositional differences among farmed finfish, conclusions reached about the product quality of one species cannot be automatically applied to another species. Further research is needed to clarify the influence of dietary ingredients on the quality of each aquaculture species of interest.
Enhancing palatability of plant feedstuffs: Research regarding palatability of various feedstuffs may indicate why feed intake often is reduced when certain feedstuffs are included in fish diets, and may also suggest how processing methods are to be improved for optimizing palatability.
Further Reading
 |
- | You can view the full report by clicking here. |
