The current state of aquafeeds: The use of Fish Meal and Fish Oil, vegetable and plant food replacements, and nutritional supplements in aquaculture feeds
Robert W Silver
University College London / University of Miami, FL
This paper seeks to define the global trends in aquaculture feeds – aquafeeds – now a major research investment due to the state of the world’s fisheries. With the production of the world fisheries in question, 2009/2010 marked a monumental shift when aquaculture overtook fisheries as the leading source of fish. Just like terrestrial farms, aquaculture centers are attempting to produce the most volume of fish while imputing the least amount of energy (Fish Conversion Rate –FCR and Fish In/Fish Out -FIFO). The crux of this debate has centered on fishmeal and fish oil use in industrial aquafeeds. This paper will overview some of the paradigms surrounding fish meal, fish oil and acceptable alternatives.
The importance of aquafeeds in aquaculture is infinite. As with all animals, the inputs are directly related to growth and reproduction. Aquaculture is similar to terrestrial farming in the sense that it is dependent on the inputs. With aquaculture, the inputs can be controlled to the minutest detail. This allows researches to find the optimal balance for protein, carbohydrates, and fats as well as individual amino acid, lipid, or mineral requirements for all species. Most of the modern research in the area of aquafeeds is centered on finding the perfect economic and biological balance for the feeds. In terms of cost, feeds are also the most important aspect of aquaculture since feed purchases is generally the largest operating cost in all aquaculture (FAO 2006)
In this paper, the first section will intend to relay the economic importance of implementing the most economically and biologically viable aquafeed. This section will focus on the growing world demand for fish and the dwindling natural resources. From there, the paper will cover much of the current research in aquafeeds. The most prominent advancements in aquafeeds recently have been the insertion of vegetable and plant products into aquafeeds, thus limiting the strain on the oceans resources for food. Also prominent in the research is the inclusion of various supplements. These studies generally focus on the use of dietary additions such as individual amino acids in vegetable feeds. From supplements, the paper will discuss the importance of fat and oil supplementation outside of straight up substitution of fish oil with plant oils. Continuing through the research, variables such as feeding by age and size, generally in relation to waste is discussed. Finally, the modern topic of gene expression in fish based on their diet and its affects on the overall health and quality of the fish in junction with various fish feeds will be examined. The paper will conclude with general conclusions from the plethora of studies and research in the field over the past few years.
Economic importance of replacing fish meal and fish oil
As stated in the FAO (2006) report, feed represents the largest operating cost in fishery management. For this reason, aquaculture centers around the world desire to reduce cost on feed while keeping fish yields as high as possible in order to make the most profit. Given the current state of the global fisheries (Worm et al 2009; FAO 2010), reducing expenditures on feed is going to be easiest if fishmeal and fish oil is cut out of the aquafeed regime. Overall, the FAO and various scientists have commented on the decrease in fish stocks. The following graph (figure 1) by the FAO (2010) displays the trends in fish stocks. The conclusion from the graph is that fishing cannot compensate for the growth of the population. Humans have answered this dilemma with aquaculture. However, the issue of what to feed the fish, the high-economic yield marine carnivorous fish especially, is the new issue.
figure 1: Absolute and per caput food fish production from marine capture for the world and world excluding China (1950-1999) (From FAO Fisheries Department, 2010)
In order to continue aquaculture advancement while dismissing reliance on global fisheries, feed inputs have evolved to include industrial compounded aquafeeds, farm-made aquafeeds, or natural food organisms. The FAO (2008) reported that if the aquaculture sector is to continue its 8.5% growth per year, then feed would have to grow as well. The major question is from where will the feed come from (Naylor et al 1998; FAO 2006; Tacon et al 2006).
Over the past few years, the prediction for the global fishmeal and fish oil industry is that consumption will decrease. Since 2000, the prices for global fishmeal and fish oil has increased from US$694 per tonne in July 2005 to almost US$1700 per tonne in March 2008 (Tacon and Metian 2008). In response to these trends, Tacon and Metian (2008) compiled six reasons for the decrease in fish meal and fish oil consumption:
1) The decrease of the worlds fish stock
2) Increasing market price of small pelagic fish due to increasing fishing costs
3) Increases in the cost of transportation and natural resources
4) Decreases in the size of fish meal stocks
5) The increase in fish meal and fish oil prices will require the feed manufacturers to evolve and remain profitable
6) The sustainability issue (Naylor et al 1998)
As indicated by these reasons, the consumption of fishmeal is predicted to decline from 4300 thousand tonnes in 2005 to 2385 thousand tonnes in 2020. This global decrease will not be exhibited by fishmeal only: fish oil consumption by marine organisms is expected to decreases 15.5% over the same time period (Tacon and Metian 2008). In relation to the entire aquaculture sector, fishmeal and fish oil consuming fish make up roughly 33% of the total sector (Welch et al 2010). In their report, Tacon and Metian (2008) composed a table estimating the fishmeal and fish oil consumption for various species throughout the world. Figure 2 below is a cut out of the table highlighting shrimp aquaculture and the future of fishmeal and fish oil consumption.
figure 2: Estimated global use and demand of fish meal and fish oil in shrimp aquaculture from 1995 to 2020 (Tacon and Metian 2008)
These results back up previous studies done by the International Fishmeal and Fishoil Organization in 2006. IFFO studied the Fish Conversion Rate (FCR) and Fish In/Fish Out ratio (FIFO). The top consumers of fish meal and fish oil are shrimp (above) and marine fin fish. Trends like the ones exhibited above of decreased consumption have been found in Chilean salmon, where fish meal have fallen from 45% to 28% in over two years (Anon 2006).
As a result of the reason mentioned above, aquaculture centers are now forced to seek feeding alternatives. If it is more economically viable to feed salmon a 50% maize based diet, then that is what most aquaculture systems will do since it all comes down to making a profit. The use of terrestrial food stuffs in aquatic organisms is now the most economically viable option (Welch et al 2010)
The use of vegetables and plant matter as replacements for fish meal and fish oil in aquaculture feeds
Due to much research on various species (see Welch et al 2010 for details) there has been a significant reduction in fishmeal and fish oil inclusion rates in favor of agricultural feed staples such as grain and soybeans. Various studies researched the most economically viable ratio of fishmeal and agricultural diet such as Espe et al (2007) and Kousoulaki et al (2009). Currently, it is known that fish can be produced on agricultural feeds primarily and even solitarily, however, major questions still revolve on quality of the product and adverse environmental affects (Welch et al 2010).
This section is going to look at the magnitude of research on the replacement of fishmeal and fish oil with plant meal and vegetable oils. The first part of this section will look at the overall effects of substituting or removing fishmeal and fish oil from aquafeeds and replacing it with agricultural feeds. Various case studies such as the Chinese sucker (Yuan et al 2010), salmon (Torstensen et al 2010), rainbow trout (Yamamoto et al 2010) and others will be closer examined. From there, this section will go on to discuss some of the other implications of changing diet as well as talk about another solution of fish meal and fish oil that is similar to agricultural products but is not – krill meal.
Finding the balance between plant and vegetable feed and growth is the key to create the most profit. As long as plant and vegetable feed is cheaper than its aquatic counterpart, it will be more beneficial to feed the farmed fish as much plant matter as possible. However, it is not as simple as replacing all of a fish’s diet with maize meal. Yamamoto et al (2010) found that feeding rainbow trout (Oncorhychus mykiss) with fishmeal-free diets that were instead constructed out of non-fat soybean meal led to various health fallacies in the fish. In their study, the most common diseases and physiological abnormalities were reduction of biliary bile acid content, disintegration of microvilli, presence of large vacuoles in the mucosal folds of distal intestine, and atrophy of hepatacytes. The most prominent result and probably the most telling of feeding farmed fish a 100% soybean diet was the retardation of growth.
In the Chinese sucker (Myxocyprinus asiaticus), Yuan et al (2010) replaced 30% of the typical aquafeed diet (fishmeal and worms) with various diet components. The table below (figure 3) details the ADC (apparent digestibility content) for white fishmeal, brown fishmeal, fermented soybean meal, extruded soybean meal, soybean meal, cottonseed meal, and rapeseed meal.
figure 3: ADCs (%) for dry matter, crude protein, crude lipid, energy and phosphorus of the reference diet and the test ingredients for Chinese sucker (Yuan et al 2010)
The conclusion from this data is that the fish meals were best digested by the suckers but the fermented and extruded soybean meals were not too far behind (74.5% ADC vs 69.4% ADC). Finding the best economical balance between the two diets is what researches have to do in order to make the most profit from aquaculture.
Two more studies that compare various ratios of fishmeal and plant meal in aquafeeds are on the Pacific white shrimp (Litopenaeus vannamei) and another on rainbow trout (Panserat 2008; Gonzales-Felix et al 2010). Penserat (2008) studied the affect of varying the diet in rainbow trout by its effects on gene expression. Gene expression is a very important area of research in regards to this field of study and will get a second look in the penultimate section. In the study, rainbow trout were fed various ratios of vegetable and plant based diet from birth until culling one year later. The results displayed that the higher level of vegetable and plant meal used in the feeding the trout, the higher differences in gene expression present. Most of the variations in gene expression were centered on the genes for fatty acid metabolism. This decrease in expression is an exhibition once again of the necessity for a ratio that is most economically viable, yet still produce healthy fish is necessary for aquaculture advancement.
In Pacific white shrimp aquaculture, the more general assay of FCR was determined to have no changes in response to variations in diet (Gonzales-Felix et al 2010). This displays that while exchanging a fish meal diet for a plant meal diet might yield a fish of the same size, the internal health could pose an issue as displayed by Panserat (2008) above. The shrimp study also defends this claim since the fatty acid composition of the shrimp’s tail was analysed it was shown that no matter what type of oil was used (even if it was high EPA/DHA Linolic acid), it was not able to be converted in the tail from PUFA to HUFA. Despite the fact that linolic acid is high in n-3 fatty acids, it does not provide the same benefits of fish oil inside the shrimp (Gonzales-Felix et al 2010).
Results similar to the one above are not universal. Philips et al (2010) found that feeding sea urchins manufactured feeds produced higher quality gonads. This is relevant because it displays how some organisms require different solutions. However, this study did not test the health and quality of the gonads, just the index. In total, all of these studies prove that understanding which species require and respond to which feeds is a key element in growth, production, efficiency, as well as other topics like limiting waste (Allen et al 2000; Zhou et al 2004). Some researchers feel that the key is understanding the digestibility of certain meals (Yuan et al 2010) while others focus solely on FCR (Gonzales-Felix et al 2010). Studies like Gomes-Requeni et al (2004) focused on amino acid utilization while Vilhelmsson et al (2004) determined feed effectiveness off of metabolism efficacy. As more information is discovered, the implementation of the ideal ratios of feed will quickly follow.
For example, Torstensen et al (2010) found that the ideal replacement of fishmeal with plant meal in Atlantic salmon would be 80% plant protein, of which 35% was vegetable oil. This result yielded the greatest growth of salmon without many negative health effects. However, the study also showed decreased consumption of the vegetable feed as well as reduced digestibility. This is important in terms of environmental sustainability because decreased consumption might lead to more wasted meal at first, and decreased digestibility will lead to increased waste throughout the fish’s life. Similar conclusions can be made from the study on trout and fishmeal free diets. Yamamoto et al (2010) found lower digestibility of the soy based carbohydrates and lipids yet where able to improve them with fermentation. It was also found that supplementing the trout’s diet with bile salts such as bovine gall powder and cholytaurine would counter the negative side effects of the soy based diet. Supplements like these will be covered in a following section.
One variation of the fishmeal fish oil replacement diet is the prospect of only replacing the fish oil with some form of vegetable oil in the diet. This has been studied extensively and will be the major topic of focus in the next section. Another variation in the diet has been the adaptation of krill meal. Just like plant meal, krill meal is cheaper than fishmeal and now is easier to purchase due to adjustments in European Union laws. Hansen et al (2010) found that as long as the krill were de-shelled prior to salmonoid consumption, growth and health factors remain the same. However, the fishmeal did outperform the de-shelled krill in nutrient absorption and cholesterol levels. Krill meal poses another option for research to follow; yet many obstacles remain before krill meal can be implemented as plant meal has been so far.
Replacement of fish oil only: cause and effects
Instead of replacing both the fishmeal and fish oil, various studies have been conducted on replacing only the fish oil with various forms of plant and vegetable oil (Richard et al 2006; Gonzales-Felix et al 2010; Ng et al 2010; Senadheera et al 2010; Jordal et al 2010). This section will look at these various case studies and will formulate a conclusion based on the results in attempt to summarise the current state of research.
In a study on rainbow trout, the gene expression of various lipogenic enzymes and LDL cholesterol receptor genes were investigated based on variations in lipid source. Richard et al (2006) used a control diet of 100% fish oil and a 100% vegetable oil blend of rapeseed, palm oil and linseed. In this case, the replacement of fish oil with the vegetable blend did not affect growth rate or muscle lipid content. These results were also found in studies on Atlantic salmon (Rosenlund et al 2001), book char (Guillou et al 1995), gilthead sea bream, (Izquierdo et al 2003) and in other studies with rainbow trout (Caballero et al 2002). Overall, the use of vegetable instead of fish oil in this collection of studies was founded to be non-effective at changing growth and survival rate. However, Richard et al (2006) did find decreases in cholesterol in the vegetable oil fed fish. This negative side effect of vegetable oil supplementation is a minor problem, but as stated above, finding the correct ratio of oils as well as meals should cause the most economically available solution to become available.
Ng et al’s (2010) study also was conducted on rainbow trout. The replacement of fish oil with palm fatty acid distillate (PFAD) in this case led to increases in fatty acid digestibility. Once again, this indicates how a ratio of various oils will provide the greatest growth and health for the fish. As mentioned above, the Pacific white shrimp were able to increase growth at a similar rate on a diet based on plant linolic acid. The only side effect is a decrease in PUFA conversion.
A very similar study to the one on Pacific white shrimp was done on Murray cod in respect to growth performance and fatty acid profile (Senadheera et al 2010). The results of this study proved two things: 1) that alpha linolenic acid is a better oil substitute than linoleic acid and 2) that “topping off” a fish with fish oil over the latter part of its lifetime will yield better results, but is still dependent on the ALA/LA ration given to the fish over its lifetime. Conclusion number two is more important in terms of finding the most economically viable solution to the fish oil problem. These results indicate that as long as a fish is fed a vegetable oil based diet that is remotely along similar fatty acid content ratios to a fish oil based diet (specifically n-3 and n-6 ratios), then substituting fish oil back into the diet for a finishing strategy would yield an even better omega fatty acid profile. This addresses one of the key critiques of fish oil substitution as well as with bovines and maize. In bovines, a diet based on maize in the US or beet silage in the UK can be topped off with grass feeding to achieve more “organic” markings with out much composition changes of the animal fat’s omega balance. In fish, this problem is not as large because the fatty acid turnover in the muscles is faster. Therefore, a diet based on vegetable ALA/LA for most of its life but topped off with fish oil will most likely be the most economically viable (Senadheera et al 2010).
Once again, studying the affects of diet on gene expression was used as an indicator of diet preference. Jordal et al (2005) compared a 100% fish oil diet to a 75% rapeseed oil diet in Atlantic salmon. Lipid metabolism genes were those studied again with the result beings a large amount of variation in expression, but very little changes in performance or growth. These minor changes exhibited in these studies can be compounded or added to other dietary changes. One of the easiest forms of change is to add simple supplements. The next session covers the addition of supplements into aquafeeds.
Supplements in aquafeeds: What and when
In many of the case studies used above, there were some minor side effects in the full or partial implementation of fishmeal and fish oil with plant and vegetable based meal and oils. One way to improve upon the finished product and thus market value and profit would be to introduce minor supplementation into the diet during the organism’s life cycle. Minor dietary supplements such as amino acids like glutamate or lipids like cholesterol can be auxiliary to a diet based on vegetable meal and yield better results. This section will cover a few of the supplements that can be used in aquafeeds as well as variations in adding the supplements, similar to the example of topping off above.
In aquatic organisms, especially crustaceans, cholesterol plays an important role in endocrine function and lipid synthesis. Tan et al (2010) and Irvin et al (2010) studied the affects of cholesterol on catfish and spiny lobster respectively. The conclusion from these studies verified previous studies that cholesterol supplementation is necessary in catfish and crustaceans when they are fed soy (plant) based diets. However, Irvin et al did not find a correlation between increased cholesterol consumption and survival rate but still recommends 4.0 g/kg DM cholesterol.
Amino acid supplementation, the other area of supplementation studied extensively in the literature, produces results similar to those above. Various amino acids were studied such as arganine and glutamate (Oehme et al 2010) lysine (Deng et al 2010). Lysine supplementation in Pacific threadfin (Polydactylus sexfilis) at levels between 2.23% and 2.43% had higher specific growth rate, feed efficiency, protein deficiency ratio and protein retention than those fed diets containing less lysine. Arganine and glutamate supplementation also improved growth in Atlantic salmon by increasing the feeding rate and insulin like growth factors (IGF). However, the study does bring up a good point in the importance of seasons.
Oehme et al’s (2010) study on glutamate and arganine found that the salmon only experienced the benefits during the long winter months of Scandinavia. The increased rate of feeding and IGF in the winter month’s displays how cycling supplements based on season could provide the most economically efficient way of growing fish.
Variables outside of the feed: Age and size of the fish
Feeding levels and ratios change throughout a species lifetime. Several examples can be found in the literature of differences from birth to death throughout all of nature. Humans are a prime example of this from growing on a milk-based diet and transferring into a predominantly animal fat and protein diet as we aged. In aquaculture, similar trends (though different foods obviously) are present. Diatoms and polysaccharide requirements in abalone (Haliotis diversicolor supertexta) vary over age, Senegalese sole (Solea senegalensis) were found to have various phenlalanine and tyrosine requirements in the days following hatching, and sea horses (Hippocampus guttalatus) were discovered to lose various fatty acids at different rates during ontogeny in comparison to the rest of the life cycle (Chao et al 2010; Pinto et al 2010; Faleiro and Narciso 2010). The most important concept from these studies is that diet needs change over time. Understanding the differences throughout an organism’s life cycle is necessary in order to optimize growth from food.
Another external factor that is influenced by the size and age of the fish is the preferred size particles of the feed. The feed particle size the optimally promotes digestions and produces growth will most likely also be the feed size that limits waste (Azaza et al 2010). Limiting waste is one of the reoccurring themes of this essay that will need to be addressed due to the worlds current trends of fighting for zero imprint aquaculture.
Continuing research in the area of fish growth based on age is also being applied to the implementation if intermittent fasting. Intermittent fasting – extended amounts of time without food – is commonly used to promote growth and health in terrestrial animals and has been discussed in fish (Kheder et al 2010). This area of research can be used to optimize food uptake by promoting digestion with the fast and thus limiting waste. Intermittent fasting has also been shown in humans to promote quality gene expression. There have not yet been tests on fasting and gene expression in fish, yet as displayed above, gene expression is a very important topic that is discussed in the next section.
Gene Expression: Understanding the true consequences of replacing a natural diet with unnatural vegetable and plant meal
Throughout the previous text, the concept of gene expression has appeared multiple times. Gene expression is the biological process of different genetic traits being expressed (genotype to phenotype) depending on external factors instead of internal factors (nature and nurture). So far, studies generally use gene expression as a means of testing the side effects of changing feed (Jordal et al 2005; Richard et al 2006; Panserat 2008; Senadheera et al 2010; Panserat 2009;). This section will recap some of the methods used in the above studies as well as the implications. The following portion will discuss the importance of gene expression over typical growth tests for positive feed replacements.
The various methods used to understand individual species gene expression vary from identifying mitochondrial proteins and transcription factors to general gene expression indicators (for a species with a completed genome, this is easy). The latter of the two methods was used by a majority of the studies. Panserat et al (2008; 2009) analysed the expression of 9000 genes in determining the effects of vegetable based diets on rainbow trout. The latter study (2009) was the first to look into hepatic transcriptome after a total replacement of fishmeal with vegetable meal. Of the 9000 genes analyzed, 176 were over or under expressed. Most of these genes involved the trout’s metabolism control system, yet few negative side effects were found. The authors believe that the relative consistency in gene expression despite the radical change in diet is associated with the macronutrient breakdown. The rainbow trout were still receiving similar levels of protein, carbohydrates and fat. In the prior study however, the results were not as clearly in favor of the shift in diet.
The other study conducted by Panseret et al (2008) was also on gene expression of rainbow trout based off of a non-fish oil diet. In this study, a lifetime of vegetable oil feed was the reason for around 100 genes (of the 9000) to be expressed differently. However, among these genes was an under-expression of the FAS enzyme gene used in fatty acid biosynthesis. The authors believed that the high concentration of linoleic and linolenic fatty acid was the cause of the retarded growth of the trout. Similar results were found in a study that was conducted on Murray cod (Senadheera et al 2010). The authors also focused on alpha linolenic acid and linoleic acid in the aquafeeds. As noted above, higher concentrations of ALA were required to achieve the fatty acid profile desired in fish (EPA/DHA).
Topping off with fish oils can reverse a lifetime of vegetable oil side effects (Penseret et al 2008; Senadheera et al 2010). However, this topping off may yield the desired EPA/DHA ratio in fish lipids, it will not be able to reverse malignant gene expression. This represents a field that needs to be researched further in the field. Another area of research that needs to be considered is that of gene expression over generations. One generation of vegetable feed might only cause minor variances in gene expression as displayed by the studies above, yet it has the potential to increase over generations. If various genes are exhibited during the early stages of development, issues can arise in these most important stages of development. The juvenile period is especially important since the production of high-quality juveniles is still one of the fallacies of marine aquaculture. This is generally due to a lack of knowledge in the field of larval feeds. Understanding gene expression at various stages of development would give aquaculturists a leg up in developing a closed system of reproduction (Pinto et al 2010).
Aquaculture is the future. It is the blue revolution and one of, if not the most prominent investment opportunity of the modern generation. Due to the symbolic nature of aquaculture as the future of food and fish, much research has gone into the perfection of the technique. This essay focused on the recent advances in diet research in aquafeeds. The current focus on aquaculture feeds is a result in the current drive of sustainability. As a result, a larger focus is being put on the environmental impacts of food and measures such as the Life Cycle Analysis, Ecological Footprinting, Energy Flow Analysis, Virtual Water Flow Anlaysis and Primary Productivity Required have been developed and are currently being applied to aquaculture feeding (Welch et al 2010). As discussed in the section on the economics of fishmeal and fish oil, it is not expected that fish based products will continue to be produced at the current rate. So far, the major replacement option as been vegetable and plant based meals. Hopefully the information presented in this paper will provide insight into many of the questions surrounding the implementation of aquafeed replacement in modern aquaculture.
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