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The search for new dietary ingredients for total or partial replacement of fish oil has assumed a growing importance and is currently the subject of advanced scientific research. Plants are recognized to be a good source of both peptides and oils for fish aquafeeds. The present paper reviews the effects played on fish growth and physiology by some plant products, highlighting also their possible detrimental effects related to the presence of anti-nutritional factors. Although complete replacement of fish meal in feeds for cultured species is still difficult, the results obtained by the current studies performed on plant products are promising and encourage further research on this issue.
Keywords: Fish nutrition; Fish meal replacement; Plant proteins; Physiological effects
The aquaculture field has recently experienced a significant increase and in future the amount of global productions is expected to provide an important supply of seafood to satisfy the growing consumers demand [1,2]. To improve the sustainability and profitability of current productive practices, however, many issues require a substantial revision, especially those related to fish nutrition. Fish meal and fish oil are one of the most important ingredients for aquafeeds, however both are a limited resource; in fact, in parallel the impact on ocean fisheries of increased aquaculture production is likely to expand further, since farming carnivorous species requires large inputs of wild fish for feed . Consequently, a reduced use of fish meal and oil represents a challenge for fast growing and sustainable aquaculture production.
The decrease in the availability and the increase in the prices of fish meal and fish oil have stimulated the search for sustainable alternatives for aquaculture feeds. To this regard, plant protein sources are increasingly used to satisfy the growing demands of the aqua-feed industry, since they represent good candidates for partial or total replacement of fish meal in fish diets . The attention addressed to alternative protein sources for fish meal replacement is also shown by recent research funded by the European Community in this field (see for example the Project PEPPA-Perspectives of plant protein use in aquaculture: biological, environmental and socio-economic consequences, funded under the EC FP5-Life quality). Moreover, several studies have explored the possibility that plant proteins could be used for total or partial replacement of fish meal, with the aim not only of testing the nutritional value of fish products, but also of taking into consideration their eventual effects on fish health. Due to good performance and fish quality, the suitability of plant protein sources has been shown for many carnivorous fish . However, it is well recognized that high dietary level of plant proteins (>40% of total protein) for partial replacement of fish meal reduces feed efficiency and growth performances [5,6]. Total replacement of fish meal is feasible when amino acid-supplemented diets are used [7-9].
Among plant protein sources, the most frequently used are legumes such as soybean, pea and lupin [7-11]; corn gluten meal  as well as cereal concentrates, including maize and wheat [13,14] that have already been tested for European sea bass, turbot, Atlantic salmon, and carp nutrition.
When plant proteins are used as alternative to replace fish meal, the presence of anti-nutritional factors (ANFs) represents one of the most important features that need to be addressed . ANFs are mainly alkaloids that affect palatability and can be washed out using water or through feed extrusion. Other anti-nutritional factors are oligosaccharides, phytate, saponin and protease inhibitors . ANFs play a limiting effect on fish growth; moreover they may cause pathomorphological changes in the intestinal epithelium of fish . Intestinal inflammatory processes induced by ANFs have been documented for soybean meal-fed rainbow trout , Atlantic salmon , whereas no significant histological alterations have been reported for lupin kernel meal-fed rainbow trout .
Lupin (Lupinus albus) is a non-starch leguminous; its seeds have a good potential for aquaculture diets due to their its nutritional value (high protein content: 30-40 g/100 g), availability and low price. Lupin seeds have a crude protein (CP) content between 31 and 42%, which is higher than the content of most other grain legumes . Lupin seed meal may be a good alternative vegetable protein of high nutritive quality when used at levels up to 30% or 40% in rainbow trout diets.
Another widely used plant protein source for aquafeeds is soybean . Soybean meal (SBM) has been one of the most studied alternatives to fish meal, but it has several limitations, including the presence of anti-nutritional factors, low level of methionine and adverse effects on the intestinal integrity of some carnivorous species . Another source of soybean is soy protein concentrate (SPC), produced using SBM fractionation, which is a highly refined ingredient, since most of the anti-nutritional factors present in SBM have been removed during processing . SPC has been tested in several juvenile fish species, such as rainbow trout, Oncorhynchus mykiss , turbot, Scophthalmus maximus , Japanese flounder, Paralichthys olivaceus  or kingfish, Seriola lalandi , showing that adequate inclusion levels of SPC in diet are different depending on the studied species.
González-Rodríguez et al.  have studied the effects of total and partial replacement of fish meal with soy protein concentrate (SPC) in juvenile tench. Fish fed from 0% to 45% replacement diets have shown significantly lower feed conversion ratios and higher protein productive values than those fed diets with higher replacement levels. At higher replacement levels (from 55% to 100%), fish have shown significantly lower growth.
Rice protein concentrate (RPC) is a good raw material for fish nutrition due to its high protein (75% crude protein) and lipid content (11% ether extract). It is known to provide a source of easily available amino acids and nutrients, so that its inclusion as a dietary ingredient for fish has attracted attention [25,26]. In rainbow trout no adverse effects on growth performance traits (up to 20% inclusion) were observed .
In blackspot seabream (Pagellus bogaraveo) juveniles, experimental trials were carried out at the CNR-IAMC in order to investigate the effects induced on the digestive enzymes by the administration of two diets containing different levels (20% and 35%) of RPC as a protein source, in partial replacement of fish meal [27,28]. compared to a control group fed a diet without RPC.
No significant differences in nutrient apparent digestibility values of crude protein, dry matter and gross energy were observed among the fish groups . The histological analysis confirmed that RPC did not induce intestinal mucosa alterations in this fish species or changes in the histochemical enzyme activities of the different intestinal regions, except for the acid phosphatase in the distal portion. The diet containing the highest level of RPC (35%) caused a significant increase of pepsin in the stomach and of trypsin in the intestine. The same stimulating effect was also observed with the diet containing 20% of RPC. The contents of chymotrypsin, carboxypeptidase A and B values were significantly enhanced in the intestine of fish fed the diet containing 35% of RPC. Therefore, data obtained in blackspot seabream suggested the inclusion of this compound as a dietary ingredient for this species due to its positive effects on protein digestibility.
In nutritional research, another interesting issue concerns the replacement of fish oil with vegetable oil, regarded as important dietary ingredient. Concerning the use of plants or grain seeds as a source of polyunsaturated oils, some trials were carried out with blackspot seabream to test the use of linum and echium oils in replacement of fish oil. A first experiment was performed in blackspot juveniles, in order to investigate the effect on the digestive enzymes of blackspot seabream of linum and echium seed oils, included in the dietary composition, as a source of polyunsaturated fatty acids . Three different groups were fed with different experimental diets, indicated as “fish”, “linum” and “echium”. Fish” diet consisted of a standard diet containing fish oil (fish flour 55%, corn flour 20%, oil 7.5%, crude starch 6.5% extruded soybean flour 5.5%, crude fibre 5.5%, mineral salts 3%); “linum” and “echium” diets had been obtained by replacing the fish oil contained in the “fish” diet with linum and echium oils. A control group was fed a commercial dry diet for carnivorous fish. Diets were administered as 1.2% of the total body biomass calculated weekly. Fish fed a diet supplemented with fish oil showed a significant reduction of pepsin content in the stomach, compared with fish fed echium oil. This depressive effect could induce alterations in the preliminary, gastric, breakdown of dietary components, resulting in the elongation of digestive times. In the same group (fish fed with fish oil), trypsin content in the intestine decreased, although not at a significant level, compared with the control group. Diet containing linum oil caused a decrease in chymotrypsin values in the pyloric caeca and in the intestine; in this organ, an increase in lipase content occurred, while the carboxypeptidase B levels were significantly reduced. Compared with the control, both linum and fish oils increased significantly amylase levels, particularly in the stomach at starvation. This result suggested that the vegetable nature of the linum oil was effective in stimulating the digestive capabilities of the carbohydrate component.
In conclusion, nutritional issues must be considered to achieve aquaculture sustainability. In this context, the search for new dietary ingredients for total or partial replacement of fish oil assumes a growing importance and is currently the subject of advanced scientific research [30-32]. At present, fish meal remains the primary protein source in aquafeeds for marine species at the fry or fingerling stages. Complete replacement of fish meal in feeds for marine species is still difficult and will require further research efforts to be obtained. Nevertheless, the results obtained by the current studies performed on plant products are promising and encourage further experimentation to meet seafood production and environmental sustainability.