Estimation of Phosphorus and Nitrogen Waste in Rainbow Trout (Oncorhynchus mykiss, Walbaum, 1792) Diets Including Different Inorganic Phosphorus Sources

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By: María Consolación Milián-Sorribes, Ana Tomás-Vidal, David S. Peñaranda, Laura Carpintero, Juan S. Mesa, Javier Dupuy, Andrés Donadeu, Judit Macías-Vidal and Silvia Martínez-Llorens. Environmental management of intensive aquaculture is essential for the achievement of sustainable aquaculture in the coming years. With this aim, a decrease in phosphorus (P) and nitrogenous (N) discharge from aquaculture effluents would reduce water eutrophication, and therefore, its environmental impact. The main strategy to achieve this reduction is through the optimization of diet formulation.

Environmental management of intensive aquaculture is essential for the achievement of sustainable aquaculture in the coming years. With this aim, a decrease in phosphorus (P) and nitrogenous (N) discharge from aquaculture effluents would reduce water eutrophication, and therefore, its environmental impact. The main strategy to achieve this reduction is through the optimization of diet formulation.

Despite the fact that fish can absorb minerals from water through their gills, fish require an additional source of P in their diet, since P is usually a limiting mineral in most natural waters, and its absorption rate from water is low.

P in fishmeal, mainly in the form of hydroxyapatite or bone phosphate as well as inorganic supplements, is relatively available to rainbow trout (Oncorhynchus mykiss). In contrast, approximately two-thirds of the P in plant sources is bound to phytate, being only partially available to fish, presumably due to low levels of intestinal phytase, the selection of the inorganic P source is an important issue, and it will be based on its solubility and digestibility, which may be affected by the calcium level in the diet, as well as changes in pH under gastrointestinal conditions.

In general, monobasic phosphates from monovalent cations are more digestible and soluble, followed closely by monocalcium phosphate (MCP) and, beyond this, by tricalcium phosphates or bone apatite. This is generally applicable to marine and freshwater fish with a stomach, such as rainbow trout.

A low absorption and retention of nutrients will mean a higher discharge into water, affecting the sustainability of the production. Aquaculture effluents with high levels of P and N contribute to the pollution of the aquatic ecosystem through the eutrophication of natural fresh water. Consequently, aquaculture faces a dilemma: feed must meet P levels, but at the same time, feeding practices must comply with environmental guidelines to minimize the P load in the aquatic environment.

The mixture of chemical products ends up directly influencing the degree of bioavailability of the phosphate species in each species, which is defined as the degree to which a nutrient ingested from a particular source is absorbed and remains available for the animal’s metabolism.

The aim of the present study was to evaluate in vivo the P availability and excretion level of diets including four different inorganic P sources in rainbow trout and excretion level of diets including four different inorganic P sources in rainbow trout. As a novelty, new formulations of inorganic phosphates were assessed from nutrition and environmental point of view for its inclusion in fish diets.

Experimental Procedure

Around 200 rainbow trout with an average weight of 130 g were moved by an authorized service from the fish farm “El Zarzalejo” located in Zamora (Spain) to the Fish Nutrition Laboratory belonging to ICTA-UPV (Valencia, Spain).

Four supplemental inorganic P sources were chosen for determining the apparent P availability and nonfecal P excretion. Before formulating the diets, each ingredient was individually weighed and analyzed in triplicate, then mixed.

To obtain the feces, a Latin square experimental design of 4 treatments × 4 tanks × 4 trials was followed (consisting of four trials or periods, and in each of them, four tanks were fed with one of the feeds, so that the fish in all the tanks were fed with the two experimental feeds).

The feeding rate was once per day at 10.00 a.m. from Monday to Saturday, with a starvation day on Sunday. Feeding was carried out at a restricted rate, according to the average weight of the fish and at water temperature, and the ration per tank was calculated according to the biomass of each fish.

The same experimental design was followed as that described in the digestibility test. Fish diets, feed ingredients and feces were analyzed based on the Association of Official Analytical Chemists (AOAC) procedures.

Due to the critical importance of being able to distinguish between the different phosphates with higher bioavailability in each species, a predictive program has been developed considering the different chemical species that compound each phosphate, by means of different chemical parameters. Each of these chemical species has been assigned a digestibility weighting value based on previous in vivo studies.

The combination of the chemical balance with the weightings leads to Predictive Equations for Digestibility Comparison (EPCD) of commercial phosphates. In vivo digestibility estimation requires specialized settings, expensive operating costs and a high number of fish, and it is difficult to perform and achieve the desired experimental working conditions.

In this regard, EPCD can help to successfully evaluate the effect of diets on nutrient digestibility providing very reliable information. Prior to analysis, the normal distribution was checked through a Kolmogorov–Smirnov test, while the homogeneity of variances using a Levene test.

Results and Discussions

Both treatments, MAP and SCP-2%, showed higher values than SCP-5% and MCP in terms of P digestibility (92 and 90%, respectively). The MSP and DCP values were in concordance with those obtained for shrimp but considerably lower than the values of 90–98% and 46–71%, respectively, reported for fish. The 73% value for the MCP is higher than the 49% reported for shrimp. These differences in digestibility are mainly due to the different degrees of solubility between phosphates

This difference in solubility is very relevant, since it reflects the ability of P to solubilize at neutral pH, simulating the intestine (absorption of P). This fact is even more important in fish, since they lack a true stomach in which a first stage of solubilization begins prior to digestion.

The needs of P in the diet depend on several factors, including the bioavailability of the element, food intake, the requirement for new tissue synthesis and the amount of endogenous loss, among others. These factors being dependent on the life cycle and size, and even environmental factors such as salinity and temperature.

It is difficult to explain the differences observed in the present experiment regarding N digestibility, because the protein source was the same in all treatments and only the inorganic phosphate source differed between diets. One possible explanation could be the differential buffering capacity (BC) of inorganic phosphates, which could affect the pH, and consequently, modulate digestive enzyme activities, and therefore, raising the protein and mineral content.

A high correlation (>0.9) was obtained between the results of the in vivo P digestibility assay and the estimated results using the EPCD index. This index could become a useful tool to formulate practical diets. Nevertheless, it is noteworthy that the only factor not taken into account in the EPCD index is the variability stemming from the different particle sizes of phosphate; therefore, an inclusion of this factor might be needed for products with a different granulometry or the comparison only of products with similar textural properties.

Due to the critical importance of being able to distinguish between the different phosphates with higher bioavailability in each species, a predictive program has been developed considering the different chemical species that compound each phosphate.

The P excretion into the water system provided similar values independent of the diet, with a maximum at 6 h after feeding, similar to previous studies. he authors indicate that when fish consume excess available P, the excess P is mainly eliminated through the gills and kidneys as non-fecal soluble P. Excretion of soluble non-fecal P also depends primarily on P source. Therefore, differences in P excretion observed in the present study might be due to the available P contained in the diets.

Conclusions

Considering N and P as the most relevant nutrients for inducing water eutrophication, it would be relevant to minimize these components into the column water. Currently, there are a wide range of commercial inorganic phosphate available; however, the most used in aquafeed are the monoammonium, monocalcium and monosodium phosphates due to their good availability in fish, although the new formulation (SCP-2%) can improve the wastes generated in the aquaculture production, which is crucial to improve the environment.

The SCP-2% source (AQphos+) presents a phosphorus digestibility comparable to MAP (without significant difference at a statistical level), but with lower P and N excretion, and thus, it is more environmentally friendly. Therefore, SCP-2% as a phosphorus source is more advantageous from a nutritional, environmental, and industrial point of view (biofilters and recirculation systems in fish farms).

“In the rainbow trout intestine, there is a sodium-dependent inorganic P carrier that is closely regulated by dietary P. Intestinal absorption rates of inorganic P decrease as dietary P levels increase, suggesting that the effectiveness of phosphate transport systems in retaining dietary P may decrease when dietary P levels are very high.”

Soluble P is the most flexible component of effluent P. There is no excretion of soluble P at low levels of dietary P, but soluble P becomes the main route of excretion as the available P concentration increases above hypothetical levels of P requirements for the species.

Analyzing the overall data on daily excretion rate, the results corroborate that fish fed MAP feed had significantly higher N excretion, probably due to the ammonium content of MAP, as has been reported in previous studies.

This excess excretion must be considered when sizing biofilters in fish recirculation systems, or in the case of open systems, since it will mean a greater release of ammoniacal N into the natural environment, which will lead to greater eutrophication of the water. in addition to optimizing dietary P, the use of diets with an optimal level of digestible N will become another key to sustainable aquaculture.

Considering fecal and soluble P losses, even though data on P retention are not available, it was possible to carry out an approximation of P and N losses in each of the treatments.

Global Feed have sponsored this article

This is a summarized version developed by the editorial team of Aquaculture Magazine based on the review article titled “Estimation of phosphorus and nitrogen wastes in rainbow trout (Oncorhynchus mykiss, Walbaum, 1792) diets that include different sources of inorganic phosphorus” developed by: María Consolación Milián-Sorribes, AnaTomás-Vidal, David S. Peñaranda, Laura Carpintero, Juan S. Mesa, Javier Dupuy, Andrés Donadeu, Judit Macías-Vidal y Silvia Martínez-Llorens. The original version was published in June 2021 through Animals, under the use of a creative commons open access license.

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