Fishmeal digestibility may not be as good as previously thought

The digestibility of an ingredient is part of an important set of criteria to determine the quality and value of an ingredient for a target animal. For prawns, fishmeal is often seen as the gold standard in ingredient quality due to its desirable characteristics including high protein content, ideal amino acid profile, chemo-attractiveness and high protein digestibility. The digestibility of an ingredient refers to the proportion of a nutrient (e.g. protein) in the diet relative to the nutrients which remain in the faeces.

The nutrients that are absorbed from the diet as it passes through the digestive system are seen as being digested while the nutrients excreted in the faeces are seen as being undigested. An indigestible marker can be added to the diet to determine changes in nutrient proportion between the diet and faeces. In our trials, we use yttrium as the indigestible marker. As the nutrient is absorbed, the relative concentration of the indigestible marker increases and thus we are given an estimate of apparent digestibility of a nutrient in a diet.

Although this method is straight forward for terrestrial animals, there is a caveat when using this method for aquatic animals as the faeces are excreted into a watery environment. The method of faecal collection becomes critical to the accuracy of apparent digestibility estimates as nutrients from faeces can leach into the water and influence the proportion of nutrients and indigestible marker.

Two standard methods have emerged to collect excreted faeces from crustaceans for apparent digestibility calculations. Faeces can be collected onto a screen or filter paper where it is most often rinsed with freshwater to remove marine salts. Alternatively, faeces can be siphoned into a smaller vessel (e.g. a bucket), where seawater is decanted and the contents centrifuged with freshwater to remove marine salts. Although careful consideration is given to minimise immersion time of faeces using these methods, effects of leaching on faeces can occur (Smith and Tabrett, 2004).

Dissecting faeces from the animal is a method that negates the effects of nutrient leaching on measuring apparent digestibility. In this study we compared the apparent digestibility of ingredients using three faeces collection methods including siphoning onto a filter screen (FS), collection via bucket settlement (FB) and dissection. Various ingredients were selected that were either rich in protein, starch or lipid to assess the effect of nutrients with high water-solubility (e.g. protein and amino acids) compared to nutrients of low water-solubility (e.g. starch and lipid) on calculations of apparent digestibility using each faeces collection method. Protein-rich ingredients included fishmeal, soybean meal and whole squid meal. Starch ingredients included wheat flour and pregelatinized wheat starch. Lipid ingredients included fish oil and canola oil. For full details of diet formulation, experimental design and analysis, see Truong et al., 2022.

Apparent digestibility of protein was influenced by the faecal collection methods as shown in Figure 1. The dissection method, which avoids the effects of leaching, consistently produced lower values of apparent digestibility for protein. Using this method, fishmeal has a protein digestibility of 51% which was considerably lower than either the FS and FB methods. The FB method has the longest period of faecal immersion and includes centrifugation, and so it follows that FB method would have the highest value of apparent digestibility of protein. Using this method, fishmeal has a protein digestibility of 82% which is the common expectation for fishmeal. When compared with soybean meal, protein digestibility using the dissection method did not differ with fishmeal (50% vs. 51%) while the FS and FB method indicated a larger difference: 62% vs. 70% and 75% vs. 82%; respectively. Interestingly, faecal collection method had a minimal effect on protein digestibility of whole squid meal which was considerably higher than fishmeal and soybean meal: 81% using the dissection method to 92% using the FB method. This suggests that the effect of nutrient leaching from faeces on protein digestibility may confound the true value of protein-rich ingredients.

Apparent digestibility of starch and lipid ingredients did not differ between faecal collection methods. Nutrients which have low susceptibility to leaching in water are more likely to be more stable in excreted faeces and enabling for more accurate predictions of apparent digestibility. Of the three methods, the dissection method provides more conservative values of protein digestibility for protein-rich ingredients where the method circumvents the effect of nutrient leaching from faeces. Although further investigation of the dissection method is required before it can be adopted as a standard procedure, it provides valuable insight towards the true value of ingredients used for prawn aquaculture feeds.

CSIRO Aquaculture contact:

Ha Truong – Research Scientist