Digestibility of BSF meal: nutritious alternative to fishmeal

Insect meal is emerging as one of the important sources of alternative protein that can replace fishmeal. One of the potential insects that can be used as aquaculture feed ingredient is the black soldier fly larvae (BSFL). The study by the Malaysian University of Sabah examines the potential of BSFL meal as the protein source in practical diets for hybrid grouper, (E. fuscoguttatus X E. lanceolatus) by analyzing a number of digestibility indicators. 

Why black soldier fly
As a carnivorous species, the hybrid grouper requires high amounts of protein in its diet. BSFL can potentially become a cost-efficient and a more sustainable alternative to fishmeal. This species offers a number of advantages as compared to other insects. 

First, BSFL provides high contents of protein - about 40%, a well-balanced essential amino acid profile and a number of minerals such as potassium, calcium, iron, magnesium and selenium and several vitamins. Secondly, BSFL production is highly sustainable and fits into the concept of circular economy, as it feeds on different types of organic waste - food waste, animal manure etc. 

The study
The experiment studied a group of total 180 hybrid groupers purchased from a local fish farm. The fish were randomly assigned to one of the four types of diet: a reference diet and three types of test diet. 

A reference diet (RD) was developed to meet the nutrient requirement for hybrid grouper (50% crude protein and 12% crude lipid) using Danish fish meal and soybean meal as protein sources. The test diets included three different types of BSFL meals:
  1. Spray-dried BSFL (SPR)
  2. Oven-dried BSFL 1 (OVN1) 
  3. Oven-dried BSFL 2 (OVN2). 

The test diets contained 70% of the RD mixture and 30% test ingredients. Chromic oxide was added at 0.5% inclusion as the digestibility marker. The actual feed was prepared by mixing all the ingredients into a moist dough to make oven-dried pellets of 3 mm. 
The trial lasted 12 weeks, during which the fish were fed twice a day at 07:00 and 16:00 until apparent satiation level.

The indicators
The study measured the performance of the diets based on the the following indicators: 
  1. Total weight gain (WG, %)
  2. Specific Growth rate by day (%)
  3. Survival rate (%) of the fish in experiment 
  4. Feed intake, measuring how much of the feed provided the fish actually ate
  5. Feed conversion ratio, showing how much feed is needed to produce a 1 kg increase in weight

To gain data necessary to calculate the indices, the experimental diets, ingredients and faeces were analyzed for proximate composition according to the Association of Official Analytical Chemist methods. In particular, the levels of crude protein, dry matter, ash and crude lipids content were analyzed. In addition, each BSFL meal was sent for amino acid analysis. 

The study measured the apparent digestibility coefficient (ADC) by determining chromic oxide concentrations in the diets and faeces using acid digestion method. The digestibility coefficient was calculated for dry matter, crude protein and crude lipid. 

After 12 weeks of trial, the experiment showed a number of interesting findings. 

(1) Nutritional composition of BSFL meal
The amount of protein in fishmeal (FM) was the highest among all the ingredients. Among the three BSFL meals, the highest amount of protein (48.20%) was found in SPR. The ash content in BSFL ranged from 7.26 to 8.27% as compared to 12.37% in FM. The crude lipid content of all BSFL meals (25.69–38.36%) was higher than that of FM (10.34%). 

(2) Growth performance
The test diet based on oven-dried BSFL meal 1 was found to provide the highest growth in terms of total weight gain and growth rate - 124% and 2.83% respectively. The same test diet also demonstrated the highest feed intake, indicating higher palatability of the BSFL-based feed. This might have offset the lower values of feed conversion ratio for the test diet OVN1: a mere 0.98, which was the lowest value among the 4 diets of the study. 

(3) Digestibility 
BSF-based diets proved to have high rates of digestibility. The test diet based on oven-dried BSF meal 1 was found to have the highest apparent digestibility coefficient for dry matter and crude protein - 84.20% and 92.56%, respectively. Compared to the same indicators for fishmeal-based diet - 85.88% and 94.32%, a BSF-based diet appears to offer a reliable nutrition alternative. 

DM - dry matter; CP - crude protein; CL - crude lipids

The growth performance of fish observed in this study showed that BSFL meal can be incorporated at least up to 30% in the diets of juvenile hybrid grouper without giving any negative effects on the growth performance. When compared to other similar studies, BSF meal offers higher digestibility values for dry matter, crude protein and crude lipid than poultry-by-product meal and soy protein concentrate based-diets that were tested on grouper. Generally, fish fed with insect-based meal diet have a greater tendency to digest dry matter, protein and lipid efficiently than those fed with ingredients from terrestrial animals and plants. 

All in all, although fishmeal offers higher contents of protein and a slightly higher digestibility than BSFL meal, the insect-based diet resulted in higher gain weight - both total and in daily rate. This could be potentially linked to the higher feed intake of BSFL-based feed, as indicated by the study, pointing to better palatability of an insect-based diet for fish. The finding supports the notion that a diet close in its composition to a natural diet of a given species provides better results in terms of feed intake and gain weight. Indeed, in nature omnivorous fish species often feed on insects found on the bottom of water bodies whereas juvenile stages of carnivorous species eat insects before switching to fish-based diets. Thus, a BSFL-based diet can be regarded as a cost-efficient, sustainable alternative to fishmeal in fish farming.