Introduction
The last decade, there has been a growing need for new aquafeeds ingredients that meet nutritional requirements while reducing environmental impact (Gasco et al., 2023). Two promising alternatives to fishmeal are insect and microalgae meals. Insect-based diets are rich in proteins and lipids (Gasco et al., 2023), while microalgae provide key nutrients such as n-3 highly unsaturated fatty acids, essential for proper fish growth (Concei����o et al., 2010). Several studies have shown that these alternative protein sources can also improve fish stress response and disease resistance (Yadav et al., 2020). This study evaluated the use of Hermetia illucens (HI) and Spirulina (SP) meals –individually and in combination– as feed ingredients for Oncorhynchus mykiss (rainbow trout), with the aim of identifying optimal inclusion levels to enhance growth performance, nutrient digestibility, and the health of the digestive system.
Materials and Methods
Seven isonitrogenous, isolipidic, and isoenergetic diets were formulated: a control diet (CTRL) containing 20% of fish meal, and six diets where the fish meal was 100% replaced with Hermetia illucens meal (HI100), 100% Spirulina meal (SP100), 75% HI and 25% SP (MIX1), 66.6% HI and 33.3% SP (MIX2), 33.3% HI and 66.6% SP (MIX3), and 25% HI and 75% SP (MIX4). To evaluate nutrient digestibility, 1% of indigestible marker was added to all the diets. For the growth trial, a total of 441 fish (initial body weight of 200.5 ± 2.6 g) were randomly allotted to 21 400L-tanks (3 replicates/diet, 21 fish/tank). Feed was hand-distributed twice daily, seven days a week. Daily rations were set as a % of the tank biomass and adjusted every 14 days based on fish growth and water temperature. The trial lasted 88 days. At the end, growth performance indices were calculated, and 10 fish per tank were slaughtered to determine the somatic indices (n=6) and the histomorphological changes of the intestine, liver, and spleen (n=4). The Apparent Digestibility Coefficients (ADCs) were measured by performing a digestibility trial, where 165 fish (initial body weight 239.6 ± 0.09 g) were randomly allotted to 11 250L-tanks. The experiment was conducted in two consecutive runs, with diet reallocation ensuring three replicates per diet across different tanks. A 7-day adaptation period was applied prior to each fecal collection phase. The fish were fed ad libitum and the feces were collected twice daily from each tank for four weeks using a continuous automatic device (Choubert et al., 1982).
Results
Growth performance and somatic indices were not affected by diet (p>0.05). The histomorphology evaluation of the anterior intestine showed that the fish fed with SP100 and MIX4 had smaller villi (length and surface), lower number of goblet cells/100μm, and thinner mucosal, submucosal, and muscularis layers when compared to the CTRL, HI100, and MIX1 diets (p<0.05). However, there was no evidence of the effect of the diets on the pathology of the digestive organs (p>0.05). Finally, the ADC of dry matter was not affected by the diet (p>0.05), while the fish fed with the SP100 diet was less able to digest crude protein compared to the CTRL diet (p<0.05) and both the diets with the highest inclusion levels of Spirulina (SP100 and MIX4) affected negatively the digestion of the ether extract compare to the CTRL, HI100, and MIX1 diets (p<0.05).
Discussion
The present study provides novel insight into the effect of the total replacement of the fish meal with HI and/or SP meals in rainbow trout. The growth performance of the fish was not different between the CTRL and the experimental diets, thus being in agreement with previous studies (Sheikhzadeh et al., 2019a, Biasato et al., 2022). Even though low inclusion levels of Spirulina can have a positive effect on the digestive system and digestibility of the fish (Zhang et al., 2024), in the present study, the diets with the highest inclusion percentages of Spirulina (SP100 and MIX4) displayed deterioration of the intestine morphology and liver's health, and lower digestion of the protein and the lipids. The results showed that the diets with the highest inclusion levels of Hermetia illucens meal (HI100 and MIX1) were the optimal ones in terms of the health of the inestine and the digestion of the fish.
Acknowledgment
Financial support has been provided by PRIMA, a program supported by the European Union, under grant agreement No 2231, project CIPROMED (PRIMA Call 2022 Section 1 Agri-food IA).
References
Biasato, I., Chemello, G., Oddon, ���, Gasco, L., (2022) Hermetia illucens meal inclusion in low-fishmeal diets for rainbow trout (Oncorhynchus mykiss): effects on the growth performance, nutrient digestibility coefficients, selected gut health traits, and health status indices. Animal Feed Science and Technology (Vol 290, 115341). https:// doi.org/10.1016/j.anifeedsci.2022.115341.
Choubert, G.; De La Noue, J.; Luquet, P., (1982) Digestibility in fish: Improved device for the automatic collection of feces. Aquaculture. (Vol 29, 185–189).
Gasco, L., Renna, M., Bellezza Oddon, S., Rezaei Far, A., Naser El Deen, S., Veldkamp, T., (2023) Insect meals in a circular economy and applications in monogastric diets, Animal Frontiers. (Vol 13, 81–90).
Concei����o, L. E. C., Y��fera, M., Makridis, P., Morais, S., & Dinis, M. T., (2010) Live feeds for early stages of fish rearing. Aquaculture Research.( Vol. 41, 613–640). https://doi.org/10.1111/j.1365-2109.2009.02242.x
Sheikhzadeh, N., Mousavi, S., Hamidian, G., Firouzamandi, M., Oushani, A.K., Mardani, K., (2019a) Role of dietary Spirulina platensis in improving mucosal immune responses and disease resistance of rainbow trout (Oncorhynchus mykiss). Aquaculture. (Vol 510, 1–8).
Yadav, G., Meena, D.K., Sahoo, A.K., Das, B.K., Sen,R., (2020) Effective valorization of microalgal biomass for the production of nutritional fish-feed supplements. Journal of Cleaner Production. (Vol 243, 118697).
Zhang, W.Q., Deng, Y.Y., Yang, Z.X., Kong, Q., Liu, P.Q., Liao, H.P., Tang, H.J.,( 2024) Effects of partial replacement of fishmeal with Spirulina platensis powder and addition of Spirulina platensis polysaccharide on growth, nutrition, antioxidant capacity and gut microbiota of Micropterus salmoides. Aquaculture. (Vol 586, 740802). https://doi.org/10.1016/j.aquaculture.2024.740802