Partial dietary fish meal replacement with soybean meal supplemented with papain alters growth, hematological, serum biochemical indices, antioxidant activities and immune response of Sea bass, Dicentrarchus labrax

This study examined how varying the proportions of fishmeal (FM) to soybean meal (SBM) with or without the inclusion of an external enzyme called papain, impacted the digestive enzyme performance, digestibility, growth, immune function, blood parameters, and antioxidant responses in sea bass (Dicentrarchus labrax). Six isonitrogenous (448.17 g kg⁻¹ crude protein) and isocaloric (18.79 MJ kg⁻¹ energy) experimental diets were formulated. The diets were divided into three groups as follows: The first is FSB1 which contains (FM: SBM = 1:1), the second is FSB2 (FM: SBM = 3:1), and the last FSB3 which contains (FM: SBM = 3:2) based on protein content. Every group was complemented with papain enzyme at 0 and 0.5 g kg ⁻¹ diet. Sea bass weighing 3.68 ± 0.02 g was distributed into 18 circular tanks (200 L) for a duration of 84 days, with three tanks per treatment (20 fish per treatment). After the trial, the group that consumed FSM2 with 0.5 g of papain kg⁻¹ recorded the highest specific growth rate, weight gain, final body weight, and the most efficient feed conversion ratio. Fish that were given diets supplemented with exogenous papain exhibited higher digestibility coefficients for crude protein, lipids, dry matter, and digestible energy compared to those fed diets without papain. However, groups that were fed different levels of FSM and papain showed better digestive enzyme activities (trypsin, chymotrypsin, amylase, and lipase), with the best results seen in the group that received the FSM2 diet along with papain. Fish that didn’t receive papain with FSM3 had poorer blood health indicators such as lower red and white blood cells, hemoglobin, lymphocytes, and hematocrit. However, adding papain to diets, especially with FSM1 and FSM2 improved globulin, total protein, and albumin levels in the blood. Sea bass fed diets containing FSM2 with papain increased immune markers (C3, C4, IgG, and IgM), enhanced antioxidant activity, and lower MDA levels (a marker of oxidative stress) in the liver and muscles. Overall, papain improved digestive enzyme activity, immune function, growth, blood parameters, digestibility, and liver antioxidant enzymes in Sea bass nourished with diets containing diverse proportions of fishmeal and soybean meal.