| Summary: | In Senegalese sole (<i>Solea senegalensis</i>) aquaculture, infectious diseases represent a major challenge, leading to significant economic losses (Arijo et al, 2005). According to FAO / WHO (2001), probiotics are defined as "live micro-organisms which, when administered in adequate amounts, confer a health benefit on the host". Previous studies working with different fish species provided evidence on the effectiveness of various probiotic bacteria in promoting the fish welfare (Castex et al., 2009; Hernandez et al., 2010; Merrifield et al., 2011).
In the current study, we evaluated the effects of dietary probiotic supplementation intercropped with plant protein as fishmeal replacement on the growth performance and immunological status in sole (<i>Solea senegalensis</i>). Fish were fed for 10 weeks on six isonitrogenous and isolipidic diets (55% protein and 8% lipid, dry matter basis), formulated to have 35% or 72% of fishmeal replacement by plant protein (PP35 or PP72) intercropped with three probiotic supplementation (NO – unsupplemented, PRO1 and PRO2), in a 2 × 3 factorial design. The probiotics were tested at a dosage recommended by the manufacturer. PRO1 was a multi-species probiotic bacteria (<i>Bacillus</i> sp., <i>Pediococcus</i> sp., <i>Enterococcus</i> sp., <i>Lactobacillus</i> sp.) supplemented at 1.8 × 10<sup>10</sup> CFU kg<sup>-1</sup> diet (CFU - colony forming unit) and PRO2 was an autolyzed yeast (<i>Saccharomyces cerevisiae</i>) supplemented at 4g kg<sup>-1</sup> diet. Zootechnical parameters were measured at start and the end of the feeding trial for growth performance determination. Plasma was collected at 3, 17, 38 and 73 days of feeding dietary treatment for the determination of innate immune response (plasma lysozyme and peroxidase contents, plasma alternative complement pathway activity – ACH50) in order to detect differences between treatments as well to identify the influence of long or short-time probiotic supplementation.
Fish with an initial body weight of 33.1 ± 0.2 g grew to a maximum final body weight of 50.6 ± 1.2 g. PRO1 groups had significantly lower weight gain and higher feed conversion ratio (11.9 ± 1.9 and 2.2 ± 0.4, respectively) compared to NO groups (17.4 ± 1.9 and 1.5 ± 0.1 respectively). Growth performance from PP35 and PP72 groups did not differ, suggesting that sole can cope well with diets containing high plant protein levels, as already reported by Silva et al. (2009).
At 3 and 73 days of feeding trial, the immune parameters analysed did not present differences among treatments, indicating that these may not be the most appropriate sampling times to detect the influence of probiotic administration in Senegalese sole. Plasma peroxidase content (EU mL<sup>-1</sup>; Figure I) was not affected by PP level, but was significantly changed by probiotic administration at 38 days of feeding. PRO1 groups presented higher plasma peroxidase content (79.3 ± 18.6) comparing to PRO2 groups (40.1 ± 10.3), but were not statistically different from NO groups (57.2 ± 14.0). Plasma lysozyme content (EU mL-1; Figure II) and ACH50 (Units ml-1; Figure III) were not affected by probiotic administration. However, fish fed PP72 diets had higher lysozyme and ACH50 levels at 38 days (1018 ± 204 and 257 ± 21, respectively) than fish fed PP35 diets (646 ± 154 and 183 ± 17, respectively). Also at 17 days, fish fed PP72 diets had higher lysozyme (345 ± 82) comparing to fish fed PP35 diets (175 ± 37).
In conclusion, the results of our study suggested that high plant protein inclusion levels may affect Senegalese sole immune status, without affecting growth performance. Multi-species probiotic (PRO1) was more effective in stimulating the humoral innate immune parameters than Saccharomyces cerevisiae (PRO2). Further studies are needed to evaluate whether dietary probiotic supplementation may induce higher resistance against stress conditions and pathogens in sole.
Figure 1 – Effects on plasma peroxidase content of Senegalese sole juveniles after 3, 17, 38 and 73 days of feeding the dietary treatments. Values represent mean ± standard error (n=9). Different superscript letters indicate significant differences (P<0.05). Considering 2 factor (Diet + Probiotic): among probiotic inclusion (a, b). EU = Enzyme Unit.
Figure 2 – Effects on plasma lysozyme content of Senegalese sole juveniles after 3, 17, 38 and 73 days of feeding the dietary treatments. Values represent mean ± standard error (n=9). Different superscript letters indicate significant differences (P<0.05). Considering 2 factor (Diet + Probiotic): among diets, despite probiotic inclusion (A, B). EU = Enzyme Unit.
Figure 3 – Effects on plasma alternative complement pathway activity (ACH50) of Senegalese sole juveniles after 3, 17, 38 and 73 days of feeding the dietary treatments. Values represent mean ± standard error (n=9). Different superscript letters indicate significant differences (P<0.05). Considering 2 factor (Diet + Probiotic): among diets, despite probiotic inclusion (A, B).
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