Targeting gill health in gilthead seabream (Sparus aurata) via functional feeds: Evidence from a multilevel assessment

A study was conducted to assess the effects of natural functional feeds on the growth and gill health of gilthead seabream. Over an 80-day period, five iso-energetic and iso-proteic diets (42 % crude protein, 18 % crude fat, digestible energy: 17.3 MJ/kg) were fed to triplicate groups of fish (initial body weight: 71.7 g). The experimental diets included a control and four diets supplemented with functional additives: caprylic acid (CA) and a mix of herbal extracts, essential oils, B vitamins, iron, and zinc (MIX). These additives were tested at two dosages: 15 g/kg and 30 g/kg for CA (CA15 and CA30), and 5 g/kg and 10 g/kg for MIX (MIX5 and MIX10). Growth performance, feed utilization, and carcass composition were monitored. Welfare parameters were measured, including gill histomorphology, blood biochemistry, haematocrit, and microbiome (gills, water, tank biofilm, and biofilters) at the start (T0) and end (T1) of the trial. In addition, CA retention was assessed in plasma and gills at several postprandial time points (0.5, 4, 8, and 12 h). Growth was significantly reduced in the CA30 group, mainly due to lower feed intake. Among plasma analytes, iron levels tended to increase at T1 in diets supplemented with functional additives, particularly MIX5 and MIX10. Caprylic acid was absorbed into target tissues within 30 min postprandial and remained stable up to 12 h. The gill microbiome remained largely unchanged, with Pseudomonas spp. dominating over 60 % of the relative abundance. These findings offer valuable insights into the effect of functional diet on growth, physiological status, and gill-associated microbial communities in gilthead seabream, supporting their potential role in sustainable aquaculture practices.