# Impact of partial replacement of fish meal with polychaete meal (Hediste diversicolor) on growth, nutrient digestibility, fatty acid profiles, and fish-in: fish-out ratio in diets of European seabass, Dicenctrarchus labrax

## TL;DR

This study shows that polychaete meal can replace up to half of fish meal in seabass diets without harming growth or quality, improving sustainability.

## Contribution

The novel finding is that polychaete meal can replace up to 50% of fish meal and partly fish oil in seabass diets without negative effects.

## Key findings

- Growth performance and feed utilization remained unaffected by polychaete meal inclusion.
- Fillet long-chain PUFAs like EPA and ARA increased with higher polychaete meal levels.
- The FIFO ratio dropped below 1 at 20% polychaete meal inclusion, indicating improved sustainability.

## Abstract

As polychaete meal (PM) from Hediste diversicolor has been reported to be a promising alternative to fish meal (FM) and fish oil (FO) in aquafeeds, we hypothesized that replacing FM with PM would support growth performance, feed utilization, and flesh fatty acid profile of European seabass (Dicentrarchus labrax). Therefore, this study was planned to investigate graded PM inclusion levels (0% [control], 5% [PM5], 10% [PM10], 15% [PM15] and 20% [PM20]), selected based on feasible incorporation limits for juvenile European seabass, as a partial substitute for FM (reduced from 29.45% in the control to 14.55 in PM20) in isonitrogenous (48% protein) and isolipidic (14% lipid) diets. A seven-week trial with a total of 150 fish (initial average weight of 14.56 ± 0.01 g) in triplicate tanks assessed growth performance, feed utilization, nutrient digestibility, body composition, fatty acid profiles, and fish-in-fish-out (FIFO) ratio. Growth performance, feed utilization, and organo-somatic indices of experimental fish remained unaffected by dietary PM levels (P > 0.05). A significant linear increase in apparent digestibility coefficients (ADCs) of protein, energy, and organic matter was observed with increasing PM inclusion (P < 0.05), while lipid ADCs remained unaffected P > 0.05). Whole-body protein and lipid content increased, with a corresponding decrease in moisture as dietary PM levels increased (P < 0.05). Fatty acid profiles and lipid quality indices of the liver and fillet were variably affected, liver PUFA levels declined with higher PM, whereas fillet long-chain PUFAs (Lc-PUFAs) such as arachidonic acid (ARA) and eicosapentaenoic acid (EPA) linearly increased. The FIFO ratio fell below 1 at a 20% PM inclusion. The findings suggest that PM can replace up to 50% of dietary FM and partly FO in seabass diets without compromising growth, nutrient utilization, or fillet quality while enhancing sustainability metrics.