PHYSIOLOGICAL IMPACTS OF VALUE-ADDED FISH MEALS WITH DIFFERENTIALLY PROCESSED SIDESTREAMS IN ATLANTIC SALMON Salmo salar FEEDS DURING ADAPTATION TO COLDWATER AFTER SEAWATER TRANSFER

Introduction

Atlantic Herring Clupea harengus and Blue whiting Micromesistius poutassou are two of the most important fish species available for commercial fish meal (Fm) production in Norway. Further, side streams from processing of fish have become increasingly important for fish protein concentrate (FPC) and hydrolysate (FPH) production due to their potential as functional ingredients. Raw material handling and processing of Fm can impact their quality. Traditionally, Fm quality has been determined by raw material freshness and protein digestibility. However, focus on improving the robustness of fish during challenging periods have provided incentive to investigate other avenues to improve the quality of Fm and their side streams. In this study we examined the impact of increasing volatile compounds through gentle processing methods of the raw material and their side streams for improving the physiological robustness in Atlantic salmon smolts during adaptation to seawater transfer combined with lowering of the water temperatures.

Material and Methods

Two qualities of blue whiting Fm namely 'Superior' and 'Standard' were produced by gentle low temperature and conventional steam drying by Pelagia AS, respectively. Herring Fm of high quality was also produced by Pelagia AS, while the added value ingredients; FPH and FPC were produced from the same batch of fresh, high-quality herring side streams by Nofima AS and Pelagia AS, respectively. Seven nutritional balanced and isoenergetic diets were produced where the control diet was a standard formulated salmon feed supplemented with 15 % high quality blue whiting Fm (D1), standard quality blue whiting Fm (D2) and herring Fm (D3). Iso-protein levels of FPH and FPC (about 4 %) were added to the high quality blue whiting Fm (D4, D5) and to combined 50:50 diets of herring Fm and blue whiting Fm (D6, D7), all balanced to reach 35.5 % water-soluble protein. The nutritional added value potential of the diets was evaluated in a controlled 8-week feeding trial with Atlantic salmon immediately after seawater transfer. The fish were adapted to decreasing temperatures from 12°C in freshwater to respectively 10.5°C (at transfer), 9.5, 8.5, 7.5, and 6.5 °C until final fish sampling.

Results and Discussion

The feeding trial was carried out with salmon (initial weight: 119.5 ± 0.6 g), showing doubling of fish body weight (final weight: 237.5 ± 6.7 g) and efficient growth in all dietary groups (average SGR: 1.23 ± 0.05 %; TGF: 2,60 ± 0.12). No significant differences in feed intake, growth or feed conversion were found in the total feeding period (ns). Increased feed intake, however, was observed in the initial 2 weeks feeding period (10.5°C) in fish fed diets added FPC (D5, D7), while continued increased feed intake was observed only in fish fed the blue whiting diet D5. During the trial, the feed intake slowly decreased in all dietary groups adapting to the steadily declining temperature. Protein and amino acid (AA) digestibility tended to be higher in fish fed D4-D7 added FPH and FPC (Prot: 92.4 ± 0.9 %; AA: 93.5 ± 0.9 %), while no differences relative to the other diets (D1-D3) was found (Prot: 91.3 ± 0.8 %; AA: 92.5 ± 0.7 %), Whole body crude composition (Protein: 16.5 ± 0.2 %; Lipid: 11.8 ± 2.4 %, Ash: 2.10 ± 0.05 %) was unaffected by the diets (ns). The protein retention efficiency was high in all dietary groups, and no significant differences in PER: 3.32 ± 0.14; PPV: 49.5 ± 2.3 % or BV: 53.8 ± 2.4 % was detected (ns). Lipid digestibility was also high in all dietary groups (ADC lipid: 94.3 ± 0.9 %), and no significant differences detected (ns).

In the sampled fish (n=5/tank), the OWI welfare scores of the skin (scale loss, bleeding, wounds and cataract) revealed high quality (OWI scores ≤ 1.2) and no diet dependent differences (ns). Liver lipid (6.4 ± 0.9 %), HSI (1.44 ± 0.10 %), slaughter yield (85.1 ± 0.7 %) and total tocopherols (868 ± 153 mg/kg) were unaffected by diets (ns). The liver contents of Fe and Cu were weakly elevated in fish fed diets added FPC, while liver Zn, Mn and Se were unaffected by diets.

Analyses of free amino acids in muscle and skin showed that nearly 80 and 70 % represented maintenance of the osmotic water balance in the muscle and skin, respectively, independent of the diets, Figure 1. The levels of osmolytes such as anserine was very high in the muscle, while taurine was much higher in the skin, while both appeared to be physiologically well regulated due to the consistent levels and observed differences between skin and muscle of fish from all dietary groups. Addition of the soluble, functional compounds from herring (FPH, FPC) did not seem to affect the tissue composition, while more advanced methods would be required to explore impacts on the physiological regulatory mechanisms.

Figure 1 Free amino acids (AA) and bioactive peptides in muscle and skin

Analysis of the volatile N-compounds (TVN, TMA, TMAO) in muscle and skin also revealed some differences in compositions; i.e. the level of TVN in the muscle of fish was about twice as high as in the skin, while TMAO (an osmolyte) was present only in fish fed D1, D4 and D5, all containing blue whiting Fm, while also in fish fed the combined fish meal diet D7 added with FPC, Figure 2. Irrespective of diets, TMA was not detected.

Figure 2 Volatile N-compounds (TVN, TMAO) in muscle and skin

Summary

Fish from all diets demonstrated efficient growth, feed intake and feed conversion, high protein and lipid digestibility, efficient protein retention and no visual observed differences in skin quality by operational welfare scores (OWI). Distinct differences in the tissue composition of free amino acids and volatile N- compounds in skin and muscle is anticipated to be of physiological relevance for maintenance of osmotic balance but will require further exploring for revealing of the physiological regulatory mechanisms. Additional analyses to study dietary impacts on serum health markers and serum osmolality, amino acid composition in the skin and the muscle, and selected gene expressions in the skin and the gills (qPCR) are in progress and will be presented.