Effect of varying dietary protein and lipid levels on the growth performance of the European grayling (Thymallus thymallus)

Despite a growing interest for the European grayling (Thymallus thymallus) as a new candidate for freshwater aquaculture, little information is available on growing records and feeding behaviour of this fish species in captivity as well as on its nutrient requirements or suitable nutrient levels to be adopted in formulating complete dry feeds. As a first approach in this direction, a feeding trial was carried out to study the growth response of grayling to diets varying in macronutrient levels. Four practical-like complete feeds containing different crude protein and lipid levels (45/20; 50/16; 55/12 and 55/20% DM), were prepared. Each diet was fed twice daily to visual satiety over 20 weeks to triplicate groups of fish, each consisting of 15 specimens (age 1+, ind. wgt. 112 ± 6.5g), according to a single factor random design. Fish groups were kept indoor in 250-L tanks supplied by well water at nearly constant water temperature (12.8 ± 0.4°C), flow-through rate (5 L•min-1/tank) and exposed to an artificial day-length set at 12h. Three specimens were sampled at random from a spare group at the beginning and 3 fish per dietary group were collected at the end of the trial for whole body nitrogen analysis and calculation of nitrogen retention (N gain*100/N intake). Irrespective of the diet, voluntary dry feed intake ranged between 4.9 and 5.5 g/kgABW/d and was found to be much lower than that of salmonids reared at the same temperature. Final body weight-FW, specific growth rate-SGR and feed conversion ratio-FCR of fish fed diets high in protein (55/12 and 55/20) were similar and significantly better than those exhibited by grayling given the diets 45/20 and 50/16 which did not differ from each other (FW, 220 vs. 196 g; SGR, 0.44 vs. 0.39; FCR, 1.2 vs. 1.4, P<0.05). Nitrogen retention (32 ± 1.5%) was unaffected by the dietary treatment. The results obtained so far suggest that grayling takes advantage of high protein diets also at post-juveniles stages and that increasing dietary lipid above 12% DM results in poor protein sparing effect in this fish species.