Moderate exercise of rainbow trout induces only minor differences in fatty acid profile, texture, white muscle fibres and proximate chemical composition of fillets

These experiments studied how moderate water velocities (0.9 body lengthsecond⁻¹ (bls⁻¹)) may influence different quality characteristics of rainbow trout when compared to fish kept in standing water (<0.1bls⁻¹). Fish fed at 1.3% of their body weight per day were slaughtered at a weight of 350g after nine weeks of experiment at 15.0°C. The fatty acid composition in fillets differed only marginally between exercised fish (excF) and control fish (ctrlF) kept in standing water. ExcF fillets had a significantly lower content of fatty acids 16:0 (P<0.05) and 18:1 (n−7) (P<0.01) and a higher content of 20:2 (n−6) (P<0.05) compared to ctrlF fish, but all differences were small (<7%). The percentage of n−3 fatty acids was not significantly affected by the applied training regime and neither was the n−3 · n−6⁻¹ ratio. The percentage of n−3 fatty acids, however, decreased linearly in both groups when the lipid content in the fillet increased (R²≥0.85, P<1·10⁻⁶). Fillet texture measured instrumentally as shear force (g) after 72h of ice storage did not differ between the two experimental groups, and neither did the content of lipid, protein or dry matter in the fillet. Muscle fibre sizes have a possible role in textural characteristics and were determined by histological analyses of white, glycolytic muscle tissue. These data showed that although differences in average fibre diameters were small (excF: 75.04 (s.d.=48.96)μm; ctrlF: 74.50 (46.21)μm) the general fibre size distribution differed significantly among the two groups (P<0.01). Moreover, moderate exercise induced small but significant changes in fibre circularity (excF: circ.=0.724; ctrlF:=0.720, P<0.05) but neither muscle fibre diameter nor circularity was significantly related to fillet texture. Altogether, the results suggest that moderate water velocities have limited impact on quality of pan-sized rainbow trout but subtle changes in the fillets indicate that other training strategies may induce stronger responses.