Validating gut flushing as a non-lethal sampling technique for diet monitoring in agastric fish

Non-lethal diet sampling is essential where individual-level monitoring and welfare matter, yet evidence on season-long outcomes in agastric fishes is scarce. We tested syringe-based gut flushing in pond-reared common carp Cyprinus carpio over one growing season (April-September) using PIT-tagged fish assigned a priori to three handling histories: flushed (sedated, measured, flushed), handled-only (sedated, measured), and unhandled controls. At harvest, recapture (survival proxy) remained high and statistically similar among handled groups and growth to harvest did not differ across treatments (end-point mass: flushed 1857.3 ± 367.4 g; handled-only 1769.7 ± 311.8 g; unhandled 1731.9 ± 392.6 g). A mid-season divergence in July (handled-only > flushed in weight gain) was transient and absent at harvest. Operational performance scaled predictably: total water required to clear effluent increased with fish size (~20 mL in small carp to >90 mL in large individuals; occasional ~120 mL) and varied seasonally, rising from May to August and easing in September. Practical safeguards (rounded catheters guided between pharyngeal teeth; conservative advancement ≤2 cm beyond the teeth; controlled bolus infusions; standardised recovery) supported complete evacuations with low injury risk. Collectively, these results validate gut flushing as a field-ready, non-lethal method that can be integrated into routine dietary monitoring of agastric cyprinids without compromising survival or harvest growth, while providing simple, size- and season-based parameters to standardise protocols across studies.