Abstract Feed withdrawal is a widespread practice in Atlantic salmon (Salmo salar) aquaculture to empty the gut prior to major farming operations, while certain pathogens and suboptimal environmental conditions in production cages are known to induce prolonged fasting. However, these fasting periods may be in conflict with ethical and legal obligations to farm animals. Presently, science-based recommendations on responsible fasting times that consider fish welfare are lacking. In this study, we measured the standard metabolic rate (SMR) and metabolic rate following acute handling and confinement stress in Atlantic salmon post smolts (~575 g, ~38 cm) following 1, 2, 3 and 4 weeks of feed withdrawal and 1 week of subsequent refeeding at 12 °C. The purpose was to identify when changes in metabolic mode occurred and assess whether the capacity to respond to stress eventually was compromised, since such observations could serve as potential welfare indicators. The SMR decreased significantly from 84.4 ± 4.7 mg O2 kg h−1 in control fish to 71.0 ± 1.8 mg O2 kg h−1 following 1 week of fasting. A further significant decrease to 65.0 ± 3.7 mg O2 kg h−1 was measured after 3 weeks, while refeeding returned SMR to control levels. The increase in MO2 following acute stress was unaffected for the first three weeks of fasting. However, the 4 week group showed a reduced peak response compared to the preceding weeks (278 ± 13 vs. 310 ± 7 mg O2 kg h−1). Weight, fork length and condition factor did not change significantly during the fasting period, and the fish immediately resumed eating upon refeeding. We conclude that up to 4 weeks of feed withdrawal had negligible effects on fish welfare. Moreover, an improved aerobic scope owing to a reduced SMR may be advantageous prior to certain farm operations.

中文翻译：

---

大西洋鲑鱼幼鱼后对饲料撤除的代谢率反应

摘要 断食是大西洋鲑鱼 (Salmo salar) 水产养殖中的一种普遍做法，以在主要养殖作业之前清空肠道，而已知某些病原体和生产网箱中的次优环境条件会导致长时间禁食。然而，这些禁食期可能与对农场动物的道德和法律义务相冲突。目前，缺乏关于考虑鱼类福利的负责任禁食时间的基于科学的建议。在这项研究中，我们测量了大西洋三文鱼在断食 1、2、3 和 4 周和 1 周后的标准代谢率 (SMR) 和急性处理和禁闭应激后的代谢率（~575 克，~38 厘米）。随后在 12 °C 下重新饲喂一周。目的是确定代谢模式何时发生变化，并评估应对压力的能力最终是否受到损害，因为这些观察结果可以作为潜在的福利指标。禁食 1 周后，SMR 从对照鱼的 84.4 ± 4.7 mg O2 kg h-1 显着降低到 71.0 ± 1.8 mg O2 kg h-1。3 周后测量到进一步显着降低至 65.0 ± 3.7 mg O2 kg h-1，同时重新喂养使 SMR 恢复到控制水平。在禁食的前三周，急性应激后 MO2 的增加不受影响。然而，与前几周相比，第 4 周组的峰值反应降低（278 ± 13 与 310 ± 7 mg O2 kg h-1）。禁食期间体重、叉长和体况因素没有显着变化，重新喂食后鱼立即恢复进食。我们得出的结论是，长达 4 周的饲料停药对鱼类福利的影响可以忽略不计。此外，在某些农场操作之前，由于 SMR 降低而改善的有氧范围可能是有利的。