Feed intake measurements in fish using radioactive isotopes: II. Experiments with Atlantic salmon and rainbow trout in sea-pens

Abstract

Short-term feed intake was measured in 2419 Atlantic salmon ( Salmo salar L.) and 1232 rainbow trout ( Salmo gairdneri Richardson), weighing 0.1–4.0 kg. The fish were kept in net-pens in the sea. They were hand-fed with a moist diet containing radioactive Ag 131I for a 2-h period. The radioactive diet was either fed to fish which had been deprived of feed for 12–15 h to assess the “maximum meal size” or to fed fish to assess “the influence of previous feeding on subsequent feed intake”. The amount of radioactive feed ingested by the fish was measured with a portalbe gamma scintillation counter placed under their stomachs. The accuracy of this method when using large fish was checked, and the fish were monitored for possible radioactive contamination. The feed intake was further assessed by weighing the stomach contents of 16 salmon. The results showed that feed intake can be measured in large fish using radioactive isotopes. No radioactive contamination was found in the fish. When the water temperature was 10–13°C the intake of moist feed amounted to 6–12% of the fish weight during the first 2-h period after the deprivation period. When feed intake during this first period was high, it became low during the rest of the day. When the water temperature was 5–6°C, feed intake after the deprivation period was only 2–3% of the fish weight, and the fish ingested relatively more feed in later meals. There was no significant difference in feed intake for the two species of fish. A mean feed intake of 4.1% moist feed was recorded from the weight of stomach contents in the separate experiment which was carried out at a water temperature of 14–15°C.