The main objective of this field experiment was to investigate whether ration level affected utilization of carotenoids, macronutrients, and minerals in 1,300 g Atlantic salmon (Salmo salar) during rapid growth. Salmon fed ration levels of either 1.2% or 0.6% of biomass of a commercial diet supplemented with astaxanthin and canthaxanthin (37 and 39 mg kg −1 , respectively) in two consecutive 6-day feeding periods had carotenoid digestibility coefficients of 11.8% and 32.1% at the high and low feed rations, respectively. Thus, low carotenoid digestibility, but good macronutrient digestibility, may explain poor pigmentation and good feed conversion in rapidly growing salmon. Practical implications are illustrated.

Traditional tagging methods for fish can have issues relating to both animal welfare and economic costs. Biometric data such as iris patterns can be captured via digital cameras allowing non-invasive tagging, and inexpensive and rapid analysis. The purpose of this study was to investigate if the iris of Atlantic Salmon (Salmo salar) is a suitable biometric template for long term identification of individuals. Atlantic Salmon were individually tagged in the body cavity using PIT-tags at the juvenile pre-smolt stage and the left eye was photographed 6 times over a 533 day period. Description of changes in iris stability was assessed both qualitatively and using iris-recognition software.Identification of individual Atlantic Salmon using the iris was not successful over the entire period, as the iris pattern changed significantly with time. Over a shorter time period (4 months) with frequent samplings, iris software was able to correctly identify individual fish. The results show that iris identification has potential to replace other methods for Atlantic Salmon over short timeframes.Individual identification of fish is important for scientific research on both wild and farmed fish.Methods for individual tagging of fish are many, but mostly involve placement of external or internal tags that can be read either directly or by electronic equipment (Thorstad et al., 2013). Both the handling of the fish during the tagging process and the tagging itself can have obvious animal welfare issues and could also affect the fish in regard to what is being studied (e.g. growth, behavior or mortality, Jepsen et al., 2015). Finding non-invasive alternatives to physical tagging could reduce the amount of stress and pain fish have to undergo and improve quality of the science produced via reduced tagging effects. For some research identifying individuals using DNA is possible but requires handling the fish to obtain samples and for a large number of fish, the costs involved can be substantial. Biometric identification using patterns or other structures unique among individuals, may for many species of animals be a cost efficient and non-invasive method for mark-recapture.Biometric identification in humans is well documented with methods as fingerprints, iris, face and voice recognition systems (Jain et al., 2006). Manual photoidentification of individual animals was pioneered in the 1970 on orcas (Orcinus orca) (Bain, 1990) and since been used in a range of taxa.