Validation of a technique for determining apparent digestibility in large (up to 5 kg) Atlantic salmon (Salmo salar L.) in seacages

This study was undertaken to assess whether collection of faeces by a stripping method was suitable for measuring digestibility in large seawater Atlantic salmon of up to 5 kg. Experiments were conducted to determine the effects on the apparent digestibility coefficient (ADC) of faecal collection technique, urine and mucus contamination of faecal samples, immediate repeated sampling on individuals, stripping pressure during the collection of faeces, dorso-flexion of the tail prior to faecal sample collection, variation between individual fish and section of gut sampled. Faecal collection by rectal suction and dissection was compared with the stripping collection method. Rectal suction produced comparable results to stripping; however, ADC for crude protein was significantly (P<0.05) lower after dissection. It is proposed that the loose consistency of the faeces produced by large salmon on commercial feed causes it to redistribute along the gut after euthanasia. Urine and mucus contamination, stripping pressure, immediate repeated sampling from individuals and dorso-flexion of the tail did not significantly affect ADC. However, the ADC for organic matter (P<0.01) and crude protein (P<0.05) were significantly lower when faeces were stripped from near the anus to the anus compared with from the pelvic fin region to the anus. There was no relationship between fish size and ADC for gross energy or crude protein. The stripping method used appeared to be suitable for collecting faeces from large seawater Atlantic salmon (300-5000 g) for the purpose of calculating apparent digestibility. A robust and practical procedure that takes account of several factors that can cause major variation in data is suggested. © 2001 Elsevier Science B.V. All rights reserved.