DIGESTIBILITY OF VARIOUS FEEDSTUFFS BY POST-JUVENILE CHINOOK SALMON (ONCORHYNCHUS-TSHAWYTSCHA) IN SEA-WATER .1. VALIDATION OF TECHNIQUE

This study was undertaken to assess whether the Guelph system for measuring digestibility in rainbow trout in fresh water was suitable for determining digestibility in chinook salmon held in sea water. Feces were collected from chinook salmon by the Guelph system, the stripping technique (once per week) and intestinal dissection. Validation of the Guelph procedure also involved assessment of: (1) the digestibility coefficients based on total fecal collection versus those based on the indirect chromic oxide indicator method, (2) the extent of nutrient leaching when using the Guelph system, and (3) the effect of level of dietary intake and extent of fish handling (due to stripping of feces) on estimates of nutrient digestibility by the Guelph method. No significant differences were found between the complete collection and indicator methods for measuring apparent nutrient digestibilities. Apparent digestibility values calculated using feces collected directly from the fish by both stripping and dissection were consistently lower than the complete collection and indicator values derived using the Guelph system. The stripping technique yielded the lowest digestibility coefficients. Nutrient leaching occurred when the feces had been exposed to sea water for more than 6 h. This was attributed to incomplete entry of feces into the collection column. Fish handling and concomitant reduction of food intake depressed digestibility coefficients for nutrients and energy and increased fish mortality. It is concluded that the Guelph system of fecal collection involving the use of Cr2O3 as the indigestible marker is a potentially reliable digestibility procedure for chinook salmon in sea water. However, care must be exercised to ensure rapid and complete removal of feces to the collection column to minimize nutrient leaching.