Effect of different acidified wet feeds on protease activities in the digestive tract and on growth rate of rainbow trout ( Salmo gairdneri Richardson)

Abstract

Acidified feeds treated with hydrochloric, formic or sulphuric acid were fed to rainbow trout ( Salmo gairdneri Richardson) to test the effects on protease activities, growth and feed utilization. The acidified feeds were prepared by mixing a frozen stored control feed with fish silage made from the same feed acidified with 2.5% ( w/w) of hydrochloric acid, formic acid or sulphuric acid, respectively. To the silages was added 0.5% ( w/w) propionic acid as a fungicide. The experimental feeds contained 0, 40, 60 and 100% of the fish silages to give final concentrations of, respectively, 0, 1.0, 1.5 and 2.5% ( w/w) of each acid tested. Hydrochloric acid had no apparent effect on growth or proteolytic activities in any part of the digestive tract. Formic acid at all levels tested seems to have a depressive effect on both growth and proteolytic activities. Sulphuric acid showed similar effects, with the exception that the protease activity in the stomach was not depressed. Acidified feeds showed lower feed utilizations than the control feed, this being most pronounced for the formic acid silage.