Effects of salinity and diet on the nutritional physiology and alimentary canal histology of the Rainbow trout

Abstract

In rainbow trout (Salmo gairdneri Richardson) which had been acclimatized to the experimental salinity and temperature for at least 14 days, food intake was maximal in the intermediate salinities of 13.0 and 28.0 p.p.t. (parts per thousand), less in fresh water and 7.5 p.p.t. and minimal, by a statistically significant margin, in 32.5 p.p.t. sea water. There were marked dat-to-day fluctuations in food intake. Upon abrupt salinity increases of 7.5 or 13.0 p.p.t. , there were decreased in growth rate which were related to decreases in food intake. Recovery of food intake and growth rate to pre-increase levels was complete within 14 days. Absorption efficiency, in terms of total dry matter, total energy, and total nitrogen, was negatively related to salinity. Total nitrogen was absorbed considerably more efficiently than either energy or dry matter. Conversion efficiency (K1 and K2) was estimated, also in terms of dry matter, energy and nitrogen, in trout of the 0+ and 1+ year-groups weighing from 50 to 150g. Between 0.0 and 28.0 p.p.t. , there was little effect of salinity on conversion efficiency. Between 28.0 and 32.5 p.p.t. , however, there was a significant decline in food conversion efficiency. The salinity: conversion efficiency relationship was not significantly influenced by fish weight in the range studied. Dry matter and energy conversion were significantly lower than nitrogen conversion efficiency. Neither food intake nor conversion efficiency was significantly affected by dietary sodium chloride supplements of up to 8.5% of the total ration. Over the range of rations from 4% to 38% of dry weight per week, there was a rectilinear relationship between ration and weight increase (% increase in dry weight per week). From this relationship can be estimated the weight maintenance requirement, and the rate of weight loss during fasting. In the same experiment, conversion efficiency (K¬1) increased from 0.00 at 4% dry weight per week (weight maintenance) to 0.19 at 38%, according to the equation K1 = 0.2115 - 0.7699/x. Live weight loss between days 7 and 48 of fasting could be described by a straight line. Trout maintained in 32.5 p.p.t. sea water showed a significantly greater weight loss than those in salinities of up to 15.00 p.p.t. In the liver and white epaxial muscle, n-hexane-extracted lipids fell significantly during fasting. In the white muscle, there was a corresponding slight increase in water content (except in 32.5 p.p.t. sea water). The volume of the gall bladder contents increased during fasting. In the alimentary canal with its associated adipose deposits, there was a very highly significant correlation between lipid and water content. There was a negative relationship between salinity and the density of distribution of mucus cells in the intestinal and rectal epithelia of acclimatized trout. The luminal cross-sectional area of the intestine and rectum, and the height of the intestinal villi tended to increase with salinity. The alimentary canal showed no evidence of histological degeneration after 48 days of fasting in various salinities.