Excretory Problems in Fresh-water Fish Culture in circulating Systems

Abstract

Increasing interest in intensive fish culture in tanks has led to a need for reliable estimates of excretory productivity by fish, especially with regard to ammonia, to facilitate the design of efficient water treatment equipment for recycling or effluent disposal. Two main aspects of this are examined: (a) realistic assessment of excretory productivity in the limited but multivariable tank situation;(b) criteria for allowable persistence of dissolved excretory products in closed systems, consistent with maintenance offish growth. Assessment of ammonia productivity was investigated using rainbow trout. Laboratory facilities were designed and built, and an approach to a fuller understanding of the problem worked out using multivariate analysis, resulting in a multiple regression model which satisfactorily described the experimental situation. Under experimental conditions, the specific excretory rate of young rainbow trout could be related to environmental conditions by the following equation (parentheses denote 95% confidence limits for one observation:)- y = 5.7522 + 0.0427x1 + 0.0002x1 2 + 0.0141x22 + 0.4165x2x3 - 0.0014x1x4 (± 4.9783)   - where y = is the specific excretory rate, x1 = the fish number, x2 = temperature, x3 = is mean free path, and x4 = stocking (mass/volume). The implications of the technique are discussed, and it is proposed that the method used to derive the relationship is worthy of further application and refinement. Under similar experimental conditions, a preliminary study was conducted on the growth tolerance of trout to simulated recycled fish effluent, and a basis was provided for future investigation thereof. Small differences in growth patterns were found between treated and control fish for two strengths of simulated effluent, lending limited support to the concept of growth stimulation at low concentrations of otherwise apparently toxic substances. The details and implications of this are discussed. The overall context of the work is discussed and related to the concept of stress in fish culture. Suggestions are made for future experiments.