Accumulation and natural disintegration of solid wastes caught on a screen suspended below a fish farm cage

Abstract

A new method consisting of a screen-like device to trap and manage solid waste from below a fish cage is proposed. To examine its effectiveness, a mathematical model was developed to predict the amount of waste and its degradation over time under low-current conditions. It was also used to examine the effects of fish stocking, feed conversion ratio, screen size, mesh size and harvest rate on the total amount accumulated and time required to degrade the waste after harvest. The characteristics of waste and fish feed used to develop the model were experimentally determined as they degraded in a tank of oxygenated 8°C saline water. As the solid waste degraded, the carbon (%) and COD (mg/(Lg dry weight)) remained constant as N (%) increased and C/N decreased. Bacteria degradation consisted of activities related to both mineralization and the physical breakdown of the waste into tiny particles. After 30–40 days in cold and saline water, approximately 50% of the waste matter disappeared from the 3mm mesh screen (p<0.001). The experimental waste degradation rate (kgm−2day−1) increased with increasing specific area of waste (kgm−2) (r2=0.97). Model simulations indicated that staggering fish harvests was the most effective method for reducing waste loads and the period for total waste removal after fish harvest. Future work will focus on the fate within the environment of the tiny particles released by the degradation process and the effect of current on waste erosion rates.