The influence of fish on the mooring loads of a floating fish farm

Abstract

The influence of fish on the mooring loads of a floating fish farm is studied numerically and experimentally. Two experimental series were conducted. One case was model tests with artificial fish. Nine rigid fish models with total volume of 2.5% of the fish cage at rest were placed inside the net cage without touching the net and towed with the net cage. The other case was live fish experiments in waves and current where more than 800 salmon of length 16cm occupied about 2.5% of the fish cage volume at rest. The flow-displacement effect of a rigid fish in current is simulated by a potential-flow slender-body theory. Viscous wake effects are added. The displacement flow is clearly more important than the viscous wake flow. Both the numerical simulations and the model tests with rigid fish in current show that the fish influence on the mooring loads of the fish cage is less than 3% of the mooring load without fish. However, the measured mooring loads with live fish in current are between 10% and 28% larger than without fish. The reason is contact between the fish and the net cage. Accounting for the latter fact in the load model by changing the solidity ratio of the net in the contact area gave reasonable numerical predictions. The experiments in waves and combined waves and current showed also a non-negligible influence of the fish on the mooring loads. The waves caused some of the fish to go to the bottom of the net probably due to that the fish was uncomfortable in the wave zone. The consequence in changing the net configuration and thereby the mooring loads is similar as adding a bottom weight to the net.