Determining fish size and swimming speed in cages and tanks using simple video techniques

A video-based system which can be used to measure the average fish size in a nonintrusive fashion was developed. It performed well under a wide range of fish behaviours, types, stocking densities, ages and water quality conditions in tanks and sea cages. The design was based on principles of simple stereo geometry, incorporated fish dimensions/weight relationships and took into consideration fish movement to lower system costs. The overall system is durable, cost effective and easy to use. No artificial lighting is required. Approximately 20 min of continuous video footage are needed, and depending on the fish-image density, 30-60 min of labour are required to select suitable fish images and determine accurately the average fish size of a population. On still fish, average fish mass was measured to within 0.5%. On moving fish in sea cages and tanks with known individual salmonid masses. no significant difference was evident between measured and actual average mass. Testing the video system against the crowding/dip netting method in trout hatchery tanks containing from 7000 to 20000 fry showed the video system to be as accurate as the crowding/dip netting method for determining fish mass provided the cameras were positioned in apparently preferred fish swimming locations. The new video method can be used to economically record and study behaviour, swimming speed and size distributions under a wide range of culture conditions. (C) 1997 Elsevier Science B.V.