Measurement of fish size in atlantic salmon (salmo salar l.) cages using stereographic video techniques

Fish images were captured using stereo cameras and analyzed using a previously developed fish-sizing computer programme (FICASS) to examine the effect of sampling position within a swimming aggregation on the mean fish mass within commercial-sized cages. The resulting mean values were compared with results obtained by other methods such as physical sampling by seining/dip netting and/or harvest data. Significant differences occurred which were associated with different feeding, sorting and stocking conditions. Video sampling results generally agreed with physical sampling results if fish were swimming together near the water surface, or if fish had been previously size-sorted. Particular swimming formations and unfed fish were associated with significant methodological differences between FICASS and physical sampling. FICASS results agreed with harvest data if unfed fish had been previously size-sorted or non-size-sorted fish had been fed, and if fish swam in an apparently uniformly-spaced aggregation.