PIT tagged individual Atlantic salmon registrered at static depth positions in a sea cage: Vertical size stratification and implications for fish sampling

Individual dynamics within salmon sea cages are poorly understood. Large inter- and intra-individual variations in swimming depth and higher average body weight deeper in the cage have been observed. Sampling of fish for inspection purposes and estimates of body weight distributions based on automatic measures at a limited depth interval may thus be skewed. The present study investigates how 335 randomly PIT tagged Atlantic salmon of 3.4 ± 0.96 kg (bled weight, mean ± SD) swam near square PIT antennas (0.6 m) fixed at either 5 or 9 m depth within a 14 m deep cage holding a total of 3750 individuals. The individual variation in registration frequency was large, with 76 individuals never registered and 12 individuals registrered 30 times or more. Larger individuals were much overrepresented at 9 m depth, resulting in 8.5% overestimation of average weight at this depth when repeated registrations were not accounted for. No overrepresentation of any size class was found at 5 m depth. Half of the individuals registered at 9 m depth were also registered at 5 m depth, and 82% of the individuals registered at 5 m depth were also registered at 9 m depth. We interpret the result as fish of all sizes using a wide depth range, but larger individuals spending more time at deeper water than smaller individuals. Therefore, size estimates at a limited depth interval should be avoided.

► Caged salmon were registered when swimming near PIT antennas at 5 and 9 m depth.
► Registration frequency increased with body weight at 9 m depth.
► Mean weight was overestimated when repeated registrations were not accounted for.
► We interpret the result as time spent at deeper water increases with body size.
► Size dependent depth preferences should be considered when estimating mean size.