A non‐invasive method to assess the impact of electronic tag insertion on stress levels in fishes

The present study investigated, for the first time, a non‐invasive technique for monitoring cortisol levels in fishes that had been implanted with electronic transmitters. This technique involved the measurement of cortisol released into the water by individual fish. Carp Cyprinus carpio and roach Rutilus rutilus that had been tagged with miniature dummy acoustic transmitters responded to the surgical implantation of the tags with an immediate (1 to 4 h) increase in cortisol concentrations in the water. These amounts of cortisol were higher than in 'handled only' and 'handled and anaesthetized' groups. Estimated cortisol release rates for both species correlated well with the direct measurements in the water. In both species, water cortisol concentration and cortisol release rates returned to pre‐stress levels within 12 h of tag‐insertion, and remained low for the remainder of the experiment. The calculation of cortisol release rates highlighted a difference of a factor of about four between peak release rates in carp and roach. There was no evident long‐term cortisol stress response to the presence of a tag in the body cavity in either species.