Influence of retention and air exposure on physiology and behaviour of common carp (Cyprinus carpio L.): a field study

Specialised angling for large size common carp (Cyprinus carpio L.) is often conducted as voluntary catch-and-release angling involving various handling measures. Among the most prominent handling measures is retention in carp sacks for the purpose of photographing the fish prior to release. When carp are captured during unfavourable light conditions for photography, they are retained until the light conditions have improved. Photography is also associated with extended air exposure. So far, little is known about potential sub-lethal and lethal impacts associated with carp sack retention and air exposure on large size common carp. To address these issues, a field study was conducted from September to December 2007 at Dow’s Lake in Ottawa, Ontario, Canada. Carp were captured by angling and allocated to a control group and five treatment groups (N = 10 each). Three treatments involved retention in carp sacks for periods up to 9 h. One treatment received air exposure for 10 min following capture, and a final treatment involved air exposure for 10 min following 9 h of retention. All fish were blood sampled to assess physiological changes, and radio transmitters were attached to monitor short-term behaviour and long-term fate of carp post-release. Retention in carp sacks induced a prolonged physiological stress response in carp indicated by elevated blood plasma cortisol concentrations, and hyperglycaemia in long-term retained fish. Blood plasma lactate levels decreased during retention, suggesting recovery from playing. Observed blood plasma ion changes were minor and likely a result of playing. Long-term retention was also found to be associated with tissue damage. Physiological changes were reflected by impaired post-release behaviour in long-term retained fish indicated by reduced minimum displacement. However, recovery was fast and no mortalities occurred. Air exposure following capture did not result in physiological changes relative to fish captured only (i.e. control), whereas exposing carp to air following retention increased blood plasma lactate levels relative to retained fish. Air exposed carp required more time to leave the release site, andminimum displacement between 31 min and 60 min post-release was reduced in fish air exposed following capture compared to control fish. However, behaviour normalised quickly, and air exposure did not result in mortality. From a practical perspective, the results indicate that despite being sub-lethally affected by retention and air exposure, carp are able to cope with these stressors. From a fish welfare perspective, however, it is recommended to avoid retention in carp sacks and minimise exposure to air.