Summary
The consequences of stress on the behaviour of wild creek chub Semotilus atromaculatusoutside the reproductive period were studied using a single intra‐coelomic injection of cortisol, suspended in coconut butter, to experimentally raise plasma cortisol levels. Behaviour between cortisol‐treated, sham‐treated (injected with coconut butter) and control S. atromaculatus was compared in a mesocosm system, using a passive integrated transponder array, and in a natural stream system (excluding shams), using surgically implanted radio transmitters. While laboratory time‐course studies revealed that the cortisol injection provided a physiologically relevant challenge, causing prolonged (c. 3 days) elevations of plasma cortisol similar to that achieved with a standardized chasing protocol, no differences in fine‐scale movements were observed between cortisol‐treated, sham‐treated and control S. atromaculatus nor in the large‐scale movements of cortisol‐treated and control S. atromaculatus. Moreover, no differences were observed in diel activity patterns among treatments. Differential mortality, however, occurred starting 10 days after treatment where cortisol‐treated S. atromaculatus exhibited nearly twice as many mortalities as shams and controls. These results suggest that, although the experimental manipulation of cortisol titres was sufficient to cause mortality in some individuals, there were compensatory mechanisms that maintained behaviours (i.e. including activity and movement) prior to death. This study is one of the first to use experimental cortisol implants outside a laboratory environment and during the non‐reproductive period and yields insight into how wild animals respond to additional challenges (in this case elevated cortisol) using ecologically meaningful endpoints.
Methodology
Sampling of S. atromaculatus was performed under an Ontario Ministry of Natural Resources Scientific Collection Permit (Licence Number: 1061994). Semotilus atromaculatus were processed with adherence to the guidelines set out by the Canadian Council on Animal Care as issued by Carleton University (B10-9). All S. atromaculatus were captured using standard electrofishing techniques, with a battery-powered backpack electrofisher (Halltech Aquatic Research Inc.; model Ht-2000; www.htex.com). Pulsed-DC electrofishing is considered to be a safe methodological tool for studying S. atromaculatus and results by Gatz & Linder (2008) suggest this practice is unlikely to have meaningful biological effects on S. atromaculatus condition, growth and movements. As S. atromaculatus generally spawn in the spring, beginning at temperatures of 12·8◦ C (Scott & Crossman, 1973), sampling was undertaken outside this range to avoid any confounding effects of the reproductive period. Semotilus atromaculatus used in laboratory and mesocosm studies were transported by lorry in a 66 l cooler, equipped with four battery-powered aerators.
For all experiments, cortisol-treated S. atromaculatus were given a single intra-coelomic injection of 10 mg ml−1 of cortisol (hydrocortisone; Sigma H2882, Sigma-Aldrich; www.sigmaaldrich.com) suspended in coconut butter at 0·005 ml g−1 body mass (Gamperl et al., 1994). Sham-treated S. atromaculatus were given a single intra-coelomic injection of coconut butter at 0·005 ml g−1 body mass. Control S. atromaculatus received no injections. Semotilus atromaculatus were then separately placed in aerated recovery bins, according to their treatment and the experimental protocol. Anaesthesia was only used for implantation of radio transmitters and for cortisol injection of telemetered S. atromaculatus. No anaesthesia was used for PIT tag injection or cortisol (or sham) injection of S. atromaculatus that were not being anaesthetized for transmitter implantation as per Canadian Council for Animal Care Guidelines.