Summary
The glucocorticoid (GC) stress response is thought to be an individual trait associated with behaviour and life history strategies. Studies exploring such relationships typically assume measured hormone values to be repeatable within an individual. However, repeatability of GCs has proven variable in wild animals and underlying reasons remain unknown. We assessed individual repeatability of circulating stress-induced cortisol, the primary GC in teleost fish, and glucose concentrations in a wild teleost fish held under consistent laboratory conditions. We also tested the hypothesis that the magnitude of intra-individual variability in stress-induced cortisol concentrations (“cortisol variability”) is influenced by body condition. Wild-caught bluegill sunfish (Lepomis macrochirus) were subjected to repeated standardized stressors and blood sampled (3 times over 6 days) once cortisol concentrations peaked. Various indicators of fish condition, both whole body and physiological, were also measured. Overall, stress-induced circulating cortisol concentrations were repeatable but stress-induced glucose was not. Cortisol variability was related to Fulton's condition factor and size (eviscerated mass) where smaller fish in poor condition exhibited increased cortisol variability. The findings have implications for the interpretation of studies that examine correlates of GC concentrations as they suggest consistency in stress responsiveness is influenced by factors such as size and condition
Methodology
Research was conducted at Queen's University Biological Station in south-eastern Ontario, Canada (44°34′N, 76°19′W) and fish were captured from Lake Opinicon. In this lake, bluegill (L. macrochirus) begin forming breeding colonies in mid-May and breed through to July (Cargnelli and Neff, 2006). While fish were showing signs of sexual maturity and colonies had begun to form at the time of collection, fish used in the current study had not yet spawned. All fish were collected in mid-May 2010 by rod and reel angling using barbless circle hooks baited with a small piece of earthworm (Lumbricus sp.). Fish were landed within 10 s, immediately placed in a 50 Litre coolers filled with lake water, and transported by boat to the laboratory. Such rapid collection is common for sunfish (Cooke et al., 2003) and generally, the greater the duration of an angling event, the greater the magnitude of physiological disturbance and recovery time required (Cooke and Suski, 2005). If total handling time from hooking exceeded 10 s or if a fish sustained any injury as a result of hooking or handling, they were not used in the experiment and released. No fish was held in the cooler for more than 2 h from capture to relocation to the laboratory. Once in the laboratory, fish were held in a 100 Litre fibreglass tank for 24 h following capture to allow for recovery. Holding tanks were exposed to ambient sunlight and a flow-through system provided continuous fresh water from the lake. This short time period was chosen as we wished to ensure appropriate recovery while minimizing total holding duration. Previous physiological assessments of bluegill angled from Lake Opinicon validate that 24 h is a sufficient recovery time. Wilson et al. (2011) quantified baseline cortisol concentrations of 92 ± 108 ng·mL− 1 (mean ± SD) following a 24-h holding period in tanks identical to those used in the presented study. These values are similar to those obtained from another study obtaining baseline concentrations 2 min following capture (47.7 ± 34 ng mL− 1; range of 0.3 to 552.9 ng mL− 1; McConnachie, 2010). Generally, bluegill exhibit large inter-individual variability.
To elicit a stress response, all fish were exposed to a standardized stressor. This consisted of 3 min of air exposure while held in a small, covered container lined with moistened padding to prevent desiccation and injury. Immediately following air exposure, fish were held individually in 20 Litre buckets regularly refreshed with lake water. This artificial stressor was employed to induce maximal elevations of cortisol concentrations. Two experiments were conducted; an initial study evaluated the time course of cortisol elevation and repeatability was assessed on a separate group of fish.