Summary
The hypothalamic‐pituitary‐interrenal (HPI) or stress axis in teleost fishes produces their primary glucocorticoid, cortisol. Although generally an adaptive response, prolonged HPI axis stimulation can impair organismal performance. Previous work has shown that stressed teleosts have higher mortality to predation than unstressed conspecifics, suggesting a role for HPI axis in modulating predator–prey interactions. Our current study investigated whether elevated cortisol levels altered the predation rate of a wild teleost fish, the bluegill sunfish (Lepomis macrochirus). Wild juvenile bluegill were given intraperitoneal implants of cocoa butter (i.e., sham), or cocoa butter containing cortisol or cortisol and the glucocorticoid receptor antagonist RU486. After 24 hr, fish were tethered along the bottom of the lake and their survival under natural predation was recorded following 24 hr. A subset of fish was used to validate the efficacy of cortisol implants in this setting. No treatment effect on survival was observed, suggesting that elevated cortisol has minimal involvement in mediating predator–prey interactions in this context. However, experimental fish may have demonstrated resiliency to physiological perturbations owing to the relatively acute duration of our experimental series, and negative effects might be manifested over a more chronic period.
Methodology
Fish collection, housing, and implantation procedures
Juvenile bluegill sunfish (mean ± standard error (SE), wet mass = 5.4 ± 0.1 g) were seized from shallow nearshore habitats in Lake Opinicon (44.5590°N, 76.3280°W; Ontario, Canada) during the months of May and June of 2018 (under Ontario Ministry of Natural Resources and Forestry permit #1089028). Lake Opinicon is a shallow and weedy euphotic lake with a diverse predator community including teleost, avian, and mammalian piscivores (Keast, 1978; Keast, Harker, & Turnbull, 1978). Seining was the preferred capture method to avoid possible selection of specific behavioral phenotypes (Gutowsky, Sullivan, Wilson, & Cooke, 2017; Wilson, Binder, McGrath, Cooke, & Godin, 2011), thus ensuring a random collection of fish in our samples. Following capture, fish were sorted by hand to select individuals of 5–8 cm total length, because previous work on sunfish predation used similarly‐sized individuals with success (Gotceitas & Colgan, 1989). Fish were promptly transported to the nearby Queen's University Biological Station (Elgin, ON, Canada) where they were held in large, outdoor flow‐through tanks (~435 L) supplied with fresh lake water (19.7 ± 0.3°C; >90% O2 saturation) and exposed to natural sunlight and darkness. Animals were held under these conditions for 24 hr before implantation. All experimental series were conducted in accordance with the guidelines set by the Canadian Council on Animal Care under administration of the Carleton University Animal Care Committee (AUP# 104262 and 106523).
Bluegill were implanted with cocoa butter containing either cortisol (hydrocortisone 21‐hemisuccinate; 25 mg kg−1 body weight [BW]), or cortisol (25 mg kg−1 BW) with RU486 (Mifepristone; 50 mg kg−1 BW), or were implanted with cocoa butter alone (5 ml kg−1 BW) as a sham control. Implants were deposited into the peritoneal cavity through the ventral surface of the fish using a 1 ml syringe and an 18 G needle. The use of cortisol‐treated cocoa butter implants has been validated for use in teleost fishes (Gamperl, Vijayan, & Boutilier, 1994) and has been widely used in centrarchid fishes (e.g., Algera et al., 2017; McConnachie, O'Connor, Gilmour, Iwama, & Cooke, 2012; O'Connor et al., 2011; Zolderdo et al., 2016). The cortisol dosage used here was based on prior work (Algera et al., 2017; Brown, MacLatchy, Hara, & Eales, 1991). Cocoa butter solutions containing cortisol were prepared following the methods of Hoogenboom et al. (2011). The RU486 dose (50 mg kg−1 BW) was based on prior work, where RU486 has been used extensively as an antagonist of GRs (Bernier, Lin, & Peter, 1999; Lawrence et al., 2017; Vijayan, Reddy, Leatherland, & Moon, 1994). We did not include a no‐treatment (i.e., no implant) control group in our experimental design because we were primarily interested in the relative effects of exogenous cortisol rather than how stressors associated with fish handling may have affected predator‐induced mortality. Furthermore, prior work indicated that plasma [cortisol] is comparable between sham‐treated and non‐treated bluegill sunfish (McConnachie et al., 2012). Following implantation, fish were held in indoor, flow‐through tanks (~211 L; T = 20.9 ± 0.4 °C; O2 saturation > 90%) for 24 hr to ensure that the pharmaceutical agents reached biologically active concentrations in the blood before exposure to predation risk, as in Lawrence et al. (2017). Fish were not fed at any time while in captivity.