We compared baseline and maximal cortisol concentrations between predator exposure and prey blood samples in pumpkinseed Lepomis gibbosus, captured using a standardised fishing event underneath osprey Pandion haliaetus nests and away from osprey nests. We did not detect differences in cortisol or glucose between sites. These findings suggest that predictable sources of predation risk may not confer stress‐related costs in teleosts.
Ospreys are highly mobile fish predators and fishing grounds can be located both away from their nests (Alerstam et al., 2006) and within meters of their nests (links to videos in Appendix S1a–c). From 14 to 24 May 2015, we located three active coastal osprey nests (each containing a breeding pair), which were selected alongside paired control sites within 1 km of each other, in Lake Opinicon, a mesophotic lake located in eastern Ontario (44 33' 56.0'' N, 76 19' 23.6'' W). Nesting sites were located at the top of eastern white pine trees Pinus strobus situated on small islands separated by <10 m from the outer perimeter of the lake (except for one site which was c. 100 m). Control sites mimicked the habitats from nesting sites (i.e., littoral edges) but were situated outside the visual detection range of the nesting sites. The average distance between nest and control sites was 0.51 km and the three nest-control sites covered a 5.9 km northeast transect covering nearly the entire area of the lake. While not recorded, water temperatures generally range between 15 and 20 degrees C and are fairly unstable during this time of year.
Adult L. gibbosus were caught at all sites using standardised fishing gear: 2 m long medium action fishing rods and reels with 2.7 kg break-strength monofilament fishing line and size 6 non-offset small J-hooks baited with live worms. Once hooked, the following experimental protocol was executed for each fish: an initial blood sample (baseline) of c. 0.25 ml was collected by caudal puncture between 3 and 6 min after hooking using 1 ml pre-heparinized syringes with 25 G, 3.8 cm needles (BD; www.bd.com). A study by Lawrence et al. (2018b) indicated that baseline cortisol titres can be obtained if L. gibbosus are sampled within 8 min of hooking. However, variation in plasma cortisol titres begins to manifest in the sample as one approaches this threshold. As every attempt was made to capture the sample within this timeframe, our samples probably reflect values that are likely to be indicative of a baseline state. In support of this, our baseline cortisol values ranged between 1.28 and 49.9 ng ml-1, which is similar to resting pre-stress levels found in other teleosts, including centrarchids (Barton & Iwama, 1991). If blood was not readily available, fish were immediately released overboard. Fish were then subjected to an additional period of 3–5 min of air exposure (inclusive of time needed to take the initial blood sample), a protocol that reliably induces physiological stress in L. gibbosus (Cook et al., 2012; Gingerich et al., 2007). All fish were then held in a 20 l cooler with fresh lake water for 45 min from when they were landed, allowing elevated glucocorticoid hormone concentrations to be achieved (following Cook et al., 2012). Fish were then removed and a second blood sample was collected as above. For both baseline and stressed blood samples, blood glucose levels were measured immediately using a small aliquot (0.05 ml) of whole blood with a hand-held portable glucose meter previously validated for use in fish (Accu-check glucose meter, Roche Diagnostics; www.accu-chek.com; Stoot et al., 2014). Remaining blood samples were then kept on ice (>2 h) until further processing onshore.