Capture heats up sharks

Abstract

Catch-and-release fishing is an important component of ecotourism industries and scientific research worldwide, but its total impact on animal physiology, health and survival is understudied for many species of fishes, particularly sharks. We combined biologging and blood chemistry to explore how this fisheries interaction influenced the physiology of two widely distributed, highly migratory shark species: the blue shark (Prionace glauca) and the tiger shark (Galeocerdo cuvier). Nineteen sharks were caught by drum line or rod-and-reel angling; subcutaneous body temperature measurements were taken immediately upon capture, with six individuals also providing subsequent subcutaneous body temperature measurements via biologging as they swam freely for several hours post-release. We found that short-term capture caused shark body temperature to increase significantly and rapidly, with increases of 0.6°C-2.7°C for blue sharks (mean, 1.2 ± 0.6°C) and 0.5°C-0.9°C for tiger sharks (mean, 0.7 ± 0.2°C) and with capture-induced heating rates of blue sharks averaging 0.3°C min^-1 but as high as 0.8°C min^-1. Blue shark body temperature was even higher deeper into the white muscle. These heating rates were three to eight times faster than maximum rates encountered by our biologging sharks swimming through thermally stratified waters and faster than most acute heating experiments conducted with ectotherms in laboratory experiments. Biologging data showed that body temperatures underwent gradual decline after release, returning to match water temperatures 10-40 mins post-release. Blood biochemistry showed variable lactate/glucose levels following capture; however, these concentrations were not correlated with the magnitude of body temperature increase, nor with body size or hooking time. These perturbations of the natural state could have immediate and longer-term effects on the welfare and ecology of sharks caught in catch-and-release fisheries and we encourage further study of the broader implications of this reported phenomenon.

Keywords: body temperature; capture; catch-and-release; physiology; shark; thermal ecology.

Figure 1

Body temperature elevation of tiger sharks and blue sharks recorded immediately upon capture (panels A and B) and average elevation of body temperature, during a 30-minute period under natural conditions several hours after release (panels C and D). Dashed black lines indicate a 1:1 identity line. (Blue shark image created by Ignacio Contreras and reproduced under the Creative Commons Attribution 3.0 Unported license: https://creativecommons.org/licenses/by/3.0/legalcode).

Figure 2

Average heating rate (°C min⁻¹) for each blue shark during capture period (red circles; n = 15) and average heating rate every 30 sec for two biologged blue sharks (BSCC and BS09) during post-release phase (n = 4205). Capture datapoints (red circles) are placed arbitrarily to left of x-axis to illustrate their collection before the beginning of the time series post-release.

Figure 3

Water temperature measurements (triangles) and body temperature measurements (circles) taken at 2 and 4 cm penetration into the dorsal musculature of four blue sharks.

Figure 4

Body temperature, water temperature and depth data for tiger sharks T1 (top panel) and T4 (bottom panel) from moment of release. A 500-point moving average smoother was applied to all data.

Figure 5

(A–C): (A) Temperature differential (ΔT) against hooking time for blue sharks with no significant relationship. (B) Temperature differential (ΔT) against log (body mass) with no significant relationship. (C) Temperature differential (ΔT) against lactate upon capture for blue sharks with no significant relationship.