Thermal transfer rate is slower in bigger fish: How does body size affect response time of small, implantable temperature recording tags?

The recent miniaturisation of implantable temperature recording tags has made measuring the water temperatures fish experience in the wild possible, but there may be a body size-dependent delay in implanted tag response time to changes in external temperature. To determine whether fish body size affects the response rate of implanted temperature tags, we implanted 20 Salvelinus fontinalis (127–228 mm fork length (FL), 15.1–120.4 g) with temperature recording tags and subjected them to rapid temperature changes (±8°C in less than 2 seconds) in the laboratory. We found that thermal transfer rates, and the lag in temperature tag response rate, was positively correlated with fish size, but the direction of temperature change (colder or warmer) had no significant effect. In fish exposed to a slower rate of temperature change (2°C h⁻¹) implanted tags did not show a response lag. Understanding the limitations of this important technology is crucial to determining the utility of the data it produces and its ability to accurately measure fish thermal experience in the wild.