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

Measuring the water temperature fish actually experience in the wild is tricky. Typically, fish thermal experience in the wild is estimated by deploying water temperature loggers throughout a study area. Metrics such as mean, minimum or maximum stream temperature are then calculated and are related to information about the fish that occupy that study area, data that include species presence/absence, population size, or individual fish growth rates. However, this method isn't perfect. Even small amounts of ground water input can provide patches of coldwater habitat in otherwise warm streams that may not be accounted for. What's more, this uncertainty can arise even when a dense web of stream temperature loggers are placed over a small area.

The recent miniaturization of implantable temperature recording tags has made measuring the water temperatures fish experience in the wild possible. There's a catch though: the size of the fish might affect how quickly these temperature recording tags respond to water temperature changes. In this NE CASC laboratory study led by Matthew O'Donnell and Amy Regish, researchers implanted 20 brook trout of varying sizes with temperature recording tags and subjected them to water temperature changes. The team found that in larger fish, the temperature recording tags took longer to detect rapid temperature changes (±8°C in less than 2-seconds) than in smaller fish. In fish exposed to a slower rate of temperature change (2 °C hour^-1), implanted tags did not show a response lag, regardless of fish size.  This means implanted temperature recording tags might underestimate the temperatures fish experience when they quickly move between warm and cold areas.

Anthropogenic factors that induce stream warming-- such as dams, impervious surfaces, and deforestation, coupled with climate related alterations in flow and temperature, are likely to be more impactful in small streams where coldwater fish, such as brook trout, occur.  Broad scale persistence models predict that as stream temperatures increase, the probability that brook trout will occur in warming streams decreases. However, those regional models do not account for the possibility that brook trout can occupy cold groundwater inputs in warming streams. Miniaturized, implantable temperature recording tags provide a tool to measure the importance of cold, ground water input to brook trout persistence. If fish can effectively seek out and occupy coldwater patches, then some warm streams that may otherwise be overlooked for protection as a coldwater fishery resource, may be suitable for brook trout. Understanding the limitations of temperature recording tags is crucial to determining the utility of the data it produces and its ability to accurately measure fish thermal experience in the wild.