In-river movements
We tested relationships between movement ecology and physiology (as body temperatures) in European barbel through implanting fish with acoustic transmitters that enabled their tracking in the study river for over 12 months. These transmitters were also fitted with temperature sensors, thus the data captured for individual fish were the timing of detection on the acoustic receiver (where there were up to 60 receivers located though the river) and their body temperature at the time of recording. To supplement these tagged barbel, common bream were also implanted with the same type of transmitter to generate data on a fish species which have lower variability in individual movement patterns. The main result to date is that in European barbel, consistent differences in the movements, behaviours and habitat use of individuals meant that there were consistent differences recorded in their body temperatures during warm water periods. Some fish revealed preferences for using relatively cool water (<18 oC) whilst other fish, at the same time, were in relatively warm water (>22 oC). This indicates that there is high individual variability in the thermal preferences of individuals, with some preferentially using warmer waters.
Thermal & behavioural phenotypes
We tested for the presence of thermal and behavioural phenotypes of European barbel through laboratory trials. Initial trials tested the suitability of a series of tests for measuring behavioural phenotypes and the consistency of individual behaviours through measuring the fish responses on three occasions. All fish used in trials were marked individually to allow their tracking through experiments. Analyses indicated that the three trials provided a strong measure of the extent of individual variation in the behaviour of these fish and that this variation was repeatable across the three replicates. These trials were then applied to quantifying the behavioural and thermal phenotypes of European barbel by firstly exposing a new set of individuals to the behavioural trials and then measuring their thermal phenotype. Once the fish had completed the behavioural trials, they were used in a ‘shuttle-box’ system, in which individuals are exposed to two linked chambers and can move freely between them24. Each chamber starts at different temperatures (e.g. 17/ 22oC); as the fish moves between the chambers, their water temperatures are altered until equilibrium is reached across both chambers, which indicates the individual’s temperature preference.
Our objectives had then planned to use these fish within experimental river channels to test how the measured phenotype diversity is expressed in natural conditions. However, time lost in the project due to Covid-19 restrictions (lock-down between January and April 2021) meant this could not be completed. Instead, the time when access was lost to experimental facilities was used to complete a systematic review and meta-analysis on the extent of behavioural thermoregulation in freshwater fishes. The results revealed that across the literature, while behavioural thermoregulation is a feature of many fish populations, this has primarily been measured as a response to warming temperatures by salmonid fishes, with most studies completed in North America. The extent of the temperature difference between these habitats decreased with increasing latitude, with juvenile and non-migratory fishes tolerating greater temperature differences than adult and anadromous. Median temperature differences between the warm water and cool water refuge was 3.8oC.
