Collection and maintenance

Diploid rainbow trout fry (mean mass ± SE: 0.418 ± 0.0325 g), diploid brown trout fry (0.602 ± 0.0563 g), and triploid brown trout fry (0.470 ± 0.0311 g) were acquired from Movanagher Fish Farm, Northern Ireland. Each fish type was acquired in a single batch of 100 fish approximately 8 weeks after the official hatch date of each type, between February to April 2019, and held in the laboratory for 3–5 days prior to any behavioural testing. Four fish were tested on each day up to 12 days after the testing began, with 6 days of testing in total per fish type (giving 6 × 4 = 24 individuals tested per fish type). We chose to age-match rather than size-match to test the fry at the same developmental stage with the same experience, however, we also used mass of fish in our statistical modelling of all behaviour variables to control for any size-effects. This method in which fish size is not controlled has been used in other studies comparing personality between species⁷¹. Fry were moved to the Queen’s University Belfast Medical Biology Centre, where they were held in a 12 °C laboratory with a 12:12 light regime, with complete water changes every two days. Fry were housed in two holding tanks (39.5 × 25 × 27 cm) with approximately 50 fish in each tank for the duration of the experiment and were fed ad libitum twice daily with INICIO Plus 0.5 mm food pellets from the fish farm. Morning feeding took place after four fish had been selected for the day’s experiments so that fish used in experiments were starved for approximately 15 h from the previous day.