Fish behavior data

We conducted the experiment from 30 May to 4 June 2018, from 8 to 12 June 2019 and from 16 to 20 August 2019. Temperature during these trials was 14.3+1.0 oC in 2018 and 10.6+0.3 oC in June 2019 and 15.4+0.1 oC in August 2019. The experimental set-up involved two sets of four white opaque plastic tubs. The tubs used in June 2018 and 2019 (25x35x15 cm) were smaller than those used in August 2019 (37x54x21 cm ) to account for differences in fish size (June: 43+5 mm in 2018 and 49+9 mm in 2019; August: 80+11 mm) (Kuggis, IKEA, Sweden). Four plastic tubs were placed in each of two large fiberglass tanks (110x110 cm; 150x150 cm) with a 2.5 cm deep continuous flow of circulating water to keep the water cool. Water depth in the tubs was 5 cm for the experiments conducted in June and 8 cm in August when the trout were larger. Water was supplied from the nearby River Imsa, and the water temperature was measured daily. Two cameras (Canon XA10; Canon Inc. Tokyo, Japan) were attached to tripods and one camera was placed over each tank with its four tubs. The cameras were placed so they allowed observations of the fish from above. Simultaneous video recordings were carried out in the eight tubs (two tanks), with one trout in each tub. Thus, we conducted one replicate of each of the eight treatments (two incubation temperatures and four crossings) on each occasion. The tub in which each treatment was tested was altered between occasions, using a randomized block design. In 2018, 15-17 replicates (occasions) were performed for each treatment. In 2019, 22-24 replicates were performed for every treatment in both June and in August. A few trials were deleted due to camera malfunction or because we accidentally scared the fish during filming, and in two cases fish jumped out of their tubs into adjacent tubs. <br> Trout were carefully netted from their holding tanks and placed in one of the experimental tubs, and were subsequently video recorded for at least 21 minutes to score the undisturbed swimming pattern of each trout. Thereafter, mirrors were placed on one short side of each tub, alternating between tubs. Trout were subsequently video recorded for an additional minimum of 21 minutes to assess their aggression towards their mirror image (Johnsson et al. 2003). Directly after, the fish were removed from the experimental arenas and measured for total length. To minimize bias, the observer was unaware of the treatments assigned to each tub when behavior was quantified from the films. <br> <em>Analysis</em>- For the open field test, we quantified (1) the percentage of time the fish spent swimming and (2) the time it took them to initiate swimming (defined as when the fish moved at least half a body length per second). For the mirror test, we quantified (3) the percentage of time the fish spent within 5 cm of the mirror, regardless of activity (swimming, motionless). In addition, we quantified the (4) time spent swimming within 5 cm of the mirror. When we analyzed the films, we did not look at the entire period. Instead, we quantified behaviors during the following time intervals from the start of a trial: 0-1 min, 5-6 min, 10-11 min, 15-16 min and 20-21 min.