Experiment 1. 'Figure_1.jpg' Datasets 'Exploration2018.csv' & 'Exploration2020.csv' In experiment 1., we studied post-release exploration of the hybridized salmon in four circular seminatural streams (column 'Pool'). All fish involved were identified using passive integrated transponders (column 'PIT'). Movement, or explorative behavior, of the fish in the seminatural streams was followed using four evenly spaced antennae capable of reading the fish's PIT-tags. This movement data has been summarized in the columns 'Descending' (movement with the water stream, i.e. downstream movement), 'Ascending' (upstream movement) and 'Total' (Total movement, i.e. Ascending & Descending). Units in these columns are rounds that the fish swam in the circular streams per hour (column 'Time.of.Day' and 'Pseudotime' from the start to the end of the experiment). Centerline length of the circular streams was 26.15 m and therefore the rounds can be converted to meters by multiplying by 26.15. The experiment consisted of two separate seven-day (column 'Date') trials, conducted on 11–18 December 2018 ('Exploration2018.csv' ) and 16–30 March 2020 ('Exploration2020.csv'). Experiment 2. 'Figure_2.jpg' Dataset 'Stress_BernoulliData.csv' The fish used in experiment 2 were of two backgrounds (Column 'Background'): Indoor fish ('Indoor') had experienced only standard aquaculture conditions, having been raised with food pellets at a density of 109 individuals/m2 in indoor tanks.Outdoor fish ('Outdoor') had spent 149 days in seminatural outdoor streams (during experiment 1. in winter 2018–2019, see above) at a density of 2.7 individuals/m2. The latter fish were not fed during the outdoor-raising and they had thus gained experience in feeding on live invertebrate prey.Again, all fish can be identified using passive integrated transponders (column 'PIT'). Experiment 2 consisted of two parts: In part 1, the fish were subjected to a stress treatment and in part 2, we observed their stress recovery for 12 hours, as reflected by the commencement of feeding activity. Before the experiments, we collected the fish from a storage tank into fasting cages (column 'Fasting_cage'), where they remained for 18-23 hours prior to the stress treatment. The stress treatment (part 1) was conducted individually in four pale brightly lit channels with a rapidly changing water current (channel number is given in column 'Stress_channel'). During the treatment most fish settled to the bottom of the channels, refusing to move, but notes were made if the fish displayed 'escape behavior', i.e. started swimming against the water current at least once during the first (0–15 min from start) and/or second (15–30 min from start) half of the treatment. These notes are found in the column 'Swimming_Activity' and range from 0 to 2. Control fish were subjected only to handling stress and were thus not exposed to the actual stress treatment (column 'Stress_Control'). Immediately after the stress treatment the fish were moved individually into 24 dark, dimly lit plastic boxes (box number given by 'ID_in_experiment'). The boxes contained 30 L of water oxygenated by aquarium oxygenators and within the box, the fish were provided with four Lucilia sericata fly larvae. The boxes were checked every three hours to see when the fish commenced feeding, i.e. started to recover from the stressful treatment (part 2 of experiment). Starting and checking times are given in the columns 'Starting_Time' and 'Checking_Time' (local clocktime as a decimal number). 'Checking_Time_Absolute' is the time from the start of the experiment and 'Larvae_Eaten' is the number of larvae eaten at each checking time. Lengths and weights of the fish were measured after the stress experiments, on 21 July 2019. Temperature and Oxygen content of the incoming water are given, as measured each day.

Background We studied postrelease explorative behavior and stress tolerance of Landlocked and anadromous Atlantic salmon and their hybrids. For the research, we hybridized the Landlocked salmon of Lake Saimaa with a Baltic anadromous salmon from River Kymijoki, Southern Finland (strain originally from River Neva, Russia). These fish were hybridized in November 2017 and October 2018 in the Kainuu Fisheries Research Station, Paltamo, Kainuu, Finland (kfrs.fi).In the data the fish are treated as four strains (column 'Strain'): 1. Purebred Landlocked salmon (LLxLL), 2. hybrids, where the maternal population was landlocked salmon (LLxBA), 3. hybrids, where the maternal population was Baltic anadromous salmon (BAxLL) and 4. purebred Baltic anadromous salmon (BAxBA). Experiment 1. Post-release exploration In experiment 1., consisting of two separate trials, we studied post-release exploration of the fish in four circular seminatural streams. (Datasets 'Exploration2018.csv' & 'Exploration2020.csv', see also 'Figure_1.jpg') Experiment 2. Stress tolerance In experiment 2. we studied the stress response and recovery of the fish (Dataset 'Stress_BernoulliData.csv', see also 'Figure_2.jpg').

All data is given as .csv-tables (comma separeted values), where ';' serves as separator. They can be opened using e.g. text editor, microsoft excel or R studio.Funding provided by: Academy of FinlandCrossref Funder Registry ID: http://dx.doi.org/10.13039/501100002341Award Number: 308485Funding provided by: Elinvoimainen Järvilohi ry*Crossref Funder Registry ID: Award Number: Funding provided by: European Maritime and Fisheries FundCrossref Funder Registry ID: http://dx.doi.org/10.13039/100014510Award Number: Funding provided by: Maa- ja MetsätalousministeriÖCrossref Funder Registry ID: http://dx.doi.org/10.13039/501100006697Award Number: Funding provided by: Natural Resources Institute Finland*Crossref Funder Registry ID: Award Number: Funding provided by: Nordic Centre of Excellence for Sustainable and Resilient Aquatic Production*Crossref Funder Registry ID: Award Number: Funding provided by: Raija and Ossi Tuuliainen Foundation*Crossref Funder Registry ID: Award Number: