Behavioral experiment
To explore the mechanisms behind potential differences in growth between the selection lines and among concentrations, we also studied differences in fish activity and feeding behavior. We subsampled eight fish per selection line per concentration at age 91 dpf and conducted a behavioral experiment in three concentrations: control, low (0.5 Mn mg L-1) and high (3.5 Mn mg L-1). The test arena was a 15 L aquarium divided into two sections with a plastic, transparent divider and with a grid with 3 x 3 cm squares attached to the walls and to the bottom. Canon EOS 80D systems camera was set to record the fish movement in aquarium on both sides, while another camera (GoPro Hero +) was set to record the aquarium from the third dimension (above). Individual fish were first acclimatized for five minutes in a plastic container after which the container was removed, and the fish could swim freely. We recorded fish movements for five minutes. After that, the fish was removed from the test arena and placed in a 4 L aquarium for an hour, after which the experiment was repeated to study whether the behavior was repeatable. Finally, we measured the SL of the fish.
Approximately one month later, we conducted the feeding experiment using the same fish (age 119 dpf) as in the activity experiment. Fish were fasting 24 hours before the experiment to increase their motivation to feed. Feeding behavior was monitored in a separate aquarium (volume 15 L) and the behavior was recorded with a Canon EOS 80D camera which was set to record the fish movements on the side wall of the aquarium for five minutes. Each fish was first acclimatized for five minutes, after which a small amount of dry food was carefully dropped on the surface of the water. We then monitored feeding probability (i.e., whether the fish fed during the feeding trial or not) for five minutes. After the experiment, the fish was removed from the test aquarium and returned to its original rearing aquaria. The experiment was repeated after another 24 h fasting period. Finally, the SL of the fish was measured.