This invention is in the field of animal feed, more in particular in the field of fish feed. Surprisingly, it has been found that the mobility period of fish may be lengthened by interfering with the diet of the fish. We found that it is possible to increase the mobility period of fish to such an extent that it is even active at temperatures as low as 2 degrees Celsius. Such may be accomplished by adding naturally occurring substances to the feed of the fish. Examples of these naturally occurring substances are immune enhancing ingredients like beta-glucans and/or small amounts of phytohormones like auxin or gibberellic acid. Also, combinations of these substances, in particular the combination of beta-glucans and phytohormones, were shown to have a synergistic effect in that they lengthened the mobility period of the fish more than the individual components on their own.
