Citation: Filipsson, K.; Åsman, V.; Greenberg, L.; Österling, M.; Watz, J.; Bergman, E. Winter Behavior of Juvenile Brown Trout in a Changing Climate: How Do Light and Ice Cover Affect Encounters with Instream Predators? Fishes 2023, 8, 521. https://doi.org/10.3390/ fishes8100521 Academic Editor: Assaf Barki Received: 14 September 2023 Revised: 13 October 2023 Accepted: 16 October 2023 Published: 20 October 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). fishes Article Winter Behavior of Juvenile Brown Trout in a Changing Climate: How Do Light and Ice Cover Affect Encounters with Instream Predators? Karl Filipsson 1,2 , Veronika Åsman 1 , Larry Greenberg 1, * , Martin Österling 1 , Johan Watz 1 and Eva Bergman 1 1 River Ecology and Management, Department of Environmental and Life Sciences, Karlstad University, Universitetsgatan 2, SE-651 88 Karlstad, Sweden; martin.osterling@kau.se (M.Ö.); johan.watz@kau.se (J.W.); eva.bergman.1868@kau.se (E.B.) 2 Jakobi Sustainablility AB, SE-412 58 Gothenburg, Sweden * Correspondence: larry.greenberg@kau.se; Tel.: +46-54-7001543 Abstract: During winter, stream fishes are vulnerable to semi-aquatic predators like mammals and birds and reduce encounters by being active in darkness or under surface ice. Less is known about the behavior of fishes towards instream piscivorous fishes. Here, we examined how surface ice and light affected the anti-predator behavior of juvenile brown trout (Salmo trutta Linnaeus, 1758) in relation to piscivorous burbot (Lota lota Linnaeus, 1758) and northern pike (Esox lucius Linnaeus, 1758) at 4 ◦ C in experimental flumes. Trout had lower foraging and swimming activity and spent more time sheltering when predators were present than when absent. In daylight, trout’s swimming activity was not affected by predators, whereas in darkness trout were less active when predators were present. Trout consumed more drifting prey during the day when ice was present, and they positioned themselves further upstream when under ice cover, regardless of light conditions. Trout stayed closer to conspecifics under ice, but only in the presence of pike. Piscivorous fishes thus constitute an essential part of the predatory landscape of juvenile trout in winter, and thus loss of ice cover caused by climate warming will likely affect trout’s interactions with predators. Keywords: anti-predator; global changer; diel behavior; foraging; piscivores; predators Key Contribution: Most studies involving predation on fishes in northern temperate streams in winter have focused on prey behavior in relation to semi-aquatic predators like mammals and birds. Here, we show that the prey also modify their behavior in the presence of instream piscivorous fishes and how this interaction is affected by surface ice cover and light. 1. Introduction The global climate is gradually becoming warmer [1], with the magnitude of the effect dependent on location and time of year. In most northern latitudes, it is expected that temperature increases will be more pronounced during winter than during summer [2]. Hence, the effects of global warming during winter may be profound and have direct and indirect effects on organisms [3–5]. In aquatic systems situated in northern latitudes, the effects of warmer winters on the physical environment have already been documented. Warmer winters have resulted in reduced snow and ice cover, both temporally and spatially, as well as erratic ice cover formation and breakup throughout winter [6–10]. Surface ice cover is thought to protect ectothermic fish from many endothermic predators, and thus ongoing and expected further reductions of the period with intact surface ice cover may have negative effects for fish. Many juvenile salmonids overwinter in streams and rivers, where they are vulner- able to predation [11–13]. Semi-aquatic predators, such as mammals and birds, both Fishes 2023, 8, 521. https://doi.org/10.3390/fishes8100521 https://www.mdpi.com/journal/fishes