Fish depth redistributions do not allow maintenance of abundance in a region of rapid change

Abstract

For marine fishes, stressors such as ocean warming, biological invasions, and fisheries are suggested to drive depth redistributions. However, it is uncertain whether depth redistribution can act to offset the fitness costs associated with adverse changing conditions and allow species to maintain abundance. To better understand the relationship between global changes and depth redistribution, we synthesized bottom trawl surveys spanning two decades in the eastern Mediterranean Sea, a hotspot for warming and biological invasions, and ask three main questions: 1) have species changed their depth distributions?, 2) are patterns of depth change associated with changes in abundance? and 3) do depth and abundance changes differ by species traits and between indigenous and non‐indigenous species (NIS)? We found that patterns of depth change were highly variable. However, the relative abundances of species that exhibit deepening decreased while that of expanding species increased. Cold‐water species deepened more than warm‐water species and decreased in relative abundance. The majority of non‐indigenous fishes expanded their depth ranges over time and increased in relative abundance. The significant link between species' thermal preference and depth changes suggests that warming may drive depth redistributions. However, the variable depth change patterns imply that the hypothesis that species track climate change by deepening may be too simplistic. Finally, at least in the eastern Mediterranean, the deepening of indigenous species (IS) into colder waters was mostly associated with reduced abundance, hence deepening was not sufficient to offset the ecological impacts of global change.