Artificial floating littoral zones: a promising nursery to support Pike (Esox lucius) in reservoirs

Abstract

The use of water resources in reservoirs leads to artificial water level fluctuations sometimes with extreme amplitudes and frequencies. These artificial fluctuations homogenize littoral habitats and often make macrophytes disappear. Consequently, spawning and refuge-nursery habitats become scarce which is critical for phytophilous species such as Northern Pike (Esox lucius) whose populations decline. Quite recently, floating artificial structures have emerged as a mitigation solution. However, the design of these structures is relatively simplistic and only consists in a simple 2D-floating mat of vegetation. Their effectiveness to support fish populations, especially pike in regulated reservoir, by providing suitable habitats for spawning, refuge and nursery remains poorly documented. Here we conceived 3D artificial Floating Littoral Zones (FLOLIZ) that mimic a natural littoral zone to support both pike spawning and juvenile growth (helophytes, hydrophytes, specific shelter areas). To assess their effectiveness, three structures of 70 m2 area and 1 m deep were installed in September 2018 in a French reservoir subject to extreme water level fluctuations. Visual surveys carried out in spring and summer by snorkelling over 2019–2023 highlighted an annual recurrence of juvenile pike in FLOLIZ, but also a higher abundance than in control stations. The maximum number of individuals observed simultaneously on the same structure was 14. Even if it cannot be asserted that pike spawned in FLOLIZ, these results highlight that FLOLIZ can provide refuge and nursery habitats for juvenile fish in reservoirs with poor littoral habitats.