Remotely operated vehicle reveals fish orientate to the substrate underneath marina floating pontoons

The modern day seascape is a mix of natural habitats (saltmarsh, mangroves, seagrass, unvegetated beaches), but increasingly is being replaced with hard engineering structures (e.g. floating pontoons) that have resulted in novel habitats available for flora and fauna. While these engineered structures provide new marine habitat re-sources, from the viewpoint of biodiversity, they effectively support a sub-set of species found in natural habitats. Unlike previous studies that have used divers for fish visual census, we used a remotely operated vessel with an underwater camera to examine the fish community using floating pontoons in a marina facility in tropical central Queensland, Australia. The camera filmed continuously the edge and underside of pontoons (both surveys covered similar pontoon linear distances - July (winter) 430m; November (summer) 485m; 2021). Fifteen fish species representing 14 families were recorded, with more species observed during summer compared to the winter survey. Remarkably, across both surveys of the 243 fishes individuals that were recorded, 166 (68%) appeared upside-down in videos (i.e., oriented to the underside of the pontoon). Engaging a behaviour of swimming upside-down has been acknowledged in natural habitats elsewhere; some coral reef species living in caves or reef ledges orientate themselves to the cave ceiling or overhanging ledge to increase prey capture rate. However, this is the first study to record fishes exhibiting the same behaviour. We conclude that our less invasive video survey approach provided the opportunity to observe fishes in a state of natural behaviour, orientating themselves towards the pontoon substrate, which is probably simply a response of adhering to basic charac-teristic modes, i.e., it is the orientation relative to the substrate that is important, rather than their orientation in space per se. Indeed marina pontoons provide an additional feature in already habitat complex estuaries; these data add to why we need to consider the ecosystem services offered by marine infrastructure, and the plasticity of fish to adapt to new environments in an expanding blue economy.

Automated summary

The modern day seascape consists of both natural habitats and hard engineering structures, with the latter providing new habitat resources but limited biodiversity support. A study in tropical central Queensland, Australia observed fish exhibiting a behavior of swimming upside-down under floating pontoons, a previously undocumented phenomenon.