Water column-based assays underestimate benthic predation pressure in mid-latitude systems

Non-indigenous species (NIS) establish and thrive on floating artificial substrata along mid-latitude shores, which might serve as propagule reservoirs and stepping stones for their dispersal. However, often, the NIS are not able to colonize the adjacent seafloor, where high predation pressure by benthic predators might inhibit them. To test this hypothesis, we quantified and compared consumption rates of standardized bait (squidpops) in four water depth zones (sea surface, sub-surface, midwaters, seafloor) in five representative regions in the Southeast Pacific, covering oceanic Rapa Nui (Easter Island; 27°S, 109°W; November 2022) as well as the Chilean continental coast (29–41°S, 71–73°W; March–June 2022). We found a strong overall effect of water zonation, with significant bait consumption being limited to the seafloor and occurring only sporadically in other depth zones. Consumption frequencies also differed between experimental regions but were not influenced by latitude or mean sea surface temperature. An analogous experiment with the NIS Ciona robusta, conducted at one sampling site, showed that the occurrence or absence of predation per site and water depth zone, but not the exact consumption rates, concurred between both bait types. Our results confirm that predation in SE Pacific temperate shallow waters mainly depends on benthic predators that cannot reach higher zones of the water column. These findings have far-reaching implications, as they indicate that predation rates in mid-latitude systems might be underestimated through commonly used water column-based experiments. For a comprehensive estimation of predation pressure in a given system, future studies should consider differences between vertical water zones.