Juvenile Gobius niger avoids seagrass in the presence and uncertain absence of seagrass-inhabiting predators

We performed a series of laboratory tank experiments designed to identify possible mechanisms for the strict seagrass avoidance of juvenile Gobius niger previously observed in their natural habitat, the Novigrad Sea, northern Adriatic. Overall our results suggest that juvenile G. niger strongly prefers seagrass when predator absence is certain, avoids seagrass in the presence of predators, and chooses habitat based on both predator identity and on the degree of certainty of predator presence or absence. In the presence of predators, juvenile G. niger occupies the same habitat as adult size G. niger but avoids the habitat chosen by Zosterisessor ophiocephalus. Juvenile G. niger is most likely to switch habitat in response to addition or removal of Z. ophiocephalus, the predator with the stronger seagrass preference. A significantly lower likelihood of selecting seagrass was exhibited when predator absence was uncertain than when predator absence was certain. Our results suggest that juvenile G. niger identifies predators by species and responds differentially to perceived risk posed by each predator through differential habitat selection, which also depends on the certainty of predator absence. These results are consistent with the hypothesis that the importance of structured refuge habitats, such as seagrass, is relative: it depends on both the prey's perception of the relative predation risk posed by multiple predators in alternative habitats, and on the prey's specific antipredator response strategies. These responses could explain the avoidance of seagrass by G. niger in the Novigrad Sea.

► Small prey fish avoided seagrass habitat if an ambushing predator was present. ► Small prey fish avoided seagrass if a predator was absent but recently present. ► Small prey fish preferred seagrass if predator absence was certain. ► Our results are consistent with the predation mode hypothesis.