Weak evidence for a relationship between group size and flight initiation distance in response to underwater human presence in an exploited fish species

12 pages, 3 figures, 3 tables, supplementary material https://doi.org/10.1007/s00265-025-03563-x.-- Data availability: All data generated or analyzed during this study are included in this published article as supplementary material

Group size can influence flight initiation distance (FID), a key antipredator behavior, in many animal species. In fishes, however, the effect of group size on FID remains unclear. Two different mechanisms might explain a putative relationship between group size and FID. If fish benefited from having more vigilant individuals around, we would expect group size to be positively associated with FID (considering collective vigilance in the context of the many eyes hypothesis). By contrast, if fish benefited from a predator dilution effect, we would expect group size to be negatively associated with FID. Importantly, such relationships should be critically sensitive to background risk levels. We capitalized on FID observations inside and outside marine protected areas and simulated risk by having a risky spearfisher or a non-risky snorkeler swim towards white seabream (Diplodus sargus) to estimate the relationship between FID and group size. Model selection provides weak evidence that group size positively influences FID of white seabream when group size was modelled two ways: categorically (alone vs. shoal), and continuously (range 2–15). While the results suggest that overall group size has a measurable impact on FID, the presence of spearfishers or snorkelers had a weak effect on the relationship of group size and FID, which seems to be more sensitive exclusively to the protection level (inside/outside marine protected areas). Our findings align with previous studies showing mixed results on the relationship between group size and FID. This study underscores the complexity of antipredator behaviors in natural settings and suggests that multiple interlinked factors, rather than group size alone, drive FID in fishes. Future research should integrate field observations, laboratory experiments, and modeling to study the ecological influence of group size on FID more comprehensively

Open Access funding provided thanks to the CRUE-CSIC agreement with Springer Nature. VS is supported by a “Ramón y Cajal” research fellowships (RYC2021-033065-I) granted by the Spanish Ministry of Science, Innovation, and Universities (MICIU), and he also acknowledge the project “HumanFear” funded by the MICIU/AEI/10.13039/501100011033 and FEDER EU. Finally, VS also acknowledge the “Severo Ochoa Centre of Excellence” accreditation (CEX2019-000928-S)

Peer reviewed