Seascapes of fear and competition shape regional seabird movement ecology

The study addresses the significant gap in understanding how fear effects of predators influence prey distributions in marine environments, particularly at large spatial scales and in conservation contexts. The Seascape of Fear (SoF) hypothesis posits that prey animals modify their behavior and distribution in response to perceived predation risk, creating a seascape shaped by fear. While the SoF has been explored in terrestrial systems, its application to marine ecosystems, especially at regional scales, remains limited. The Benguela upwelling ecosystem off South Africa provides an ideal setting to test this hypothesis due to the presence of both Cape gannets (prey) and Cape fur seals (predator), along with significant human impact from fisheries. The researchers predicted that gannets would avoid seals, particularly at night when vulnerability is highest, leading to diel spatial shifts in their foraging patterns. They also anticipated a trade-off between avoiding seals and accessing foraging resources, especially during the day when foraging needs are paramount. The study aims to contribute to a better understanding of seascape ecology of gannets, incorporating the impact of both predator avoidance and resource competition.

The introduction to the paper highlights a lack of research on fear effects in marine environments at large spatial scales, contrasting with the well-established terrestrial 'landscape of fear' concept. The authors mention that previous work has primarily focused on coastal reef habitats, with limited studies on regional-scale impacts. The existing literature acknowledges the influence of predation risk on prey behavior and its cascading effects on population dynamics and ecosystems. However, the study notes the scarcity of research integrating fear effects into marine seascape ecology, especially in the context of ecosystems perturbed by climate change and overfishing, where such understanding is crucial for effective conservation strategies.

The study used multi-year electronic tracking data from 197 Cape gannets equipped with GPS loggers, monitoring their movements from Malgas Island, South Africa, between October and November from 2008 to 2015. The data included location information at 30-second to 2-minute intervals. Concurrently, data from 25 Cape fur seals tracked using Argos satellite transmitters were used to represent the distribution of the predator. To estimate the nighttime resting locations and daytime foraging locations, the researchers defined behavioural states based on bird speed and used resource selection functions (RSFs) to model the spatial distribution of seals and assess their habitat preferences. The analysis incorporated environmental variables like bathymetry, slope, and distance to shore. The influence of fisheries on gannet movements was assessed using vessel logbook records from 2008 to 2015. The study employed a binomial choice framework to model gannet foraging habitat selection during the day, accounting for individual variability and considering variables like distance to shore, seal occurrence, and fisheries catches (purse-seiners and trawlers). This allowed for an assessment of the trade-off between predation risk and competition for resources. A mixed-effect logistic regression was used to analyze the probability of daytime foraging versus nighttime resting based on environmental features like seal occurrence, sea surface temperature, and fisheries catches. This comprehensive approach integrated tracking data with fisheries data to produce a holistic understanding of the gannets' movement ecology in response to both biotic and abiotic factors.

The study validated the hypothesis that both fear and competition shape regional gannet spatial ecology. Cape fur seals were active both day and night, with their occurrence decreasing with distance from the coast. Gannets exhibited two main nighttime strategies: returning to the colony or resting at sea. Gannets that rested at sea did so in offshore areas with significantly lower seal occurrence (reducing predation risk by 25%), highlighting avoidance behavior. Daytime foraging locations showed a higher risk of seal encounter compared to nighttime, but gannets selected areas that still minimized seal co-occurrence. There was evidence of competition with fisheries: gannets were more likely to forage near areas with high purse-seiner catches of small pelagic fish but avoided areas with high trawler catches of hake. In summary, the findings show that Cape gannets balance foraging needs with predation risk, adjusting their spatial distribution and behavior diurnally to minimize the chances of encountering seals. The study presented data on nighttime metrics, such as time spent at sea, and daytime foraging habitat use, including distance to shore, distance to colony, seal occurrence, and fisheries catches. Figure 4 showed the probability of daytime foraging versus nighttime resting in relation to seal occurrence, sea surface temperature, and fisheries catches. These data points reinforce the key finding of the interplay between fear, competition, and the gannets' foraging patterns.

The results strongly support the integration of the 'seascape of fear' concept into marine ecological studies. The study highlights the importance of considering both predation risk and competition with fisheries when examining seabird movement ecology. The observed diel shifts in gannet distribution underscore the significant influence of fear on spatial patterns, especially at night when vulnerability to seals is higher. The study contributes novel insights into the interplay between fear and competition in shaping the spatial distribution and behavior of marine species. The findings demonstrate that incorporating the 'seascape of fear' concept is essential for effective conservation strategies, particularly in the design of marine protected areas (MPAs). Understanding the spatial distribution of both prey and predators is crucial for effective MPA design, as neglecting fear effects can lead to inadequate protection of threatened species.

This study provides the first evidence of a regionally-scaled 'seascape of fear' affecting seabird movement ecology, highlighting the importance of integrating fear effects into the management of marine ecosystems. The findings demonstrate that Cape gannets balance foraging needs with predation risk, leading to diel shifts in distribution to minimize interactions with their predator. The study's results underscore the need to incorporate the 'seascape of fear' into the design of MPAs to ensure effective conservation of threatened species. Future research should expand the scope of SoF studies to include trophic networks within coastal ecosystems for a more comprehensive understanding of the spatiotemporal distribution of aquatic animals and population consequences.

The study focused on a specific time period (October-November) and location (Malgas Island). Extrapolating these findings to other seasons, locations, or species may require further research. The data on seal distribution relied on a subset of tracked seals, and the accuracy of this representation of seal distribution could influence the results. The study's reliance on fisheries logbook data introduces potential reporting biases.