Tropical oceans are often described as “marine deserts”, where resources are patchily distributed and predators, such as seabirds, must cover wide areas to find food. However, oceanic islands like the Fernando de Noronha Archipelago, Brazil, can enhance productivity, creating areas with higher prey concentrations for seabirds. The “Island Mass Effect” occurs when the topography of islands and seamounts interferes with the circulation of ocean currents. In Fernando de Noronha, this effect increases productivity to the west of the archipelago, on the side opposite to the prevailing currents. Nevertheless, previous studies have observed a concentration of planktivorous fish to the east of the archipelago, potentially due to the accumulation of zooplankton generated by the input of bottom particles brought up by upwelling events.

 

But do the foraging strategies of the archipelago’s seabirds reflect this phenomenon over time? Do they concentrate their foraging to the east, resulting in small variations in diet over time?

 

Masked Booby (Sula dactylatra) by Júlia Jacoby

 

To address this question, we used tracking data and stable isotope analyses of carbon and nitrogen from Masked Boobies (Sula dactylatra) breeding in Fernando de Noronha in 2017, 2018, 2019, and 2022. Some birds were equipped with GPS-only loggers, while others carried GPS devices coupled with pressure sensors that record diving events. For the GPS-only tracks, we estimated dives using a deep neural network trained and validated against data from pressure-sensor devices. With both recorded and estimated dives, we calculated annual utilization distributions using Gaussian Kernel Density Estimation (KDE).

 

Isotopic data, used to infer variation in foraging areas (carbon) and trophic position (nitrogen), allowed us to identify interannual changes in trophic niches and to estimate shifts in prey species contributions to the diet. Combining these datasets enabled us to test whether birds maintain predictable foraging patterns over time.

 

Across all years, boobies consistently foraged to the east of the archipelago, supporting the idea that the Island Mass Effect contributes to predictable prey availability. Despite this consistent foraging direction, there were interannual variations in foraging proximity. In 2022, birds foraged closer to the colony, coinciding with dietary changes marked by an increased contribution of false herring (Harengula clupeola) and a decrease in four-winged flying fish (Hirundichthys affinis). This suggests that, although the Island Mass Effect enhances prey predictability, other less predictable environmental factors, such as sea surface temperature or water column stratification, may influence prey aggregation near the archipelago and/or alter the prey species composition.

 

 

Figure from Jacoby et al. (2025): Foraging distributions using 25 %, 50 %, 75 %, and 90 % contours of the Kernel utilization distribution based on recorded and estimated dives of males (M; blue) and females (F; orange) of masked booby during breeding seasons at the Fernando de Noronha Archipelago in 2017, 2018, 2019 and 2022. n = number of foraging trips.

 

These findings highlight the adaptability of boobies to variable environmental conditions and demonstrate the flexibility of their foraging strategies. They reinforce the role of seabirds as indicators of spatiotemporal variation in fish communities, which are key resources for the functioning of oceanic ecosystems. Furthermore, in the context of future climate change scenarios, long-term monitoring of foraging strategies provides a valuable tool for assessing rapid ecosystem responses to environmental change.

 

This study was the result of a collaboration between the Brazilian institutions Universidade Federal do Rio Grande do Sul (UFRGS) and Universidade Federal do Rio Grande (FURG) and the French institutions Institut de Recherche pour le Développement (IRD) and Centre d’Études Biologiques de Chizé.

 

Data from this paper are available to request on the Seabird Tracking Database, datasets 2401, 2403, 2405 and 2407

Read the full article here: Jacoby, J., Roy, A., Lanco, S., Barbraud, C., Delord, K., Bugoni, L., Nunes, G. T. (2025) Flexible foraging strategies of a tropical seabird in the western Atlantic Ocean. Journal of Experimental Marine Biology and Ecology, 586, 152099. https://doi.org/10.1016/j.jembe.2025.152099