Abstract
The dynamics of the subtropical pelagic ecosystems of the Northeast Atlantic are still poorly known due to the high costs associated with sampling large oceanic areas. Top predators can be used as alternative low-cost samplers and indicators of the temporal variability of such systems. To study the variation in the composition of pelagic species through time in the broad Canary current region, we analysed foraging trips and regurgitations of Cory's shearwaters Calonectris borealis nesting on Selvagens islands, in 2008–2011 and 2016–2018. Fisheries data, oceanographic variables and the North Atlantic Oscillation were explored as possible explanatory variables for trends in behaviour and diet. Cory's shearwaters' diet, complemented by fisheries data, revealed marked changes in the composition of the pelagic fish communities. In 2016 there was a peak in the abundance of the Atlantic chub mackerel Scomber colias, followed by an explosive increase in the abundance of the Longspine snipefish Macroramphosus scolopax in 2017 and 2018, as deduced from the diet composition of the Cory's shearwater, and supported by fisheries data, in the broad oceanic area surrounding the Selvagens islands. Oceanographic variables did not show fluctuations correlated with these marked shifts in pelagic fish availability, the causes of which remain largely unknown. This study highlights the importance of the Atlantic chub mackerel and of the Longspine snipefish in the Madeira/Canary region and exemplifies the efficiency of avian predators in revealing rapid changes in pelagic communities of oceanic domains. Such trends and variations need to be better monitored and understood to measure the impact of ongoing global changes and to sustainably manage the marine environment and resources.
Highlights
The high-seas of the subtropical eastern North Atlantic are a nutrientpoor region, contrasting with the richer waters of the strong coastal upwelling of West Africa associated with the Canary Current (Cropper et al, 2014)
Even though predators at high trophic levels dominate the pelagic community with a large number of species in this region, all trophic levels are characterized by a low biomass when comparing with the more productive coastal region (Pitcher et al, 2007)
We counted a total of 2775 prey individuals in regurgitations, 79% of which were successfully identified to species, genus or family level, resulting in 47 identified species/genus of 32 identified families
Summary
The high-seas of the subtropical eastern North Atlantic are a nutrientpoor region, contrasting with the richer waters of the strong coastal upwelling of West Africa associated with the Canary Current (Cropper et al, 2014). Even though predators at high trophic levels dominate the pelagic community with a large number of species in this region, all trophic levels are characterized by a low biomass when comparing with the more productive coastal region (Pitcher et al, 2007). Among these forage fish are the Atlantic chub mackerel Scomber colias (hereafter chub mackerel) and the Blue jack mackerel Trachurus picturatus which are some of the most abundant and with higher productivity rates of this type of ecosystem (Hermida and Delgado, 2016). Their daily vertical migration creates an important link between the depths and the epipelagic domain (Salvanes and Kristoffersen, 2001)
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