Abstract
Biological production in the oceanic zone (i.e. waters beyond the continental shelves) is typically spatially patchy and strongly seasonal. In response, seabirds have adapted to move rapidly within and between ocean basins, making them important pelagic consumers. Studies in the Pacific, Southern and Indian Oceans have shown that seabirds are relatively abundant in major frontal systems, with species composition varying by water mass. In contrast, surprisingly little was known about seabird distribution in the oceanic North Atlantic until recent tracking showed that relative abundance and diversity peak in the Sub-polar Frontal Zone, west of the Mid-Atlantic Ridge, now proposed as a Marine Protected Area. However, absolute seabird abundance, distribution, age and species composition, and their potential environmental drivers in the oceanic temperate NW Atlantic remain largely unknown. Consequently, we systematically surveyed seabirds and environmental conditions across this area by ship in June 2017, then modelled the density of common species as functions of environmental covariates, validating model predictions against independent tracking data. Medium-sized petrels (99.8%), especially Great Shearwaters (Ardenna gravis, 63%), accounted for the majority of total avian biomass, which correlated at the macroscale with net primary production and peaked at the sub-polar front. At the mesoscale, the density of each species was associated with sea surface temperature, indicating zonation by water mass. Most species also exhibited scale-dependent associations with eddies and fronts. Approximately 51, 26, 23, 7 and 1 % of the currently estimated Atlantic populations of Cory's Shearwaters (Calonectris borealis), Great Shearwaters, Sooty Shearwaters (A. grisea), Northern Fulmars (Fulmarus glacialis) and Leach’s Storm-petrels (Oceanodroma leucorhoa) occurred in the area during our survey, many of which were undergoing moult (a vital maintena nce activity). For some species, these estimates are higher than suggested by tracking, probably due to the presence of immatures and birds from untracked populations. Our results support the conclusion that MPA status is warranted and provide a baseline against which future changes can be assessed. Moreover, they indicate potential drivers of seabird abundance and diversity in the oceanic zone of the North Atlantic that should be investigated further.
Highlights
The oceanic zone is the largest habitat on Earth
In order to obtain an approximate estimate of the moult status of birds in the study area, whenever possible we examined birds using binoculars and recorded whether they were clearly in active primary moult
We modelled counts per segment nj,sas Generalised Linear Models (GLMs) or Generalised Additive Models (GAMs) with the form
Summary
The oceanic zone (i.e. waters beyond the continental shelves) is the largest habitat on Earth It remains relatively poorly understood but is undergoing increasingly rapid human exploitation (Crespo et al, 2018; St. John et al, 2016). Do seabirds consume a similar amount to human fisheries, they are bycaught in those fisheries, leading to widespread and unsustainable population declines (Croxall et al, 2012; Cury et al, 2011; Dias et al, 2019; Gremillet et al, 2018) They are likely to be vulnerable to the large-scale effects of climate change (Dias et al, 2019; Rodríguez et al, 2019). In order to conserve seabirds effectively, it is necessary to understand relationships with environmental drivers and underlying processes driving seabird abundance and distribution (Gremillet & Boulinier, 2009)
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