Associations of non-breeding shearwater species on the northeastern Newfoundland coast

Abstract

Prey aggregations are not uniformly distributed, driving predator species to aggregate in specific areas of high food availability. On the east coast of Newfoundland, capelin Mallotus villosus, a small forage fish, migrate inshore to spawn during the summer, providing an abundant food source for marine predators. During this period, non-breeding great shearwaters Ardenna gravis (GRSH) and sooty shearwaters A. grisea (SOSH), both long-distance migratory sea birds, aggregate in coastal Newfoundland, but it is unclear what drives their distributional patterns within this region. Using at-sea surveys, we investigated whether the density and distributional patterns of GRSH or SOSH were influenced by sea surface temperature, depth and fish (prey) density as well as the number of the other seabird species or other shearwater species (i.e. GRSH or SOSH). The presence and number of GRSH and SOSH were positively influenced by the density of the other sympatric shearwater species but were not influenced by the densities of other seabird species. These findings suggest that the benefits of foraging in close association may outweigh costs. Fish density was less important in explaining the presence and number of GRSH and SOSH than depth, as both species were mainly found together in shallow areas (< 50 m) along the coast. As fish density was primarily distributed in shallow areas, reflecting predictable locations of and migratory routes to capelin spawning sites, depth (or distance from shore) and the distribution of other shearwaters may provide important cues to locate regions of high prey availability in coastal Newfoundland.

Moult chronology and strategies of sympatric Great (Ardenna gravis) and Sooty (A. grisea) Shearwaters based on stable isotope analysis

“…The isotopic niche breadth of both shearwater species in the study area has been shown to narrow throughout the summer seasonalso suggesting convergence on a similar prey type/ diet (Gulka et al 2017, which has been found for other marine predator species in coastal Newfoundland (Gulka et al 2017, Jenkins & Davoren 2021. Additionally, these two species also show high spatial overlap at sea (Carvalho & Davoren 2019.…”

“…Identifying key moulting locations will be important for marine spatial planning for effective conservation of these species in the future. We identified an overlapping area where both Sooty and Great Shearwaters complete their moult in coastal Newfoundland (Carvalho & Davoren 2019, which has been previously described as a 'biological hotspot' where many other seabird, whale and predatory fish species aggregate over persistent Capelin spawning aggregations (Davoren 2013). Protecting this moulting area would minimize disturbance and the impacts of threats (e.g.…”

“…If one species completes moult in advance of another similar, sympatric species, this species may depart moulting/ non-breeding grounds if food availability decreases, which would allow niche partitioning. Interestingly, prey availability decreases in the study area once Capelin spawning is complete (Carvalho & Davoren 2019), and Sooty Shearwaters, the slightly smaller (40-51 cm;Carboneras et al 2020a) and less competitive species compared with Great Shearwaters (43-51 cm; Carboneras et al 2020b, have more advanced moult and, thus, tend to leave the area in early August (P. C. Carvalho pers. obs.).…”

“…1981, Ronconi et al . 2010a) and prey fish spawning sites (Davoren 2013, Carvalho & Davoren 2019), it is possible that moulting regions overlap.…”

“…Bugoni et al 2015), whereas Sooty Shearwaters appear to moult offshore in the North Atlantic, based on observations of newly moulted feathers upon arrival in inshore areas (Brown 1988). Given that both species also forage together in the North Atlantic at naturally occurring prey sources associated with physical oceanographic features, such as upwellings (Brown et al 1981, Ronconi et al 2010a) and prey fish spawning sites (Davoren 2013, Carvalho & Davoren 2019, it is possible that moulting regions overlap.…”