Seabird diving behaviour reveals the functional significance of shelf-sea fronts as foraging hotspots

Abstract

Oceanic fronts are key habitats for a diverse range of marine predators, yet how they influence fine-scale foraging behaviour is poorly understood. Here, we investigated the dive behaviour of northern gannets Morus bassanus in relation to shelf-sea fronts. We GPS (global positioning system) tracked 53 breeding birds and examined the relationship between 1901 foraging dives (from time-depth recorders) and thermal fronts (identified via Earth Observation composite front mapping) in the Celtic Sea, Northeast Atlantic. We (i) used a habitat-use availability analysis to determine whether gannets preferentially dived at fronts, and (ii) compared dive characteristics in relation to fronts to investigate the functional significance of these oceanographic features. We found that relationships between gannet dive probabilities and fronts varied by frontal metric and sex. While both sexes were more likely to dive in the presence of seasonally persistent fronts, links to more ephemeral features were less clear. Here, males were positively correlated with distance to front and cross-front gradient strength, with the reverse for females. Both sexes performed two dive strategies: shallow V-shaped plunge dives with little or no active swim phase (92% of dives) and deeper U-shaped dives with an active pursuit phase of at least 3 s (8% of dives). When foraging around fronts, gannets were half as likely to engage in U-shaped dives compared with V-shaped dives, independent of sex. Moreover, V-shaped dive durations were significantly shortened around fronts. These behavioural responses support the assertion that fronts are important foraging habitats for marine predators, and suggest a possible mechanistic link between the two in terms of dive behaviour. This research also emphasizes the importance of cross-disciplinary research when attempting to understand marine ecosystems.

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

“…Other studies have found that SST can predict seabird aggregations (Hyrenbach et al 2006, Nishizawa et al 2015, especially in frontal zones (Abrams 1985, Begg & Reid 1997, which was not the case in this study. In our study area, the average SST does not vary substantially during the period surveyed, and fronts driving prey distribution did not appear to be present, as documented in other areas (Hoefer 2000, Bost et al 2009, Scales et al 2014, Cox et al 2016. In contrast, capelin aggregate mostly near the seabed in a layer of > 0°C water within deep (> 200 m) trenches during their inshore spawning migration on the Newfoundland Shelf (Mowbray 2002) and then occupy depths with > 0°C water while inshore during the spawning season (Davoren et al 2006), suggesting that capelin distribution would be more correlated with bottom temperature than with SST.…”

Environmental heterogeneity amplifies behavioural response to a temporal cycle

“…The interactions between the tidal cycle and distance to the colony or depth might appear to explain relatively small amounts of model variance. However, they are comparable to the variance explained by oceanic fronts (Cox et al 2016), which are now widely accepted to be important features enhancing prey availability to marine top predators (Scales et al 2014, Cox et al 2016). Furthermore, for animals with such finely balanced energy budgets (Collins et al 2016), the fact that these behavioural changes occur repeatedly within central-place foraging suggests that the ability to adapt to predictable resource changes in otherwise variable environments could make all the difference between breeding success and failure.…”

Personality predicts foraging site fidelity and trip repeatability in a marine predator

“…In predictable environments, returning to previous foraging locations should be favoured; conversely, in unpredictable habitats the probability of a previous location being profitable again is low, and consequently, animals should show lower site fidelity and greater reliance on environmental cues to locate prey (Switzer, 1993;Weimerskirch, Le Corre, Jaquemet, & Marsac, 2005). The marine environment is characterized by both persistent oceanographic features (bathymetric structures and fronts) which generate predictable prey patches, as well as highly dynamic tidal and weather processes which result in spatiotemporally variable resource distributions (Cox et al, 2016;Scales et al, 2014). High and low reliance on environmental cues may represent alternative foraging tactics that can both be profitable within the same macro-scale habitat (Carroll, Harcourt, Pitcher, Slip, & Jonsen, 2018).…”