Convergent Foraging Tactics of Marine Predators with Different Feeding Strategies across Heterogeneous Ocean Environments

Abstract

Advances in satellite tracking and archival technologies now allow marine animal movements and behavior to be recorded at much finer temporal scales, providing a more detailed ecological understanding that can potentially be applicable to conservation and management strategies. Pelagic sharks are commercially exploited worldwide with current concerns that populations are declining, however, how pelagic sharks use exploited environments remains enigmatic for most species. Here we analyzed high-resolution dive depth profiles of two pelagic shark species with contrasting feeding strategies to investigate movement patterns in relation to environmental heterogeneity. Seven macropredatory blue (Prionace glauca) and six plankton-feeding basking (Cetorhinus maximus) sharks were tagged with pop-off satellite-linked archival tags in the North Atlantic Ocean to examine habitat use and investigate the function of dives. We grouped dives of both species into five major categories based on the two-dimensional dive profile shape. Each dive-shape class presented similar frequency and characteristics among the two species with U-and V-shaped dives predominating. We tested the spatial occurrence of different U-and V-shape dive parameters in response to environmental field gradients and found that mean depth and mean depth range decreased with increasing levels of primary productivity (chlorophyll "a"), whereas ascent velocities displayed a positive correlation. The results suggest that a planktivore and a macropredator responded behaviourally in similar ways to environmental heterogeneity. This indicates fine-scale dive profiles of shark species with different feeding strategies can be used to identify key marine habitats, such as foraging areas where sharks aggregate and which may represent target areas for conservation.

Diving behavior of the reef manta ray (Mobula alfredi) in New Caledonia: More frequent and deeper night-time diving to 672 meters

“…On the other hand, our results show that six manta rays remained at maximum depths for more than 10 minutes. These dive patterns are more akin to U-shaped profiles, suggesting the exploitation of aggregated prey [38,42]. As fishes, manta rays diving is not limited by the ability to store oxygen, but more probably by the low temperatures at these depths.…”

“…Analysis of dive profiles can provide valuable information on diving behaviour [38]. Classification of dive profiles has been mainly conducted on air-breathing marine animals such as seabirds [39], sea turtles [40], or seals [41], as well as a few studies focused on predatory fish [42,43].…”

“…Dives with very short or no bottom time, called "V-Shaped" dives, are possible indicators of travelling and/or prey searching behaviour [38][39][40][41][42]. By comparison, U-shaped or square-shaped dives profiles with distinctively longer bottom times are thought to suggest foraging activities [38][39][40][41][42]. Asymmetrical V-shaped dives were described for reef manta rays by Braun et al [8].…”

Seasonal changes in basking shark vertical space use in the north-east Atlantic

“…Sims et al (2003) suggested that vertical movements were consistent with those associated with foraging, with animals likely feeding year round on zooplankton. These behaviours, however, differed with habitat type: when in deep stratified waters of the continental shelf-edge, basking sharks exhibited normal diel vertical migration (DVM; dusk ascent-dawn descent), but when in shallow, inner-shelf waters basking sharks conducted reverse diel vertical migration (rDVM; dusk descent-dawn ascent; Sims et al 2005;Stéphan et al 2011;Queiroz et al 2017). Basking sharks were also shown to switch behaviours to a tidal rhythm when encountering boundaries between thermally stratified and mixed waters (Shepard et al 2006).…”

Global spatial risk assessment of sharks under the footprint of fisheries

“…, and was calculated here based on the SST data and using maximum gradient maps by determining where for each pixel a geodetic-distance-corrected maximum thermal gradient was identified; (iii) sea water salinity (psu) (SAL), an important determinant of habitat use in some sharks 1,38 ; (iv) sea surface height above geoid (m) (SSH) that influences shark presence 5 and catches by fisheries 6 ; (v) ocean mixed layer depth thickness or thermocline depth (m) (MLD) that affects pelagic shark foraging behaviour 39 ; (vi) mass concentration chlorophyll a in sea water (mg m -3 ) (CHL) as a proxy for productivity that often characterises preferred habitats of sharks5,39 ; (vii) mole concentration of phytoplankton expressed as carbon in sea water concentration (mmol m -3 ) (PHY) as a direct measure of productivity; (viii) net primary production of biomass expressed as carbon per unit volume in sea water per day (g m -3 d -1 ) (NPP) quantifying productivity; and (ix) mole concentration of dissolved molecular oxygen in sea water (mmol m -3 ) (DO) that can strongly influence shark space use 1 . Environmental datasets i to v were downloaded from Copernicus Marine Environment Monitoring Service (CMEMS) Global Ocean Physics Reanalysis product (https://bit.ly/2MOJeSy; downloaded November 2017) and datasets vi to ix from CMEMS Global Ocean Biochemistry Hindcast product (https://bit.ly/2TwNbOq; downloaded November 2017).…”