Association of whale sharks (Rhincodon typus) with thermo-biological frontal systems of the eastern tropical Pacific

Abstract

Satellite tracking of 27 whale sharks in the eastern tropical Pacific, examined in relation to environmental data, indicates preferential occupancy of thermo-biological frontal systems. In these systems, thermal gradients are caused by wind-forced circulation and mixing, and biological gradients are caused by associated nutrient enrichment and enhanced primary productivity. Two of the frontal systems result from upwelling, driven by divergence in the current systems along the equator and the west coast of South America; the third results from wind jet dynamics off Central America. All whale sharks were tagged near Darwin Island, Galápagos, within the equatorial Pacific upwelling system. Occupancy of frontal habitat is pronounced in synoptic patterns of shark locations in relation to serpentine, temporally varying thermal fronts across a zonal expanse > 4000 km. 80% of shark positions in northern equatorial upwelling habitat and 100% of positions in eastern boundary upwelling habitat were located within the upwelling front. Analysis of equatorial shark locations relative to thermal gradients reveals occupancy of a transition point in environmental stability. Equatorial subsurface tag data show residence in shallow, warm (>22°C) water 94% of the time. Surface zonal current speeds for all equatorial tracking explain only 16% of the variance in shark zonal movement speeds, indicating that passive drifting is not a primary determinant of movement patterns. Movement from equatorial to eastern boundary frontal zones occurred during boreal winter, when equatorial upwelling weakens seasonally. Off Peru sharks tracked upwelling frontal positions within ~100–350 km from the coast. Off Central America, the largest tagged shark (12.8 m TL) occupied an oceanic front along the periphery of the Panama wind jet. Seasonal movement from waning equatorial upwelling to productive eastern boundary habitat is consistent with underlying trophic dynamics. Persistent shallow residence in thermo-biological frontal zones suggests the role of physical-biological interactions that concentrate food resources.

Longest recorded trans-Pacific migration of a whale shark (Rhincodon typus)

“…There was also considerable variation between individuals in the frequency of detections. One large female tagged at the Galapagos Islands surfaced off the shelf break of Peru after an interval of 4 months, which represents a movement consistent with that of other female R. typus that provided more frequent positions (Hearn et al 2013(Hearn et al , 2016Ryan et al 2017) and is related to thermal frontal systems instead of ocean current flow directions (but see Wilson et al 2006;Sleeman et al 2010). These results show that lengthy periods without transmissions do not necessarily imply tag shedding.…”

Multi-method assessment of whale shark (Rhincodon typus) residency, distribution, and dispersal behavior at an aggregation site in the Red Sea

“…This possibility is intriguing given the evidence that thermoregulation is a strong driver of R . typus migration [67], vertical behavior [68], and physiology [69]. The Red Sea is thermally homogenous at depth with maximum surface temperatures of ~30°C and minimum temperatures at depth of ~22°C [70].…”

Enhancing insights into foraging specialization in the world's largest fish using a multi‐tissue, multi‐isotope approach

“…), while open ocean movements suggest utilization of prey concentrated by fronts (Ryan et al. ). Careful isotopic analyses of R. typus may thus be particularly informative for understanding and comparing diets in and outside aggregations, especially given the species’ threatened status and oceanic migrations which make lethal sampling and sustained feeding observations undesirable and difficult, respectively.…”

“…The diet of R. typus outside of coastal aggregations is generally unknown. Fatty acid analyses have suggested that they may depend on deep-water zooplankton and fishes during long-distance migrations (Rohner et al 2013, Marcus et al 2016, while open ocean movements suggest utilization of prey concentrated by fronts (Ryan et al 2017). Careful isotopic analyses of R. typus may thus be particularly informative for understanding and comparing diets in and outside aggregations, especially given the species' threatened status and oceanic migrations which make lethal sampling and sustained feeding observations undesirable and difficult, respectively.…”