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.
Highlights
Environmental settingWithin the eastern tropical Pacific is one of the most prominent structures in the global marine environment—the equatorial Pacific upwelling system (Fig 1a)
The goal of this study is to evaluate whether, within the vast tropical Pacific ecosystem, whale sharks exhibit preferential habitat occupancy in relation to the three types of productivity enhancing ecosystems and their associated thermo-biological frontal systems
The whale sharks tracked in this study were associated with three ecosystems of the eastern tropical Pacific: (1) the open ocean equatorial upwelling system, (2) the eastern boundary upwelling system of the South Pacific, the Humboldt Current System, and (3) the Central American wind jet system nearest the equator
Summary
Within the eastern tropical Pacific is one of the most prominent structures in the global marine environment—the equatorial Pacific upwelling system (Fig 1a). Wavelike structure in SST along the equatorial upwelling (Fig 1a) is due to tropical instability waves / vortex trains [4,5]. These dynamics determine patterns of poleward advection of cold water, nutrients, and enhanced phytoplankton biomass [6,7,8,9], as well as accumulation of plankton in frontal convergence zones through physical-biological interactions [4,10,11,12]. The whale shark tag deployment site for this study is located in the equatorial Pacific ecosystem, at the northern extent of the Galapagos Islands (Fig 1a)
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