Abstract
BackgroundPredators can impact ecosystems through trophic cascades such that differential patterns in habitat use can lead to spatiotemporal variation in top down forcing on community dynamics. Thus, improved understanding of predator movements is important for evaluating the potential ecosystem effects of their declines.Methodology/Principal FindingsWe satellite-tagged an apex predator (bull sharks, Carcharhinus leucas) and a sympatric mesopredator (Atlantic tarpon, Megalops atlanticus) in southern Florida waters to describe their habitat use, abundance and movement patterns. We asked four questions: (1) How do the seasonal abundance patterns of bull sharks and tarpon compare? (2) How do the movement patterns of bull sharks and tarpon compare, and what proportion of time do their respective primary ranges overlap? (3) Do tarpon movement patterns (e.g., straight versus convoluted paths) and/or their rates of movement (ROM) differ in areas of low versus high bull shark abundance? and (4) Can any general conclusions be reached concerning whether tarpon may mitigate risk of predation by sharks when they are in areas of high bull shark abundance?Conclusions/SignificanceDespite similarities in diet, bull sharks and tarpon showed little overlap in habitat use. Bull shark abundance was high year-round, but peaked in winter; while tarpon abundance and fishery catches were highest in late spring. However, presence of the largest sharks (>230 cm) coincided with peak tarpon abundance. When moving over deep open waters (areas of high shark abundance and high food availability) tarpon maintained relatively high ROM in directed lines until reaching shallow structurally-complex areas. At such locations, tarpon exhibited slow tortuous movements over relatively long time periods indicative of foraging. Tarpon periodically concentrated up rivers, where tracked bull sharks were absent. We propose that tarpon trade-off energetic costs of both food assimilation and osmoregulation to reduce predation risk by bull sharks.
Highlights
Because movement promotes energy flow across habitat boundaries [1,2], ecological and evolutionary processes are inherently linked to movement, including ecosystem function and biodiversity [3]
The bulk of the migratory front arrives in late spring and departs the area by early-summer
We propose several hypotheses relating to optimal foraging strategies of both tarpon and bull sharks to explain these observed patterns
Summary
Because movement promotes energy flow across habitat boundaries [1,2], ecological and evolutionary processes are inherently linked to movement, including ecosystem function and biodiversity [3]. We conducted a joint tagging study of bull sharks and Atlantic tarpon in southern Florida to describe their spatial distribution, habitat use and movement patterns relative to one another.
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