Eating between the lines: functional feeding response of bonnetheads (Sphyrna tiburo)

Abstract

Mobile mesopredators can have strong influences in the structuring of marine food webs in shallow coastal ecosystems. Few quantitative estimates of the effects of predation on prey populations by highly mobile mesopredators exist, yet they are necessary to evaluate the impact mesopredators have on prey resources. Quantifying a predator’s functional response provides valuable information on how a predator’s per capita consumption rate of prey can influence community structure and prey populations. We examined the functional response of bonnetheads (Sphyrna tiburo) to one of its few natural prey items brown shrimp (Farfantepenaeus aztecus) to determine if per capita feeding rates would increase with increases in prey density. We simulated natural conditions in outdoor mesocosms and offered live prey at varying densities. The functional response of a predator can take one of three forms: linear (type I), rise to an asymptote (type II), and a sigmoid shape (type III). Bonnetheads consumed prey proportional to prey density and demonstrated a response that was in between a type I (R2 = 0.847, p < 0.01) and type II (R2 = 0.877, p < 0.01) functional response. Bonnetheads showed an increased rate of consumption with initial low prey densities but high densities of available prey did not consistently result in increased consumption rates. At the maximum prey density offered, bonnetheads consumed less prey than at intermediate densities. Bonnetheads may not have as strong of an influence on prey populations at high prey densities compared to predation impacts at low densities. Bonnethead per capita feeding rates did not conform to a type I, type II, or type III functional response and our results suggest that bonnetheads follow a functional response continuum more closely. This study highlights the importance and difficulty in understanding the feeding ecology of highly mobile mesopredators.