Risky business for a juvenile marine predator? Testing the influence of foraging strategies on size and growth rate under natural conditions

Abstract

Mechanisms driving selection of body size and growth rate in wild marine vertebrates are poorly understood, thus limiting knowledge of their fitness costs at ecological, physiological and genetic scales. Here, we indirectly tested whether selection for size-related traits of juvenile sharks that inhabit a nursery hosting two dichotomous habitats, protected mangroves (low predation risk) and exposed seagrass beds (high predation risk), is influenced by their foraging behaviour. Juvenile sharks displayed a continuum of foraging strategies between mangrove and seagrass areas, with some individuals preferentially feeding in one habitat over another. Foraging habitat was correlated with growth rate, whereby slower growing, smaller individuals fed predominantly in sheltered mangroves, whereas larger, faster growing animals fed over exposed seagrass. Concomitantly, tracked juveniles undertook variable movement behaviours across both the low and high predation risk habitat. These data provide supporting evidence for the hypothesis that directional selection favouring smaller size and slower growth rate, both heritable traits in this shark population, may be driven by variability in foraging behaviour and predation risk. Such evolutionary pathways may be critical to adaptation within predator-driven marine ecosystems.

Evidence for rapid recovery of shark populations within a coral reef marine protected area

“…Growth rates of juveniles could have increased both through the loss of conspecific competitors and the ability for smaller juveniles to occupy high-quality habitats where they might have been otherwise excluded or predated on by larger adults (e.g. Hussey et al 2017). Additionally, targeted fishing for sharks also captures large mesopredatory teleosts, which are likely to act as competitors to reef sharks (Barley et al 2017a, b;Roff et al 2016;Ruppert et al 2013).…”

Size frequency, dispersal distances and variable growth rates of young sharks in a multi‐species aggregation

“…This results in a wide range of growth rates that are to date the broadest reported in young wild sharks. In a recent study , variable growth rates of N. brevirostris were attributed to different foraging strategies, with slow growing individuals (1.4 cm year −1 L PC ) foraging predominantly in mangrove habitats (predator safe) and fast‐growing individuals (9.5 cm year −1 L PC ) more likely to forage over predator exposed seagrass beds (Hussey et al , ). Unlike Bimini, where a resource abundant (Newman & Gruber, ) and mono‐species habitat implies a lack of competitive patterns, St. Joseph is an oligotrophic and potentially resource‐limited habitat.…”

Effects of anticoagulants on stable‐isotope values (δ13C and δ15N) of shark blood components

“…The use of blood fractions, specifically red blood cells (RBC) and blood plasma, has increased over recent years (Vander‐Zanden et al ., ). RBC and plasma are especially useful to assess trophic positions and foraging behaviours of juvenile sharks (Hussey et al ., ; Kinney et al ., ; Matich et al ., ; Matich, Kiszka, et al ., ), because they incorporate trophic interactions over different time scales and can be collected non‐lethally. RBC stable‐isotope values reflect energy sources ( e.g ., foraging and maternal provisions) over extended periods (multiple months), while plasma stable‐isotope values represent more recent trophic interactions (weeks to months; McMeans et al ., ; Vaudo et al ., ; Matich et al ., ).…”