Effects of nursery habitat use on ecology and population genetic structure in the scalloped hammerhead shark (Sphyrna lewini)

This study investigated nursery habitat use in the scalloped hammerhead shark (Sphyrna lewini) in order to better understand the role of nurseries in the ecology of this shark and other sharks with similar life histories. A 28-month mark-recapture study of juvenile sharks was conducted within a Hawai'i nursery (Kane'ohe Bay, O'ahu). During the study 4,120 sharks were tagged, indicating that ∼ 7700 (+/- SD 2240) hammerhead sharks are born into the bay each year. Retention over a year period within the bay is low (8--15%). A primary cause of low retention appears to be mortality from starvation. Estimated daily ration requirements from food conversion experiments (115 kJ kg-1 day-1) were consistent with previous hypotheses of high metabolic rates in these young sharks, which may help explain high observed mortality rates. Despite their apparent lack of foraging success, juvenile S. lewini use Kane'ohe Bay as a nursery for up to a year (maximum time-at-liberty = 324 days). Growth rates of recaptured sharks and size distribution of the population both supported this estimate of residence time. Nursery habitat appears to be important in the life history of S. lewini, but not necessarily as a source of plentiful prey; predator avoidance may be more valuable than food availability in nursery habitats. Female philopatry to nursery areas was tested with a global genetic survey of S. lewini (N = 271). Analyses of the mitochondrial DNA (mtDNA) control region revealed three lineages worldwide, with significant population subdivision (highest observed to date in a shark, phist = 0.467, p < 0.001). Estimates of divergence time and migration rate between adjacent populations indicate that structure within oceans is primarily due to genetic discontinuities across oceanic gaps in distribution. Nursery populations linked by continuous coastline have high connectivity, but oceanic dispersal by females appears rare. Although philopatric loyalty to natal nurseries cannot be ruled out, oceanic barriers appear to have a much stronger influence on the genetic architecture of this species and may indicate a mechanism for recent evolutionary radiations in the genus Sphyrna.