Movement ecology of young-of-the-year blue sharks Prionace glauca and shortfin makos Isurus oxyrinchus within a putative binational nursery area

Managing pelagic shark stocks requires information about the movement patterns and fisheries interactions of all age classes, including young-of-the-year (YOY; age-0), which, for many species, is particularly scant. This study investigated the vertical and horizontal movements of YOY blue sharks Prionace glauca and shortfin makos Isurus oxyrinchus in the Southern California Bight (SCB) ecoregion using pop-up satellite archival tags (PSATs) and fishery data. Geolocation estimates from PSATs (n = 5 for each species) occurred entirely within the SCB ecoregion and exclusive economic zones of the USA and México (mean tracking period: 46.1 ± 24.8 d). YOY blue sharks spent 96.4% of their time at <40 m depth (60.4% <2 m) and exhibited reverse diel vertical migration (greater nighttime depths). YOY shortfin makos spent 97.0% of their time at <40 m (24.8% <2 m) and exhibited a ‘midday dip’ of 5-10 m below the surface, centered around solar noon. These differences in vertical movements likely reflect foraging tactics. Additionally, observer data from the California drift gillnet fishery suggest juvenile blue sharks and shortfin makos move seasonally along the coast, northward from late summer to early fall and southward from late autumn to early winter. Age classes for both species exhibited latitudinal segregation, with smaller blue sharks and larger shortfin makos found farther north. The SCB ecoregion has long been regarded as a nursery area for both species; however, our data, in combination with previous studies, suggest this important habitat is much more extensive, comprising the highly productive continental margins spanning approximately 27.7-46.2°N for blue sharks and 23.4-34.5°N for shortfin makos.