title: Growth dynamics of juvenile loggerhead sea turtles undergoing an ontogenetic habitat shift.
abstract: Ontogenetic niche theory predicts that individuals may undergo one or more changes in habitat or diet throughout their lifetime to maintain optimal growth rates, or to optimize trade-offs between mortality risk and growth. We combine skeletochronological and stable nitrogen isotope (δ15N) analyses of sea turtle humeri (n = 61) to characterize the growth dynamics of juvenile loggerhead sea turtles (Caretta caretta) during an oceanic-to-neritic ontogenetic shift. The primary objective of this study was to determine how ontogenetic niche theory extends to sea turtles, and to individuals with different patterns of resource use (discrete shifters, n = 23; facultative shifters n = 14; non-shifters, n = 24). Mean growth rates peaked at the start of the ontogenetic shift (based on change in δ15N values), but returned to pre-shift levels within 2 years. Turtles generally only experienced 1 year of relatively high growth, but the timing of peak growth relative to the start of an ontogenetic shift varied among individuals (before, n = 14; during, n = 12; after, n = 8). Furthermore, no reduction in growth preceded the transition, as is predicted by ontogenetic niche theory. Annual growth rates were similar between non-transitioning turtles resident in oceanic and neritic habitats and turtles displaying alternative patterns of resource use. These results suggest that factors other than maximization of size-specific growth may more strongly influence the timing of ontogenetic shifts in loggerhead sea turtles, and that alternative patterns of resource use may have limited influence on somatic growth and age at maturation in this species.
authors.affiliation: Department of Fisheries and Wildlife, Oregon State University, 104 Nash Hall, Corvallis, OR, 97331, USA. matthew.ramirez@oregonstate.edu.
authors.forename: Matthew D
authors.initials: MD
authors.lastname: Ramirez
authors.affiliation: NOAA National Marine Fisheries Service, Southeast Fisheries Science Center, Beaufort Laboratory, 101 Pivers Island Road, Beaufort, NC, 28516, USA.
authors.forename: Larisa
authors.initials: L
authors.lastname: Avens
authors.affiliation: NOAA National Marine Fisheries Service, Southwest Fisheries Science Center, 8901 La Jolla Shores Drive, La Jolla, CA, 92037, USA.
authors.forename: Jeffrey A
authors.initials: JA
authors.lastname: Seminoff
authors.affiliation: NOAA National Marine Fisheries Service, Southeast Fisheries Science Center, Beaufort Laboratory, 101 Pivers Island Road, Beaufort, NC, 28516, USA.
authors.forename: Lisa R
authors.initials: LR
authors.lastname: Goshe
authors.affiliation: Department of Fisheries and Wildlife, Oregon State University, 104 Nash Hall, Corvallis, OR, 97331, USA.
authors.forename: Selina S
authors.initials: SS
authors.lastname: Heppell
mesh_terms: D000818:Animals; D017753:Ecosystem; D005247:Feeding Behavior; D009587:Nitrogen Isotopes; D009792:Oceans and Seas; D014426:Turtles
chemical_list: D009587:Nitrogen Isotopes
keywords: Growth rates; Life history variation; Ontogenetic niche theory; Skeletochronology; Stable nitrogen isotopes