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Changes in Diel Diving Patterns Accompany Shifts Between Northern Foraging and Southward Migration in Leatherback Turtles

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Changes in Diel Diving Patterns Accompany Shifts Between Northern Foraging and Southward Migration in Leatherback Turtles 754 Changes in diel diving patterns accompany shifts between northern foraging and southward migration in leatherback turtles M.C. James, C.A. Ottensmeyer, S.A. Eckert, and R.A. Myers Abstract: Diel diving patterns have been widely documented among plankton-feeding marine vertebrates. In many cases, these patterns have been interpreted as a response to the diel vertical migrations of prey. The leatherback turtle, Dermochelys coriacea (Vandelli, 1761), is a large marine predator that exploits gelatinous plankton in disparate foraging areas. Indi- viduals of this species spend extended periods at northern latitudes before moving southward through pelagic waters. To identify and compare potential diel patterns of diving behaviour in temperate areas, where foraging has been ob- served, versus during southward migration, 15 subadult and adult leatherbacks were equipped with satellite-linked time–depth recorders off Nova Scotia, Canada. We observed variation in nocturnal versus diurnal behaviour, both at northern latitudes and during migration; however, diel differences in both diving and surface activity were much less pronounced while leatherbacks were in the north. We interpret the difference in leatherback diel diving regimen to re- flect a response to changing resource conditions at these times, with leatherbacks foraging throughout the day and night at high latitudes, then changing to a bimodal pattern of diving during southward migration, with generally lon- ger, deeper diving occurring during the night versus during the day. By quantifying diel changes in leatherback behav- iour, we provide the first surface time correction factors based on multiple individuals for use in estimating abundance from aerial surveys. Re´sume´ : Les patrons journaliers de plonge´e chez les verte´bre´s marins planctonophages ont e´te´ bien e´tudie´s. Souvent, les patrons sont interpre´te´s comme des re´actions aux migrations verticales journalie`res des proies. La tortue luth, Dermochelys coriacea (Vandelli, 1761), est un pre´dateur marin de grande taille qui exploite le plancton ge´latineux dans des aires d’ali- mentation disparates. Les individus de cette espe`ce passent de longues pe´riodes dans les latitudes nordiques avant de se de´- placer vers le sud dans les eaux pe´lagiques. Dans le but d’identifier et de comparer les patrons journaliers potentiels du comportement de plonge´e dans les zones tempe´re´es ou` on observe de l’alimentation (par rapport aux patrons observe´s du- rant la migration vers le sud), nous avons muni 15 tortues subadultes et adultes d’enregistreurs de temps et de profondeur relie´sa` des satellites au large de la Nouvelle-E´ cosse, Canada. Nous avons observe´ une variation entre les comportements nocturne et diurne, tant dans les latitudes nordiques que durant la migration; les variations journalie`res d’activite´ en plon- ge´e et en surface sont, cependant, beaucoup moindres lorsque les tortues sont dans le nord. Nous interpre´tons les diffe´ren- ces dans le re´gime journalier de plonge´e des tortues luth comme des re´actions aux conditions changeantes des ressources a` ce moment; aux latitudes e´leve´es, les tortues s’alimentent tout au long de la journe´e et de la nuit; durant la migration vers le sud, elles adoptent un patron bimodal de plonge´e, avec ge´ne´ralement des plonge´es plus profondes et plus prolonge´es du- rant la nuit que durant le jour. En quantifiant les changements journaliers du comportement des tortues luth, nous fournis- sons les premiers facteurs de correction du temps passe´ en surface base´s sur l’observation de plusieurs individus pour servir a` estimer l’abondance a` partir d’inventaires ae´riens. [Traduit par la Re´daction] Introduction diverse range of taxa that includes small benthic herbivores (Rogers et al. 1998) and large pelagic vertebrates (Musyl et A common pattern among marine organisms, diel vertical al. 2003; Weng and Block 2004). However, such diel move- migration (DVM), normally involves descent to deeper ments, which are often mediated by light levels, have been waters at dawn, followed by an ascent towards the surface most widely reported in zooplankton (e.g., Forward 1988). at dusk (Hays 2003). DVM has been documented across a DVM has been interpreted as a means by which organisms avoid predators or maximize feeding efficiency, with a Received 19 October 2005. Accepted 3 March 2006. Published trade-off between these motivations shaping DVM in some on the NRC Research Press Web site at http://cjz.nrc.ca on species (Loose and Dawidowicz 1994; Bollens 1996). Inter- 14 June 2006. pretation of movement data collected from several plankti- M.C. James,1 C.A. Ottensmeyer, and R.A. Myers. Department vores has, therefore, been enhanced by understanding DVM of Biology, Dalhousie University, 1355 Oxford Street, Halifax, in their prey. It has been suggested that diel dive patterns in NS B3H 4J1, Canada. whale sharks, Rhincodon typus Smith, 1828, may represent S.A. Eckert. Marine Laboratory, Duke University, 135 Duke the tracking of zooplankton prey (Graham et al. 2005) and Marine Lab Road, Beaufort, NC 28516-9721, USA. diel changes in the depth preferences of basking sharks, Ce- 1Corresponding author (e-mail: [email protected]). torhinus maximus (Gunnerus, 1765), are consistent with a Can. J. Zool. 84: 754–765 (2006) doi:10.1139/Z06-046 # 2006 NRC Canada James et al. 755 response to DVM in zooplankton (Sims et al. 2003). Simi- iour of leatherback turtles equipped with satellite tags off lar findings have been reported in Atlantic redfishes, genus Nova Scotia, Canada. In particular, we aimed (i) to identify Sebastes Cuvier, 1829 (Gauthier and Rose 2002). Among potential consistent differences in day and night behaviours, predators of gelatinous zooplankton, the ocean sunfish, (ii) to compare diel behaviour patterns between northern Mola mola (L., 1758), also exhibits patterns of DVM that areas where foraging is known to occur and more pelagic may parallel those of their prey (Cartamil and Lowe 2004). waters during southward movements of leatherbacks, and Patterns of DVM observed in planktivorous species may (iii) to estimate the proportion of time leatherbacks spend at not only reflect changing prey availability, but also selec- and near the surface to identify optimal areas for aerial cen- tion for environmental conditions such as ambient light sus of this species and to guide the choice of sighting cor- (Nelson et al. 1997). In addition, patterns of DVM in rection factors for use in aerial surveys. planktivores can be variable across a species’ range. For example, assumption of ‘‘normal’’ or ‘‘reverse’’ (ascent at Methods dawn, descent at dusk) DVM in basking sharks appears to be habitat-specific (Sims et al. 2005). During the summers of 2001–2003, we captured leather- The leatherback turtle, Dermochelys coriacea (Vandelli, back turtles at the surface in waters off Nova Scotia, Can- 1761), is a large marine vertebrate that exploits gelatinous ada, using a breakaway hoop net operated from a 10.5 m zooplankton (primarily members of phyla Cnidaria and Cte- commercial fishing boat (for methods see James et al. nophora) (Bleakney 1965; den Hartog and van Nierop 2005a). We attached satellite-linked time–depth recorders 1984). The leatherback is the only marine turtle to specialize (SLTDR; models SSC3 (n = 13) and SDR-T16 (n =2); on gelatinous prey as an adult. It is also unique among sea Wildlife Computers, Redmond, Washington) to the carapace turtles in its ability to inhabit cold northern waters (James using a custom-fitted harness made of nylon webbing and and Mrosovsky 2004), with some adult and subadult leather- polyvinyl tubing (modified after Eckert 2002). Harnesses in- backs completing annual round-trip migrations from tropical corporated corrodible links to ensure their eventual release. waters to temperate waters (James et al. 2005a) to forage on Leatherbacks were repeatedly doused with buckets of sea large jellyfish (James and Herman 2001). Distinct ‘‘phases’’ water while aboard and were normally released within of the migratory cycle have been delineated by dramatic 30 min of capture. All procedures were in accordance with changes in suites of behaviours (James et al. 2005b), but the principles and guidelines of the Canadian Council on only at a resolution of 24 h periods. Diel dive patterns have Animal Care, approved by the Dalhousie University Com- been described for female leatherbacks during internesting mittee on Laboratory Animals, and licensed by Fisheries intervals in tropical waters (Eckert et al. 1989, 1996; Eckert and Oceans Canada. 2002) and during departure from nesting areas (Hays et al. SLTDRs collected and relayed data on time at depth, time 2004). However, analyses of diel patterns of diving behav- at temperature, maximum dive depth, and dive duration that iour have been reported neither for leatherbacks in northern were binned within 14 user-defined data ranges over 6 h foraging areas nor for leatherbacks during southward migra- collection periods. Periods were set so that one consistently tion from these habitats. encompassed night and one encompassed day — night: Unfortunately, very little is known about the distribution 2100–0300; morning: 0300–0900; day: 0900–1500; evening: and movements of gelatinous zooplankton (Graham et al. 1500–2100; Atlantic daylight time. Time at depth and time 2003), particularly in the open ocean (cf. Harbison et al. at temperature reflected all time when SLTDRs were sub- 1978). Instead, most research has focused on the biology of merged, whereas dives were registered only when leather- these organisms in coastal and shelf waters (Brewer 1989; backs descended below 4 m (n = 12 tags) or 6 m (n =3 Purcell 1992; Olesen et al. 1994; Buecher et al. 2001) and tags). While SLTDRs simultaneously record data from dif- in aquaria (Mackie et al. 1981; Mills 1983; Costello and ferent channels (e.g., depth, temperature), data are transmit- Colin 1994; Hansson 1997).
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