Efficient abstracting of dive profiles using a broken-stick model

• Published in: Methods in Ecology and Evolution
• Main Authors: Photopoulou, T., Lovell, Philip, Fedak, M.A., Thomas, L., Matthiopoulos, J.
• Other Authors: NERC, University of St Andrews. School of Biology, University of St Andrews. Marine Alliance for Science & Technology Scotland, University of St Andrews. Scottish Oceans Institute, University of St Andrews. Sea Mammal Research Unit, University of St Andrews. Statistics, University of St Andrews. Centre for Research into Ecological & Environmental Modelling
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2016
• Subjects: Animal telemetry; Broken-stick model; CTD-SRDL; Dive profile; Dive type; Dive zone index; Elephant seal; Data abstraction; QL Zoology; QH301 Biology; Environmental Science (miscellaneous); DAS; QL; QH301; Ner; Elephant Seal; Elephant Seals
• Online Access: https://hdl.handle.net/10023/7972; https://doi.org/10.1111/2041-210X.12328; http://onlinelibrary.wiley.com/doi/10.1111/2041-210X.12328/suppinfo; http://www.smru.st-and.ac.uk/Instrumentation/Overview/

This work was supported by SMRU Ltd (now SMRU Marine) in the form of a PhD fellowship (T.P.). Completion of the manuscript was supported by a National Research Foundation Scarce Skills Postdoctoral Fellowship at the University of Cape Town, South Africa (T.P.). The CTD-SRDL data presented in this manuscript were collected as part of a project funded by the Natural Environment Research Council (NERC) grants NE/E018289/1 and NER/D/S/2002/00426. For diving animals, animal-borne sensors are used to collect time-depth information for studying behaviour, ranging patterns and foraging ecology. Often, this information needs to be compressed for storage or transmission. Widely used devices called conductivity-temperature-depth satellite relay data loggers (CTD-SRDLs) sample time and depth at high resolution during a dive and then abstract the time-depth trajectory using a broken-stick model (BSM). This approximation method can summarize efficiently the curvilinear shape of a dive, using a piecewise linear shape with a small, fixed number of vertices, or break points. We present the process of abstracting dives using the BSM and quantify its performance, by measuring the uncertainty associated with the profiles it produces. We develop a method for obtaining a confidence zone and an index for the goodness-of-fit (dive zone index, DZI) for abstracted dive profiles. We validate our results with a case study using dives from elephant seals (Mirounga spp.). We use generalized additive models (GAMs) to determine whether the DZI can be used as a proxy for an absolute measure of fit and investigate the relationship between the DZI and the dive shape. We found a strong correlation between the residual sum of squares (RSS) for the difference between the detailed and abstracted profiles, and the DZI and maximum residual (R4), for dives resulting from CTD-SRDLs (69% deviance explained). On its own, the DZI explained a lower percentage of deviance which was variable for abstracted dives with different numbers of break points. We ...