Comparative analysis of methods for inferring successful foraging areas from Argos and GPS tracking data

Ac Dragon,P Monestiez,A Bar-Hen,C Guinet

doi:10.3354/meps09618

Abstract

Identifying animals' successful foraging areas is a major challenge, but such comprehensive knowledge is needed for the management and conservation of wild populations. In recent decades, numerous specific analytic methods have been developed to handle tracking data and to identify preferred foraging areas. In this study, we assessed the efficiency of different track-based methods on Argos and GPS predators' tracks. We investigated (1) the consistency in the detection of foraging areas between track-based methods applied to 2 tracking data resolutions and (2) the similarity of foraging behaviour identification between track-based methods and an independent index of foraging success. We focused on methods that are commonly used in the literature: empirical descriptors of foraging effort, Hidden Markov Models (HMMs) and first passage time analysis. We applied these methods to satellite tracking data collected on 6 long-ranging elephant seals equipped with both Argos and GPS tags. Seals were also equipped with time depth recorder loggers from which we estimated an independent index, based on the drift rate and the changes in the seals' body condition, as a proxy for foraging success along the tracks. Favourable foraging zones identified by track-based methods were compared to locations where the body condition of the seals significantly increased. With or without an environmental covariate, HMMs were the most reliable for identifying successful foraging areas on both high (GPS) and low (Argos) resolution data. Areas identified by HMMs as intensively used were congruent with the locations where seals significantly increased their body condition given a 4 d metabolisation lag.

Highlights

In natural habitats, individual fitness directly depends on animals’ ability to acquire resources in an efficient way (Stevick et al 2002)
We focused on methods that are commonly used in the literature: empirical descriptors of foraging effort, Hidden Markov Models (HMMs) and first passage time analysis
For GPS data, track-based methods had similar ranges of time spent on intensive foraging (27.5% ±13.7)

Summary

Introduction

Individual fitness directly depends on animals’ ability to acquire resources in an efficient way (Stevick et al 2002). While foraging, predators often display movement patterns at multiple spatial and temporal scales that are assumed to match the spatial structure of their prey environment (Fauchald 1999). In a prey-aggregated environment such as the open ocean, it is predicted that a predator having already captured a first prey would intensify its foraging in the patch (Charnov 1976, Parker & Stuart 1976). This results in a behaviour called the area-restricted search (ARS, Kareiva & Odell 1987) and is defined by a decrease in speed and an increase in sinuosity of movement (Benhamou & Bovet 1989). Track-based measurements of free-ranging predators are thought to provide information on their foraging behaviour

Results

Discussion

Conclusion