The optimal spatial scale for the analysis of elephant seal foraging as determined by geo-location in relation to sea surface temperatures

Abstract

The optimal spatial scale for the analysis of elephant seal foraging as determined by geo-location in relation to sea surface temperatures. -ICES Journal of Marine Science, 59: 770-781.There is increasing emphasis put on the correlation of marine predator behaviour and foraging performance with the bio-physical properties of the ocean environment. However, spatial error in the estimated position of animals and the accuracy of interpolated, physical oceanographic data require the assessment of the appropriate spatial resolution at which to assess relationships. We recorded surface temperature data from 17 archival tags attached to female southern elephant seals at Macquarie Island during the post-lactation foraging trip of 1999-2000. Archival-tag temperature data were associated with twice-daily, at-sea positions derived from light levels (i.e., ''geo-location''). We compared these surface temperatures and their associated spatial error to satellite-derived Multi-Channel Sea Surface Temperature (MCSST) data to assess at what spatial scale the agreement between the two data sources was highest. We considered scales from 50 50 km through to 500 500 km grid cells, at 50 50 km increments. Averaged over all individuals and assessed in fortnightly time periods, we found a peak in agreement between the mean surface temperature recorded by the archival tags and the MCSST data at 350 350 km grid cells (122 500 km 2 ). We used logistic regression model selection to examine the effects of spatial scale, time period, latitude, longitude, and MCSST variance per grid cell on the agreement between the data sources. The most parsimonious model included all main effects, and a significant interaction between time period and latitude. There was also a trend for reduced agreement at higher latitudes, and there was a peak in temperature agreement in the general region around 180 longitude. There were two peaks in agreement during the early and latter parts of the post-lactation foraging trip, resulting probably from the more northerly latitudes travelled during these times and the improved MCSST coverage in the summer months. The 350 350 km scale recommended by this study should be appropriate for the study of coarse-scale associations between marine predators located using geo-location and physical oceanographic data during summer. However, the approach remains valid for predators located at sea using other positioning methods.

Validation of geolocation estimates based on light level and sea surface temperature from electronic tags

“…In addition, having an error measurement now provides an opportunity to define the movements of Atlantic bluefin tunas, salmon sharks, blue sharks and a variety of other electronically tagged animals with a quantified error distribution, when using the specified tags. This will improve the analysis of the movements of these animals in relationship to oceanographic features (Bradshaw et al 2002).…”

“…Latitude is then estimated from day length while longitude is estimated from the time of local noon or midnight (Wilson et al 1992, Hill 1994, Hill & Braun 2001, Ekstrom 2004. The accuracy of the geolocation estimates influences the optimal spatial scale on which to assess the relationship between the tagged animals and physical oceanographic conditions (Bradshaw et al 2002). Therefore, improving these geolocation estimates and determining their error distributions are important priorities in the field of electronic tagging (Gunn & Block 2001).…”

Linking foraging behaviour of the northern elephant seal with oceanography and bathymetry at mesoscales

“…We did not explicitly address the question of an appropriate spatial scale for examination of the tracking data with remotely sensed oceanography, as has been advocated in other studies (Guinet et al 2001, Bradshaw et al 2002. However, the temporal and spatial scales of datasets used (SSTg = weekly, 18 km; SLA = 5 d, 1.0°; behaviour = 2 d), oceanographic features considered (mesoscale features ~100s km) and average error in animal positions (Le Boeuf et al 2000a, Vincent et al 2002 were comparable and thus appropriate.…”

Resource partitioning through oceanic segregation of foraging juvenile southern elephant seals (Mirounga leonina)

“…This time-dependent model of the variance in geolocation estimates (Sibert et al 2003) was used to provide realistic estimates of in situ movement parameters from geo-location positions while the seals were at sea. Lightderived geo-location data have inherent spatial errors up to ±350 km (van den Hoff 2002; Bradshaw et al 2002) and other parameters derived from them retain these errors.…”