Migrations of juvenile southern elephant seals from Macquarie Island

摘要

I analysed tag resight and at-sea location data collected for juvenile (> 1 - 3 year-old)udsouthern elephant seals (Mirounga leonina) from Macquarie Island to establish theseudparticular objectives:ud1. Identifying regions where the seals disperse to and/or migrate between.ud2. Establishing a spatial scale for plotting and correlating geolocations with oceanudfeatures and zones.ud3. Recording the important foraging areas essential to the survival of these seals.ud4. Quantifying the degree to which the seals share their marine habitat withuddiffering age groups and sexes of elephant seals, and with commercial fisheries.udPermanently marking seals provides information relating to haulout locality andudtiming, migration duration and age related dispersal. Southern elephant sealsudmarked at Macquarie Island disperse to distant locations where they are sometimesudseen during their moult and, for juveniles, a mid-year haulout period (July-August).udMost (87%) of the seals resighted were within 1 OOO km of Macquarie Island, andudmost, commonly at Campbell Island (700 km to the north-east). The sex and ageudclasses most likely to disperse there were males less than two years old. Maleudelephant seals of all ages were resighted significantly more often than females, theudratio being 2:1 (P > 0.05). Migration duration tended to increase with the seal's ageudbut migration distance was underestimated from resight observations whenudcompared with known telemetry records. Emigration from the Macquarie Islandudpopulation appears limited. From the resight effort at Campbell Island during 1995udthe maximum proportion of the juvenile population from Macquarie Island toudhaulout at Campbell Island was in the order of0.0053.udI tested the location estimation performance of geolocation time-depth recordersud(GLTDR) against a stationary site and free-ranging elephant seal locationsudestablished by satellite and global positioning system (GPS) telemetry. I foundudsignificant variation in latitude, but not longitude estimation, between individualudGLTDRs within the same deployment period. This indicates a high level ofudconfidence in longitude estimation if corrections for latitude are being made usingudremotely sensed data (e.g., sea-surface temperature) and/or seal behaviors such asuddive depth and haulout. There was variability in location accuracy depending on season and latitude as previous studies have found. At-sea GLTDR derived locationudestimates improved at higher latitudes where day length changes are greatest, sealudtravel rates decrease and locations are clumped in presumed foraging areas orudhaulout sites. Geolocation estimates improved as a consequence of deploymentudperiod, longer stationary periods produced better estimates with greater confidence.udThere was also considerable variation in daily location estimates of location duringudnon-equinox periods at a stationary site. These were related to some measuredudweather conditions.udAt-sea geolocations should be analysed using a scale of no less than± 2.5° (5°udgrid square) at sub-Antarctic latitudes, and at least ± 2° (4° grid square) at higherudlatitudes during non-equinox periods if a high level of confidence (95%) in positionudestimation is required. For lesser degrees of confidence (68%), the scale can beuddecreased to± 1.5° (3° grid square) at sub"'.'Antarctic latitudes and probably at higherudlatitudes also, given that the accuracy of geolocation increases with increasedudlatitude. I conclude that geolocation is a very useful method for tracking the pelagicudmovements of most land breeding marine birds, reptiles and mammals that makeudlong (ea. 1 OOO km) migrations to foraging grounds. The locations can be correlatedudto meso-scale ocean features and similarly scaled commercial fishing areas.udTwenty-two juvenile southern elephant seals aged 7 to 14 months were tracked asudthey departed Macquarie Island during 1993 and 1995. Migratory behavior andudareas of concentrated activity, presumed to be pelagic foraging grounds, wereudestablished from location data gathered by attached geolocation time-depthudrecorders. The seals ranged widely from the island preferentially migrating to theudsouth-east of Macquarie Island. Survivorship of seals that had been handled andudfitted with GLTDRs was similar to free-ranging seals of the same age that had noudinstruments attached. Thus, the methodology used for this study appears to have nouddetrimental effect on survival and this gives confidence in the results of this study.udAlthough not statistically tested, the foraging activity of seals appeared to beudassociated with meso-scale oceanographic frontal systems and near bathymetricudfeatures, especially waters near the Antarctic Polar Front (APF), the Southernudboundary of the Antarctic Circumpolar Current (SACC), the Marginal Ice-edgeudZone (MIZ), and the Campbell Plateau. Seals commonly foraged in waters greater than 3 OOO m deep where the sea-surface temperatures were between 1.0 and 4.0° Cudunless they foraged over the Campbell Plateau where the sea-surface temperaturesudwere about 8° C and the depth less than 2 OOOm.udThe foraging areas for juvenile seals overlap those of adult female seals but theudoverlap is minimal in time and space because of the differing haulout behavior anduddiving ability of the age classes. Adult male seals forage too far to the south toudoverlap with juvenile seals of either sex but there appears to be an ontogeny ofuddifferential foraging migrations with male seals from Macquarie Island similar toudthat for northern elephant seals.udSouthern elephant seals from Macquarie Island forage within managed fisheryudareas and in the high seas, an area of unregulated fishing. The Convention on theudConservation of Antarctic Marine Living Resources (CCAMLR) areas 58.4.1 andudespecially 88.1 were important foraging areas for juvenile elephant seals fromudMacquarie Island. Currently the overlap between fisheries and elephant seals fromudMacquarie Island appears minimal, but there is a lack of knowledge of the diet ofudsouthern elephant seals while at sea and in particular as they forage at the Antarcticudcontinental margin.