Península Valdés (PV) is the most austral calving ground for the SW Atlantic population of Eubalaena australis. Recent studies indicate that E. australis often feeds in PV mainly in late September and October. A microscopic analysis of food chitin remains found in five whale faeces was performed in the present study in an attempt to obtain baseline knowledge about trophic ecology and degree of use of plankton food available for whales in PV during spring (September–December). The remains in faeces from stranded and live individuals included copepods, other zooplankton and centric diatoms, all of which were characterized. Copepod remains were found to be dominant. Scanning electron and confocal laser scanning microscopes were used for comparative analyses between the mandibular gnathobases found in whale faeces and those obtained from preserved specimens. Mandibular gnathobases were the same in structure and morphometry as those obtained from preserved Calanus australis (copepodites 4–6). The positive relationship observed between the total length and width of the mandibular gnathobases edge of C. australis and those found in faeces allowed us to infer the developmental stages of the copepods ingested by E. australis. Our results indicate – for the first time – the relevant role of C. australis copepodite 5 as main prey for E. australis in PV during the calving season. Copepodite 5 of C. australis accumulates energy-rich lipids. This is energetically attractive for whales and it is the potential reason why E. australis feeds mainly on dense patches dominated by this developmental stage of C. australis.

This paper reports on sightings and dispersion of individual southern elephant seals (Mirounga leonina) along the Atlantic and Pacific coasts of South America based on reports of tagged/marked seals from Patagonia and of animals of unknown origin. From 154 sightings, encompassing at least 354 individuals, we found that individuals dispersed to subequatorial latitudes on both sides of the continent, and to more temperate sites, in the Magellanic region of Tierra del Fuego. Nineteen sites were visited by tagged seals from the established colony of Península Valdés (PV, Argentina). PV and the smaller seal population of the Falklands/Malvinas were regularly connected by adults of both sexes. There were more sightings of males than females. No incipient new breeding colonies were found along the Atlantic coast of South America. Some observations coincided with places where elephant seals had been recorded or exploited in the seventeenth and eighteenth centuries. A shortage of suitable habitat for expansion and proximity to predictable food could act as a stabilizing process preventing colonization of new areas from PV. Dispersion data, coherent with population genetics, support a Patagonian elephant seal stock.

The study describes the annual cycle of southern elephant seals for the increasing colony of Península Valdés, and compares it to the haul-out pattern reported for stable or decreasing sub-Antarctic colonies of the species. Data were collected during censuses of the entire colony (nine breeding seasons, two moulting seasons and one autumn–winter haul-out), monthly surveys of coastline and mark-recapture of pups. Pup production during the study period grew from 12 113 to 14 621 animals (1995–2003). Most relevant events of the cycle were similar for all colonies studied, independent of the geographical location, size or population trend. Compared with other locations where southern elephant seals breed and moult, distinct features for Península Valdés were: i) breeding occurs earlier in the spring, and ii) most adult females reproduce and moult in the same colony. The increasing number of animals ashore was correlated with the expansion in pup production. Population trends and stability of demographic events could be related to a consistent physical environment (ocean fronts) during foraging phases of the annual cycle.