Sex-specific variation in the use of vertical habitat by a resident Antarctic top predator

Abstract

Patterns of habitat use are commonly studied in horizontal space, but this does not capture the four-dimensional nature of ocean habitats (space, depth, and time). Deep-diving marine animals encounter varying oceanographic conditions, particularly at the poles, where there is strong seasonal variation in vertical ocean structuring. This dimension of space use is hidden if we only consider horizontal movement. To identify different diving behaviours and usage patterns of vertically distributed habitat, we use hidden Markov models fitted to telemetry data from an air-breathing top predator, the Weddell seal, in the Weddell Sea, Antarctica. We present evidence of overlapping use of high-density, continental shelf water masses by both sexes, as well as important differences in their preferences for oceanographic conditions. Males spend more time in the unique high-salinity shelf water masses found at depth, while females also venture off the continental shelf and visit warmer, shallower water masses. Both sexes exhibit a diurnal pattern in diving behaviour (deep in the day, shallow at night) that persists from austral autumn into winter. The differences in habitat use in this resident, sexually monomorphic Antarctic top predator suggest a different set of needs and constraints operating at the intraspecific level, not driven by body size.

Weddell seal behaviour during an exceptional oceanographic event in the Filchner-Ronne Ice Shelf in 2017

“…Both the female and the three males in our study displayed a mixture of pelagic and benthic dives; however, pelagic dives were more common that benthic dives for both sexes. This is in contrast to what Photopoulou et al (2020) found, where males performed only benthic dives on the shelf and females left the shelf and moved northward. We found no obvious sex-specific differences nor any seasonal shifts in behaviour, but given that our sample size was small, this conclusion is not definitive.…”

“…hunting time per dive) was the greatest. This is a novel observation and in contrast to the earlier findings from the Weddell Sea (Nachtsheim et al 2019, Photopoulou et al 2020). Seals were found within the outflow, at the mouth of the trough, over the sill or travelling N-S over the central part or over the eastern side at the boundary of ISW or within it.…”

“…This period corresponds with the strongest inflow of MWDW and warmest temperatures along the bottom (Ryan et al 2017, 2020). Photopoulou et al (2020) and Nachtsheim et al (2019) reported seal diving behaviour in 2011 and 2014, respectively, on the western side of the Filchner trough in MWDW. Although two seals were tagged in the western part and one in the southern part in the present study, they all converged and remained in the eastern part later in the season.…”

“…Weddell seals have been intensively studied in the western Ross Sea and East Antarctica (e.g. Stirling 1969, Harcourt et al 2000, Burns & Kooyman 2001, Hindell et al 2002, Lake et al 2005, Wheatley et al 2006, Heerah et al 2013); however, our understanding of their ecology in the Weddell Sea in the wintertime is still sparse (Langley et al 2018, Nachtsheim et al 2019, Photopoulou et al 2020). Here, we document the behaviour and habitat utilization of four Weddell seals in the Filchner-Ronne Ice Shelf region in 2017.…”

“…Several studies documented important foraging efforts in the MWDW or Modified Circumpolar Deep Water (MCDW) by southern elephant seals (Biuw et al 2007, Labrousse et al 2015, Hindell et al 2016) and by Weddell seals (Heerah et al 2013, Nachtsheim et al 2019, Photopoulou et al 2020). Photopoulou et al (2020) and Nachtsheim et al (2019) reported seal diving behaviour in 2011 and 2014, respectively, on the western side of the Filchner trough in MWDW. Here, we report the behavioural response(s) of four Weddell seals to the specific oceanographic conditions of the region in 2017, which had longer and warmer MWDW inflow than usual.…”