SEaOS data has provided a wealth of detail about the favoured ocean conditions, diving behaviours, and migration strategies of the southern elephant seal, and the range and distribution of seals from different colonies.
Some Helpful Definitions:
The Arctic Circumpolar Current (ACC)
A strong eastward flowing current surrounding Antarctica
Regions with strong currents (jets) where there are strong vertical gradients of ocean temperature and/or salinity.
Sub Antarctic Front (SAF)
The most northerly of the frontal systems in the region. Centered around about 51°S.
It appears that migration patterns of seals from the different colonies studied are quite distinctive. Three broad classes of behaviour have been observed:
Frontal or Inter-Frontal Pelagic:
Remaining in deep ocean within the region of the Antarctic Circumpolar Current (ACC), often travelling along or between frontal systems in the ocean, but seldom entering truly Antarctic waters.
Crossing frontal systems and travelling south through Antarctic waters,
often eventually remaining close to or on the continental shelf. Other studies have shown that benthic foraging is also common on the Patagonian Plateau east of Argentina, and south of New Zealand.
Pelagic Ice Edge:
Again, crossing Antarctic waters southward, but generally remaining in the region of pelagic (deep sea) pack ice.
These different strategies were favoured to varying degrees by seals from different colonies, as summarized briefly here:
How do these different migration patterns affect the diving behaviour of seals in different areas? The vertical bars in the chart below represent individual dives (blue and yellow are dives for which different types of information is available). The slight shading represents local day and night.
A one week snapshot of the dive record of a seal (drag scrollbar to view more).
This seal shows a very clear so-called ‘diurnal pattern’, with shallow dives at night and deep dives during the day. Not all seals show such distinct diurnal behaviour. It appears to be most common among seals in the frontal/inter-frontal regions. On the continental shelves, seals almost always dive to the bottom.
A longer snapshot of about 4 months of a seal travelling along and between fronts in the ACC.
The coloured field represents water temperatures recorded by the tag, going from blue (cold) to red (warm).
The dark blue and yellow dashed lines represent average night-time and daytime dive depths respectively.
Early in the trip, night-time dives appear to track the temperature minimum layer at night. In late April, the seal seems to spend the nights just underneath a region of sharp temperature decrease, and the seal tracks this region as it deepens in late May when the seal crosses the Sub-Antarctic Front northwards.
Such areas of sharp gradients in water properties can often act as barriers for many smaller organisms, and they are therefore likely to represent areas of increased food densities from which the seals may profit.
The shape of an elephant seal's dive
is called the dive profile.
Once we know where seals go, what they do and what it is like there, the next question is: what are the consequences, terms of feeding and body condition, of being in these locations? The shapes of some dive profiles can help us understand this.
As the seal's condition improves with feeding, it will gain more body fat, and so become more bouyant. During a particular type of dive, called ‘drift dives’, seals swim downwards, then drift passively for a long time, before swimming actively back to the surface. We are not sure why they do this, but it is possible that seals rest while drifting. Resting during dives may avoid the risk of being attacked by sharks or killer whales while resting at the surface. Whatever the reason, we can use the drift rate to estimate body condition.
A seal with relatively less body fat tends to drift downwards more than a fatter animal, and studying changes in dive profiles therefore allows researchers to gain a general understanding of where and when a seal is feeding successfully.
drag slider to vary body fat:
The graph shows how the drift rate of a seal from Macquarie Island changed over the course of a journey lasting 230 days.
After about 50 days out from Macquarie the rate of drift (during this part of the seal's dives) begins to gradually rise as the seal puts on fat.
From around 175 days there is a gradual decline indicating a loss of fat, presumably the seal is not feeding.