Walruses are homebodies

If you think the walrus you see the second time you pass the haul-out site at Sarstangen looks familiar, you may be right. Here’s how researchers know that walruses often revisit favoured sites.

The walruses that live in Svalbard and Franz Josef Land are a genetically distinct population. Hunted nearly to extinction, they were protected in 1952 and have recovered since then. Aerial surveys of the part of the population that lives in Svalbard show that their numbers have more than doubled from around 2 600 in 2006 to around 5 500 in 2018. Svalbard’s walrus population is heavily dominated by males, although the number of mother–calf pairs has been increasing recently, particularly in northeastern Svalbard.

Seals can be tracked by gluing biologging devices to their fur. Walruses have little fur, but are unique in having tusks—a perfect place to attach biologging instruments. Accordingly, many studies have used tusk-mounted tracking devices to trace walruses throughout their range, shedding light on their movements and diving behaviour. Previous tracking studies of the Svalbard–Franz Josef Land population have revealed that many male walruses migrate from Svalbard towards Franz Josef Land in the autumn/winter, most likely because females can be found there. Several breeding areas have been identified deep in the sea ice between Svalbard and Franz Josef Land. Additionally, a few long (> 1 year) tracking data sets have shown that some walruses return to more or less the same foraging grounds from one summer to the next. However, we don’t know whether this is a general phenomenon, nor whether individuals return to the same breeding grounds the following year.

Although walrus tusks are an ideal platform for mounting instruments, there have been some logistical challenges. The main problem has been a high risk of mortality when the walruses were sedated. For our study, we developed a novel method to immobilise the walruses. It involves use of a new reversal agent in addition to intubation and ventilation with 100% oxygen during handling and recovery. This procedure was extremely successful: no walruses were lost.

For our study, a new Global Positioning System (GPS) tracking device custom-designed for walrus deployments was developed in collaboration with Sirtrack (now Lotek.com) in New Zealand. The devices acquire a GPS fix every hour and are powered by lithium batteries with an expected lifetime of 5 years. GPS data collected by the loggers is downloaded remotely to receiver stations (mobile or stationary) via radio signals at distances up to 500 metres. Stationary receiver stations were erected at eight different walrus haul-out sites in Svalbard. With these new dataloggers we could follow walruses for several years, studying their movements, choice of habitat, when they went where, and whether they returned to the same place.

GPS loggers were deployed on 40 male walruses at various sites in Svalbard. Information about the 12 walruses whose loggers reported GPS data for more than a year was analysed. On average we had positions for 933 days for each animal; the longest record was 6.2 years. The walruses displayed clear seasonal migratory behaviour and distinct overlap in both summer and winter areas in consecutive years. Individuals migrating from south of Svalbard all returned to the south in the summer, and those from north of Svalbard all returned to their northern summer grounds. In winter they all migrated east or northeast to the areas around Kvitøya and Victoria Island, as also seen in previous studies. Summer core habitats were characterised by shallow water depths (median 18–89 m). Winter areas had deeper water (median 91–144 m). Sea ice concentrations varied considerably from year to year and place to place, but winter areas generally had more sea ice than summer areas. All summer core areas had relatively high sea ice concentrations early in the summer, but were ice-free by midsummer.

The total distance a walrus travelled per year ranged from 5 225–10 406 km and this distance was remarkably similar on an individual basis across years. Also, the timing of an individual’s seasonal migrations was remarkably similar from year to year, but varied greatly between individuals. While neither sea ice nor total daylight hours had clear impact on migration timing, sea ice did impact movement behaviour. Walruses took more tortuous paths when sea ice concentrations were high, both summer and winter. 

Site fidelity normally develops when animals experience areas where vital resources are predictably available (here: food and potential mates). Ongoing climate change is expected to drastically change the environment, which might affect the predictability of resources vital to walruses. It remains to be seen if these animals will be able to shift from their seemingly quite fixed routines.