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Antarctic fur seal monitoring on Bird Island

Studies of Antarctic fur seal demography started in 1981 and were established as a long-term monitoring project in 1983. Three main types of studies are currently carried out:

  • demography, life histories and population dynamics;
  • female seasonal performance and attendance (to their pups);
  • diet
A male fur seal holding territory on Bird Island South Georgia. The bull male is shown guarding a hareem of 4 females.
A male fur seal holding territory on Bird Island South Georgia. The bull male is shown guarding a hareem of 4 females.

Other opportunistic studies include telemetry deployments, to study movements and distribution at sea, and collection of data on seal entanglement in man-made debris (fishing gear, packaging plastic bands, etc). Some of these data are contributed to the Convention for the Conservation of Antarctic Marine Living Resources (CCAMLR) Ecosystem Monitoring Program.

Demography, population dynamics and life histories

Demographic and population studies at Bird Island are conducted at an enclosed beach, roughly the size a tennis court (approx. 400m2) called the Special Study Beach (SSB). It has a raised walkway from which the seals can be accessed to without risk to the scientists and with minimal disturbance to the seals. Seals on the beach during the breeding season are recorded and marked with small blobs of paint. For some animals permanent marking is required using plastic cattle-ear type tags attached the front flippers and implanted PIT (Passive Integrated Transponder) tags. Many individuals are also identified between years by using genetic markers obtained from DNA in skin samples.

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Fur seal counts at SSB

In a typical breeding season between mid-November and early January around 750 pups born on this beach, plus 160 territorial males and over 900 females are recorded. Pups are particularly vulnerable in their first few months of life; in years with low food supply mothers struggle to find food and pups may starve, other risks include accidental suffocation and injuries inflicted by other seals, and birds.

During the season, at least 10 newborn pups are weighed, genetically sampled, and PIT-tag implanted, every day. When they have grown larger and are moulting into their adult coats 300 PIT-tagged pups are also given flipper tags. This combination of marks provides the best long-term method of identification so that we can follow the successful pups throughout their lives. However, only a small fraction will survive into adulthood and return to breed at the beach; like many other seals, Antarctic fur seals tend to return to breed on the beaches where they were born.

Productivity

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Fur seal pup production at SSB

Of the seals breeding at Bird Island, elephant seals are “capital breeders”. This means that females accumulate fat stores before pupping and convert these into fat-rich milk, so that they can remain ashore with their pups throughout the six weeks that they suckle. In contrast, smaller fur seals are “income breeders”, which means that in order to provide milk for their pups over their 4-month suckling period they need to constantly top up their energy reserves. This dependence on the local marine environment to rear their offspring, makes fur seal females important indicators of the state of the seasonal environment and, notably of the local availability of Antarctic krill. In relation with this, two important types of data are collected at Bird Island:

  • Monthly serial fur seal pup weights
  • Female pup attendance data.

Monthly serial fur seal pup weights.

Using fur seal pup weights at birth collected at the Special Study Beach, combined with weights of 100 pups randomly sampled when they are 1, 2 and 3 months old, allows measurement of pup growth every year. Years where growth is slow will produce lower than average monthly mean pup weights. The contrast of this performance index between years is an important indicator of the local environment and food quality.

Female attendance patterns.

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Fur seal foraging trip duration

In order for nursing females to provide milk for their pups over their 4-month suckling period, they need to top up their energy reserves constantly. To do this, they alternate feeding trips at sea (1–14 days) with short periods on land (1–4 days) giving milk to their pups. When there is a lot of food available these feeding trips are short, but in years when food is scarce mothers’ trips are longer as they have to spend more time at sea finding enough food to eat.

We record the trip durations of 20 females each year using small VHF transmitters attached to the seal’s fur. When the mother is on land a receiver can “hear” the signal transmitted by the device and it logs her presence, conversely when she is at sea no signal is heard. These logged data can be compared with previous years and, in combination with our other information collected, they can tell us about the local food resources, and especially of the Antarctic krill which makes for 75% or more of their summer diet.

Diet studies.

What fur seals eat can tell us a lot about both, what impact they are have on the local food supply, and what changes are occurring within the prey populations themselves. As we cannot directly observe what the seals are eating we have to analyse their faeces (scats). We use prey parts that are not digested e.g., fish ear bones (otoliths) and krill carapaces, to work out what size and species of prey the seals eat.

Antarctic krill length frequency
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Antarctic krill in fur seal scats

Krill, which dominate the seal’s diet, are central to the Antarctic marine food web and are important to most of the other predators breeding at Bird Island. By analysing seal scats every week (in summer) and bi-weekly (in winter, when fewer seals are around) we can monitor the size of krill all year round showing us patterns in their growth and recruitment.

Fish length and mass

When possible we identify the fish species consumed by the seals using the otoliths (fish ear bones) in scats. By measuring the size of these structures we can estimate the mean size and weight of the fish ingested. This information can help us understand how much food there is available to fur seals and how the prey composition of the fur seal diet varies over time with changes in the local ecosystem.

Other Antarctic fur seal studies

Tracking and ecology

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Fur seal foraging trip from Bird Island, South Georgia. The blue line shows movements at the water surface and dives are represented by coloured bars: blue (deep 100m +), green (40 – 100m) and red (<40m). The seal swims out from the shallow continental shelf (orange) to deeper oceanic water (blue).

The majority of a seal’s life is spent at sea and, as we cannot easily observe them during this time, we have to use other methods to monitor them. These mostly involve the use of small electronic tags that are attached to the animals. The most common type we use is called a PTT (platform transmitter terminal or satellite tag) which transmits a signal to orbiting satellites to tell us where the animal is located. In recent years Global Positioning System (GPS) tags have been developed giving much more accurate information of the seals’ movements. We can also record when the animals are diving using a Time-Depth Recorder (TDR). Combining all this information we can build up a 3D picture of the seals’ behaviour when it is out at sea. Such information can tell us how seals find and catch their prey and which areas of the ocean are important to them.

To study the seals for longer periods BAS have developed small Global Location Sensing devices (GLS), which use sunset/rise times and local noon to calculate where an animal is located. Although information from these devices is less accurate than that from GPS or PTT tags, they are smaller (approximately the size of a £1 coin) and cheaper. They also last for much longer which means we can track individuals over many years and even trace the movements of weaned pups departing on their first trips from Bird Island.

Seal entanglement

Every year a number of fur seals turn up entangled in fishing gear (nets, ropes) or other man-made debris, such as discarded packaging plastic bands, engine rings, etc. The collection of these items is opportunistic, and requires capture and disentanglement of the seals whenever they show up. A list of the entangling instances observed and entangling materials recovered is produced each year and submitted to CCAMLR.