Higher Predators – Bird Island – Seal monitoring
There are three seal species that commonly come ashore (haul out) at Bird Island, to breed, moult, and rest: Antarctic fur seal (Arctocephalus gazella), southern elephant seal (Mirounga leonina) and leopard seal (Hydrurga leptonyx).
The most abundant, and the main focus of studies at Bird Island, is the Antarctic fur seal, which is not a ‘true’ seal but an otarid (eared seal), a group that includes fur seals and sea lions. Antarctic fur seals are important predators of Antarctic krill and fish species dependent on the krill in the Scotia Sea ecosystem. Studies of fur seal ecology tell us about what is happening in the marine environment; because of its size, abundance and accessibility, the Antarctic fur seal is an excellent research species to investigate diet, physiology, behaviour, genetics, life history and population dynamics.
Antarctic fur seals breed between mid-November and early January and approximately 70,000 pups are born on Bird Island. Mothers rear their pups for up to four months and weaning occurs in April. Mothers combine short (2–14 days) feeding trips to sea with periods ashore (1–4 days) suckling their pups on energy-rich milk. During winter, many weaned pups and older juveniles stay around the island so that there is not a single month of the year without fur seals present. Antarctic fur seals were almost extinct by the early 20th Century, as they were hunted for their luxurious fur. Bird Island was one of the places where the recovery of the species started, after the discovery of a small breeding colony in 1930. Antarctic fur seal research on the island started in 1958 and continued during intermittent studies until 1981, when the British Antarctic Survey established their permanent year round base and continuous annual monitoring was possible.
Antarctic Fur Seals
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);
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.
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.
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
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.
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
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.
Tracking and ecology
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.
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.
Leopard seals have been monitored at Bird Island since the early 1980s and in 1993 the first individuals were given identification tags to initiate a long-term study. In 2005 the tagging programme was augmented with photo-identification, to improve individual recognition rates. Modelling the prey choice, based on opportunistic diet data collection, together with the arrival, residence and departure times of monitored individuals we obtain estimates of seasonal abundance of leopard seals and prey consumption. Three main sources of data are produced as part of this programme.
Total seasonal counts
All observations of leopard seal individuals (identified or unidentified) are recorded on a daily basis during the austral winter and early spring, when the seals disperse from the pack ice and temporarily use Bird Island to find prey and rest ashore.
Seasonal number of individuals
Using the individually unique fur patterns of leopard seals through photo-identification, and reading the plastic tags given to a number of individuals every year, we produce the minimum observed number of individuals.
Opportunistic diet data collection
Using observations on feeding behaviour and scats we study the seals’ diet throughout the winter. The diet data are used in studies of prey-preference variability (between individuals and over time), and to assess predation impacts on the main prey populations (gentoo penguins and fur seals).
|Fur seal counts at SSB||Fur seal pup production at SSB|
|Fur seal pup weights||Fur seal foraging trip duration|
|Antarctic krill in fur seal scats||Counts of leopard seals at Bird Island|