This multi-year sea ice means that most of the Weddell Sea has never been surveyed for seals before, even more so in the seals’ breeding season in spring, when sea ice concentrations are still too high to make the areas accessible to ships. The prospect of satellite images allowing us to find seals in areas never searched before, during a time of the year hardly ever surveyed, was very exciting. But up to this point, satellite images have only been used to find seals and penguins in Antarctica that occur on fast-ice (ice attached to the Antarctic continent). It is fairly easy to spot a dark blob on a flat white surface. However, this was the first attempt to use volunteers to find seals in Antarctic breeding on the sea ice.
By nature, sea ice is uneven. Think of it as blocks of ice floating in a glass of water: they are constantly smashing up against each other, causing the edges to form ridges and melt pools forming in the middle. When the sea ice refreezes, the ridges are locked in and the melt pools freeze over creating a mesh of different shades, colours and surface topography. We weren’t sure whether this would work – we didn’t know if people would be successful at finding seals on the jumbled sea ice.
When we looked over the results the first time, we knew very quickly that it would not be possible to get a very accurate estimate of crabeater seal numbers. But, people were able to see them, and with quality control measures in place, we could model their potential distribution based on their preferred habitat throughout the Weddell Sea. In other words, we could say: “we know where they are based on these environmental conditions, based on those environmental conditions, where else can they be?”
Crabeater seals were mostly found away from the ice edge and into the pack ice, over areas where the water is a 1000 m or deeper, and away from the continental shelf edge. This is typical Antarctic krill habitat – the prey that makes up 90% of a crabeater seal’s diet. This was surprising and a novel finding seeing as crabeater seals spend their entire breeding season on the ice, and do not go into the water to hunt. Why would they then prefer to be close to their main food source, if they are not eating during this time? We suggest that it is because they are such specialised predators and it would be easier for them to be close to their preferred food source once they are done breeding and to replenish the body condition lost while breeding. Hauling out further away from their food resources mean that they would spend more time searching for prey after fasting for several weeks while breeding.
Crabeater seals also preferred to haul out in areas where the ice concentration was more variable – but not too variable – throughout breeding season. Ice concentration variation is a measure of leads and cracks opening up for the crabeater seals to access the deeper parts of the ice-covered Weddell Sea. They need to be deeper into the pack-ice because it is harder for predators to find the exposed mom and pup. However, the ice cannot be to unpredictable and break up when a female is still nursing her pup on an ice floe – it needs to be stable for the 2-3 weeks that it takes for a female to successfully wean her pup.
Although we were not able to count all the seals, we were able to make some new and important discoveries about their breeding habitat. Knowing crabeater seal distribution is valuable conservation information for the planning of the Marine Protected Area in the Weddell Sea. There are several world class scientists working on ways to improve these methods to eventually get to better population estimates than seven to 30 million individuals. One option would be to do a ground validation count; that is to take satellite photos at the same time while doing ship- and helicopter-based surveys. Of course, this means that there will be several more expeditions where I’ll be standing on the bridge for hours on end watching the shadow of the ship gliding across the ice in front of me. A prospect that I am certainly not complaining about.
Photographers, from top to bottom, left to right: Kyle Mortara; Liam Quinn, Ben Lai, Dave Johnston, Mia Wege, Ursula Rack, Ben Lai.
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October 08, 2020 at 04:14PM
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Counting seals from space: Studying the crabeater seals of Antarctica - Oceanographic - Oceanographic Magazine
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