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Left: blue whale 'blowing'. Photo: NOAA.
Middle: swarm of krill; millions are needed to feed a whale. Photo: R.Pedersen.
Right:Krill are crustaceans, the same group as shrimps, crabs and lobsters. Photo: Mar-Eco.
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The blue whale is the largest animal ever known to have lived on Earth; they can weigh over 150 tonnes. Despite their huge size they feed almost entirely on zooplankton. Their favourite is 'krill', a tiny shrimp-like animal, which they filter from the sea with their huge baleen plates - flat, flexible plates with frayed edges, made of keratin (like hair and nails).
Blue whales are found from sub-polar to sub-tropical latitudes. In spring they move towards the pole, taking advantage of the high zooplankton production in summer. In autumn they move to warmer waters to reduce their energy expenditure at a time when there is little food. This is when they mate and breed.
Blue whales (National Geograhic)
Whales and seals (British antarctic Survey)
More about blue whales and measures to protect them (NOAA)