Just like humans, individual Galapagos sea lions differ in their food preferences.
Researchers closely studied these animals near the islet of Caamaño, in the centre of the Galápagos archipelago. Some individuals look for food at the bottom of the sea (so-called benthic foragers), while others prefer to hunt in the water column (pelagic foragers). Among the latter group some hunt predominantly at night and others during the day.
Sea lions with different hunting styles can be easily recognized by the length of their whiskers. Looking for food at the bottom of the sea, for example fish buried in the sand, damages the whiskers and therefore these are shorter in benthic foragers, even though whiskers keep growing throughout the animal’s life.
But foraging style has much greater consequences than just whisker length.
Pelagic foragers are more sensitive to changes in the average yearly surface sea temperatures, which at the study site varied between 22.2 and 25.4°C over 15 years. At higher temperatures pelagic foragers had fewer pups, who were in poorer condition and had slightly lower survival chance compared to when the water temperature was lower. This is probably a consequence of the availability of the prey of these animals. Colder waters are usually rich with nutrients and can sustain more marine life and therefore provide more fish for the sea lions, while higher temperatures are linked with lower abundance of pelagic fish.
On the other hand, benthic foragers are less sensitive to changes in sea surface temperatures. Their prey seems to be less affected by the sea’s surface water temperature. However, benthic fish tend to provide less calories, and searching for food at the bottom of the sea seems to cost more energy. All-in-all, benthic foragers do better than pelagic foragers when temperatures are higher, but worse when they are lower. In other words, each strategy is more suited to a particular temperature. Thus, presence of a variety in feeding styles can, at least to some extent, help the population to persist in variable environmental conditions.
Unfortunately, independent of temperature, neither benthic nor pelagic foragers produce enough pups to compensate for the mortality experienced by these animals. Overfishing and entanglement in fishing nets contribute to the population decline of the Galapagos sea lions. With the predicted rise in the surface sea temperatures and further decline in reproduction of pelagic feeders situation of this already endangered species will become even worse.
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Photo: Brian Gratwicke – https://www.flickr.com/photos/briangratwicke/8500601944/, CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid=26337749