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Diseases can kill Stellers or make it difficult for them to have healthy babies. Some scientists have looked into whether there was a widespread sickness around the time that the Steller sea lions started dying off. They’ve even considered sexually transmitted diseases! So far, they haven’t found any evidence, and it’s likely that this is not a major cause of the problem.
Some scientists are looking into whether fisheries are taking too much of the fish that sea lions like to eat. It’s hard to tell, however, whether a large catch of fish means that we're taking too much from the ocean or just that there’s a lot of fish in the water. To fully understand what’s going on, the scientists must figure out exactly what sea lions eat, how much they need and how large the fish populations are.
Steller sea lions are easily scared away from the places where they mate and raise their babies. Scientists are concerned that when people like fishers, scientists and whale watchers keep visiting, the Stellers will abandon these places before the pups are ready, making them more vulnerable to other threats.
Another theory is that changes in the ocean ecosystem are throwing the food web out of whack. As a result, the populations of nutritious fish might be declining or moving further away, where the sea lions can't safely or easily get to them.
It is also possible that the harvesting, entanglement and shooting of baby sea lions in the 1960s and 70s led to really low populations in the 80s. The scientists considering this theory have reached the conclusion that not enough pups died to affect the population so drastically.
Transient killer whales eat marine mammals. After whaling took out the bigger whales that killer whales like to dine on, the orcas may have started to eat more sea lions. This theory is very controversial, and many top marine mammal scientists are arguing fiercely over it.
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