Blue whales, the largest animals on Earth, can reach lengths of over 30 meters (100 feet) and weigh up to 200 tons. These baleen whales primarily feed on tiny, shrimp-like crustaceans called krill. Despite their immense size, blue whales are listed as endangered by the International Union for Conservation of Nature (IUCN) and protected under the Endangered Species Act (ESA) in the United States. They face numerous threats that contribute to their mortality and hinder their recovery.
The Legacy of Whaling
Commercial whaling was the most significant historical factor in the decline of blue whale populations. Industrial-scale whaling decimated these marine giants in the early 20th century. Whalers targeted them for meat and blubber, with harvesting peaking between 1930 and 1931, when approximately 30,000 blue whales were taken.
This cumulative impact led blue whales to the brink of extinction, with hundreds of thousands killed in the first half of the 20th century. Recognizing the severe depletion, the International Whaling Commission (IWC) banned all blue whale hunting in 1966. Though commercial whaling is now prohibited, the historical devastation continues to affect blue whale populations, as their slow reproductive rate means recovery is gradual.
Direct Threats from Human Activity
Direct human activities present immediate dangers to blue whales. Two prominent causes of mortality are ship strikes and entanglement in fishing gear, often resulting in severe injury or death.
Ship strikes occur when large vessels, such as cargo ships and cruise liners, collide with whales. As global shipping traffic increases and migration routes overlap with busy shipping lanes, the risk of fatal encounters rises. Collisions inflict blunt force trauma or propeller wounds, leading to internal hemorrhaging, broken bones, or deep lacerations.
Entanglement in fishing gear is another major direct threat. Blue whales can become ensnared in active or discarded equipment. Entanglement prevents whales from swimming, feeding, or surfacing for air, leading to drowning, starvation, or exhaustion. The gear can also cause deep cuts, infections, and amputations.
Environmental Stressors and Indirect Impacts
Beyond direct threats, environmental changes and indirect human-caused factors weaken blue whales and hinder population recovery.
Ocean noise pollution, generated by shipping, seismic surveys, and naval sonar, disrupts whale behavior. This constant underwater din interferes with communication, navigation, and finding prey or mates. Such disruptions can lead to stress, disorientation, and altered migration routes, pushing whales into dangerous areas or away from feeding grounds.
Prey depletion, primarily of krill, poses a significant challenge to blue whale survival. Overfishing by commercial fisheries reduces available food for whales, while climate change impacts krill populations by altering ocean temperatures and acidity. A decline in krill can lead to malnutrition, reduced reproductive success, and starvation.
Climate change also contributes to habitat degradation by altering ocean temperatures and currents. These changes can shift prey distribution, forcing blue whales to undertake longer migrations or forage in less productive areas. Chemical pollution, with substances like PCBs and heavy metals, can accumulate in whale tissues, compromising immune systems and reproductive capabilities.
Natural Causes of Death
Natural causes of death for blue whales are relatively rare compared to human-induced mortality. Their immense size makes healthy adults largely invulnerable to predation.
Orcas, or killer whales, are the only known natural predators. While attacks on adult blue whales are uncommon, documented instances exist of orca pods cooperatively attacking and killing both calves and, more rarely, adult blue whales. These predatory events typically involve large groups of orcas targeting vulnerable individuals.
Like all living organisms, blue whales can succumb to disease or old age. However, evidence for these as widespread mortality factors is limited. Whale strandings, where whales become beached, can also occur naturally due to illness, disorientation, or navigational errors, though human factors can also contribute to such events.