Whilst the cumulative effects of global warming are as yet uncertain, what is known is that global warming will affect most of the species living on planet Earth, including cetaceans. Cetaceans are whales, dolphins and porpoises, and they inhabit most of the world’s oceans and some freshwater rivers and lakes in the equatorial zone. Evidence gathered over the last twenty to thirty years has shown that changes in the cetaceans’ habitat will impact on their range, behavior and sustainability. If the species is to survive, they must adapt to climate change. The following is a discussion of how global warming will affect cetaceans, and in particular, whales.
Global warming will impact on cetaceans in a number of ways. The loss of the sea ice will potentially have the greatest impact on whales and will result in a reduction of suitable habitats for feeding, resting and breeding (Burek 2008). Changes in the availability of food sources due to shifts in prey species and food web changes may result in the loss of body condition, making cetaceans more susceptible to endemic pathogens and changes in food preferences could result in exposure to contaminants which cetaceans are unable to tolerate. A decrease in sea ice is likely to open the Artic to increased human activity, resulting in the exposure of cetaceans to the possibility of increased collisions with boats, acoustic pollution and entanglement in fishing equipment (Burek 2008). The timing of breeding and migration and changes to migration routes may also be affected by changes in sea ice.
As highly derived long-lived (i.e., K-selected) species, Arctic marine mammals are ill equipped to respond quickly to rapid climate change (Moore 2008). Different species of whales respond in different ways to the impact of climate change. The Right Whale (Eubalaena genus) relies on a species of zooplankton (C. marshallae) as its source of food (Zerbini 2015). In the Bering Sea, temperature fluctuations influence the diversity and biomass of zooplankton on the ice shelf and there is a correlation between extensive sea ice cover over the shelf and the abundance of C. marshallae (Zerbini 2015). A warming climate will result in less extensive ice cover and therefore a reduction in abundance of C. marshallae. When zooplankton concentration is too low, Right Whales do not feed and without dense patches of this zooplankton, female whales cannot bulk up to prepare for calving, carry a pregnancy to term or produce enough milk (Zerbini 2015). Changes in zooplankton abundance could affect Right Whale ranges.
A reduction in sea ice in the Antarctic could expose two types of Killer Whales (Orcinus orca) to the risk of losing sources of food. One species of Killer Whale eats fish which school under the ice to protect against predators. These whales tend to stay in one area as they do not have to travel in search of food. The other type feeds on terrestrial mammals such as seals and penguins, found on ice floes (Roach 2008). They also prey on Chinook salmon, whose population has been reduced by a decrease in the amount of food available, caused by ocean warming and increased temperatures in some rivers (Ford 2010). It is likely that Killer Whale populations will be able to adapt to a different supply of food, as they are able to eat most species of marine animals and are very aggressive predators.
Even though it appears that global warming will negatively impact cetaceans generally, some evidence exists that adaptations by both the whales and their food sources may mitigate some of these negative impacts. Studies of fin (Balaenoptera physalus) and humpback whales (Megaptera novaeangliae) in the Gulf of St. Lawrence have shown the ability of these two species to time their migration to their feeding grounds in response to changes in the break-up of ice and rising sea surface temperatures (Ramp 2015). This ability to adapt their seasonal movement to the shift in productivity in one of their prime feeding grounds may explain how they have adapted to past climate fluctuations (Ramp 2015). The ability to adapt easily to changes in habitat is an encouraging indicator of the resilience of the species.
Other studies have found that the Gray Whale (Eschrichtius robustus), which feeds on krill and herring at the ocean’s surface, employs an alternative feeding regime and behavior pattern in response to changes in their habitat and employ more generalist filter feeding modes when their primary food source disappears (Boswell 2011). These whales demonstrate “ecological plasticity”, which scientists believe to be common among marine mammals, and this trait may be what saves them from decline (Boswell 2011).
Research on Antarctic krill (Euphausia superba), a major food source for cetaceans, found that, rather than being dispersed evenly under the ice sheet, krill congregated tightly to a zone just under the edge of the sea ice (Brierley 2002). It is believed that this behavior is a compromise between being located close to its food source while still providing refuge from predators. Any reduction in the volume of sea ice which may result from global warming will not necessarily result in a reduction in krill biomass. This means that even if the icecap is reduced in size, a major food source for whales will not be consequently reduced (Brierley 2002).
The multiple responses of various species of whales to global warming are indicators of the adaptability of the species. The reduction in ice cover at the poles is the largest threat to the survival of cetacean species, as it governs the feeding, habitat, migration and breeding behaviors of most species of whales. The disappearance or reduction of food sources will impact on cetaceans, although it is thought that this may not be as serious as issue as initially believed. Whales have been found to be historically very adaptable to climate changes and some species have adapted by altering their responses to habitat change. It can only be hoped that their response to the current period of global warming will enable them to avoid extinction.
Works Cited
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Moore, S.E. and Huntington, H.P. “Arctic Marine Mammals and Climate Change: Impacts and Resilience.” Ecological Applications 18.2 (2008): S157-S165.
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