Chinook salmon, the southern residents' largest food source, has a population that has significantly decreased in the past century. In the late 1800s and early 1900s, it is estimated that the Chinook salmon had a population of up to 700 thousand in the Puget Sound. Currently, the estimates for Chinook salmon population in the Puget Sound are at most 280 thousand (this information may not be accurate because the data is from 2008, so the current population may be lower). Other salmon populations, such as pink, coho, and sockeye, have also decreased by a large amount (it is possible that the population of chum salmon has actually had a slight increase).
Reduced prey population causes orcas to spend more of their time looking for food. This increases mortality rates, possibly due to starvation, and reduces reproductive rates because there is less time available for mating and socialization. Increased mortality rates and decreased reproductive rates due to reduced prey availability are likely the main causes for the decline in southern resident population.
The causes for reduced salmon population are primarily a result of human activity. The main issues created by humans that result in reduced salmon population are overfishing, water contamination, and climate change. There are also human-made issues that do not lower the salmon population. but prevent salmon from being available to southern residents. For example, obstructions such as dams prevent salmon from travelling to locations that southern residents have access to. Another side effect of dams is that they cause large amounts of salmon to congregate near them, which allows pinnipeds (seals and sea lions) to consume them, further reducing their population.
In addition to reducing the population of prey, water contamination also affects the southern residents directly. There are several chemical compounds that contaminate the water and can harm orcas, including organochlorines, PCBs (polychlorinated biphenyls), PCDDs (polychlorinated dibenzodioxins), PCDFs (polychlorinated dibenzofurans), DDT (dichlorodiphenyltrichloroethane), DDE (dichlorodiphenyldichloroethylene), PCNs (polychlorinated napthalines), PBDEs (polybrominated diphenyl ethers), PAHs (polycyclic aromatic hydrocarbons), and methyl mercury chloride.
The exposure of orcas to these contaminants can cause a variety of problems that harm the southern residents. Organochlorines may cause orcas to have hindered reproduction, hormonal dysfunction, and skeleton deformation. PCBs can weaken the orca's immune system, resulting in a higher likelihood of infectious diseases. DDTs and PCBs may also cause orcas to die before six months of age. PBDEs, a type of flame retardant, may lead to immunotoxicity and neurotoxicity. There are numerous other chemical compounds that may have contaminated the water that could also harm orcas, and their effects on marine life are being researched.
The main effect of vessels on southern resident orcas is the sound generated by engines and sonar. Loud sounds can disrupt echolocation and communication with other orcas, and may also damage an orca's hearing. Although orcas do have decent vision, echolocation is much more useful to for finding prey. Disruption of echolocation can severely limit the consumption of prey, and can be very detrimental to orcas when combined with reduced prey availability. Because loud sounds can also disrupt communication with other orcas, it may further reduce reproduction rates.
Vessels can also strike southern residents and injure or even kill them. Vessel collision with orcas is rare, but it does sometimes occur. Vessel collision is most damaging to southern residents when they come into contact with the boat's propeller.
Whale watching boats intentionally get as close as possible to orcas, so the loudness of the sound produced by the boat engines is increased. While whale watching boats can bring harm to the southern residents, whale watching also has some positive effects, like educating the public about orcas and the problems that affect them. The economic benefits of whale watching also give governments another reason to protect the southern residents.
The main reason that the population of southern residents is so low today is because, before it was outlawed in 1971, over 100 southern residents were captured to be put in aquariums. Before the capture of southern residents was common, it is estimated that the southern resident population was over 200. Between the 1960s and 1971, however, at least 107 southern resident orcas were captured. The most significant of these captures was the Penn Cove capture of 1970, where about 80 orcas were trapped and about 45 were successfully captured. Since 1971, whaling was made illegal in the United States, and no southern residents have been captured since.
Recovery Plan for Southern Resident Killer Whales by the National Marine Fisheries Service, 2008
Center for Whale Research web site
All uncredited images and sounds are in the public domain.