Frequent Questions—San Pedro Basin Whale Falls

Surveys by UC San Diego’s Scripps Institution of Oceanography of the sea floor off Southern California earlier this year identified a large number of whale remains. Often called “whale falls,” they are the remains of whales that died and sank to the sea floor. These whale falls provide an opportunity to explore the role of carbon capture in climate change mitigation.

Large scavengers and tiny marine organisms typically consume the soft tissue of carcasses, leaving skeletons as the only trace of the whales. Images from the Scripps survey documented at least six whale falls in the San Pedro Basin. This area of ocean floor is located between Catalina Island and the mainland off Long Beach Harbor in Los Angeles. Sonar surveys by a remotely operated vehicle suggest that there may be more than 60 total whale falls in the approximately 350-square mile area Scripps examined. If true, it would be one of the greatest concentrations of whale falls ever found. The initial information offers few clues about the species of whales involved, why they died, or how old the remains may be. Scripps plans to publish the data through the scientific peer-review process to allow for further investigations.

NOAA Fisheries was not involved in the Scripps survey. However, we are responsible for protecting whales and other species protected by the Marine Mammal Protection Act and the Endangered Species Act. Depending on what we can learn about the whale falls, the findings may provide additional information about factors affecting whales off Southern California. We wanted to answer some initial questions based on what we know so far.

Does the discovery of the whale falls reveal an unrecognized source or level of whale mortality that could be threatening the species?

NOAA Fisheries and its partners regularly survey whales and other marine mammals off the U.S. West Coast. These surveys consistently indicate that populations of most major whale species, including humpback, blue, gray, and fin whales, are increasing. Stock assessments for the different species describe these increases, which reflect the continuing recovery of whale numbers since the commercial whaling era. Vessel collisions and entanglements in fishing gear as well as natural causes from disease to old age do kill some whales every year. However, their populations have generally continued steady growth. We work with partners on designating shipping lanes that reduce the risk of ship strikes and voluntary ship slow downs to reduce the severity of strikes.

Fin whales off the West Coast, for example, increased an average of roughly 7.5 percent per year (PDF, 8 pages) from 1991 to 2014 up to a total of at least 8,000 whales in 2018. Humpback whales off the West Coast include two main stocks, one that winters off mainland Mexico and the other that winters off Central America.The Central American stock includes about 1,500 whales, increasing about 4.7 percent annually (PDF, 12 pages). The Mexican stock includes about 3,480, increasing about 8 percent per year (PDF, 12 pages). Scientists assumed in the stock assessment for blue whales (PDF, 8 pages) that the roughly 1,800 blue whales off the West Coast have increased about 4 percent per year. Gray whales were the first large whale species to recover from near extinction in the whaling era and were taken off the endangered list in 1994. Population numbers have since gone through ups and downs, including a recent Unusual Mortality Event that contributed to a population decline of several thousand animals. Gray whales have rebounded from similar declines in past decades, however.

This consistent increase in whale numbers demonstrates that ship collisions and other causes of whale mortality do not appear to be derailing their recovery. Further research on the whale falls may help unravel what kind of whales they are and whether and when they died of natural or other causes. The mortalities do not appear to have affected the species at a population level.

What could cause the death of so many whales in that area?

While we have learned a lot about the factors that influence whale survival, we don’t know very much about what happens once whales die. The 60 whale falls the Scripps survey found may approximately double the total number of modern whale falls known around the world. However, very little of the rest of the ocean’s seafloor has been surveyed at the level of detail that the Scripps team surveyed the San Pedro Basin. We also do not know how long the remains have been there. That leaves us without much sense of how many whale falls are typical for such an area, or even how many would represent an unusual number of whale falls. Even with the Scripps survey, we do not know how long the whale falls have been collecting. The tissue of a whale carcass typically decomposes within about 2 years, and skeletons can remain for many more. One possible course of scientific investigation could be roughly estimating the age of the whale falls based on sediments that have collected on the skeletons, or by radiometric dating of the whale bones. As many of the remains are deteriorated, however, we may never know why the individual whales died.

Research published in 2003 (PDF, 44 pages) provided one of the first worldwide looks at whale falls, both at individual sites and their role in the marine ecosystem. Led by Craig Smith of the University of Hawaii, the scientists at the time estimated around 10,000 whale fall carcasses around the northeast Pacific Ocean. That would translate into many thousands along the California Coast. They projected that more than 600,000 whale falls are in various stages of decay around the world at any one time. Scientists now think about 90 percent of large whale carcasses sink into the deep sea, falling to the bottom where they may never be found.