Sea Otter Mortality from White Sharks in California
Brian Hatfield (USGS), Michael Harris (CDFW), and Jack Ames (CDFW) also contributed to this piece.
In 1968 the California Department of Fish and Wildlife (CDFW) began a program to attempt to document and examine all beach-cast sea otter carcasses in California and to assign a cause of death to each case, when possible. In general, these works revealed that death in approximately 8-11% of stranded sea otter carcasses could be attributed to shark bite and that most of the shark-bitten otters were recovered in the northern half of the otter’s range, specifically the Monterey Peninsula and the Point Ano Nuevo areas.
In recent years there has been a significant increase in both the total numbers of shark-bitten otters and the proportion of all recovered carcasses for which shark-bite was the primary cause of death (12% between 1985-2000 vs. 21% between 2001-2010)(see figure). The trend has varied across the sea otters geographical range in central California: the northern range-end (near Point Ano Nuevo) has been a consistent shark-bite “hot spot” since 1980, while there has actually been a decrease in the proportional occurrence of shark-bite mortality around Monterey Peninsula. The most striking change has been an increase in shark-bitten cases in the south half of the otter’s range: 6% of deaths were caused by sharks prior to 1990, vs. 30% of recovered carcasses since 2001. In addition to these changing spatial patterns, there has also been a shift in the sex composition: only 17% of shark-bite mortalities were female prior to 1990, but this value has increased to 37% in the last 5 years. The reported trends in shark-bite mortality have important implications for the recovery of this ESA-listed population.
The level of shark induced mortality, including an increase in the number of females being attacked, is likely having an effect on the recovery of this threatened species. What is not clear is the reason or reasons behind the increase. Is there an increase in the white shark population? If so, increasing populations of elephant seals and CA sea lions, moving the large mesh net fishery farther offshore in the 1980s (which caught white sharks incidentally), and legal protection for white sharks enacted in 1994 might all be responsible, in part, for an increase.
There are many unanswered questions regarding shark-bitten sea otters. We still don’t know if white sharks ever eat sea otters or if other species of shark attack sea otters (to date, there is no evidence for any other species of shark biting sea otters). What age-class/sex of white sharks are responsible for the recent increase? Are juvenile sharks that are switching from fish to marine mammal prey responsible for the increase? Why are so many victims of shark attack (maybe all) not eaten?
Stay tuned. Publications are in progress to evaluate both the temporal and demographic trends in sea otter mortality from white sharks and to analyze the population level impacts. Links to these and other related publication will be posted.