A 2008 US Department of Commerce analysis (SPLASH) noted that the many challenges to determining the recovery status included the lack of accurate population estimates, the unexpected complexity of population structures and their migration. The report was based on data collected from 2004 to 2006. At the time, the North Pacific population was some 18,302. The estimate is consistent with a moderate rate of recovery for a depleted population, although it was considered to be a "dramatic increase in abundance" from other post-1960s estimates. By comparison, Calambokidis et al. estimated 9,819, covering 1991–1993. This represents a 4% annual increase in population from 1993 to 2006. The sanctuary provided by US national parks, such as Glacier Bay National Park and Preserve and Cape Hatteras National Seashore, became major factors in population recovery.