Report: The effects of salmon fisheries on Southern Resident Killer Whales

A recent report by an independent science panel reviewed data on the effects of salmon fisheries on Southern Resident Killer Whale populations. The report was released on November 30, 2012 and was commissioned by NOAA Fisheries and Oceans Canada.

Photo courtesy of NOAA
Photo courtesy of NOAA

Southern Resident Killer Whales (SRKW) are listed as an endangered species in both the U.S. and Canada. The National Marine Fisheries Service (NOAA Fisheries) and Oceans Canada commissioned three workshops to address the question of how Chinook salmon fisheries may impact SRKW population growth. An independent science panel prepared the report, in which they discuss data from the workshops and recommend further research.

According to the report, the SRKW population has been increasing slowly, with periods of fluctuation, since the 1970s, but the growth rate has been slower than expected. Birth and death rates would indicate a growth rate of 1% per year, but currently the number of whales is increasing only 0.71% per year due to a skewed sex ratio. A rate of 2.3% per year is required to delist the whales in the U.S.

Although there is strong evidence that SRKW rely mainly on Chinook salmon during the summer, they feed on chum when Chinook are less available in the fall and winter. Poor condition, which carries a strong risk of mortality and can be caused by nutritional stress, has been observed among the SRKW population between 1994 and 2008, but it is not clear whether nutritional stress is a contributing factor or the primary factor, or whether disease could be the cause.

Current Chinook populations are much reduced from their historical size, and many populations are dominated by younger and smaller fish. In examining the data presented in the workshops, however, the panel cautions against the assumption that reduction in fisheries would result in a parallel increase in the number of Chinook available to SRKW. Because of other competing predators such as harbor seals, sea lions, and Northern Resident Killer Whales, whose range overlaps with that of the SRKW, the total number of salmon available would not be equal to the number given up by the fisheries. In addition, some stocks that are currently harvested are less important to SRKW, and most fisheries harvest a mix of immature and mature fish, some of which would not survive to adulthood, when they are most critical to whale populations. Although the panel commended the presenting scientists for their data, they concluded that there were too many unknown or uncertain factors to establish a linear relationship between Chinook abundance and SRKW survival.

The panel did consider the strategies most likely to increase Chinook abundance overall, which include restoring freshwater habitat and reducing migration mortality downstream.

View the the full report, including links to supplementary materials.

View maps tracking SRKW population movements.