Comment from Andy Stock

The petition finding notes that there is considerable uncertainty concerning the dynamics and human impacts on the potential white shark DPS in the Northeastern Pacific. The petitions discuss pollution, marine debris, decreased prey resources due to human activities and ocean acidification as threats to the proposed DPS. While the impacts of these individual stressors may be unknown or not be sufficient for a listing as endangered or threatened, their combination is a considerable risk. The cumulative effects of multiple stressors are little understood, but there is evidence that synergistic effects of co-occurring stressors are common at the population level (e.g. Crain et al., 2008, Ecology Letters 11(12)). If a population is subject to many different stressors, the likelihood of synergistic effects is consequently high. Furthermore, while the potential direct and indirect effects of ocean acidification on white sharks are unknown, ocean acidification has been identified to be a major threat to California Current pelagic ecosystems based on expert judgment (Teck et al., 2010, Ecological Applications 20(5)), indicating that in spite of knowledge gaps, there is reason for concern. In addition, the possibility of stress caused by global warming and naturally occurring regime shifts in the Pacific (e.g. Chavez et al., 2003, Science, Vol. 299 no. 5604) as well as the uncertain but small (hundreds) number of mature individuals in the potential DPS should be considered. In order to ensure survival of the potential DPS, it is thus important to protect it from any managable stressors. While much of the information to justify a listing of the proposed DPS as “endangered” may be unknown, the variety of threats and exposure to anthropogenic and natural changes could justify a listing as “threatened”, and the risk for becoming endangered should be evaluated for the combination of the potential anthropogenic and natural stressors described above.