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Status Review of the Northeastern Pacific Population of White Sharks (Carcharodon Carcharias) Under the Endangered Species Act

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Status Review of the Northeastern Pacific Population of White Sharks (Carcharodon Carcharias) Under the Endangered Species Act Status Review of the Northeastern Pacific Population of White Sharks (Carcharodon carcharias) Under the Endangered Species Act Heidi Dewar, Tomoharu Eguchi, John Hyde, Doug Kinzey, Suzanne Kohin, Jeff Moore, Barbara L. Taylor, and Russ Vetter National Oceanic and Atmospheric Administration National Marine Fisheries Service Southwest Fisheries Science Center 8901 La Jolla Shores Dr La Jolla, California 92037 1 2 EXECUTIVE SUMMARY Background In June and August of 2012, the National Marine Fisheries Service (NMFS) received two petitions requesting that the northeastern Pacific (NEP) population of white sharks (Carcharodon carcharias) be listed as endangered or threatened under the Endangered Species Act (ESA). In September 2012, NMFS published a 90-day finding (77 FR 59582) announcing that both petitions presented substantial scientific or commercial information indicating that the NEP white shark population may warrant listing under the ESA and that the agency would conduct an ESA status review of the NEP white shark population. NMFS formed a Biological Review Team (BRT) made up of scientists from the Southwest Fisheries Science Center having diverse backgrounds in population modeling, genetics, conservation biology, fisheries management, and shark biology and ecology to conduct the status review. The BRT considered a variety of scientific and technical information and conducted its own independent analyses. This document reports the results of its comprehensive status review of the NEP white shark population. This report was peer reviewed by three independent scientists. Approach of the BRT While recent years have seen a dramatic increase in the information available on the behaviors and movements of NEP white sharks, the BRT acknowledged that there is considerable uncertainty surrounding many aspects of abundance, trends in abundance, threats, life history, and habitat use for all age and sex classes. Such uncertainties are expected for a naturally uncommon highly migratory species. The BRT decided to treat the uncertainty explicitly by defining where it exists and using a process called Structured Expert Decision Making (SEDM) to weigh various plausible scenarios, taking into account all available data on the NEP white shark population but also considering information on white sharks from other regions and from similar species. For each SEDM exercise, the BRT developed a range of values, based on the best available data, across which plausibility points were allocated. Some of the results are provided in the “Executive Summary” and are identified as plausibility points. The exercises for which SEDM was used are presented in Appendix A. The BRT’s objectives in taking this approach were to make plausible decisions in complex situations involving uncertainties, to insure that the decision making process was as transparent as possible and to provide assurance that the team was basing its decisions on a common understanding of the evidence. Status of Northeastern Pacific Population of White Sharks White sharks are globally distributed and are the largest of the predatory sharks. In the NEP, white sharks are most commonly observed as adults and subadults at near shore i aggregations that are typically associated with pinniped rookeries at Guadalupe Island (Domeier et al. 2012) and in central California (Klimley et al. 1992, Jorgensen et al. 2010). Recently, satellite tagging results have shown that they also exhibit an extensive offshore phase that differs by sex (Jorgensen et al. 2010, Domeier and Nasby-Lucas 2013). Early life history stages are more frequently encountered over the continental shelf in the southern California Bight and in Sebastián Vizcaíno Bay in Baja California Norte (Klimley 1985, Weng et al. 2007b, Domeier 2012, Santana-Morales et al. 2012). As with most of the large sharks, white sharks are relatively slow growing and have low overall fecundity, making them vulnerable to over exploitation. While they are not targeted in current fisheries they are taken incidentally in both artisanal and industrial fisheries (Lowe et al. 2012, Santana-Morales et al. 2012). Fisheries mortality, in addition to a historic reduction in the prey base across life history phases, may have reduced the number of white sharks in the NEP, although the data are insufficient to determine if and by how much the population may have declined. Low population numbers for subadult and adult white sharks in the NEP were recently reported by Chapple et al. (2011). The task of the BRT was to review the status of the white shark population in the NEP and assess its risk of extinction. To address these issues the BRT: 1) compiled the best available information relevant to the status review, 2) determined whether the NEP white shark represents a distinct population segment (DPS), and 3) examined the abundance estimates and current and projected population status. The BRT utilized the best available information to assess the risk of extinction to the population. Determination of Distinct Population Segment The BRT examined both the discreteness and significance of the NEP white shark population in relation to the global taxon to assess whether it met the criteria of a DPS. An examination of the behavioral and genetic data suggests that the NEP white shark population is markedly separated from other white shark populations. Photo identification (photo-ID) and electronic tagging data reveal a high level of site fidelity and no apparent movements outside the NEP (Domeier and Nasby-Lucas 2008, Jorgensen et al. 2010, Anderson et al. 2011, Nasby-Lucas and Domeier 2012). Genetic data reveal a monophyletic clade with no shared haplotypes between the NEP white shark population and those in Japan, Australia/New Zealand and South Africa (Jorgensen et al. 2010, Tanaka et al. 2011). This monophyletic pattern supports the inference that little movement occurs among these regions. Based on the genetic and tagging data, the BRT concluded that the NEP white shark population was discrete in relation to the global taxon. The BRT also examined the significance of the NEP white shark population with respect to the global white shark taxon. The presence of a unique monophyletic clade with no shared haplotypes suggests that the NEP white shark is unique. In addition, the NEP represents a large portion of the global range of white sharks and its loss would result in a significant gap in the taxon’s range. Thus, the BRT concluded that the NEP white shark population is significant to the global white shark taxon. Overall, the BRT concluded that the NEP white shark population meets the DPS criteria for discreteness and ii significance and qualifies as a DPS. Some uncertainties exist, however. Of note is that the use of mtDNA data only assesses female-mediated gene flow and samples were only collected from central California. In addition, satellite tagging efforts have not been conducted in Japan, which could inform mixing across the North Pacific. Risk Assessment To determine whether the NEP white shark population is at risk of extinction either now or in the foreseeable future the BRT examined: 1) threats to the population, 2) direct and indirect information on population trends, 3) the population abundance and associated biases, and 4) the impacts of fishery mortality on the population under a range of estimated (or assumed) abundance and mortality levels. Finally, the BRT considered all of the available information related to extinction risk and assessed the overall extinction risk facing the NEP white shark population now and in the foreseeable future. Threats: Key to assessing extinction risk for the NEP white shark population was to examine potential natural and anthropogenic threats to the population. In addition to current and future threats, the BRT examined historic threats which provided insight into the potential for past population declines. Threats included: 1) fisheries mortality in U.S., Mexican and international waters, 2) loss of prey due to overharvesting, 3) small population effects, 4) disease and predation, 5) habitat degradation linked to contaminants, and 6) global climate change. The BRT conducted SEDM assessment to characterize the severity of threats and the associated certainty. The BRT examined fisheries mortality across a range of fisheries and in all white shark age classes. In the NEP, white sharks are and have been caught in a range of fisheries with varying risk over time. Offshore, the greatest threat was likely posed by the high seas drift net fisheries which overlapped, although minimally, with the habitat of adult white sharks in their offshore phase. These fisheries were banned in the early 1990s and were considered only a historic risk. Near shore, white sharks across all size classes have been taken primarily in net based fisheries. Similar to offshore, changes in regulations in U.S. coastal net fisheries have also reduced the overall catch of white sharks from peak values in the early 1980s. Current catch estimates for the last 10 years in the U.S. net based fisheries are ~28 young-of-the-year (YOY) and juvenile white sharks per year, of which ~50% are presumed mortalities. Additional catch of YOY and juvenile white sharks is documented in Mexico, primarily in Sebastián Vizcaíno Bay. The estimated annual white shark landings for Baja California Norte in 2011 was ~185 individuals all of which are assumed to be mortalities. New fisheries regulations banning shark fishing from May through July off the Pacific Coast of Mexico have
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