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The recovery of southern sea otters appears to have taken an upturn, according to results from the annual California sea otter survey released by the U.S. Geological Survey. Yet despite an overall increase in sea otter abundance, sharks have been "taking a bite" out of the portion of the population that could fuel expansion into new areas.

"There's much more to the story here than the main finding would suggest," said Dr. Tim Tinker, a research ecologist who leads the USGS sea otter research program. "We are looking into various factors that may be affecting the survey results, including a boom in urchin abundance from Big Sur to Monterey that may explain the uptick in numbers in the range center, and high levels of shark bite mortality that are likely responsible for continued declines at the north and south ends of the range."

This year's survey results suggest an increasing trend over the last five years of almost 2 percent per year and the population index, a statistical representation of the entire population calculated as the three-year running average of census counts, has climbed to 3,054 from 2,711 in 2010. The growth is accounted for by an unexpected jump in numbers in the center of the sea otter's range, an area that spans from Monterey south to Cambria.

While the population index continues to trend upward, the northern and southern subsets of the population continue a five-year decline, dropping 2 percent and 3.4 percent per year, respectively, numbers consistent with increased shark bite induced mortality in these same areas.
Since the 1980s, USGS scientists have computed the annual population index and evaluated trends in the southern sea otter, "Enhydra lutris nereis," a federally listed threatened species found in California. For southern sea otters to be considered for removal from threatened species listing under the Endangered Species Act, the population index would have to exceed 3,090 for three consecutive years.

The figure presents a graph of three lines, titled "Number of Southern Sea Otters in Three Coastal Regions of California."

The horizontal axis is labeled "Year," and the years 2000 through 2015, in increments of three years, are indicated. The vertical axis is labeled "Number of otters counted," and the numbers zero through 2,500, in increments of 500, are indicated.

One line represents north, one line represents central, and one line represents south. The line representing north begins on the vertical axis at approximately 250. It rises very gradually until just before 2003, where it begins to rise steadily until it reaches approximately 500 in 2005. The line then dips slightly until 2006 before it begins to gradually rise again until it reaches about 500 in 2014. It then dips gradually to slightly less than 500 in 2015.

The line representing south begins on the vertical axis, slightly above 500. It falls gradually until it reaches approximately 400 in 2003. Then it begins to rise steadily until it reaches approximately 700 in 2007. It then levels off until 2012 before falling to approximately 500 in 2013. Finally it rises gradually to approximately 550 in 2015.

The line representing central begins on the vertical axis at approximately 1,400. It rises steadily until it reaches approximately 1,600 in 2006. It then levels off until 2009 before decreasing to approximately 1,450 in 2010. It remains level until 2012 and then rises steeply until it reaches approximately 1,900 in 2015.

The lines on the graph represent the 3-year running average of the southern sea otter population in the north, central, and south coastal regions.

Adapted from Brian Hatfield and Tim Tinker, "Spring 2015 California Sea Otter (Enhydra lutris nereis) Census Results." Published in 2015 by US Geological Survey.

The passage characterizes the main finding of the 2015 sea otter survey conducted by the US Geological Survey as _________

(A) initially promising, because it shows that the southern sea otter will not become an endangered species.
(B) very significant, because it validates the recent efforts of sea otter conservation groups working along the California coast.
(C) ultimately inconclusive, because it does not incorporate data from sea otter population surveys conducted before 2010.
(D) somewhat misleading, because it does not make it clear that the southern sea otter population trends varied throughout the range.

答案和解析

答案：D

解析：

Choice D is the best answer. According to the passage, the US Geological Survey found that the population of the southern sea otters "appears to have taken an upturn." The passage characterizes this finding as somewhat misleading, however, because the population of the southern sea otters is increasing only in the center of California's coastal range, while sharks are causing "declines at the north and south ends of the range."