Counting Noses: Not an Easy Feat

Counting Noses: Not an Easy Feat

By Kim Steinhardt and Gena Bentall


How many southern sea otters are there? Is the number rising or falling? Where are they on the road to recovery? What are the biggest threats they face?

These and many more questions like them are central to the sea otter conservation story, a story that began with humans hunting them to near extinction by the early 1900s. Researchers, conservation activists, and marine, fishery, and wildlife management specialists all seek to have a solid foundation of reliable data on which to develop and support science-based policy decisions affecting the recovery of sea otters.

The most southerly of the three sea otter subspecies, the southern sea otter, once populated the coast all the way south into Baja California before being hunted down to a remnant few by the turn of the century. This keystone species was reduced to perhaps 30 or 40 individuals by 1911, and even now—more than one hundred years later—the southern sea otter still numbers only a little over 3,000. This small population is distributed along the central California coastline from about Pigeon Point in San Mateo County, to the north, to Gaviota State Beach in Santa Barbara County, to the south, with a small translocated population at San Nicolas Island, off Ventura County. Growth at the center of this mainland range (Seaside to Cayucos) is likely to be limited, as the sea otter populations there are thought to be at carrying capacity—the number the current habitat resources can support. Sustained growth and recovery will require expansion in the numbers of sea otters at the north and south range fronts.

In order to effectively direct conservation efforts and scientific study, we must have a better understanding of whether the numbers of sea otters in California are growing, declining, or staying steady.

Researchers track population trends with an annual census in which they survey the numbers of sea otters in California. Teams of well-trained and experienced counters go out into the field each spring and make a coordinated and thorough count, seeking sea otters along the coast of California on foot and from airplanes. Counting every sea otter nose along hundreds of miles of their coastal range—from rugged Año Nuevo to the sandy dunes of Pismo Beach, the wild seas of Pt. Conception, and the remote island of San Nicolas—is a bit of a tricky undertaking.

This annual effort, coordinated by the U.S. Geological Survey Western Ecological Research Center, involves participants from California Department of Fish and Wildlife, Monterey Bay Aquarium, US Fish and Wildlife Service, Sea Otter Savvy, and others.

Each annual survey results in a “raw number”—the actual number of sea otters counted. Many factors may influence the accuracy of the raw number from year to year, including kelp species composition and abundance, viewing conditions, observer experience, and the movements and distribution of the sea otters themselves. Any of these factors, alone or in combination, may affect the observers’ ability to visually locate sea otters. For example, in years when there is a very low abundance of giant kelp (Macrocystis pyrifera) canopy, sea otters tend to be more widely scattered, increasing the likelihood that observers will miss some otters as they scan. For this reason, no single year’s raw count is a reliable indicator of growth or decline from the previous year. The official data point for each year is based on an average of counts from three consecutive years. For example, the data point for 2017 is based on the averaged raw counts from 2015, 2016, and 2017. This running three-year average is referred to as the “population index,” and it is the metric used by the U.S. Fish and Wildlife Service to track the status of the population.

Census Table11 1015x1024 Counting Noses: Not an Easy Feat

Viewed alongside each other for multiple years, the population index data points indicate trends of growth or decline in the southern sea otter population, but that is not to say anomalously high or low raw counts aren’t worthy of notice and concern. In the case of the last two years—2016 and 2017—we have extremes at both ends of the spectrum: a record high raw number in 2016 and a steep drop in 2017. Both are reason to be attentive. While the current population index shows a 3 percent decline from last year, we will have to wait until 2018 to know if the raw count from 2017 indicates a more concerning longer-term trend.

To understand the importance researchers and policymakers place on the running average, we need to recognize the limitations of the individual yearly raw number. A sharp increase or decrease in the raw count may in fact prove to be an indicator of real trends in the population over the course of several years, or it may turn out to be the fleeting result of observer error due to poor conditions or other factors influencing the accuracy of the count that year. To get a meaningful view of population trends, we need to look beyond what’s right in front of our noses and take in the bigger picture.

Counting noses: a tricky business indeed.

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