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POPULATION DYNAMICS  

AND BIOLOGY OF THE CALIFORNIA SEA                          OTTER AT THE SOUTHERN END OF ITS RANGE

Principal Investigators: James Estes (UCSC), Terrie Williams (UCSC),        Daniel Costa (UCSC), Katherine Ralls (Smithsonian Institution), and           Donald Siniff (University of Minnesota)

 

Background

Sea otters (Enhydra lutris) were hunted to near extinction during the Pacific maritime fur trade (Kenyon 1969).  Further hunting was prohibited by international treaty in 1911, at which time a dozen or so remnant colonies survived.  The southern sea otter (Enhydra lutris nereis) is descended from one of these remnant colonies that survived along the Big Sur coastline of central California, and contained perhaps as few as 50 individuals at the beginning of the 20th century (Riedman and Estes 1990).  While sea otter populations elsewhere in the North Pacific Ocean recovered at rates of 17-20% yr-1, the California population has never grown at more than one-third this rate (Estes 1990) and is currently listed as Threatened under the Endangered Species Act.  Concerns over oil and gas development were the principal reason for listing, and the Southern Sea Otter Recovery Team (SSORT) has based its recommended criteria for de-listing or up-listing (to Endangered) on oil spill risk analysis (FWS 2000).  Our current understanding of the dynamics of the southern sea otter population is largely based on MMS-funded studies conducted during the 1980s (Siniff and Ralls, 1988), at a time when the population was increasing.  The population continued increasing until the mid-1990s, at which time the U.S. Fish and Wildlife Service anticipated de-listing by 2000.  About 1995, however, the population dynamics changed for unknown reasons and the population began to decline; annual population counts have steadily decreased since 1995 (USGS-BRD, unpublished data).  At the present rate of decline, the population will reach the SSORT's recommended criterion for up-listing to Endangered within two to three years. 

 

Although the California sea otter population is declining, the geographic range of the population continues to expand both to the north and south.  Range expansion to the south, which is characterized by a seasonal redistribution of several hundred individuals (believed to be largely non-territorial males) has brought sea otters into closer association with the potential effects of oil and gas development, and increased conflicts with Southern California shellfisheries.  Most of these otters congregate during the winter/spring near Cojo Cove, about 5 kilometers southeast of Pt. Conception.  These otters are thought to rejoin the more northern population during the summer and autumn, but exactly where they go is unknown.  Because these otters travel seasonally, and because both range expansion and population decline could conceivably be due to a single causal factor such as depletion of food resources to the north - for example, the demonstrated relationship between foraging behavior and thermoregulation in sea otters (Costa and Kooyman 1984) means that reduced prey availability could increase energetic demands beyond daily foraging capabilities, forcing otters to seek habitats more consisted with their physiological capabilities - it is impossible to understand the population dynamics of the southern sea otters without better understanding the reasons for the population decline as a whole. 

 

Objectives

 

Our proposed research has three main objectives:  1) to describe the population dynamics, behavior and seasonal movement patterns of sea otters at the southern end of their range;  2) to better understand the reason for the current population decline; and  3) to examine the inter-relationships between nutritional requirements, thermal stability and activity patterns, and the ways in which these relationships determine habitat requirements of the California sea otter. 

Update

Monitoring of study animals by radio telemetry was conducted by fieldworkers based at Piedras Blancas field station, San Simeon CA.  Data collection on movement patterns, activity budgets and foraging behavior/diet is collected  and all data are entered immediately into the wild sea otter database (WSOD).  At this time the database includes over 12,000 re-sightings of study animals (recording location, reproductive status, etc.); approximately 6,000 records for activity budgets (behavior, location, body temperature); and over 27,000 observational records of feeding dives (dive success, prey type, dive location).  

As was the case during the first two years of the study, the majority of males captured in the southern study area have made regular long-distance movements of 100 - 400 km to locations throughout the range (Figure 1).  In some cases these movements span the entire current range of the southern sea otter population.  Study animals that were captured in the northern study area tend to make far fewer long-distance movements (Figure 1), although a slight increase in then frequency of long-distance movements was observed during the late winter months of 2003.

 

 

Figure 1.  California sea otter range.  Red dots represent San Simeon study animals and purple dots represent Point Conception study animals. 

 

 

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