Changes in Stranding and Survival Patterns of Young California Sea Lions (Zalophus californianus) in a Central California Rehabilitation Center in 2015: Associations with Anomalous Oceanographic Conditions
IAAAM 2016
Cara L. Field1*; Tenaya Norris1; Eugene DeRango1,2; Hanna Buechi1; Shawn Johnson1; Frances Gulland1
1The Marine Mammal Center, Sausalito, CA, USA; 2Sonoma State University, Dept. of Biology, Rohnert Park, CA, USA


California sea lions (CSL; Zalophus californianus) breed and forage along the California coast and are highly dependent upon local prey availability, thus annual population size is associated with regional oceanographic conditions.1,2 Malnutrition is a common cause of stranding for California sea lions under 2 years of age.3 Changes in stranding numbers may provide insight into changes in sea lion prey availability associated with anomalous oceanographic conditions in the California Current System (CCS; 34.97-40.00°N). Young CSL are born on offshore islands in June and wean at 9–12 months of age, from May to July the following year.1,2 Data on live stranded CSLs admitted to The Marine Mammal Center in central California demonstrate most strandings occur around this weaning period from May–July (1997–2014 = 138 ± 40 individuals) with few strandings from January–April (1997–2014 = 33 ± 7 individuals). However, from January–April 2015, a greater number of young CSL stranded (n = 415) than is typical for an entire calendar year (1997–2014 = 216 ± 49 individuals). Weights of pups ( = 13.1 ± 0.2 kg) and yearlings ( = 19.3 ± 0.5 kg) that stranded during January–April of 2015 were less than those of pups that stranded January–April of 1997–2014 (pups = 14.8 ± 0.2 kg; yearlings = 23.7 ± 0.5 kg).

Once admitted into rehabilitation, CSLs at TMMC are tube fed a herring-based formula if they are not eating, followed by whole herring once they begin to eat, and are given supportive care that often includes anthelmintics and antibiotics. Despite the same standard of care, pup and yearling survival was lower in 2015 in comparison to previous years. Significant factors affecting survival of pups undergoing rehabilitation in 2015 included weight at admission, and month, county, and cause of stranding. Cause of stranding also significantly affected yearling survival in 2015. The early temporal shift and increase in stranded CSL pups in spring 2015 indicates lactating females were unable to adequately provision their pups, rather than pups failing to thrive post-weaning, as is likely the case later in the year. As dams are experienced foragers, the inability to provide enough nutrition to nursing pups suggests greater changes to prey availability than previous years with persistent, associated with large-scale warming anomalies in the CCS.4


The authors thank the numerous volunteers and staff of The Marine Mammal Center for their care of the stranded marine mammals.

* Presenting author

Literature Cited

1.  Melin SR. Winter and Spring Attendance Patterns of California Sea Lion (Zalophus californianus) Females and Pups at San Miguel Island, 1991–1994 [Master of Science thesis]. Seattle, WA: University of Washington; 1995.

2.  Lowry MS, Stewart BS, Heath CB, Yochem PK, Francis JM. Seasonal and annual variability in the diet of California sea lions, Zalophus californianus, at San Nicolas Island, California, 1981–1986. Fishery Bulletin. 1991;89:331–336.

3.  Greig DJ, Gulland FMD, Kreuder C. A decade of live California sea lion (Zalophus californianus) strandings along the central California coast: causes and trends, 1991–2000. Aquatic Mammals. 2005;31(1):11–22.

4.  Peterson W, Robert M, Bond N. The warm blob continues to dominate the ecosystem of the northern California Current. PICES Press. 2015;23:36–38.


Speaker Information
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Cara L. Field, DVM, PhD
The Marine Mammal Center
Sausalito, CA, USA

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