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NOAA Technical Memorandum NMFS-AFSC-368

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Short-term survival rates of branded Steller sea lion pups


Survival rates of western Steller sea lion (Eumetopias jubatus) pups (total N = 621) were estimated during 2000, 2002, and 2004 at one rookery on Marmot Island and during 2003 and 2005 at two rookeries on Ugamak Island for up to 73 days following hot-iron branding. Estimated daily apparent survival rates increased and standard errors of the estimates decreased with increasing durations of in-season monitoring. An asymptotic apparent survival rate (φa) of 0.9995 d-1 was estimated from sightings of marked pups on 7-30 occasions between 1 and 73 days after branding, as well as in subsequent years through 2015. Sex and pup mass at the time of branding were not strong factors affecting survival. Extrapolations of φ beyond the in-season period yielded 12-week and 1-year survival rates of 0.960 and 0.837, respectively. During six of the seven in-season monitoring periods, numbers of dead pups counted by observers following branding were less than those estimated to have died based on our calculated survival rate. Two more dead pups were counted than predicted at Marmot in 2000, but the actual cause(s) of any of the pup deaths are not known.

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