Individual study: Surrogate-reared southern sea otter Enhydra lutris pups have greater survival rates in the wild than pups reared without surrogates, California, USA
Nicholson T.E., Mayer K.A., Staedler M.M. & Johnson A.B. (2007) Effects of rearing methods on survival of released free-ranging juvenile southern sea otters. Biological Conservation, 138, 313-320
The range of the endangered southern sea otter Enhydra lutris, is restricted to a 200 km stretch of the Californian coast, with a population estimated at only 2,735 individuals in 2005. Due to its rarity, attempts are ongoing to rehabilitate abandoned otter pups. Since 1984, Monterey Bay Aquarium's Sea Otter Research and Conservation (SORAC) program has treated more than 70 stranded pups, those rehabilitated for release in the early years were reared by methods relying heavily on human care. From 1986 to 2000, 67% of unsuccessful pup releases were due to failures to reintegrate with the wild population and avoiding interactions with people. In an attempt to address this, SORAC initiated a surrogate program, pairing pups with captive adult females. It was hoped that surrogate-reared pups would develop better foraging skills and would have greater success re-adapting to the wild, compared with pups rehabilitated by the earlier human-intensive methods.
Behaviour development during different captive rehabilitation strategies, release success, and survival in the wild was compared between surrogate-reared male pups (n = 5, 2001-2003) and non-surrogate reared male pups (n = 6, 1998-2000). Survival rates in the wild of male pups raised in 1986 to 1998 (n = 20), surrogate-reared male pups from 2004 (n = 2) and of free-ranging juvenile males (n = 12) were also compared.
Males were chosen for comparison as since autumn 2001 when the surrogate program was implemented, 88% of stranded pups were male; so only male pups were rehabilitated using the surrogate method during 2001 to 2003 (this study period). Since 1984, male and female pups have stranded with similar frequency, thus the prevalence of male pup strandings during the study period, are considered most likely the result of chance.
Monitoring: Before release, the now juvenile otters, were implanted with a radio-transmitter and colour-coded flipper tags were attached. During the first month after release, daily surveys were undertaken to locate animals from shore; then less frequently but assisted by ground-based radio-tracking. Wider-ranging radio-tracking surveys for 'missing' otters were conducted from the air.
Release success: Release success was judged by whether or not the released juvenile foraged proficiently in the wild and avoided contact with humans; individuals that survived a month or more were considered successful. Those that exhibited foraging skills but interacted with humans were regarded as a mixed outcome. Where possible, individuals failing to adapt were bought back into care.
Surrogate-reared pups began foraging independently on live-prey 2–3 weeks earlier, and had higher survival rates (71% vs. 31%) in the wild than pups reared without surrogates. Despite higher surrogate-reared survival, they had a lower rate of annual survival in the wild as juveniles (27% survival/year) compared to wild juveniles (75-88%).
The authors conclude that the surrogate program was less labour-intensive and more effective than earlier rehabilitation methods. Work is ongoing to improve reintroduction techniques.
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