Abstract

Since establishment of The Marine Mammal Center (TMMC) during 1975, the number of pinnipeds admitted for rehabilitation has increased. Despite the increasing number of stranded animals, only a few studies have been conducted to assess the survival of these animals after release. Such studies are essential to determine the efficacy of different rehabilitation techniques, and to determine the ability of rehabilitated seals and sea lions to forage, dive, and reproduce following release. Post-release monitoring depends upon the ability to track animals in the wild using radio and satellite telemetry, which may be limited by 1ogistics such as animal size, ecology, molting, and equipment design and cost. Since 1995, The Marine Mammal Center has evaluated the post-release survival of 35 rehabilitated pinnipeds using different means of telemetry.

During 1995 and 1996, a total of 24 rehabilitated harbor seal pups (Phoca vitulina) were tagged with VHF radio transmitters, which were believed to be suitable for this project because previous studies have indicated that harbor seals are relatively sedentary animals that tend to remain coastal and exhibit haul-out site fidelity. The objectives of the study were to compare the movements, diving behaviors, survival, and health between rehabilitated harbor seal pups and a control group of wild harbor seal pups. Stranded harbor seal pups recovered from Pebble Beach, California, were admitted to TMMC during pupping season. Blood samples were collected from all pups before and after rehabilitation to examine complete blood counts and serum chemistries. Radio transmitters were attached to the heads of rehabilitated pups (n=15 during 1995, n=9 during 1996) that were released at Pebble Beach. Twenty six wild pups were captured at Pebble Beach and 15 (n=10 during 1995, n=5 during 1996) were radio-tagged. Blood samples also were collected from wild pups. Maximum distance dispersed by rehabilitated pups (mean=98.13 km, SD=59.16, range=30.50 to 178.00 km, n=15) did not significantly differ from that of wild pups (mean=i19.80 km, SD=66.37, range=26.50 to 180.50 km, n=Il; t=0.88, P>0.05). Furthermore, mean duration of dives (mean=1.23 min, SD--0.44) and surface intervals (mean=0.44 min, SD=0.10) of rehabilitated pups (n=18) did not differ significantly from mean duration of dives (mean=1.35 min, SD=0.55) and surface intervals (mean=0.40 min, SD=0.04; P>0.05) of wild pups (n=7). For two months after release, at least 72.72% of the rehabilitated pups had survived, whereas at least 73.33% of the wild pups had survived. Although mean blood values of rehabilitated pups were within ranges of that established for clinically healthy harbor seal pups, many of the values differed significantly from mean blood values of wild pups. Significant differences between mean blood values, however, may not be biologically significant. These data have been used to determine optimum release criteria for harbor seal pups.

Telemetry also may be used to assess the survival of rehabilitated animals with clinical lesions with unknown effects on free-living animals. A number of rehabilitated animals with eye lesions are capable of feeding and reproducing in captivity, but their survival in the wild is unknown, making the release of such animals a controversial issue. Six (n=2 during 1996, n=4 during 1997) visually impaired California sea lions (Zalophus californianus) were tagged with VHF radio transmitters to monitor their movements after release. Signals from these animals suggest some visually impaired animals may survive in the wild, but a larger sample size is needed to determine the effect of ocular lesions on survival of rehabilitated sea lions.

One northern fur seal (Callorhinus ursinus) pup and one elephant seal (Mirounga angustirostris),which suffered from partial temporomandibular arthrodesis, were radio-tracked after release using VHF radio transmitters, but were never relocated. Lack of data may have been a result of mortality, or may have been due to the extent of the animal's movements. Fur seals and elephant seals are pelagic species, which undergo seasonal migrations. Radio telemetry is thus of little use in wider ranging species.

Satellite telemetry, therefore, was used to assess the movements and survival of two Steller sea lion (Eumetopias jubatus) pups that were hand-reared from approximately two weeks of age. After 10 months in captivity, both animals were clinically healthy and weighed 123 and 69 kg. Type III Satellite linked time-depth recorders (Wildlife Computers, Redmond, WA) were attached to the backs of the two sea lions and they were released near Southeast Farallon Island (37° 42'6"N, 123° 0'1"W). One animal traveled as far north as Yachats, Oregon (44° 40'3"N, 124° 30'0"W; approximately 825 km), whereas the other remained near the Farallon Islands. Depth and duration of dives were significantly greater for the heavier sea lion than for the lighter one, and transmissions were received from the heavier animal for four months, but only for five weeks from the lighter one. These data suggest that excess weight gain during rehabilitation may be beneficial for short-term survival following release.

Satellite telemetry also was used to monitor an adult Guadalupe fur seal (Arctocephalus townsendi), which stranded at Point Lobos State Reserve, Carmel, California during January of 1998. The fur seal, probably a victim of El Nino, was underweight at 34 kg when first admitted and maintained in captivity until she weighed 63 kg. The animal was released March 5th and satellite telemetry is currently being used to monitor her progress.

The goal of wildlife rehabilitation is to re-introduce healthy animals back into the wild population that will survive and eventually reproduce. Currently, it is logistically difficult to determine whether this objective is being met because most of the animals admitted to TMMC are pups and juveniles and the methodologies described above can only be used for a short period proceeding release. In the future, more long-term studies should be conducted as technology advances.