Efficacy of subcutaneous VHF implants and remote telemetry monitoring to assess survival rates in harbor seals

Abstract

A long-term study to compare harbor seal (Phoca vitulina) survival rates between 2 study areas was conducted in Alaska, in Glacier Bay (GB) and Prince William Sound (PWS). Very-high-frequency (VHF) transmitters with 3.5- to 5-year batteries were subcutaneously implanted into 277 harbor seals; 122 in PWS from 2003 to 2005, and 155 in GB from 2004 to 2006. The presence of radiotagged seals was remotely monitored using VHF dataloggers, which transmitted data via the Geostationary Operational Environmental Satellite (GOES) system. The GOES site is administered by the National Oceanic and Atmospheric Administration, National Environmental Satellite Data and Information Service; data were accessible to researchers via telnet. Initial data-logging was plagued with ambient electronic interference (“noise”); subsequent equipment revisions substantially reduced the noise and resulted in 8,129 total “seal-days” (≥ 1 detection calendar day−1 seal−1) of telemetry detections (X̄ = 29 days/seal ±3.4 SE; maximum 424 days spanning 4.15 years). Although 84% of radiotagged seals were detected at least once during the year they were radiotagged, the proportion of tagged seals detected in subsequent years dropped to 59%, 28%, 0.9%, and 3% in the 1st through the 4th year, respectively. Tag failure and tag rejection, including 1 tag rejected 11.75 months after implantation surgery, were documented. There was almost no evidence of health problems at the time of implanting the tags in the harbor seals. Survey effort outside the study areas did not detect evidence of high emigration rates. Although subcutaneously implanted transmitters could be radiotracked during and after molt and the long battery life provided multiyear data on a subset of individual seals, this method of subcutaneously implanting radiotags was not effective for assessing the long-term survival rate of free-ranging harbor seals.