Remote monitoring of survival and short-range year-round movements of harbor seals in Prince William Sound, Alaska

Harbor seals (Phoca vitulina richardii) declined by greater than 63% in Prince William Sound (PWS) beginning in the mid 1980s.
The long-term population trend documented an average annual rate of decline of 2.0%/yr (1990-2006) although recent data indicate the population
is stabilizing and showing signs of increase. Fluctuations in population abundance can be evaluated by estimating survival and reproduction,
and quantifying the effect that proximate factors (e.g., diet, condition, general health, contaminants, etc.) have on those vital rates.
In 2003, the Alaska Department of Fish and Game, Harbor Seal Research Program initiated a vital rates study in PWS in which 122 harbor seals
(70.5% females, 61.5% less than 1 year old) received subcutaneously-implanted 5yr VHF transmitters with mortality sensors. At capture we obtained
biological samples from each individual to assess age, genetics, body condition and health, diet, immunocompetency, reproductive condition,
disease exposure and contaminants. Our objective was to analyze survival data from telemetry resights, relative to data on diet and health
status of the individuals, to determine which factors differentiate between seals that survive and those that do not; potentially elucidating
key factors contributing to the population dynamics of seals in PWS . To increase our resight rates, in 2005 with NPRB funding we established
six monitoring stations that remotely assess presence/survival of radio-tagged seals year-round. After a season of equipment revisions to improve
data quality, we determined that remote monitoring provided reliable resight data, logging greater than 2000 “seal days” (at least one location
per calendar day) for PWS seals in 2005-07. Resight rates were greater than 85% in the year of tagging but unacceptably low in subsequent years.
We initiated Sound-wide telemetry surveys in pupping and molt seasons in 2006 to attempt to relocate emigrants with little success, and found
some evidence of transmitter failure and rejection. Analysis of resight data indicates that harbor seals show high site fidelity. When assessing
interannual survival, little is gained by year-round monitoring; 96% of seals located in the off season are also resighted during pupping/molt season
if radio signals are not censored by then. This study also determined that the use of subcutaneously implanted transmitters cannot be recommended
in harbor seals due to low resight rates. These datasets were archived as part of the North Pacific Research Board legacy project recovery effort undertaken by Axiom Data Science and NPRB in 2025. The goal of the recovery effort was to assess the NPRB-funded data projects from 2002 to 2014 and archive final data packages that were ready for publication to increase long-term accessibility and discoverability. Data packages were archived as is given limited funding and resources.