Trends in Abundance and Current Status of Harbor Seals in Oregon: 1977–2003

Abstract

The distribution and abundance of harbor seals (Phoca vitulina richardii) in Oregon were monitored from 1977 to 2003 by aerial photographic surveys. Harbor seals on shore were counted each year during the reproductive period. Mean annual counts of non‐pups (adults and subadults) were used as an index of population size and the trend in the counts was modeled using exponential (density‐independent) and generalized logistic (density‐dependent) growth models. Models were fit using maximum likelihood and evaluated using Akaike's Information Criterion. The population dynamics of harbor seals in Oregon were best described by the generalized logistic model. The population grew following protection under the Marine Mammal Protection Act of 1972 until stabilizing in the early 1990s. The estimated absolute abundance of harbor seals (all age classes) during the 2002 reproductive period was 10,087 individuals (95% confidence interval was 8,445–12,046 individuals). The current predicted population size for harbor seals in Oregon is above its estimated maximum net productivity level and hence within its optimum sustainable population range. We speculate that recent increases in ocean productivity in the eastern Pacific Ocean may lead to an increase in carrying capacity and renewed growth in Oregon's harbor seal population.

Long‐term patterns in harbour seal site‐use and the consequences for managing protected areas

“…However, data on the consistency of site-use at the intermediate timescales of interest to conservation biologists are sparse. Fine-scale studies of haul-out site use have typically been conducted over only a few years, and longer time-series of abundance estimates generally present aggregated data at larger scales (Jeffries et al, 2003;Brown et al, 2005;Lonergan et al, 2007) or use a sub-set of sites for trend monitoring (Frost et al, 1999;Small et al, 2003). The results from this study are the first to present data on fine-scale patterns of harbour seal haul-out site use over a period of two decades.…”

“…Harbour seals are the most widespread of pinnipeds. Their populations show marked regional variations in their dynamics, with some populations increasing over recent decades (Jeffries et al, 2003;Brown et al, 2005) while others have declined (Bowen et al, 2003;Lonergan et al, 2007;Small et al, 2008). In Europe, harbour seals are protected under the European Commission Habitats Directive (Council Directive 92/43/EEC), requiring the designation of Special Areas of Conservation (SACs) to protect both their terrestrial haul-out sites and associated foraging areas (Baxter, 2001;Cunningham et al, 2009).…”

Killer Whales and Marine Mammal Trends in the North Pacific—a Re‐examination of Evidence for Sequential Megafauna Collapse and the Prey‐switching Hypothesis

“…1990), Washington (Jeffries et al . 2003), and Oregon (Brown et al . 2005); they may also be leveling off in California.…”

Killer Whales, Whaling, and Sequential Megafaunal Collapse in the North Pacific: A Comparative Analysis of the Dynamics of Marine Mammals in Alaska and British Columbia Following Commercial Whaling

“…3B; Olesiuk et al 1990, Olesiuk 1999). Harbor seals also appear to have increased in California, Oregon, Washington, and southeast Alaska ( Jeffries et al 2003, Small et al 2003, Angliss and Lodge 2004, Carretta et al 2004, Brown et al 2005).…”