Space use of Pacific harbor seals (Phoca vitulina richardii) from two haulout locations along the Oregon coast

Abstract

Background: There are approximately 10,000-12,000 Pacific harbor seals (Phoca vitulina richardii) inhabiting the Oregon coast, and unlike other species of pinnipeds in this region, are reliably present year-round. Despite this, and drastic rebounds in population since the enactment of the Marine Mammal Protection Act, limited data is available for the present period regarding their space use at sea, and within estuarine, riverine, or bay areas within the state.

Objective: To examine site-based differences in space use for 24 adult Pacific harbor seals captured and outfitted with satellite transmitters at two predominant haulout sites on the Oregon Coast, USA.

Design: We captured 24 adult harbor seals from two haulout sites on the Central Oregon coast between September 2014-16 and fitted them with external Wildlife Computers SPOT5 satellite transmitters to track movement. Using state-space modeled locations derived from satellite telemetry data, we evaluated spatial behavior of these animals using a correlated random walk model via R package crawl. Kernel density estimation was subsequently used to calculate home range and core area for each animal. Percent use of open ocean habitat versus use of estuaries, rivers and bays was quantified, as was an initial examination of presence within five newly-established marine reserves in Oregon. Examination of haulout site-related differences in spatial behavior were examined for seals captured in Netarts and Alsea Bays, Oregon and haul out behavior related to time of day, season, and tidal level was also investigated.

Results: The average individual home range for seals was 364.47 ± 382.87 km2 with seals captured in Alsea bay demonstrating a significantly higher home range area than those captured in Netarts Bay. Alsea bay seals also tended to range farther from shore than Netarts Bay animals. The average calculated core area for seals encompassed on average 29.41 ± 29.23 km2 per animal, however the home range of one animal was so small, core area could not be calculated. Use of marine reserves was limited for animals in this study, representing less than 2% of locations with a majority occurring in Cape Perpetua Marine Reserve and North Marine Protected Area. Seals were more likely to haul out during low tides and periods of low light (dusk, night and dawn), and hauling out behavior increased in winter months.

Significance: These findings demonstrate the first major documentation of space use of harbor seals in the state for nearly three decades, and lends itself to future comparison and formation of mechanistically-based hypotheses for behavior of a common marine mammal in the highly productive northern California Current System.

Fig 1. 50% and 95% utilization distributions (general probably distribution of encounter for all 24 tagged harbor seals at a 2km x 2km (CVh) resolution.

Name of 11 bays and estuaries with seal presence labeled in black letters, with capture sites depicted by bold text. 50% UD labeled in fuchsia, 95% UD labeled in chartreuse (Coordinate system: GCS WGS 1984).

Fig 2. Map of 95% to 10% utilization distribution isopleths (general probability distribution of encounter) at a 2km x 2km (CVh) resolution for seals captured in Alsea Bay (n = 16).

Name of 11 bays and estuaries with seal presence labeled in black letters, with capture sites depicted by bold text. Utilization distributions are as shown in legend, with warm colors depicting higher probability of encounter (Coordinate system: GCS WGS 1984).

Fig 3. Map of 95% to 10% utilization distribution isopleths (general probability distribution of encounter) at a 2km x 2km (CVh) resolution for seals captured in Netarts Bay (n = 8).

Name of 11 bays and estuaries with seal presence labeled in black letters, with capture sites depicted by bold text. Utilization distributions are as shown in legend, with warm colors depicting higher probability of encounter (Coordinate system: GCS WGS 1984).