Assessment of Photographs from Wildlife Monitoring 

Cameras in Drakes Estero, Point Reyes National 

Seashore, California

By William A. Lellis, Carrie J. Blakeslee, Laurie K. Allen, Bruce F. Molnia, Susan D. Price, 

Sky Bristol, and Brent Stewart 

Open-File Report 2012–1249

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2012 

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Suggested citation:

Lellis, W.A., Blakeslee, C.J., Allen, L.K., Molnia, B.F., Price, S.D., Bristol, Sky, and Stewart, Brent, 2012, 

Assessment of photographs from wildlife monitoring cameras in Drakes Estero, Point Reyes National Seashore, 

California: U.S. Geological Survey Open-File Report 2012–1249, 24 p., available only at 

http://pubs.usgs.gov/of/2012/1249.

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iii

Contents

Background ................................................................................................................................................................... 1

Photograph Analysis...................................................................................................................................................... 1

Video Analysis ............................................................................................................................................................... 2

Scientific Value .............................................................................................................................................................. 3

Habitat........................................................................................................................................................................ 3

Displacement ............................................................................................................................................................. 4

Disturbance................................................................................................................................................................ 4

Conclusions ................................................................................................................................................................... 5

References Cited........................................................................................................................................................... 6

Appendix 1. Summary analysis of 3,140 photographs from 75 potential disturbance events to hauled out harbor 

seals in Drakes Estero..................................................................................................................................................19

Figures

1. Sample of photographs from Point Reyes monitoring cameras ........................................................................... 8 

2. A series of photographs of a seal flushing event on March 31, 2008, at the Oyster Bar site 

within Drakes Estero ............................................................................................................................................ 9 

 3. A series of photographs of a seal flushing event on April 13, 2008, beginning at 12:01 p.m.

at the Oyster Bar site within Drakes Estero........................................................................................................ 10

 4. A series of photographs of a seal flushing event on April 13, 2008, beginning at 12:29 p.m. 

at the Oyster Bar site within Drakes Estero........................................................................................................ 11

 5. A series of photographs of a seal flushing event on April 14, 2008, at the Oyster Bar site 

within Drakes Estero.......................................................................................................................................... 12

 6. A series of photographs of a seal flushing event on April 23, 2008, at the Oyster Bar site 

within Drakes Estero .......................................................................................................................................... 13

 7. A series of photographs of a seal flushing event on May 6, 2008, at the Oyster Bar site 

within Drakes Estero.......................................................................................................................................... 14

 8. A series of photographs of a seal flushing event on May 15, 2008, at the Oyster Bar site 

within Drakes Estero.......................................................................................................................................... 15

 9. A series of photographs of a seal flushing event on May 31, 2008, at the Oyster Bar site 

within Drakes Estero.......................................................................................................................................... 16

10. A series of photographs of a seal flushing event on June 2, 2008, at the Oyster Bar site 

within Drakes Estero.......................................................................................................................................... 17

11. A series of photographs of a seal flushing event on June 11, 2008, at the Oyster Bar site 

within Drakes Estero.......................................................................................................................................... 18

Tables

1. Flushing events of Drakes Estero seals identified in 2008 National Park Service photographs........................... 7 
1 

Assessment of Photographs from Wildlife Monitoring 

Cameras in Drakes Estero, Point Reyes National 

Seashore, California

By William A. Lellis,1 Carrie J. Blakeslee,1 Laurie K. Allen,1 Bruce F. Molnia,1 Susan D. Price,1 

Sky Bristol,1 and Brent Stewart2

Background

Between 2007 and 2010, National Park Service (NPS) staff at the Point Reyes National 

Seashore, California, collected over 300,000 photographic images of Drakes Estero from remotely 

operated wildlife monitoring cameras. The purpose of the systems was to obtain photographic data to 

help understand possible relationships between anthropogenic activities and Pacific harbor seal (Phoca 

vitulina richardsi) behavior and distribution. 

The value of the NPS photographs for use in assessing the frequency and impacts of seal 

disturbance and displacement in Drakes Estero has been debated. In September 2011, the NPS 

determined that the photographs did not provide meaningful information for development of a Draft 

Environmental Impact Statement (DEIS) for the Drakes Bay Oyster Company Special Use Permit. 

Limitations of the photographs included lack of study design, poor photographic quality, inadequate 

field of view, incomplete estuary coverage, camera obstructions, and weather limitations. 

The Marine Mammal Commission (MMC) reviewed the scientific data underpinning the Drakes 

Bay Oyster Company DEIS in November 2011 and recommended further analysis of the NPS 

photographs for use in characterizing rates and consequences of seal disturbance (Marine Mammal 

Commission, 2011). In response to that recommendation, the NPS asked the U.S. Geological Survey 

(USGS) to conduct an independent review of the photographs and render an opinion on the utility of the 

remote camera data for informing the environmental impact analyses included in the DEIS. 

In consultation with the NPS, we selected the 2008 photographic dataset for detailed evaluation 

because it covers a full harbor seal breeding season (March 1 to June 30), provides two fields of view 

(two cameras were deployed), and represents a time period when cameras were most consistently 

deployed and maintained. The NPS requested that the photographs be evaluated in absence of other data 

or information pertaining to seal and human activity in the estuary and that we focus on the extent to 

which the photographs could be used in understanding the relationship between human activity

(including commercial oyster production) and harbor seal disturbance and distribution in the estuary. 

Photograph Analysis

The NPS provided 333,042 digital photographs of the Drakes Estero taken by remote cameras 

between 2007 and 2010. These same photographs are available to the public on the Point Reyes 

 

1 U.S. Geological Survey

2 Hubbs-Seaworld Research Institute, San Diego, Calif.
2 

National Seashore Reading Room Web site (NPS Reading Room, 2012; Web address listed in 

References Cited). Included in the collection were 165,282 photographs taken in 2008 from two sites 

within Drakes Estero: 100,457 from a site referred to as the Upper Estero Far (UEF) and 64,825 from a 

site referred to as the Oyster Bar (OB). These photographs were taken between March 14 and June 23, 

2008, at an interval of one per minute during daylight hours (approximately 720 photographs per day at 

each site). Some of these photographs were duplicates. 

Our initial plan was to analyze a random subsample of 10 percent of all 2008 photographs and 

assess each individual photograph for quality and information that could be used to study seal 

disturbance and displacement. Such information would include photograph clarity and resolution, 

obstructions, field of view, light, weather conditions, stage of tide, presence and number of seals, human 

activity, nonhuman activity, and evidence of seal disturbance. Seal disturbance was classified as head 

alert (increased vigilance), flushing on land (change in position), or flushing into water (abandon site). 

(See Marine Mammal Commission (2011, p. 13-16) for additional discussion.) 

Initial review of a portion of the intended subsample indicated that many photographs were of no 

obvious value to understanding seal behavior during haulout because of inadequate light, inadequate 

observing conditions due to weather (fog, rain, wind), obstructions (plants), too wide a field of view, 

misdirection of camera, wrong tidal stage (no exposed sand bars for haulout), and (or) no seals within 

camera view (fig. 1). A smaller portion of the photographs contained potentially useful information such 

as exposed sandbars, presence of hauled out seals, and (or) potentially disturbing stimuli such as boats, 

people, birds, or other unidentified objects in the water, sky, or on land. 

Within the photographs that contained hauled out seals, the distance of the camera from the seals 

was often too great, the angle of the camera too low to the water, and resolution of the photographs too 

low to allow an accurate count of the number of individual seals within groups hauled out on the sand. 

Nor was it possible, in most cases, to distinguish any behaviors among individual seals, such as head 

alerts, that could definitively be ascribed to increased vigilance in response to a stimulus. These same 

limitations also prevented determination of any distinguishing features that would allow for 

identification of specific boats or people, or activities in which they were engaged. Attempts to improve 

resolution through photographic enhancements were unsuccessful. 

Video Analysis

Our initial approach of analyzing a random subsample of all photographs had two significant 

limitations. The first was that information contained within each photograph lacked the context of how 

it fit into events that occurred immediately before and after that moment in time. This lack of sequential 

information effectively prevented determination of seal flushing (change in number of seals over time) 

or the reasonable establishment of cause and effect between seal behavior and human or nonhuman 

activity. The second limitation was that examination of individual photographs was extremely time 

consuming and yielded little usable information for the given effort. As such, we changed our approach 

to animation of photographs into daily videos to allow more rapid screening and to add sequential 

context to each photograph. 

To make the videos, we retrieved the 2008 photographs from the Point Reyes National Seashore 

Reading Room Web site using a utility that can copy a site’s pages, images, movies, and other files. 

Each image on the Web site was dated and time-stamped. Although the NPS cameras were programmed 

to take one photograph every minute for 12 hours (720 photographs per day), camera malfunction, 

battery failure, changes in camera programming, or other technical issues resulted in some days 

containing more or less than 720 photographs. Missing photographs for specific dates and times were 

verified as nonexistent with Point Reyes staff. 
3 

After retrieval, we added the file name to each image as a watermark and imported the files into 

Apple iPhoto for production. The interval between frames (that is, photographs) was set to 1 second 

with no transition effects, in order to create a time-lapse style video. No photographs were edited from 

their original form or deleted during production. Videos were exported as MPEG4 files, which retained 

each photograph’s original resolution. All videos can be downloaded from the USGS Applied Earth 

Systems Web site (Web address listed in References Cited).

Each video was reviewed by at least two observers working together to identify and record data 

of possible interest in an analysis of seal disturbance, including time of day when the sand bars appeared 

and disappeared, time of day when seals hauled onto or left a sandbar, human activity when seals were 

present, nonhuman activity when seals were present, and any abrupt changes in the size or location of a 

group or the number of seals hauled out on a sand bar. Within each video, human activity was recorded 

only during times when seals were hauled onto sandbars (that is, boat traffic was not recorded during 

high tide or when seals were not present on the sand bars). 

In total, we produced 191 videos from the 2008 photographic collection (103 UEF, 88 OB). No 

hauled out seals were detected in any of the UEF videos, because of low resolution and wide field of 

camera view, so we conducted no further analyses of those pictures. Within the OB videos, we 

identified 75 different events (appendix 1) in which human activity was visible in the photographs while 

seals were hauled out, or there was an unusual amount of nonhuman stimuli (birds), or there was a 

sudden change in the number or position of hauled seals. Human activity during seal haulout included 

boats (44 events, 34 of which had people visible on the sand bars while the boat was stationary), camera 

maintenance (21 events), and kayaks (2 events). We detected camera service by either a change in 

camera angle or a reset in the image number during a daily photograph sequence. 

Photographic sequences of each event, plus the 10 photographs before the start of each event and 

the 10 photographs after the end of each event (3,140 photographs total) were analyzed for incidence 

and cause of seal disturbance. Ten of the 75 events were classified as containing behaviors indicative of 

disturbance in the form of flushing (table 1, figs. 2-11, appendix 1). Two flushing events were 

associated with the presence of a kayak, two were associated with birds landing in the area, two were 

associated with boat activity, and four occurred when no obvious stimuli were visible within the field of 

view of the camera.

Scientific Value

Using the analysis we conducted of the 2008 Drakes Estero photographs as a representative 

sample of all 4 years of monitoring, we considered the scientific value of these photographs without 

other supporting information for use in analyses to determine the impacts of human and nonhuman 

activities on seal habitat, displacement, or disturbance. 

Habitat

Fitness is defined as a measure of an individual seal’s ability to survive and reproduce and is 

influenced by many factors, including suitable haulout habitat for resting, molting, and reproduction, 

particularly for females and pups during the spring breeding season (Marine Mammal Commission

2011, p. 13). Suitable haulout sites provide quick access to deep water for shelter, protection from storm 

events and predators, and minimization of disturbance and harassment. In that regard, monitoring 

cameras can provide site-specific information on habitat persistence over time; physical impact of 

weather, storms, and waves; occupancy rate; frequency and severity of harassment from predators such 

as coyotes and elephant seals; frequency and severity of disturbance from human and nonhuman 
4 

sources; and a general sense of degree of comfort seals have with a site (degree to which they maintain a 

resting position during haulout). 

The 2008 Drakes Estero photographs can provide information on habitat persistence and use at 

the OB site. The photographs have adequate resolution, time and date-stamping, field of view, and span 

of operation to determine daily timing and duration of sand bar exposure, storm damage and wave 

conditions, frequency and timing of site usage, and how weather and tidal cycles affect site occupation. 

Data on sand bar exposure could be related to local tidal gages to develop predictive models of daily 

habitat timing and availability within the estuary.

Limitations of the 2008 photographs for habitat monitoring include lack of information during 

darkness, limited information during low visibility conditions such as fog, inconsistent or limited ability 

to count animals or estimate age for use in occupancy estimates, lack of information on concurrent use 

of other haulout sites, and inadequate resolution to identify specific predators on land or in the water 

(see 3/31/08 and 5/1/08 in appendix 1). Habitat monitoring could be improved by installation of highdefinition cameras, multiple cameras with different focal lengths and field of view, and cameras capable 

of detecting animals during darkness. 

Displacement 

Displacement is defined as the avoidance of an otherwise preferred haulout site based on 

experience or perception of a possible threat (Marine Mammal Commission, 2011, p. 15). The 2008 OB 

monitoring camera provides a view of an area that simultaneously contains both hauled seals in the 

foreground and human activity in the form of boat traffic in water and foot traffic on submerged and 

exposed sandbars in the background. No seals were observed to be hauled out at any time in the area of 

human traffic in the photographs examined during this analysis. Thus, two questions on seal 

displacement can be raised for this site: are seals being completely displaced from the distant sandbars 

due to direct human activity and are seals being partially displaced from the closer sandbars due to 

indirect human activity? Answering these questions requires accurate counts of hauled seals over time 

and distance and a means of comparing occupancy rates during periods of human activity and no human 

activity. 

Wildlife monitoring cameras can be used to study displacement by providing data on seal 

abundance and distribution over time in the presence and absence of human activity. The resolution of 

the 2008 OB photographs, however, is too low to provide consistently accurate counts of individual 

seals for this purpose. In addition, resolution diminishes with distance from the camera, thus creating an 

inherent bias to detect more seals in the foreground (site of haulout) than in the background (site of 

human activity). Monitoring to study seal displacement could be improved by installation of higher 

resolution cameras with greater image capture rate to increase accuracy of seal counts and by 

installation of cameras at multiple locations or with different focal lengths to remove distance sampling 

bias. Implementation of a statistically valid experimental design that controls human activity relative to 

variations in seal haulout activity over season, tide, and weather would also improve accuracy of 

displacement studies.

Disturbance

Disturbance is defined as an event or stimulus that alters a seal’s behavior or use of estuary 

habitat for resting, molting, or reproduction (Marine Mammal Commission, 2011, p. 13). Disturbed 

seals may show a continuum of responses to disturbance, including vocalizations, increased vigilance 

such as raised head (head alert), change in position on land (flush toward water), flush into water and 

return, and flush into water and not return (abandon site). 
5 

The 2008 OB photographs lack sound, so they do not provide any information on vocalizations. 

Within some photographs, there is enough resolution to detect changes in head position in individual 

seals (see OB-05-15-08 IMG_ 1599-1601). However, the ability to detect change is inconsistent across 

photographs and position of seals within the photographs, so for practical purposes the resolution is too 

coarse in the 2008 OB photographs to document the more subtle indicators of seal disturbance. The 

photographs can be used to document the more coarse indicators of disturbance, including flush toward 

water (see OB-06-11-2008 IMG_1155-1158), flush into water and return (see OB-04-13-2008 

IMG_2190-2219), and abandon site (see OB-03-31-2008 IMG_0018-0050). Documentation of 

disturbance events would be greatly enhanced with increased resolution and multiple camera angles.

Within the 2008 OB videos, we identified 10 incidents of seal disturbance that involved a 

flushing event (table 1, figs. 2-11, appendix 1). This does not include all incidents of disturbance, 

because we could not include vocalizations, nor could we consistently detect head alerts and other 

postural changes indicating increased vigilance. It is also possible that we missed incidents of flushing, 

particularly those involving changes in position on land within large groups of seals at distances farthest 

from the camera. 

Correlation of these flushing events with specific stimuli was difficult due to lack of associated 

sound, coarse resolution, and limited field of view on land, water, and air. Three types of stimuli that 

could be directly connected, or at least associated with a flushing level of disturbance in the OB seals 

are kayaks passing in proximity (see OB-04-13-2008 IMG_2186-2200 and OB-04-13-2008 IMG_2218-

2224), seabirds landing among or close to the seals or passing nearby (see OB-04-14-2008 IMG_0354-

0359 and OB-04-23-2008 IMG_1315-1322), and boat traffic at nearby sandbars (see OB-05-15-2008 

IMG_1590-1605 and OB-06-11-2008 IMG_1153-1163). However, there are numerous incidents of 

increased seabird activity in the photographs with no indication of flushing-level disturbance to seals.

We recorded 40 incidents of boat visits to the adjacent sandbar (many with related foot traffic) that did 

not seem to cause a flushing-level disturbance in the hauled seals, and at times there are multiple 

sources of potential disturbance stimuli occurring simultaneously. We found no evidence that activities 

related to maintenance of the remote camera system directly caused any flushing-level disturbances in 

the seals, although the relationship between camera maintenance and bird movement could not be 

ascertained by these photographs. 

Conclusions 

Based on our analysis of 165,282 photographs taken in 2008 from two remote cameras within 

Drakes Estero, we conclude that the protocols used by the NPS camera monitoring program did provide 

some data that could be used to document gross haulout patterns of seals and some instances of 

reactions to potential stimuli in the Drakes Estero. Data are limited to seal use of the Oyster Bar site 

related to time, tide, and weather and to some coarse detection of disturbance as measured by flushing 

of seals from resting positions toward or into the water. The length of time that seals abandoned the 

haulout sites after flushing could also be quantified in these photographs. 

Camera focus was generally too poor and image resolution was too low to allow accurate 

counting or aging of seals or to provide enough anatomical detail to quantify postures associated with 

increased vigilance to potentially disturbing stimuli. The methods and equipment used did not allow 

discrimination between visual and auditory elements of potentially disturbing stimuli, and the field of 

view was too narrow to discriminate causation from correlation between seals and observed visual 

stimuli for most disturbance events. A wide-angle camera system with higher image resolution 

capabilities, or a network of linked high-resolution cameras coupled with audio recording systems 

would help determine whether movements or subtle changes in the behavior and posture of harbor seals 
6 

is directly caused by human disturbance. The systems would also provide better opportunities for 

recording normal haulout patterns and behaviors. 

Direct monitoring by on-site observers would allow better documentation and evaluation of seal 

behaviors and the variables that influence them, provided that the observers themselves do not create 

additional potential for seal disturbance, such as flushing of birds into the seal haulout area. A video and 

audio monitoring system that could broadcast continuously by radio frequency, cellular telephone, or 

satellite to a remote site would reduce the chances that operation of photographic equipment could 

confound the observations. That system would need to resolve the same issues of focus, field of view, 

angle, and resolution that have limited the utility of the time-lapse camera system used in 2008. 

The first order limitation of all these methods is that they only document the brief response or 

non-response of harbor seals to a single potentially correlative stimulus. Larger scale questions on the 

significance of disturbance events to seal behavior within Drakes Estero, or the relationship of localized 

seal disturbances to overall population structure and viability, require rigorous investigation and 

hypothesis testing. If hypothesis testing and discrimination of causation from correlation is the intent of 

further effort at Point Reyes, then development of a more rigorous and comprehensive study design to 

incorporate several behavioral and environmental monitoring methods is needed. 

References Cited

Applied Earth Systems Informatics Research (AESIR), 2012, PRNS time lapse videos: Web site 

accessed September 2012 at 

https://my.usgs.gov/confluence/display/aesir/PRNS+Time+Lapse+Videos. 

Marine Mammal Commission, 2011, Mariculture and harbor seals in Drakes Estero, California: A 

report by the Marine Mammal Commission, November 22, 2011, available at 

http://mmc.gov/drakes_estero/pdfs/drakes_estero_report.pdf. 

National Park Service Reading Room, 2012, Photographs—Drakes Estero wildlife monitoring cameras 

– 2008: Web site accessed September 2012 at 

http://www.nps.gov/pore/parkmgmt/planning_reading_room_photographs_wmc_de_2008.htm. 
7 

Table 1. Flushing events of Drakes Estero seals identified in 2008 National Park Service photographs. Videos can be retrieved from the USGS Web site 

(https://my.usgs.gov/confluence/display/aesir/PRNS+Time+Lapse+Videos), and image files can be retrieved from the NPS Reading Room Web site 

(http://www.nps.gov/pore/parkmgmt/planning_reading_room_photographs_wmc_de_2008.htm). 

[m4v is a MPEG4 file; min, minute]

Date Video File Name Time

NPS Web 

site Folder

Image File 

Name Notes

03/31/08 OB-03-31-2008-Large.m4v 1:57 p.m.

March 31-

April 2 IMG_0026

Unidentified black object appears in single image (IMG_0023) 3 min prior to 

all seals flushing into the water; seals do not return to haulout site for the 

remainder of the tide.

04/13/08 OB-04-13-2008-Large.m4v 12:04 p.m.

April 10-

April 14 IMG_2195

Kayak becomes visible in vicinity of seals at 11:55 a.m. (IMG_2186); 9 min 

later all seals flush into the water as kayak passes haulout site; seals return to 

site 6 min after flushing (IMG_2201.)

04/13/08 OB-04-13-2008-Large.m4v 12:32 p.m.

April 10-

April 14 IMG_2223

Kayak becomes visible in vicinity of seals at 12:28 p.m. (IMG_2219); 4 min 

later all but 2 seals flush into the water.

04/14/08 OB-04-14-2008-Large.m4v 6:20 p.m.

April 14-

April 17 IMG_0355

Unidentified birds land on sandbar at 6:19 PM (IMG_0354); 1 min later some 

seals flush into the water; some seals remain on sandbar

04/23/08 OB-04-23-2008_Large.m4v 1:14 p.m.

April 23-

April 25 IMG_1319

Boat becomes visible at 1:09 p.m. (IMG_1314); people walk on sandbar; bird

activity near seals increase; 5 min after boat becomes visible birds land on 

sandbar and some seals flush into the water; some seals remained hauled out 

during event.

05/06/08 OB-05-06-2008_Large.m4v 10:19 a.m.

May 5- 

May 7 IMG_0675

Boat becomes visible at approximately 8:57 a.m. (IMG_0593); many seals 

flush around 80 min later; some seals return within 10 min; boat remained on 

sand bar > 7 hours.

05/15/08 OB-05-15-2008_Large.m4v 2:07 p.m.

May 13 -

May 15 IMG_1603

Boat becomes visible at 1:55 p.m. (IMG_1591); people walk on sandbar; boat 

leaves area at 2:06 p.m. (IMG_1602); some seals flush into water 1 min later.

05/31/08 OB-05-31-2008-Large.m4v 7:32 a.m.

May 29-

June 2 IMG_0951 All but one seal flush into water; no visible human activity.

06/02/08 OB-06-02-2008-Large.m4v 7:11 a.m.

May 29-

June 2 IMG_2370

All seals flush into water; no visible human activity; some seals returned to 

site approximately 1 hour later (IMG_2438)

06/11/08 OB-06-11-2008-Large.m4v 11:25 a.m.

June 9- 

June 12 IMG_1157

A portion of hauled seals flush toward water at 11:25 a.m., 3 min prior to boat 

arrival (IMG_1160); people seen walking on sandbar; seals remained hauled 

out during event.
Figure 1. Sample of photographs from Point Reyes monitoring cameras, 2008, showing different 

conditions in the estuary. A, Nighttime. B, Foggy. C, Windy. D, Calm, with grass obstruction, 

E, Exposed sandbars, no seals. F, Exposed sandbars with seals hauled out. 

8
Figure 2. A series of photographs of a seal flushing event on March 31, 2008, at the Oyster Bar 

site within Drakes Estero. A, Seals hauled out onshore (lower right corner). B, Seals hauled out 

onshore with the appearance of an unidentified black object on the shore opposite to the seals. 

C, Black object is gone and seals remained hauled out. D, Seals remained hauled out. E, All 

seals flush from the haulout site. F, No seals evident. Photographs were taken at 1-minute 

intervals. 

9
Figure 3. A series of photographs of a seal flushing event on April 13, 2008, beginning at12:01 

p.m.at the Oyster Bar site within Drakes Estero. A, Seals hauled out along shore as a kayaker 

approaches. B, Seals remained hauled out as kayaker comes closer. C, Kayaker continues to 

approach hauled out seals without movement of seals. D, Some seals begin flushing from shore 

in the presence of the kayaker. E, All seals have flushed and kayaker remains in view. F, All 

seals remained flushed as kayaker leaves haulout site. Photographs were taken at 1-minute 

intervals. 

10
Figure 4. A series of photographs of a seal flushing event on April 13, 2008, beginning at 12:29 

p.m. at the Oyster Bar site within Drakes Estero. A, Seals hauled out along shore as a kayaker 

approaches. B, Some seals begin to leave hauled out area as kayaker comes closer. C, Kayaker 

is no longer in view and seals continue to move. D, More seals have flushed into the water, with 

a few remaining seals hauled out. E, A few seals remain on shore. F, Some seals begin to return 

to haulout site. Photographs were taken at 1-minute intervals.

11
Figure 5. A series of photographs of a seal flushing event on April 14, 2008, at the Oyster Bar 

site within Drakes Estero. A, Seals hauled out along the shore. B, Seals still hauled out along 

the shore. C, Unidentified birds begin to land on sandbar near some hauled-out seals. D, A 

group of seals near the birds flush into the water. E, Birds remain on shore where some seals are 

still hauled out. F, Bird and seal activity does not appear to change. Photographs were taken at 

1-minute intervals. 

12
Figure 6. A series of photographs of a seal flushing event on April 23, 2008, at the Oyster Bar 

site within Drakes Estero. A, Seals hauled out along the shore while a boat with people 

associated with it is docked on the opposite channel (far upper left corner). B, Boat, people, and 

seal activity do not appear to change. C, Bird activity near the seals increases. D, Some birds 

land on the haulout site near the seals. E, Some seals begin to flush from the haulout site as birds 

continue to be active near and on the shore. F, More seals flush from the haulout site as bird 

activity continues (boat and people remain on opposite shore). Photographs were taken at 1-

minute intervals. 

13
Figure 7. A series of photographs of a seal flushing event on May 6, 2008, at the Oyster Bar site 

within Drakes Estero. A, Seals hauled out along the shore while a boat is present along opposite 

shore (boat arrived approximately 80 minutes prior to photograph). B, Seal and boat activity do 

not change. C, About a third of the seals flush from the haulout site. D–F, Seal and boat activity 

do not change. (No people were visible within the camera view during the flushing event.) 

Photographs were taken at 1-minute intervals.

14
Figure 8. A series of photographs of a seal flushing event on May 15, 2008, at the Oyster Bar 

site within Drakes Estero. A, Seals hauled out along the shore and a boat docked along the far 

back channel (upper right corner). B, Slight increase in seal activity; boat remains along the far 

back channel. C, Some seals flush into the water with their heads visible; the boat has left the 

channel. D, Some seals remain in water, moving around. E, Seals begin to return to shore. F, 

Most of the seals have returned to the haulout site. Photographs were taken at 1-minute 

intervals. 

15
Figure 9. A series of photographs of a seal flushing event on May 31, 2008, at the Oyster Bar 

site within Drakes Estero. A, Seals hauled out along the shore while the tide is rising. B, Seals 

becoming slightly inundated by the tide but remain hauled out. C, All seals but one flush from 

the shore, with no visible stimuli present. D, A single seal remains hauled out on the shore. E–

F, No change in seal activity. Photographs were taken at 1-minute intervals. 

16
Figure 10. A series of photographs of a seal flushing event on June 2, 2008, at the Oyster Bar 

site within Drakes Estero. A, Seals hauled out along the shore. B, No change in seal activity. C, 

All seals flush from haulout site, with no visible stimuli. D, No seals present on shore. E–F, No 

change in activity. Photographs were taken at 1-minute intervals.

17
Figure 11. A series of photographs of a seal flushing event on June 11, 2008, at the Oyster Bar 

site within Drakes Estero. A, Seals hauled out along the shore. B, Not change in seal activity. 

C, A sudden, brief movement of seals toward the water’s edge. D, Seals remain near water’s 

edge. E, No change in seal activity. F, Boat enters frame landing on the shore opposite to the 

hauled out seals; seal activity does not change. Photographs were taken at 1-minute intervals. 

18
Appendix 1. Summary analysis of 3,140 photographs from 75 potential 

disturbance events to hauled out harbor seals in Drakes Estero.

19
APPENDIX 1 STIMULI AND DISTURBANCE EVENTS 

Date Stimulus

Number of

Photos

Reviewed

Evidence of

Seals Flushing

Connection Between Stimulus

and Seal Flushing Boat

Camera

Serviced Kayak Birds Unknown

Flushing

Events Comments

03/26/08 camera serviced 93 no 1 0

Camera service; 

disturbance to se

03/27/08 boat 40 no 1 0

Boat visits area; 

no evidence of d

03/27/08 boat 21 no 1 0

Boat visits area; 

disturbance to se

03/27/08 camera serviced 36 no 1 0

Camera serviced

servicing; no sea

servicing; no evid

03/31/08 boat 89 no 1 0

Boat visits area; evidence of distu

03/31/08 camera serviced 23 no 1 0

Camera serviced

angle; poor came

to seals

03/31/08 black object 24 yes Black object on opposite bank? 1 1

Black object on ssmall group (<10

human stimuli; m

04/05/08 birds 21 no 1 0

Lots of gulls and no evidence of d

04/05/08 boat 53 no 1 0

Boat visits area; 

water rising and 

evidence of distu

04/07/08 camera serviced 24 no 1 0

Camera serviced 

resolution; no ev

04/08/08 boat 25 no 1 0

Boat present; no 

partially submerg

evidence of distu

04/10/08 boat 181 no 1 0

Boat visits area; 

seabirds present

to seals

04/10/08 camera serviced 22 no 1 0

Camera serviced 

disturbance to se

04/10/08 boat 53 no 1 0

Boat visits area; disturbance to se

04/11/08 boat 31 no 1 0

Boat visits area; 

and plants obscu

falls; no evidence 

04/11/08 boat 36 no 1 0

Boat visits area; 

disturbance to se

04/12/08 boat 24 no 1 0

Boat visits area; 

partially submergseals

20
APPENDIX 1 STIMULI AND DISTURBANCE EVENTS 

Date Stimulus

Number of

Photos

Reviewed

Evidence of

Seals Flushing

Connection Between Stimulus

and Seal Flushing Boat

Camera

Serviced Kayak Birds Unknown

Flushing

Events Comments

04/12/08 boat 35 no 1 0

Boat visits area; mostly submerge

seals

04/12/08 boat 30 no 1 0

Boat visits area; habitat being expno evidence of d

04/12/08 boat 85 no 1 0

Boat visits area; 

focus and resolu

seals

04/13/08 kayak 32 yes yes 1 1

Kayak visits area

small group of sepresent; kayak a

meters or less; a

within 10 minute

04/13/08 kayak 24 yes yes 1 1

Kayak visits areameters of small g

into water

04/14/08 camera serviced 21 no 1 0 Camera moved; 

04/14/08 birds 22 yes Birds landing? 1 1

Birds arrive in ar

few seals in sma

arrive from area 

presence in the e

04/16/08 boat 32 no 1 0

Boat visits area; 

camera focus anare partially submto seals

04/16/08 boat 29 no 1 0

Boat visits area; 

camera focus; no 

04/17/08 camera serviced 21 no 1 0

Camera serviced

gulls and seabirdsubmerged; no e

04/23/08 boat/birds 37 yes

Not clear, boat present but

flushing seems related to birds

landing 1 1 1

Boat visits area; other seabirds pr

and boat beyond

seabirds continuseem to be respo

04/23/08 boat 36 no 1 0

Boat visits area; 

partially submergseals

21
APPENDIX 1 STIMULI AND DISTURBANCE EVENTS 

Date Stimulus

Number of

Photos

Reviewed

Evidence of

Seals Flushing

Connection Between Stimulus

and Seal Flushing Boat

Camera

Serviced Kayak Birds Unknown

Flushing

Events Comments

04/24/08 boat 36 no 1 0

Boat visits area; camera focus an

water submergeseals

04/25/08 camera serviced 27 no 1 0

Camera serviced 

and resolution; n

04/26/08 boat 52 no 1 0

Boat visits area; 

and resolution; n

04/26/08 boat 42 no 1 0

Boat visits area; 

disturbance to se

04/29/08 boat 27 no 1 0

Boat visits area; 

disturbance to se

04/29/08 camera serviced 21 no 1 0

Camera serviced 

disturbance to se

04/30/08 boat 37 no 1 0

Boat visits area; a few seals prese

evidence of distu

05/01/08 boat 78 no 1 0

Boat visits area; 

haulout habitat asubmerged; no e

05/01/08 black object 21 no 1 0

Unknown dark o

seabirds present

05/01/08 camera serviced 21 no 1 0

Camera serviced

resolution; low tno evidence of d

05/02/08 boat 41 no 1 0

Boat visits area; 

resolution; tide f

seals

05/03/08 boat 37 no 1 0

Boat visits area; camera focus; tid

no evidence of d

05/03/08 boat 45 no 1 0

Boat visits area; 

few seals presen

disturbance to se

05/03/08 birds 21 no 1 0

Birds swimming 

and seabirds roo

disturbance to se

05/05/08 boat/camera 63 no 1 1 0

Boat visits area; 

maintenance; tid

focus; no eviden

05/05/08 boat 94 no 1 0

Boat visits area; 

camera focus; no 

22
APPENDIX 1 STIMULI AND DISTURBANCE EVENTS 

Date Stimulus

Number of

Photos

Reviewed

Evidence of

Seals Flushing

Connection Between Stimulus

and Seal Flushing Boat

Camera

Serviced Kayak Birds Unknown

Flushing

Events Comments

05/06/08 boat 195 yes

1/3 of seals flushed, cause notclear, likely not related to boat 1 1

Boat visits area; hauled out flush; 

minutes; not clea

flushing; gulls an

water

05/06/08 boat 23 no 1 0

Boat visits area; 

few seals presen

evidence of distu

05/07/08 boat 254 no 1 0

Boat visits area; 

camera focus; la

evidence of distu

05/13/08 camera serviced 12 no 1 0

Camera serviced

evidence of distu

05/13/08 boat 68 no 1 0

Boat visits area; 

camera focus; lo

near camera side

05/15/08 boat 31 yes 1 1

Boat visits area; 

focus; some sealleaves the area

05/15/08 camera serviced 21 no 1 0

Camera serviced

evidence of distu

05/19/08 camera serviced 23 no 1 0

Camera serviceddisturbance to se

05/22/08 camera serviced 23 no 1 0 Camera serviced

05/22/08 boat 40 no 1 0

Boat visits area; evidence of distu

05/23/08 boat 57 no 1 0

Boat visits area; focus; gulls and s

evidence of distu

05/23/08 camera serviced 62 no 1 0

Camera serviced 

camera focus; se

of disturbances t

05/27/08 camera serviced 22 no 1 0

Camera serviced 

lots of gulls and ono evidence of d

05/27/08 boat 35 no 1 0

Boat visits area; 

camera focus an

of disturbance to 

05/29/08 boat 15 no 1 0

Boat visits area; 

camera focus; tid

disturbance to se

23
APPENDIX 1 STIMULI AND DISTURBANCE EVENTS 

Date Stimulus

Number of

Photos

Reviewed

Evidence of

Seals Flushing

Connection Between Stimulus

and Seal Flushing Boat

Camera

Serviced Kayak Birds Unknown

Flushing

Events Comments

05/29/08 camera serviced 10 no 1 0

Camera serviced

no evidence of d

05/30/08 boat 26 no 1 0

Boat visits area; 

camera focus; no 

05/31/08 unknown 21 yes

Not clear, no obvious stimulus

apparent in slide sequence 1 1

Tide rising; small 

submerged but n

nearby are undis

06/02/08 unknown 21 yes

Not clear, no obvious stimulus

apparent in slide sequence 1 1

Low tide; very po

seals all flush int

disturbance; lots 

06/02/08 camera serviced 21 no 1 0

Camera serviced 

no evidence of d

06/03/08 boat 39 no 1 0

Boat visits area; 

visibility to some 

seals; no evidenc

06/04/08 boat 38 no 1 0

Boat visits area; mostly submergeevidence of distu

06/05/08 boat 47 no 1 0

Boat visits area; 

focus; no eviden

06/05/08 camera serviced 21 no 1 0 Camera serviced

06/06/08 boat 49 no 1 0

Boat visits area; slowly rising with 

no evidence of d

06/11/08 boat 34 yes

Minor flushing before boatarrival, cause unknown 1 1

Boat visits area; 

focus; rafting birtoward water's earrives but none 

disturbance to se

06/12/08 boat 46 no 1 0

Boat visits area; 

high tide; few se

disturbance to se

06/12/08 camera serviced 23 no 1 0

Camera serviced

with few seals pr

evidence of distu

Total 3,140 44 21 2 4 4 10

24