U.S. Department of the Interior

U.S. Geological Survey

Data Series 1093

Abundance and Productivity of Marbled Murrelets 

(Brachyramphus marmoratus) Off Central California 

during the 2017 Breeding Season
Cover: Juvenile marbled murrelet (Brachyramphus marmoratus) off Santa Cruz, 

California. Photograph by David Presksta, Bureau of Ocean Energy Management, 

August 1, 2017.
Abundance and Productivity of Marbled Murrelets 

(Brachyramphus marmoratus) Off Central California 

during the 2017 Breeding Season

By Jonathan J. Felis, Josh Adams, and Emily C. Kelsey

Data Series 1093

U.S. Department of the Interior

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

RYAN K. ZINKE, Secretary

U.S. Geological Survey

James F. Reilly II, Director

U.S. Geological Survey, Reston, Virginia: 2018

For more information on the USGS—the Federal source for science about the Earth, its natural and living 

resources, natural hazards, and the environment—visit https://www.usgs.gov or call 1–888–ASK–USGS.

For an overview of USGS information products, including maps, imagery, and publications, 

visit https://store.usgs.gov.

Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the 

U.S. Government.

Although this information product, for the most part, is in the public domain, it also may contain copyrighted materials 

as noted in the text. Permission to reproduce copyrighted items must be secured from the copyright owner.

Suggested citation:

Felis, J.J., Adams, J., and Kelsey, E.C.., 2018, Abundance and productivity of marbled murrelets (Brachyramphus 

marmoratus) off central California during the 2017 breeding season: U.S. Geological Survey Data Series 1093, 12 p., 

https://doi.org/10.3133/ds1093.

ISSN 2327-638X (online)
iii

Contents

Abstract...........................................................................................................................................................1

Introduction.....................................................................................................................................................1

Methods...........................................................................................................................................................2

At-Sea Survey Methods ......................................................................................................................2

Abundance Estimation Methods........................................................................................................4

Juvenile Ratio Estimation Methods ...................................................................................................5

Marbled Murrelet Abundance and Productivity Results ........................................................................7

Abundance Estimation Results...........................................................................................................7

Productivity—Juvenile Ratio Results................................................................................................7

Discussion.....................................................................................................................................................10

Summary........................................................................................................................................................11

Acknowledgments.......................................................................................................................................11

References Cited..........................................................................................................................................11

Figures

1. Map of Conservation Zone 6 showing survey routes and marbled murrelet

(Brachyramphus marmoratus) detections from Half Moon Bay to Santa Cruz,

central California, 2017 ................................................................................................................3

2. Graph showing modeled detection probability of marbled murrelets

(Brachyramphus marmoratus) sighted within perpendicular distance less than 

or equal to 120 meters of vessel for all surveys, Conservation Zone 6, central

California, 2017 ..............................................................................................................................7

3. Graph showing mean annual marbled murrelets (Brachyramphus marmoratus)

at-sea abundance estimates, Conservation Zone 6, central California, for all 

years for which survey data were available during 2001–17 ................................................8

4. Graph showing date-corrected marbled murrelets (Brachyramphus marmoratus)

hatch-year to after-hatch-year ratios, plus or minus standard errors, Conservation

Zone 6, central California, for all years for which survey data was available during

1996–2017 .......................................................................................................................................9

Tables

1. Observer view condition classifications and descriptions for marbled murrelets

(Brachyramphus marmoratus) surveys ....................................................................................2

2. Marbled murrelett (Brachyramphus marmoratus) survey dates, route direction,

effort, observations, and density/abundance estimates for all surveys,

Conservation Zone 6, central California, 2017 .........................................................................4

3. Annual at-sea marbled murrelets (Brachyramphus marmoratus) estimates for

surveys drawn in both directions, surveys only drawn from the north, and surveys

only drawn from the south, Conservation Zone 6, central California, 1999–2017 .............8

4. Annual estimates of hatch-year to after-hatch-year ratios, date-corrected ratios,

and standard errors for marbled murrelets (Brachyramphus marmoratus) from

at-sea surveys done during the breeding season, Conservation Zone 6, central

California, 1996–2003 and 2007–17 ............................................................................................9
iv

Conversion Factors

International System of Units to U.S. customary units

Multiply By To obtain

Length

meter (m) 3.281 foot (ft) 

meter (m) 1.094 yard (yd)

kilometer (km) 0.6214 mile (mi)

Datum

Horizontal coordinate information is referenced to the World Geodetic System of 1984 (WGS 84). 
Abundance and Productivity of Marbled Murrelets 

(Brachyramphus marmoratus) Off Central California 

during the 2017 Breeding Season

By Jonathan J. Felis, Josh Adams, and Emily C. Kelsey

Abstract

Marbled murrelets (Brachyramphus marmoratus) have 

been listed as “Endangered” by the State of California and 

“Threatened” by the U.S. Fish and Wildlife Service since 1992 

in California, Oregon, and Washington. Information regarding 

marbled murrelet abundance, distribution, population trends, 

and habitat associations is critical for risk assessment, 

effective management and evaluation of conservation efficacy, 

and ultimately to meet Federal- and State-mandated recovery 

efforts for this species. During June–August 2017, the U.S. 

Geological Survey Western Ecological Research Center 

continued previously established, long-term (1999–2016), 

at-sea surveys to estimate abundance and productivity 

of marbled murrelets in U.S. Fish and Wildlife Service 

Conservation Zone 6 (central California—San Francisco Bay 

to Monterey Bay). Using conventional distance sampling 

methods, we estimated marbled murrelet abundance using 189 

detections of 321 individuals observed on nine unique surveys. 

The abundance estimated for the entire study area using 

all surveys in 2017 was 530 birds (95-percent confidence 

interval, 384–732 birds). Estimated abundance from 2017 is 

comparable to most prior years of study, except for 2008 and 

2015, which had anomalously low abundances. We estimated 

productivity (calculated as the hatch-year [HY] to after-hatchyear [AHY] ratio) in 2017 using three detections of three 

individuals observed in six surveys. After date-correcting HY 

and AHY counts to account for birds expected to be absent 

from the water while inland at nests, the date-corrected 

juvenile ratio was 0.022 ± 0.014 standard error. We created a 

synthesized database of all marbled murrelet survey data from 

1999 to 2017 to allow scientists and managers to evaluate 

established survey methods and assess trends in abundance 

and productivity estimates. Future modifications of survey 

design could help reduce variance in abundance estimation.

Introduction

The marbled murrelet (Brachyramphus marmoratus) is 

a small, compact wing-propelled, diving seabird of the family 

Alcidae. Marbled murrelets inhabit North American nearshore 

marine waters from Alaska to central California. In California, 

marbled murrelets nest from April to August in forests within 

80 km of the coast. The southernmost known breeding area 

for marbled murrelets occurs south of San Francisco in 

forested areas of the Santa Cruz Mountains near Point Año 

Nuevo, California, and is separated from the nearest northern 

California population by 240–320 km. An estimated 174–699 

individuals comprise the annual breeding population of 

marbled murrelets in this disjunct area (Henry, 2017). During 

their breeding season of April to August, the distribution of 

marbled murrelets extends from Half Moon Bay to Santa 

Cruz, and is centered near Point Año Nuevo, San Mateo 

County (Henry, 2017). Sightings of marbled murrelets south 

of Santa Cruz in Monterey Bay during the breeding season are 

infrequent (Ralph and Miller, 1995; Henkel, 2004).

In 2017, the U.S. Geological Survey Western Ecological 

Research Center (USGS-WERC) partnered with California 

State Parks to continue long-term, at-sea surveys to estimate 

abundance and juvenile (that is, hatch-year) productivity 

of marbled murrelets in U.S. Fish and Wildlife Service 

Conservation Zone 6 (central California—San Francisco Bay 

to Monterey Bay). Marbled murrelets have been listed as 

“Endangered” by the State of California and “Threatened” by 

the U.S. Fish and Wildlife Service since 1992 in California, 

Oregon, and Washington. Abundance of marbled murrelets 

has been estimated at sea off central California since 1999 

(excluding 2004–06; Henkel and Peery, 2008; Peery and 

others, 2009; Peery and Henry, 2010; Henry and others, 

2012; Henry, 2017) and is funded by the U.S. Fish and 

Wildlife Service Natural Resource Damage Assessment and 
2 Abundance and Productivity of Marbled Murrelets Off Central California during the 2017 Breeding Season

Restoration Program under the guidance of the Luckenbach 

Oil Spill Trustee Council. Information regarding marbled 

murrelet abundance, distribution, population trends, and 

habitat associations is critical for risk assessment, effective 

management and evaluation of conservation efficacy, and 

ultimately to meet Federal- and State-mandated recovery 

efforts for this species.

USGS-WERC continued at-sea surveys in 2017 to assess 

abundance and productivity for two primary purposes: (1) to 

maintain efforts to quantify the status of marbled murrelets 

in central California (FWS Conservation Zone 6), and (2) to 

help evaluate potential benefits and marbled murrelet response 

to ongoing corvid control in coastal California State parks. 

Additionally, marbled murrelet distribution data at sea may 

help resource managers designate critical at-sea habitat for the 

species. In this report, we describe our methods, and provide 

summaries of survey effort/routes and results estimating 

marbled murrelet abundance and productivity (juvenile ratio) 

for 2017. 

Methods

At-Sea Survey Methods

In 2017, USGS-WERC completed nine at-sea surveys 

for marbled murrelets between Half Moon Bay and Santa 

Cruz, California (fig. 1). We conducted surveys during the 

previously established survey window (June 1–August 24; 

Henry, 2017) and allocated surveys to two periods within this 

window—three surveys during June 1–July 10 and six surveys 

during July 11–August 24. Surveys were almost exclusively 

conducted when viewing conditions were excellent to good 

(table 1). We used data from all surveys to estimate marbled 

murrelet abundance; however, we only used the six surveys 

during the second survey period to estimate juvenile ratio 

(following Henry, 2017). 

Historically, survey routes were designed as continuous, 

about 100-km zigzag transect lines to sample nearshore 

(200– 1,350 m from coast) and offshore (1,350–2,500 m 

from coast) strata, with approximately four times greater 

effort within the nearshore stratum owing to greater marbled 

murrelet densities known to occur nearshore (see Henry, 2017, 

and references therein). Routes originally were drawn starting 

at a random distance (200–2,500 m) from shore, and an equal 

number of routes were drawn using starting points at the north 

and south ends of the survey area. Survey routes drawn from 

the south resulted in a greater amount of habitat surveyed 

in south-facing, leeward bays that often had greater relative 

abundances of marbled murrelets than more exposed stretches 

of the coast (Henry, 2017).

In 2017, we identified 10 unique survey routes (5 each 

drawn from north and south) used by Henry (2017) during 

2013–16 surveys and randomly selected our survey routes 

from this pool (without replacement) for each survey; 

ultimately, we used 8 survey routes (4 drawn from the north 

and 4 from the south) to complete 9 surveys (one route was 

unintentionally surveyed twice). We conducted all surveys 

by following the selected route from north (Pillar Point 

Harbor, Half Moon Bay) to south (Soquel Point, Monterey 

Bay) using a Global Positioning System (GPS). When the 

survey route intersected land or crossed hazardous areas 

(for example, the extreme nearshore or passage between 

Año Nuevo Island and the mainland), we maintained survey 

effort while safely navigating to the next transect segment. 

We conducted surveys from a small boat using line-transect 

methods (Becker and others, 1997; Peery and others, 2006; 

Henry, 2017). Two observers, standing on either side of a 

6-m open skiff (R/V Lucy M, also used during 2013–16) 

traveling 12–15 knots, recorded the observation time, angle 

off the transect line, and the distance to all groups of marbled 

murrelets detected. Observers counted marbled murrelets 

as a group when individuals were within 2 m of each other, 

or if they showed behavior indicative of group status (for 

example, co-diving or vocalizing with one another; Strong 

and others, 1995). Observers recorded the age-class of each 

marbled murrelet based on three plumage classifications: 

(1) “after-hatch-year” (AHY), (2) “hatch-year” (HY), or 

(3) “unknown.” Behavior was recorded as “resting” on the 

water or “flying,” with flight direction noted. Prior to each 

survey, observers calibrated distance estimation using a laser 

rangefinder on buoys in the harbor. To facilitate estimations 

of sighting angles, we placed marks along the bow of the boat 

in 10-degree increments. The vessel occasionally paused or 

deviated from the transect line to properly identify marbled 

murrelet age-class; no additional observations were counted 

during these deviations. 

Table 1. Observer view condition classifications and 

descriptions for marbled murrelet (Brachyramphus marmoratus) 

surveys.

View condition Description

5 - Excellent Glassy

4 - Very Good Wavelets and (or) minor glare

3 - Good Small waves/wavelets and (or) minor glare; still 

able to reliably detect murrelets within about 

150 meters of line

2 - Fair Waves and/or moderate glare; chance of missing 

murrelets within about 150 meters of line

1 - Poor High wind waves and (or) high glare; murrelets 

very difficult to detect
Methods 3

sac18-4097_fig01

Map image is the intellectual property of Esri and is used herein under license.

Copyright © 2018 Esri and its licensors. All rights reserved.

WGS 1984 Web Mercator Auxiliary Sphere

37°

30'

37°

00'

0 2 4 6 8 MILES

0 2 4 6 8 KILOMETERS

122°30' 122°00'

Half Moon Bay

Santa

Cruz

Point Año Nuevo

EXPLANATION

Marbled murrelet observations

Transect lines

After hatch year or unknown

Juvenile

CALIFORNIA

OREGON

NEVADA

IDA

H

O

WASHINGTON

Area

of map 

Figure 1. Conservation Zone 6 showing survey routes and marbled murrelet (Brachyramphus marmoratus) detections from Half 

Moon Bay to Santa Cruz, central California, 2017.
4 Abundance and Productivity of Marbled Murrelets Off Central California during the 2017 Breeding Season

Observers recorded all observations and observation 

times in digital voice recorders, including survey start and 

end times, ocean conditions (Beaufort Sea state), viewing 

conditions (table 1), and time periods when effort was paused 

for any reason (for example, vessel paused or deviated from 

the transect line to identify marbled murrelets age-class). 

Observers reviewed their own recordings and transcribed and 

tabulated their sighting data into a single spreadsheet that was 

examined for quality assurance and quality control and merged 

into a combined spreadsheet. We acquired a continuous 

1-second GPS track during each survey using a handheld 

GPS unit; this track was used to geo-reference observations 

based on matching date/time using custom scripting in R 

(R Core Team, 2016). We created a spatial representation 

of strata in ArcGISTM based on the same coastline shapefile 

used in previous years (California Department of Fish and 

Wildlife, 2004), and calculated linear effort for each survey 

in each stratum consistent with previous years by using the 

hypothetical survey route delineated by the zigzag segment 

nodes. During the first survey (USGS07S), no data were 

collected by one observer (that is, one side of the boat) during 

two short segments near the beginning of the transect line. 

Therefore, we accounted for this by reducing linear effort in 

each stratum according to the proportion of distance surveyed 

by only one observer. This resulted in 13 and 11 percent 

reductions in linear effort within the nearshore and offshore 

strata, respectively, on survey USGS07S (table 2). 

Abundance Estimation Methods

We calculated perpendicular distance for each detection 

(sine of the sighting angle × observation distance) and 

inspected the distribution of perpendicular detection distances 

to select a truncation distance where detections approached 

zero, beyond which we excluded observations from analysis. 

Consistent with previous years, we included sightings of flying 

birds in our analysis, despite the potential that flying birds 

likely have a different probability of detection and including 

these could affect abundance estimates. 

Table 2. Marbled murrelet (Brachyramphus marmoratus) survey dates, route direction, effort, observations, and density/abundance 

estimates for all surveys, Conservation Zone 6, central California, 2017. 

[Route direction: Direction from which the route was drawn; all routes were surveyed from north to south. Abbreviations: km, kilometer; km2, square 

kilometer; 95% CI, 95-percent confidence interval of the abundance estimate]

Survey 

date

Route 

direction Route name

Transect 

length 

(km)

Number 

of 

groups

Mean 

group 

size

Number 

of 

individuals

Number 

of 

juveniles

Nearshore 

density birds 

per km2 (95% CI)

Offshore density 

birds per km2

(95% CI)

Abundance 

birds 

(95% CI)

06-07-2017 South USGS07S 188.0 11 1.45 16 0 1.74 (1.30–2.33) 1.45 (0.04–57.07) 334 (140–6,217)

06-19-2017 North USGS02N 103.9 14 1.57 22 0 2.03 (1.52–2.71) 1.68 (0.62–4.54) 388 (224–759)

06-22-2017 North USGS09N 99.1 15 1.07 16 0 1.91 (1.61–2.26) 0 199 (168–236)

07-12-2017 South USGS10S 102.1 22 1.77 39 1 4.49 (3.86–5.22) 0 470 (404–547)

07-23-2017 South USGS07S 101.2 28 1.96 55 0 5.34 (4.43–6.44) 3.85 (1.83–8.08) 961 (655–1,519)

07-26-2017 North USGS04N 103.7 23 1.65 38 0 4.05 (3.23–5.08) 1.41 (0.52–3.80) 571 (393–929)

08-01-2017 South USGS01S 97.3 33 1.91 63 0 7.18 (5.59–9.22) 0.98 (0.88–1.10) 854 (677–1,079)

08-09-2017 South USGS05S 95.7 15 1.80 27 0 3.26 (2.76–3.84) 0 341 (289–402)

08-18-2017 North USGS06N 101.8 28 1.61 45 2 4.88 (4.09–5.83) 0.91 (0.81–1.01) 605 (512–716)

1Linear effort reduced to indicate lesser effort of one of the two observers on this survey (see section, “Methods” in text).
Methods 5

Following Henry (2017), we used the program 

DISTANCE v7.1 (Thomas and others, 2010) to model our 

detection function and estimate murrelet abundance using 

conventional distance sampling (see Buckland and others, 

2015, for a detailed description of methods for modeling 

and selecting detection functions and estimating abundance 

from line transects). Specifically, using DISTANCE v7.1, 

we pooled observations from all 2017 surveys to create a 

global detection function for 2017 surveys and applied this 

function to each survey to calculate stratum- and surveyspecific density estimates based on the linear effort sampled 

during each survey. Consistent with Henry (2017), we grouped 

perpendicular detection distances into 20-m bins, used a 

120-m truncation distance, and evaluated the half-normal 

function, with or without cosine expansion; we selected the 

detection function with the smallest Akaike information 

criterion (AIC; Burnham and Anderson, 2004) value (that 

is, most parsimonious fit). We used the mean of observed 

cluster size method to estimate cluster (group) sizes. We 

assigned marbled murrelet observations to either the nearshore 

or offshore stratum in ArcGISTM based on spatial overlap. 

We calculated survey-specific abundances by multiplying 

the stratum-specific density estimate by the total area of 

each stratum in the study area (104.65 km2 for each) and 

then summed the two stratum-specific abundance values 

for a total area abundance estimate. Consistent with Henry 

(2017), we repeated the analysis described above with the 

data partitioned by survey route draw direction to evaluate 

the effect of survey route direction on abundance estimation. 

New, direction-specific detection functions were modeled 

for these subsets of the data and used to estimate abundance 

(following Henry, 2017). We report annual abundances and 

95-percent confidence intervals (95-percent CI) estimated by 

DISTANCE v7.1.

Juvenile Ratio Estimation Methods

We estimated the juvenile ratio (the ratio of HY to AHY 

individuals) for marbled murrelet surveys conducted during 

the fledging period. The fledging period ranged from July 10, 

when an estimated 34-percent of HY birds were estimated 

to have fledged, to August 24, when HY and AHY murrelets 

become indistinguishable at sea because AHY birds begin 

pre-basic molt (Long and others, 2001; Peery and others, 

2007). Thus, we included only surveys done between July 10 

and August 24 to estimate the 2017 juvenile ratio (following 

Henry, 2017). Identification of HY birds followed techniques 

outlined by Long and others (2001) and were aided by 

reviewing photographs and resources provided by the Alaska 

murrelet group (K. Nesvacil, Alaska Department of Fish and 

Game, written commun., 2017) before surveys. We included 

only those birds confidently identified to age class to estimate 

the juvenile ratio. 

We adjusted HY and AHY counts to account for birds 

estimated to have been inland during the time of the survey. 

A certain percentage of AHY birds are still incubating young 

during the fledging period and, therefore, are not on the water 

during at-sea surveys, potentially creating a positively biased 

juvenile ratio. The proportion of AHY birds incubating is 

reported to be less than 6 percent between July 10 and July 17, 

and less than 1 percent after July 17 (Peery and others, 2004, 

2007). Therefore, to correct for the number of AHY birds 

counted at sea between July 10 and July 17, we calculated, as 

the date-corrected number of AHY individuals,
6 Abundance and Productivity of Marbled Murrelets Off Central California during the 2017 Breeding Season

Acorrected

Aobserved

DATEi

=

1 1 − − ( . 8 7145545 0. . 18445455× + 0 00045455 2

× DATEi )

, (1)

where Aobserved is the number of after-hatch-year (AHY) birds counted on survey i, and the 

denominator is 1 minus the linear regression model for the proportion of incubating AHY 

individuals estimated for the Julian Day of survey i (DATEi; Peery and others, 2007). For surveys 

after Julian Day 199, we assumed no birds were incubating and the observed number of AHY 

birds was not corrected. 

In addition to adjusting for incubating adults (to avoid positive bias in the estimated ratio), the 

juvenile ratio calculation can be negatively biased by not accounting for HY birds that have not yet 

fledged by the time of the survey. Based on 47 observed fledging events in California, Peery and 

others (2007) estimated that 75 percent of juveniles had fledged by August 24, considered herein 

to be the last day of the fledging period. Therefore, to adjust for the number of HY birds observed 

during a given at-sea survey, we calculated Hcorrected after Peery and others (2007): 

H H

DATE corrected

observed

i

= − + 1. . 5433 0 0098× , (2)

where Hcorrected is the date-corrected number of hatch-year (HY) individuals, Hobserved is the 

number of HY individuals counted on survey i, and the denominator is the regression model for 

the cumulative proportion of HY birds fledged predicted according to Julian Day (DATEi). 

We used Acorrected and Hcorrected to estimate the juvenile ratio ˆ ( ) Rt for year t,

1

1

ˆ ,

n

i

t n

i

H

R

A

=

∑

∑

(3)

where Hi and Ai are the number of hatch-year and after-hatch-year individuals for survey i, 

respectively, and n is the number of surveys done in year t (Levy and Lemeshow, 1991). 

We estimated the variance of the juvenile ratio ˆ (var( ) ˆ Ht as:

2

24 3

ˆ ˆ ˆ ˆ ˆ ˆ 1 var( ) var( ) 2 cov( , ) ˆˆˆ ˆ var( ) ˆ , ˆˆ ˆ

t t t t tt

t

tt t

H H A H HA

R

n AA A

 

=+−  

 

(4)

where ˆ var( ) ˆ Ht is the variance in the number of hatch-year (HY) individuals observed in year t,

ˆ var( ) ˆ At is the variance in the number of after-hatch-year (AHY) individuals observed in 

year t,

ˆ ˆ cov( , ) ˆ H A t t is the covariance between the numbers of HY and AHY individuals observed in 

year t, and

ˆ ˆ and H A t t are the mean number of HY and AHY individuals observed in year t, respectively 

(van Kempen and van Vliet, 2000; Peery and others, 2007; Henry, 2017).

We did all calculations to estimate juvenile ratios (uncorrected and corrected) and associated 

variance using custom scripts provided by M.Z. Perry (University of Wisconsin-Madison, written 

commun., October 26, 2017) and run with SAS/STAT® software (SAS, Cary, North Carolina). 
Marbled Murrelet Abundance and Productivity Results 7

Marbled Murrelet Abundance and 

Productivity Results

Abundance Estimation Results

We detected 198 marbled murrelet groups consisting of 

336 individuals on all surveys combined in 2017. Detections 

approached zero at 120 m; therefore, consistent with Henry 

(2017), we excluded from analysis observations greater 

than 120 m from the transect line. We included flying birds 

(11 percent of all detections less than or equal to 120 m from 

the transect line; following Henry [2017]). After removing 

eight detections greater than 120 m and one detection with no 

specified distance, we estimated marbled murrelet abundance 

using 189 detections of 321 individuals (table 2, fig. 1).

For all surveys combined (regardless of draw direction), 

the half-normal detection model with no monotonicity 

constraints (that is, no cosine adjustments) was the bestfitting model, and the observed number of sightings were 

not significantly different from the number predicted using 

this detection model (chi-squared [χ2] = 2.24, degrees of 

freedom [df] = 4, probability value [P] = 0.69; fig. 2). Surveyspecific marbled murrelet density estimates ranged from 1.74 

to 7.18 birds km-2 in the nearshore stratum and from 0.00 

to 3.85 birds km2 in the offshore stratum; survey-specific 

abundance estimates ranged from 199 to 961 individuals 

(table 2). The abundance estimated for the entire study area 

using all surveys was 530 birds (95-percent CI, 384–732 birds; 

percent coefficient of variation [CV], 15.46; table 3). The 

half-normal detection model with no monotonicity constraints 

was the best-fitting model for north drawn surveys (χ2 = 1.06, 

df = 4, P = 0.90). The half-normal detection model with a 

cosine (order 2) adjustment was the best-fitting model for 

south-drawn surveys (χ2 = 0.71, df = 3, P = 0.87). Estimated 

abundance for surveys drawn from the north (413 birds; 

95-percent CI, 247–689 birds) was less than for surveys drawn 

from the south (790 birds; 95-percent CI, 487–1,280 birds; 

table 3). Estimated abundances for all study years (1999–

2017) are shown in table 3 and figure 3. Estimated abundances 

from 2017 are comparable to most prior years of study, except 

for 2008 and 2015, which had anomalously low abundances 

(table 3, fig. 3). 

Productivity—Juvenile Ratio Results

We detected three HY marbled murrelets in 2017, one on 

July 12, and two on August 18 (table 2, fig. 1). In 2017, the 

uncorrected juvenile ratio (R) was = 0.012 ± 0.007 standard 

error [SE] and the corrected juvenile ratio ˆ ( ) R was 0.022 

± 0.014 SE (table 4). Estimated uncorrected and corrected 

juvenile ratios for all study years (1996–2017) are shown 

in table 4 and figure 4. Historical (pre-2017) juvenile ratio 

values presented in table 4 and figure 4 were obtained using 

various survey transect designs. Surveys used for juvenile 

estimates followed (1) shore-parallel transects near Año 

Nuevo for 1996–98, (2) standardized zigzag transects for 

2001–17 (a subset of the abundance estimation transects), 

or (3) a combination of shore-parallel Año Nuevo transects 

and standardized zigzag transects for 1999–2000. We present 

historical values here as calculated and reported by Henry 

(2017), but do not compare values across years.

Detection probability

Distance, in kilometers

1.0

0.8

0.6

0.4

0.2

0.0

0.00 0.02 0.04 0.06 0.08 0.10 0.12

Figure 2. Modeled detection probability of marbled murrelets 

(Brachyramphus marmoratus) sighted within perpendicular 

distance less than or equal to 120 meters of vessel for all surveys, 

Conservation Zone 6, central California, 2017. Bin width is 20 meters.
8 Abundance and Productivity of Marbled Murrelets Off Central California during the 2017 Breeding Season

sac18-4097_fig03

Year

Population estimate

2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018

1,200

1,000

800

600

400

200

0

EXPLANATION

Mean annual 

marbled murrelet 

abundance

95-percent confidence interval

95-percent confidence interval

Figure 3. Mean annual marbled murrelet (Brachyramphus marmoratus) at-sea abundance estimates, Conservation Zone 6, central 

California, for all years for which survey data were available during 2001–17. Years 1999 and 2000 are excluded because survey routes 

were only drawn from the north in those years.

Table 3. Annual at-sea marbled murrelet (Brachyramphus marmoratus) estimates for surveys drawn in both directions, surveys only 

drawn from the north, and surveys only drawn from the south, Conservation Zone 6, central California, 1999–2017. 

[All values from years prior to 2017 were referenced from Henry (2017). N: At-sea abundance estimate. 95% CI: 95-percent confidence interval of the 

abundance estimate. n: Number of surveys used in the abundance estimate]

 Year

Both directions North South

N 95% CI n N 95% CI n N 95% CI n

1999 N/A 487 333–713 5 No surveys

2000 N/A 496 338–728 8 No surveys

2001 661 556–786 15 637 441–920 8 733 583–922 7

2002 683 561–832 15 628 487–809 9 729 494–1,075 6

2003 699 567–860 12 615 463–815 6 782 570–1,074 6

2004 No surveys No surveys No surveys

2005 No surveys No surveys No surveys

2006 No surveys No surveys No surveys

2007 378 238–518 4 269 109–429 2 488 349–626 2

2008 174 91–256 4 122 61–184 1 225 131–319 3

2009 631 449–885 8 495 232–1,054 4 789 522–1,193 4

2010 446 340–585 7 366 240–559 4 560 343–925 3

2011 433 339–553 6 320 225–454 2 452 331–618 4

2012 487 403–588 6 475 373–605 3 501 359–699 3

2013 628 386–1,022 6 439 233–827 3 556 126–2,456 3

2014 438 307–624 9 444 258–765 4 434 231–817 4

2015 243 152–386 9 225 136–370 4 296 159–549 5

2016 657 406–1,063 7 510 358–726 3 720 297–1,747 4

2017 530 384–732 9 413 247–689 4 790 487–1,280 5
Marbled Murrelet Abundance and Productivity Results 9

sac18-4097_fig04

Year

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018

0.16

0.14

0.12

0.10

0.08

0.06

0.04

0.02

0.00

Date-corrected hatch-year to 

after-hatch-year ratio

EXPLANATION

Date-corrected annual hatch-year 

to after-hatch-year ratio estimate

Standard error

Standard error

Figure 4. Date-corrected marbled murrelet (Brachyramphus marmoratus) hatch-year to after-hatch-year ratios, plus or minus 

standard errors, Conservation Zone 6, central California, for all years for which survey data was available during 1996–2017. All values 

from years prior to 2017 were referenced from Henry (2017). Surveys used to estimate ratios were limited to July 10–August 24. Datecorrected estimates were corrected for the proportion of hatch-year murrelets that had not fledged and the proportion of after-hatchyear murrelets still incubating at the time the survey was done (see Peery and others, 2007). 

Table 4. Annual estimates of hatch-year to after-hatch-year ratios (R), date-corrected ratios ˆ ( ) R , and standard errors (SE) for marbled 

murrelets (Brachyramphus marmoratus) from at-sea surveys done during the breeding season, Conservation Zone 6, central California, 

1996–2003 and 2007–17. 

[All values from years prior to 2017 were referenced from Henry (2017). Surveys used to estimate ratios were limited to July 10–August 24. Corrected estimates 

were corrected for the proportion of hatch-year murrelets that had not fledged and the proportion of after-hatch-year murrelets still incubating at the time the 

survey was done (see Peery and others, 2007). Nsurveys: Number of surveys used to produce the estimate. Abbreviation: NA, not available]

 Year Nsurveys

Observed Corrected

R (SE) ˆ ( ) R (SE) 

1996 4 0.006 0.003 0.010 0.003

1997 5 0.010 0.003 0.022 0.007

1998 6 0.007 0.003 0.013 0.006

1999 10 0.016 0.005 0.033 0.010

2000 9 0.024 0.008 0.049 0.016

2001 8 0.034 0.008 0.070 0.022

2002 11 0.026 0.004 0.051 0.009

2003 8 0.024 0.005 0.049 0.011

2007 3 0.017 0.018 0.049 0.052

2008 4 0 NA 0 NA

 Year Nsurveys

Observed Corrected

R (SE) ˆ ( ) R (SE) 

2009 4 0.015 0.011 0.028 0.018

2010 3 0.037 0.018 0.081 0.039

2011 4 0.053 0.015 0.080 0.017

2012 5 0.020 0.014 0.032 0.019

2013 6 0.051 0.018 0.093 0.025

2014 6 0.049 0.025 0.081 0.035

2015 6 0.031 0.011 0.059 0.020

2016 5 0.061 0.030 0.108 0.051

2017 6 0.012 0.007 0.022 0.014
10 Abundance and Productivity of Marbled Murrelets Off Central California during the 2017 Breeding Season

Discussion

We followed survey methods and analytical procedures 

to estimate densities and abundances for marbled murrelets 

using conventional distance sampling off central California 

according to methods described in Henry (2017). Options 

for estimating density and abundance using the program 

DISTANCE v7.1 (Thomas and others, 2010) are flexible, 

and if adjusted, can yield different density and abundance 

estimations. Parameters in DISTANCE v.7.1 that can be 

adjusted include modifying the distribution of perpendicular 

distances by binning and truncation, the selection of detection 

functions fit to this distribution, techniques for determining the 

group-size estimator, and post-stratification. We maintained 

the previous decision to use a binned distribution (20-m bins) 

truncated at 120 m and AIC to select the most parsimonious 

half-normal detection function for estimation. The current 

survey design (zigzag lines drawn from the north or south) 

attempts to accommodate variance in the relative densities of 

marbled murrelets with respect to distance from shore using 

inshore (200–1,350 m) and offshore (1,350–2,500 m) strata. 

This stratification scheme places only about 75 percent of the 

survey effort in the strata where greater than 90 percent of 

marbled murrelets are estimated to occur (Becker and others, 

1997). The method used from 1999 to 2017 for estimating 

densities uses these delineations to post-stratify the survey 

results for analysis in DISTANCE. Variances in density 

estimates for the offshore strata likely are inflated because 

of the relatively few, yet variable, detections of marbled 

murrelets in this zone. 

The use of an approximately equal number of transects 

per year, drawn either from the north (n = 4) or south (n = 5), 

attempts to accommodate for the non-random distribution of 

marbled murrelet aggregation areas associated with variability 

in the shape of the coastline. Despite survey-specific density 

estimates (summed nearshore and offshore) and an annual 

estimate based on the annual pooled detections, the sets of 

north-drawn and south-drawn surveys traditionally have been 

evaluated separately using independent detection functions 

generated respectively from those surveys. Because surveys 

are always done from north to south, there is no reason to 

assume and apply different detection functions to estimate 

density for these two sets. A key concept in distance sampling 

is “pooling robustness” (Burnham and others, 2004). Pooling 

robustness, or applying a global detection function based on 

all sightings to estimate probability of detection, is appropriate 

in this study because we recognize no specific covariates 

that could affect detection differently in either nearshore or 

offshore strata, nor between the north- compared to southdrawn route directions. Although detection functions for 

north-drawn and south-drawn routes potentially have good fit, 

variations in the shape and functional response of the detection 

curves will generate different estimates. For example, using 

the global detection function, and adhering to the concept of 

pooling robustness, the average abundance for the five southdrawn surveys in table 2 is 592 murrelets, compared with 

790 birds (95-percent CI, 487–1,280 birds) when stratified 

by survey route direction. A similar comparison for northdrawn surveys is less different (441 compared to 413 birds; 

95-percent CI, 247–689 birds).

If the objective for annual assessments of marbled 

murrelet abundance is to determine trends through time, 

then the focus should be on determining density at sea with 

the greatest possible degree of precision (that is, minimal 

variance). Because the distribution of murrelets off central 

California tends to be patchy and non-randomly distributed 

between Half Moon Bay and Santa Cruz, revised density 

estimates could be achieved using alternate post-stratification 

strategies. In addition to considering different distance breaks 

to delineate nearshore/offshore strata, more than 1 decade 

of surveys has shown that although few murrelets have been 

sighted south of Davenport, this area was allocated about 

25 percent of the total survey effort. By contrast, the area near 

Point Año Nuevo consistently supports the greatest number 

of sightings. Along-shore post-stratification may result in 

density estimates with tighter confidence intervals (that is, less 

variance), which would allow for better resolution (that is, 

greater power) to detect inter-annual trends. 

Becker and others (1997) presented a review of several 

studies, as well as an informative comparison of strip and 

line-transect methods used to measure the densities of marbled 

murrelets at sea. In their analyses, the authors compared the 

means and standard deviations as well as the power to detect 

trends in at-sea densities among 100-m strips, 200-m strips, 

and line transects placed at variable distances from the coast. 

Based on Becker and others (1997), scientists adopted the 

current zigzag survey design in 1999. Almost two decades 

later, we are not aware of any attempt to refine analyses 

and examine the power to detect change through time in 

marbled murrelet density at sea using this adopted survey 

design in Conservation Zone 6. A substantial time-series of 

marbled murrelet surveys is now available (Felis and others, 

2018) that warrants a comprehensive evaluation of survey 

biases, distribution at sea, and sampling design. Additionally, 

new opportunities for more contemporary analysis beyond 

conventional distance sampling could be examined (for 

example, revised post-stratification, multiple covariate 

distance sampling, and density surface modeling; Buckland 

and others, 2015).

Although methods and protocols have largely remained 

the same, the annual marbled murrelet survey program has 

been carried out by several different entities through time and, 

until now, there has been no secure, fully documented database 

available. We created a synthesized database of all marbled 

murrelet survey data from 1999 to 2017 to allow scientists 

and managers to evaluate established survey methods and 

assess trends in abundance estimation and juvenile ratios 

(Felis and others, 2018). This database also facilitates annual 

survey logistics (for example, pre-survey planning) as well as 

repeatability of analytical methods across years and project 

teams.
References Cited 11

Summary

We estimated at-sea abundance and productivity of 

marbled murrelets (Brachyramphus marmoratus) in U.S. 

Fish and Wildlife Service Conservation Zone 6 (central 

California—San Francisco Bay to Monterey Bay) in 2017 

using vessel-based surveys and conventional distance 

sampling methods. The abundance estimated for the study 

area was 530 birds (95-percent confidence interval, 384–732 

birds). Estimated abundance from 2017 is comparable to 

most prior years of study, except for 2008 and 2015, which 

had anomalously low abundances. Estimated productivity 

(calculated as a date-corrected hatch-year [HY] to afterhatch-year [AHY] ratio) in 2017 was 0.022 ± 0.014 standard 

error. We created a synthesized database of all marbled 

murrelet survey data from 1999 to 2017 to allow scientists and 

managers to evaluate established survey methods and assess 

trends in abundance and productivity estimates.

Acknowledgments

We would like to thank our additional murrelet observers 

Max Czapanskiy (U.S. Geological Survey [USGS], University 

of California at Santa Cruz [UCSC]) and Alex Rinkert. Dave 

Benet and the UCSC small boat program provided excellent 

vessel support and maintained the R/V Lucy M. Portia Halbert 

(California State Parks) provided support and insight and 

assisted with transitioning the survey program from Bill Henry 

(Groundswell Coastal Ecology) to the USGS. Thanks to Bill 

for his patience and contributions. M. Zach Peery (University 

of Wisconsin-Madison) kindly provided data and SAS code 

to calculate juvenile ratios and associated variance. Julie Yee 

(USGS) kindly ran Zach’s code. We would especially like 

to thank our talented additional vessel operators and truck 

drivers: Jessie Beck, Kate Vylet (UCSC), Cordell Johnson 

(USGS), Jackson Currie (USGS), Spencer Gordon (California 

State Parks), and Dave Cowman (California State Parks).

References Cited

Becker, B.H., Beissinger, S.R., and Carter, H.R., 1997, 

At-sea density monitoring of marbled murrelets in central 

California—Methodological considerations: The Condor, 

v. 99, no. 3, p. 743–755, https://doi.org/10.2307/1370485.

Buckland, S.T., Rexstad, E.A., Marques, T.A., and Oedekoven, 

C.S., 2015, Distance sampling—Methods and applications: 

Cham, Switzerland, Springer International Publishing, 

277 p., https://doi.org/10.1007/978-3-319-19219-2.

Burnham, K.P., and Anderson, D.R., 2004, Multimodel 

inference—Understanding AIC and BIC in model 

selection: Sociological Methods and Research, v. 33, no. 2, 

p. 261– 304.

Burnham, K.P., Buckland, S.T., Laake, J.L., Borchers, 

D.L., Marques, T.A., Bishop, J.R.B., and Thomas, L., 

2004, Further topics in distance sampling, in Buckland, 

S.T., Anderson, D.R., Burnham, K.P., Laake, J.L., 

Borchers, D.L., and Thomas, L., eds., Advanced distance 

sampling: Oxford, United Kingdom, Oxford University 

Press, p. 307–392.

California Department of Fish and Wildlife, 2004, Generalized 

California coastline—coastn83.shp: California Department 

of Fish and Wildlife database, accessed October 20, 2017, 

at https://www.wildlife.ca.gov/Conservation/Marine/GIS/

Downloads.

Felis, J.J., Adams, J., Peery, M.Z., Henry, R.W., Henkel, 

L.A., Becker, B.H., and Halbert, P., 2018, Annual marbled 

murrelet abundance and productivity surveys off central 

California (Zone 6), 1999–2017: U.S. Geological Survey 

data release, https://doi.org/10.5066/F75B01RW.

Henkel, L.A., 2004, At-sea distribution of marbled murrelets 

in San Luis Obispo County, California: Watsonville, 

California, H.T. Harvey and Associates, 14 p.

Henkel, L.A., and Peery, M.Z., 2008, Abundance and 

productivity of marbled murrelets off central California 

during the 2007 breeding season: Moss Landing, California, 

Moss Landing Marine Laboratories, 13 p.

Henry, R.W., 2017, Murrelet at-sea abundance, productivity, 

and prey resources in Zone 6, in Halbert, P. and 

Singer, S.W., eds., Marbled murrelet landscape management 

plan for zone 6, Felton, California: California Department 

of Parks and Recreation, 235 p.

Henry, R.W., Tyler, W.B., and Peery, M.Z., 2012, Abundance 

and productivity of marbled murrelets off central California 

during the 2010 and 2011 breeding seasons: Half Moon 

Bay, California, California State Parks, 17 p.

Levy, P.S., and Lemeshow, S., 1991, Sampling of 

populations—Methods and applications (2d ed.): New York, 

Wiley, 420 p.

Long, L.L., Ralph, C.J., and Miller, S.L., 2001, A new method 

for ageing marbled murrelets and the effect on productivity 

estimates: Pacific Seabirds, v. 28, p. 82–91.

Peery, M.Z., Becker, B.H., and Beissinger, S.R., 2006, 

Combining demographic and count-based approaches to 

identify source–sink dynamics of a threatened seabird: 

Ecological Applications, v. 16, no. 4, p. 1516–1528, https://

doi.org/10.1890/1051-0761(2006)016[1516:CDACAT]2.0

.CO;2.
12 Abundance and Productivity of Marbled Murrelets Off Central California during the 2017 Breeding Season

Peery, M.Z., Becker, B.H., and Beissinger, S.R., 2007, Age 

ratios as estimators of productivity—Testing assumptions 

on a threatened seabird, the marbled murrelet: The Auk, 

v. 124, no. 1, p. 224–240, https://doi.org/10.1642/0004-

8038(2007)124[224:ARAEOP]2.0.CO;2. 

Peery, M.Z., Beissinger, S.R., Newman, S.H., Burkett, E.B., 

and Williams, T.D., 2004, Applying the declining population 

paradigm—Diagnosing causes of poor reproduction 

in the marbled murrelet: Conservation Biology, v. 18, 

no. 4, p. 1088–1098, https://doi.org/10.1111/j.1523-

1739.2004.00134.x.

Peery, M.Z., and Henry, R.W., 2010, Abundance and 

productivity of marbled murrelets off central California 

during the 2009 breeding season: Half Moon Bay, 

California, California State Parks, 16 p.

Peery, M.Z., Newman, S.H., Storlazzi, C.D., and 

Beissinger, S.R., 2009, Meeting reproductive demands 

in a dynamic upwelling system—Foraging strategies 

of a pursuit-diving seabird, the marbled murrelet: The 

Condor, v. 111, no. 1, p. 120–134, https://doi.org/10.1525/

cond.2009.080094.

Ralph, C.J., and Miller, S.L., 1995, Offshore population 

estimates of marbled murrelets in California, in Ralph, C.J., 

Hunt, G.L., Raphael, M.G., and Piatt, J.F., eds., Ecology and 

conservation of the marbled murrelet: U.S. Forest Service 

General Technical Report PSW–GTR–152 p. 353–360.

R Core Team, 2016, R—A language and environment for 

statistical computing: Vienna, Austria, R Foundation for 

Statistical Computing, https://www.R–project.org/.

Strong, C.S., Keitt, B.S., Bradford, S., McIver, W.R., 

Palmer, C.J., and Gaffney, I., 1995, Distribution and 

population estimates of marbled murrelets at sea in 

Oregon during the summers of 1992 and 1993, in Ralph, 

C.J., Hunt, G.L., Jr., Raphael, M.G., and Piatt, J.F., eds., 

Ecology and conservation of the marbled murrelet: U.S. 

Forest Service General Technical Report PSW–GTR–152, 

p. 339–352.

Thomas, L., Buckland, S.T., Rexstad, E.A., Laake, J.L., 

Strindberg, S., Hedley, S.L., Bishop, J.R.B., Marques, T.A., 

and Burnham, K.P., 2010, Distance software—Design 

and analysis of distance sampling surveys for estimating 

population size: Journal of Applied Ecology, v. 47, no. 1, 

p. 5–14, https://doi.org/10.1111/j.1365-2664.2009.01737.x.

van Kempen, G.M.P., and van Vliet, L.J., 2000, Mean 

and variance of ratio estimators used in fluorescence 

imaging: Cytometry, v. 39, no. 4, p. 300–305, https://doi.

org/10.1002/(SICI)1097-0320(20000401)39:4<300::AIDCYTO8>3.0.CO;2-O.
Publishing support provided by the U.S. Geological Survey

Science Publishing Network, Tacoma Publishing Service Center

For more information concerning the research in this report, contact the

Director, Western Ecological Research Center

U.S. Geological Survey

3020 State University Drive

Modoc Hall, Room 4004

Sacramento, California 95819

https://www.werc.usgs.gov/
Felis and others—Abundance and Productivity of Marbled Murrelets Off Central California during the 2017 Breeding Season—Data Series 1093

ISSN 2327-638X (online)

https://doi.org/10.3133/ds1093