U.S. Department of the Interior

U.S. Geological Survey

Data Series 1123

Abundance and Productivity of Marbled Murrelets 

(Brachyramphus marmoratus) Off Central California 

During the 2019 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 2019 

Breeding Season

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

Laney White

Data Series 1123

U.S. Department of the Interior

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

DAVID BERNHARDT, Secretary

U.S. Geological Survey

James F. Reilly II, Director

U.S. Geological Survey, Reston, Virginia: 2020

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Suggested citation:

Felis, J.J., Kelsey, E.C., Adams, J., Horton, C., and White, L., 2020, Abundance and productivity of marbled murrelets 

(Brachyramphus marmoratus) off central California during the 2019 breeding season: U.S. Geological Survey Data 

Series 1123, 13 p., https://doi.org/10.3133/ds1123.

ISSN 2327-638X (online)
iii

Acknowledgments

We would like to thank our additional murrelet observer: Alex Rinkert, of California State Parks. 

The University of California–Santa Cruz (UCSC) small boat program and Dave Benet (UCSC) 

provided excellent vessel support and maintained the R/V Lucy M. We would also like to thank 

our additional vessel operators and truck drivers: Cordell Johnson (USGS) and Rob Wyland 

(USGS). Finally, we are grateful to Martin Raphael and Benjamin Becker for reviewing prior 

versions of this report.
iv

Contents

Acknowledgments........................................................................................................................................iii

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 ........................................................................6

Abundance Estimation Results ...........................................................................................................6

Productivity—Juvenile Ratio Results ................................................................................................9

Discussion.....................................................................................................................................................11

References Cited..........................................................................................................................................12

Figures

1. Map of U.S. Fish and Wildlife Service Conservation Zone 6 showing survey 

routes and marbled murrelet detections from Half Moon Bay to Santa Cruz, 

central California, 2019 ................................................................................................................3

2. Graph showing modeled detection probability of marbled murrelets sighted 

within perpendicular distance less than or equal to 0.12 kilometers of vessel 

for all surveys, U.S. Fish and Wildlife Service Conservation Zone 6, central 

California, 2019 ..............................................................................................................................7

3. Graph showing mean annual marbled murrelet at-sea abundance estimates, 

U.S. Fish and Wildlife Service Conservation Zone 6, central California, for all 

years for which survey data was available during 2001–19 ..................................................8

4. Graph showing date-corrected marbled murrelet hatch-year to 

after-hatch-year ratios, plus or minus standard errors, using standardized 

zig-zag surveys and all survey transect types, 1996–2019, U.S. Fish and Wildlife 

Service Conservation Zone 6, central California ...................................................................10

Tables

1. Observer view condition classifications and descriptions for marbled murrelet 

surveys conducted from Half Moon Bay to Santa Cruz, central California, 2019 ..............2

2. Marbled murrelet survey dates, route direction, effort, observations, and 

density/abundance estimates for all surveys, U.S. Fish and Wildlife Service 

Conservation Zone 6, central California, 2019 ..........................................................................5

3. Annual at-sea marbled murrelet abundance estimates and 95-percent 

confidence intervals for surveys drawn in both directions, surveys only drawn 

from the north, and surveys only drawn from the south, U.S. Fish and Wildlife 

Service Conservation Zone 6, central California, 1999–2019 .................................................7

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

standard errors for marbled murrelets from at-sea surveys done during the 

breeding season using standardized zig-zag surveys and all survey transect 

types, U.S. Fish and Wildlife Service Conservation Zone 6, central California ..................9
v

Conversion Factors

International System of Units to U.S. customary units

Multiply By To obtain

Length

meter (m) 3.281 foot (ft)

kilometer (km) 0.6214 mile (mi)

meter (m) 1.094 yard (yd)

Area

square kilometer (km2) 0.3861 square mile (mi2)

U.S. customary units to International System of Units

Multiply By To obtain

Speed

nautical mile per hour (knot) 1.852 kilometer per hour (km/h)

Datum

Horizontal coordinate information is referenced to the World Geodetic System of 1984 (WGS 84).

Abbreviations

χ2 chi-squared

AHY after hatch year

df degrees of freedom

CI confidence interval

GPS Global Positioning System

HY hatch year

N population estimate (number of birds)

n number of surveys

P probability value

R/V research vessel

SE standard error

USGS U.S. Geological Survey

WERC Western Ecological Research Center
vi
Abundance and Productivity of Marbled Murrelets 

(Brachyramphus marmoratus) Off Central California During 

the 2019 Breeding Season

By Jonathan J. Felis, Emily C. Kelsey, Josh Adams, Cheryl Horton, and Laney White

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, evaluation of conservation 

efficacy, and ultimately, to meet Federal and State recovery 

efforts for this species. During June–August 2019, the 

U.S. Geological Survey Western Ecological Research Center 

continued previously established, long-term (1996–2019), 

at-sea surveys to estimate abundance and productivity 

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

Conservation Zone 6 (San Francisco Bay to Point Sur in 

central California). Using conventional distance sampling 

methods, we estimated marbled murrelet abundance using 

125 detections of 216 murrelets (mean group size, 1.72) 

observed on 8 surveys. The abundance estimated for the entire 

study area using all surveys in 2019 was 404 birds (95-percent 

confidence interval, 272–601 birds). Estimated abundance 

from 2019 is comparable to most prior years of study. In 2019, 

we estimated reproductive productivity (calculated as the 

hatch-year [HY] to after-hatch-year [AHY] ratio) using three 

detections of three HY murrelets observed on 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 0.025±0.020 standard error. 

We discuss changes in methodologies during 1996–2019 that 

could be addressed in re-analysis of this long-term dataset. We 

updated a synthesized database of all Zone 6 marbled murrelet 

survey data since 1999 with 2019 data to allow scientists and 

managers to evaluate established survey methods and assess 

trends in abundance and productivity estimates.

Introduction

The marbled murrelet (Brachyramphus marmoratus) is a 

small, 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 March to October in forests within 80 kilometers (km) of 

the coast (Nelson, 1997). The southernmost known breeding 

area for marbled murrelets is south of San Francisco Bay in 

forested areas of the Santa Cruz Mountains near Point Año 

Nuevo and is separated from the nearest northern California 

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

compose the annual breeding population of marbled murrelets 

in this disjunct area (Henry, 2017). During their breeding 

season (April to August), the at-sea distribution of marbled 

murrelets extends primarily from Half Moon Bay to Santa 

Cruz, with greatest abundance in the waters near Point Año 

Nuevo (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), but there 

has been less consistent survey effort in this region.

In 2019, 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 reproductive productivity of marbled murrelets 

in U.S. Fish and Wildlife Service Conservation Zone 6 

(central California—San Francisco Bay to Point Sur). 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, Felis and others, 

2018, Felis and others, 2019) and this monitoring program 

is funded by the U.S. Fish and Wildlife Service Natural 

Resource Damage Assessment and 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 recovery efforts for this species.
2 Abundance and Productivity of Marbled Murrelets Off Central California During the 2019 Breeding Season

The USGS-WERC continued at-sea surveys in 2019 to 

assess abundance and productivity for two primary purposes: 

(1) to maintain efforts to quantify the status of marbled 

murrelets in central California (U.S. Fish and Wildlife Service 

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 

identify critical at-sea habitat for the species (for example, 

Bellefleur and others, 2009). In this report, we describe our 

methods and provide summaries of survey effort and results 

estimating marbled murrelet abundance and productivity 

(juvenile ratio) for 2019.

Methods

At-Sea Survey Methods

In 2019, USGS-WERC completed eight 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: two surveys during June 1–July 9 and six surveys 

during July 10–August 24. Protocol dictates that three surveys 

are conducted in the first survey window and six in the second 

window (Henry, 2017), but in 2019 weather conditions 

prevented us from accomplishing all three surveys in the first 

window. We used data from all surveys to estimate marbled 

murrelet abundance, and we used the six surveys during the 

second survey period to estimate juvenile ratio (following 

Henry, 2017). Surveys were almost exclusively conducted 

when viewing conditions were excellent to good (table 1).

Historically, survey routes were designed as continuous, 

approximately 100-km long zig-zag transect lines to sample 

nearshore (200–1,350 meters [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 known marbled murrelet densities 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 that were 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 2019, 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 (3 drawn from the north 

and 5 from the south) to complete the 8 surveys. 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, high surf areas nearshore), we maintained 

survey effort while safely navigating as close as possible to the 

transect line. 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) traveling 12–15 knots 

(22–28 km per hour), recorded the observation time, angle 

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

murrelets detected. The skiff was operated by a third crew 

member whose sole responsibility was piloting the vessel. 

Skiff size and travel speed were consistent with those used in 

surveys conducted since 2007. From 1999 to 2003, surveys 

were conducted in a 4-m open inflatable skiff at approximately 

10 knots (18 km per hour).

Table 1. Observer view condition classifications and descriptions for marbled murrelet (Brachyramphus marmoratus) surveys 

conducted from Half Moon Bay to Santa Cruz, central California, 2019.

[~, approximately; m, meter]

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 ~150 m of line (75 m aside for each 

observer)

2—Fair Waves and (or) moderate glare; chance of missing murrelets within ~150 m of line (75 m aside for each observer)

1—Poor High wind waves and (or) high glare; murrelets very difficult to detect
Methods 3

122°20' 122°

37°20'

37°

Half Moon Bay

Santa Cruz

Point Año Nuevo

OREGON

CALIFORNIA

NEVADA

Conservation Map area Zone 6

Monterey

San Francisco

Point Sur

Bay

Half Moon

Bay

Bay

Base modified from U.S. Geological Survey and other

Federal and State digital data, 1:260,000

Universal Transverse Mercator, zone 10

North American Datum of 1983

S

a

n

t

a

C

r

u

z

M

o

u

n t

a i

n

s

Soquel Point

Pillar Point Harbor

0 5 10 MILES

0 5 10 KILOMETERS

Marbled murrelet observations

Transect lines

EXPLANATION

After hatch year or unknown

Hatch year

Figure 1. Survey routes and marbled murrelet (Brachyramphus marmoratus) detections from Half Moon Bay to Santa Cruz, 

central California, 2019.
4 Abundance and Productivity of Marbled Murrelets Off Central California During the 2019 Breeding Season

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. Distance and angle were estimated at 

the time of first detection, regardless of behavior. Prior to 

each survey, observers calibrated distance estimation using 

a laser rangefinder on buoys and other targets 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. Observers 

also recorded observations of all non-murrelet marine birds 

and mammals encountered along transects, consistent with 

surveys conducted since 2007. Marine mammal observations 

were recorded using line-transect methods as described for 

murrelets and non-murrelet bird observations were counted in 

a 75-m aside strip transect (no distances or angles recorded). 

Non-murrelet observations were archived but are not presented 

or analyzed in this report.

Observers recorded all observations and observation 

times using 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 deviated from the transect 

line to identify marbled murrelet 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 then 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 georeference observations based on 

matching date/time using custom scripting in R (R Core 

Team, 2016).

We updated a synthesized database of all marbled 

murrelet survey data since 1999 (Felis and others, 2020) 

with 2019 data to allow scientists and managers to evaluate 

established survey methods and assess trends in abundance 

estimation and juvenile ratios.

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. Historical protocol 

for this monitoring program indicates that flying birds should 

only be counted if they cross the beam of the vessel (Henry 

and Tyler, 2017). Additionally, the distance and angle to flying 

birds was only estimated when flying birds actually crossed 

the beam (90-degree angle and a distance estimate) in earlier 

years (1999–2003). In later years, re-examination of historical 

data (Felis and others, 2019) shows that flying birds, assuming 

they were only counted if they crossed the beam, were given 

a distance/angle estimate for when they were first detected. 

We maintained the methodology of 2007–19 for estimating 

distance and angle of flying birds.

We created a spatial representation of strata in ArcGISTM 

based on the same coastline shapefile used in 2007–18 

(California Department of Fish and Wildlife, 2004) and 

calculated linear effort for each survey within each stratum 

consistent with previous years by using the hypothetical 

survey route delineated by the zigzag segment nodes (table 2). 

We assigned marbled murrelet observations to either the 

nearshore or offshore stratum in ArcGISTM based on spatial 

overlap. The calculation of stratum-specific linear effort 

and the assignment of observations to strata from 1999 to 

2003 was done with an older coastline representation (Peery 

and others, 2006). We maintained the use of the modern 

stratum delineation to facilitate comparison of our abundance 

estimates to recent data (2007–19) but advise caution when 

comparing to older abundance estimates from 1999 to 2003.

Following Henry (2017), we used the program 

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

detection function and estimate marbled murrelet abundance 

using conventional distance sampling (see Buckland and 

others, 2015, for detection function modeling, model 

selection, and line transect abundance estimation methods). 

Specifically, using DISTANCE v7.1, we pooled observations 

from all 2019 surveys to create a global detection function 

for 2019 surveys and applied this function to each survey to 

calculate stratum- and survey-specific 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 at the sample (observation) level. 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 square kilometers [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 draw-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).
Methods 5

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

estimates for all surveys, U.S. Fish and Wildlife Service Conservation Zone 6, central California, 2019.

[mm/dd/yyyy, month/day/year; km, kilometer; km2, square kilometer; %, percent; CI, confidence interval; USGS, U.S. Geological Survey]

Survey date

(mm/dd/

yyyy)

Route 

direction

Transect 

length 

(km)

Number 

of 

groups

Mean 

group 

size

Number 

of 

individuals

Number 

of hatch 

year

Nearshore density 

birds per km2

(95% CI)

Offshore density 

birds per km2

(95% CI)

Abundance 

birds 

(95% CI)

06/12/2019 South 97.3 9 2.00 18 0 2.25 (1.37–3.69) 0.00 235 (143–386)

06/14/2019 North 103.9 14 1.71 24 0 2.87 (2.10–3.94) 0.00 300 (220–412)

07/18/2019 South 96.5 22 1.36 30 0 3.66 (2.80–4.78) 0.91 (0.73–1.13) 478 (369–618)

07/22/2019 North 103.7 27 1.85 50 0 5.45 (4.16–7.14) 2.62 (0.63–10.82) 845 (501–1880)

08/09/2019 South 95.7 22 1.81 40 1 5.13 (4.02–6.56) 0.00 537 (421–687)

08/10/2019 North 101.7 10 1.70 17 0 1.69 (1.26–2.26) 1.44 (0.09–24.30) 328 (141–2780)

08/15/2019 South 101.1 8 1.86 15 2 1.85 (1.23–2.77) 0.00 194 (129–290)

08/19/2019 South 101.6 13 1.69 22 0 2.69 (2.06–3.52) 0.00 282 (216–368)

We report annual abundances and 95-percent confidence 

intervals (95-percent CI) estimates for 2017–19 (current 

program of study), and historical abundance estimates 

from 1999 to 2016 (as reported by Henry 2017). Historical 

estimates were calculated similarly, with a few exceptions. 

The modeling of detection functions allowed for the inclusion 

of covariates (observer, view conditions) in annual model 

selection from 1999 to 2003, but not during following years 

(Peery and others, 2006). Additionally, detection functions 

were modeled annually except for 2008, when at-sea murrelet 

observations and number of surveys were too few to model 

a year-specific detection function. As a result, a detection 

function was modeled using pooled observation data from 

2007 to 2008 and applied to 2008 data to estimate abundance 

(Peery and others, 2008). Finally, the spatial boundaries of 

strata are different for 1999–2003 and 2007–19 analyses.

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 previously established fledging period 

ranged from July 10, when an estimated 34 percent of HY 

birds are thought to have fledged, to August 24, about the time 

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 

between July 10 and August 24 to estimate the 2019 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:

A A

DATE

corrected

observed

i



  



1

18 7145545 0 18445455

0 000454

. .

. 55 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 July 17, 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):
6 Abundance and Productivity of Marbled Murrelets Off Central California During the 2019 Breeding Season

Hcorrected = ________________ Hobserved

− 1.5433 + 0.0098 × DAT Ei

 (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 

(Rˆt) for year t,

Rˆt = 

∑1 _

nHi

∑1

nAi

 (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 (va

ˆr(H

ˆt)) as

 va

ˆr(Rˆt) = 1_

n

(

_va

ˆr(H

ˆt)

Aˆt

2

+

H

ˆt

2

va

ˆr(Aˆ _t)

Aˆt

4

−

2 H

ˆt coˆv(Hˆt

, Aˆ _t)

Aˆt

3 ) (4)

where

va

ˆr(H

ˆt) is the variance in the number of hatch-year 

(HY) individuals observed in year t,

va

ˆr(Aˆt) is the variance in the number of 

after-hatch-year (AHY) individuals 

observed in year t,

coˆv(Aˆt, H

ˆt) is the covariance between the numbers of HY 

and AHY individuals observed in 

year t, and

H

ˆt

 and Aˆt 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).

In past annual reports, we included juvenile ratios from 

1996 to 2016 as reported by Henry (2017; prior to USGS 

surveys) with our results (Felis and others, 2018; Felis 

and others, 2019). However, these pre-2017 results were 

obtained using various survey transect designs. Surveys used 

for juvenile estimates followed (1) shore-parallel transects 

(400-m from shore) from Half Moon Bay to Soquel Point for 

1996–98, (2) standardized zig-zag transects for 2001–11 and 

2014–19 (a subset of the abundance estimation transects), or 

(3) a combination of (1) and (2) for 1999–2000 and 2012–13 

(Henry, 2017; Peery and others, 2007). Because changes 

in sampling design and area of inference during the course 

of this program could complicate interannual comparison, 

we recalculated annual juvenile ratios for 1999–2000 and 

2012–13 using only standardized zig-zag transects (using data 

compiled in Felis and others, 2020) and report those results 

here in addition to historical estimates. We did all calculations 

to estimate juvenile ratios (uncorrected and corrected) and 

associated variance using R (R Core Team, 2016).

Marbled Murrelet Abundance and 

Productivity Results

Abundance Estimation Results

We detected 131 marbled murrelet groups consisting 

of 225 murrelets on all surveys combined in 2019. Murrelet 

sightings were concentrated around Point Año Nuevo. 

Sightings were sporadic between Half Moon Bay and Point 

Año Nuevo and infrequent between Point Año Nuevo and 

Santa Cruz (fig. 1). Detections approached zero at 120 m 

horizontal sighting distance; therefore, consistent with Henry 

(2017), we excluded from analysis observations that were 

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

birds (10 percent of all detections less than or equal to 120 m 

from the transect line; following Henry, 2017). After removing 

5 detections greater than 120 m and 1 detection with no 

specified distance, we estimated marbled murrelet abundance 

using 125 detections of 216 murrelets (mean group size, 1.72; 

table 2; fig. 1).

For all surveys combined (regardless of draw direction), 

the half-normal detection model with a cosine adjustment 

(order 2) was the best-fitting model, and the observed number 

of sightings was not significantly different from the number 

predicted using this detection model (chi-squared [χ2] = 2.69, 

degrees of freedom [df] = 3, probability value [P] = 0.44; 

fig. 2). Survey-specific marbled murrelet density estimates 

ranged from 1.69 to 5.45 birds per km2 in the nearshore 

stratum and from 0.00 to 2.62 birds per km2 in the offshore 

stratum; survey-specific abundance estimates ranged from 194 

to 845 murrelets (table 2). The abundance estimated for the 

entire study area in 2019 using all surveys was 404 murrelets 

(95-percent CI, 272–601 murrelets; percent coefficient of 

variation [CV], 19.48; table 3). The half-normal detection 

models with no cosine adjustments were the best-fitting model 

for north-drawn surveys (χ2 = 2.90, df = 4, P = 0.58) and 

south-drawn surveys (χ2 = 7.95, df = 4, P = 0.09). Estimated 

abundance for surveys drawn from the north (446 murrelets; 

95-percent CI, 204–971 murrelets) was more than for surveys 

drawn from the south (288 murrelets; 95-percent CI, 178–467 

murrelets; table 3), although the estimates overlapped at the 

95-percent confidence-level. Estimated abundances for all 

study years (2001–19) are shown in table 3 and figure 3.
Marbled Murrelet Abundance and Productivity Results 7

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

Distance, in kilometer

Detection probability

Figure 2. Modeled detection probability of marbled murrelets 

(Brachyramphus marmoratus) sighted within perpendicular 

distance less than or equal to 0.12 kilometers of vessel for all 

surveys, U.S. Fish and Wildlife Service Conservation Zone 6, 

central California, 2019. Bin width is 0.02 kilometers.

Table 3. Annual at-sea marbled murrelet (Brachyramphus marmoratus) abundance estimates (N) and 95-percent confidence intervals 

(CI) for surveys drawn in both directions, surveys only drawn from the north, and surveys only drawn from the south, U.S. Fish and 

Wildlife Service Conservation Zone 6, central California, 1999–2019.

[Number of surveys, n, used in each estimate is listed. Abbreviations: %, percent; CI, confidence interval; —, no data; N/A, not applicable]

Both Directions North South

Year 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 9 729 494–1075 6

2003 699 567–860 12 615 463–815 6 782 570–1074 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 8 495 232–1054 4 789 522–1193 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 628 386–1022 6 439 233–827 3 556 126–2456 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 657 406–1063 7 510 358–726 3 720 297–1747 4

2017 530 384–732 9 413 247–689 4 790 487–1280 5

2018 370 250–546 9 513 334–788 4 227 112–460 5

2019 404 272–601 8 446 204–971 3 288 178–467 5
8 Abundance and Productivity of Marbled Murrelets Off Central California During the 2019 Breeding Season

1,200

1,000

800

600

400

200

0

2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020

Year

Population estimate

Mean annual marbled murrelet abundance

95-percent confidence interval

95-percent confidence interval

EXPLANATION

Figure 3. Mean annual marbled murrelet (Brachyramphus marmoratus) at-sea abundance estimates, U.S. Fish and Wildlife 

Service Conservation Zone 6, central California, for all years for which survey data was available during 2001–19. Years 1999 

and 2000 are excluded because survey routes were only drawn from the north in those years, and no surveys were conducted 

in 2004–06. All values from years prior to 2017 were referenced from Henry (2017).
Marbled Murrelet Abundance and Productivity Results 9

Productivity—Juvenile Ratio Results

We detected three HY marbled murrelets in 2019: one on 

August 9, and two on August 15 (table 2; fig. 1). In 2019, the 

uncorrected juvenile ratio (R) was = 0.017±0.013 standard 

error (SE), and the corrected juvenile ratio (Rˆ) was 

0.025±0.020 SE. Estimated corrected juvenile ratios using 

standardized zig-zag surveys (1999–2019; calculated in this 

study) and all transect types (shore-parallel, 1996–98; a 

combination of zig-zag and shore-parallel, 1999–2000, 

2012–13; as reported by Henry, 2017) are shown in table 4 and 

figure 4.

Table 4. Annual estimates of date-corrected hatch-year to after-hatch-year ratios (R

ˆ) and standard errors (SE) for marbled murrelets 

(Brachyramphus marmoratus) from at-sea surveys done during the breeding season using standardized zig-zag surveys (1999–2019; 

calculated in this study) and all survey transect types (shore-parallel, 1996–98; a combination of zig-zag and shore-parallel, 1999–2000, 

2012–13; zig-zag only, 2001–11, 2014–16; as reported by Henry, 2017), U.S. Fish and Wildlife Service Conservation Zone 6, central 

California.

[Note that values are identical in years where only zig-zag surveys were conducted. No surveys were conducted in 2004–06. Number of surveys, n, used in each 

estimate is listed. Surveys used to estimate ratios were limited to July 10–August 24. Date-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). 

Abbreviation: NA, not applicable; +, plus]

Year Survey types 

conducted

All survey types (historically reported) Zig-zag only (calculated in this study)

n R

ˆ SE n Rˆ SE

1996 Parallel 4 0.010 0.003 0 NA NA

1997 Parallel 5 0.022 0.007 0 NA NA

1998 Parallel 6 0.013 0.006 0 NA NA

1999 Zig+parallel 10 0.033 0.010 4 0.057 0.027

2000 Zig+parallel 9 0.049 0.016 4 0.024 0.012

2001 Zig 8 0.070 0.021 8 0.070 0.021

2002 Zig 11 0.051 0.009 11 0.051 0.009

2003 Zig 8 0.049 0.011 8 0.049 0.011

2004 No surveys 0 No surveys 0 No surveys

2005 No surveys 0 No surveys 0 No surveys

2006 No surveys 0 No surveys 0 No surveys

2007 Zig 3 0.049 0.052 3 0.049 0.052

2008 Zig 4 0.000 NA 4 0.000 NA

2009 Zig 4 0.028 0.018 4 0.028 0.018

2010 Zig 3 0.081 0.039 3 0.081 0.039

2011 Zig 3 0.080 0.018 3 0.080 0.018

2012 Zig+parallel 5 0.032 0.019 3 0.029 0.022

2013 Zig+parallel 6 0.093 0.025 3 0.122 0.062

2014 Zig 6 0.081 0.035 6 0.081 0.035

2015 Zig 6 0.059 0.020 6 0.059 0.020

2016 Zig 5 0.108 0.051 5 0.108 0.051

2017 Zig 6 0.022 0.015 6 0.022 0.015

2018 Zig 6 0.047 0.032 6 0.047 0.032

2019 Zig 6 0.025 0.020 6 0.025 0.020
10 Abundance and Productivity of Marbled Murrelets Off Central California During the 2019 Breeding Season

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020

Year

0.200

0.180

0.160

0.140

0.120

0.100

0.080

0.060

0.040

0.020

0.000

Date-corrected HY:AHY ratio

EXPLANATION

All survey types (historically reported)

Standard error

Standard error

Zig-zag surveys only (recalculated in this study)

Standard error

Standard error

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

standard errors, U.S. Fish and Wildlife Service Conservation Zone 6, central California, using standardized zig-zag surveys (black 

markers; 1999–2019; calculated in this study) and all survey transect types (hollow markers; shore-parallel, 1996–98; a combination of 

zig-zag and shore-parallel, 1999–2000, 2012–13; zig-zag, 2001–11, 2014–19; as reported by Henry, 2017). Note that values are identical 

in years where only zig-zag surveys were conducted. No surveys were conducted in 2004–06. Surveys used to estimate ratios were 

limited to July 10–August 24. Date-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).
Discussion 11

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). Estimated 

abundance from 2019 was below the long-term mean 

(2001–19: 491 murrelets) but is comparable at the 95-percent 

CI level to most prior years of study. Comparison is 

challenged by changes in methodologies over time. Future 

re-calculation of annual abundance estimates could (1) model 

more robust global detection functions for distinct eras of 

this program (for example, 1999–2003 with a smaller vessel 

at slower speeds and 2007–19 with a larger vessel at faster 

speeds); (2) consistently allow for covariates (for example, 

observer, viewing conditions) to be included in detection 

function modeling; (3) remove flying birds from detection 

function modeling and potentially from annual density and 

abundance estimates because of the inconsistencies in how 

and when these birds were recorded over time; and (4) use 

consistent stratum boundaries to assign linear effort and 

observations to the nearshore and offshore strata.

The corrected juvenile ratio in 2019 (0.025) was less 

than the long-term mean from zig-zag surveys (1999–2019: 

0.055) and for all survey types combined (1996–2019: 

0.048). Using corrected juvenile ratios from all survey types 

combined, Henry (2017) suggested that productivity had 

increased during 1996–2016. The re-calculation of corrected 

juvenile ratios using comparable sampling methods (limited 

to zig-zag surveys, 1999–2019) revealed no clear patterns in 

productivity over time, although formal statistical analysis 

is warranted. Both inter-annual variability in juvenile ratio 

estimates and the variance around those ratios appeared to 

increase between earlier (1996–2003) and later (2007–19) 

study years, for unknown reasons. Future annual productivity 

estimates and analysis of trend could further investigate 

(1) changes in survey platform (for example, 1996–2003 with 

a smaller vessel at slower speeds, 2007–19 with a larger vessel 

at faster speeds) and (2) changes in transect design, wherein 

shore-parallel transects (400 m from coast; 1996–2000) 

sample areas of much greater murrelet density (Becker and 

others, 1997) than zig-zag transects (2001–19).

Future work of analyzing these annual abundance and 

productivity estimates for trend, as well as power to detect 

trend, could provide useful information for evaluating 

the effectiveness of this monitoring program. The annual 

marbled murrelet survey program has involved several 

different research groups through time; therefore, we updated 

a synthesized database of all marbled murrelet survey data 

since 1999 with 2019 data to allow scientists and managers 

to evaluate established survey methods and assess trends in 

abundance estimation and juvenile ratios (Felis and others, 

2020). This database also facilitates annual survey logistics 

(for example, pre-survey planning) and promotes repeatability 

of analytical methods across years and project teams.
12 Abundance and Productivity of Marbled Murrelets Off Central California During the 2019 Breeding Season

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

Director, Western Ecological Research Center

U.S. Geological Survey

3020 State University Drive East

Sacramento, California 95819

https://www.usgs.gov/centers/werc

Publishing support provided by the Science Publishing Network, 

Sacramento Publishing Service Center
Felis and others—Abundance and Productivity of Marbled Murrelets (Brachyramphus marmoratus) Off Central California During the 2019 Breeding Season—

U.S. Geological Survey Data Series 1123

ISSN 2327-638X (online)

https://doi.org/10.3133/ds1123