Sanctuary Integrated Monitoring Network
Monitoring Project

Marbled Murrelets: Abundance and Productivity in Central California during the 2007 and 2008 Breeding Seasons

Principal Investigator(s)

  • M. Zachariah Peery
    Moss Landing Marine Laboratories, California State University
  • Laird Henkel
    California Department of Fish and Wildlife
Start Date: June 20, 2007

The Marbled Murrelet (Brachyramphus marmoratus) is a small seabird that is federally-listed as Threatened and state-listed in California as Endangered. Potential threats to Marbled Murrelets in California include loss of old-growth forest nesting habitat, changes in prey (small fish) availability, increasing predator populations, gill-netting bycatch, and oil spills. Various oil spill trustee councils have provided funding for restoration, including protection of nesting habitat and management of predatory corvids.

Population monitoring of Marbled Murrelets typically is conducted using at-sea surveys. Annual at-sea surveys are critical in determining the success of restoration efforts and the current status of the species range-wide. Under the Northwest Forest Plan, annual at-sea monitoring occurs in California within Conservation Zones 4 and 5, from the Oregon border south to San Francisco Bay. Conservation Zone 6, from San Francisco Bay south to Monterey Bay, is not included in the Northwest Forest Plan, but population monitoring within Zone 6 was conducted from 1999 through 2003 with a combination of state, federal, and private funding. No decline was detected during this period, despite the fact that reproductive success was too low to compensate for adult mortality. To aid in determining the success of restoration efforts in the Santa Cruz Mountains, the Command Trustee Council funded at-sea surveys in Zone 6 during the 2007 breeding season. The 2007 surveys suggested that the population had declined to 378 individuals from 661-699 during the initial survey period (1999-2003).

Summary to Date

In 2007 we conducted four at-sea surveys between Half Moon Bay and Santa Cruz
(approximately 100 km), and one at-sea survey between Santa Cruz and Moss Landing (approximately 27 km). The surveys between Half Moon Bay and Santa Cruz followed zig-zag transect routes consistent with similar surveys conducted between 1999 and 2003. These surveys included between 70 and 87 km of transect in a nearshore stratum (200-1350 m from shore) and between 15 and 27 km of transect in an offshore stratum (1350-2500 m from shore); they were conducted between 20 June and 19 July 2007. The survey routes were created using random starting points. In 2007, two transects were drawn from the south and two transects were drawn from the north. The survey between Santa Cruz and Moss Landing was conducted on 20 August 2007 to assess whether a substantial number of murrelets had dispersed south out of the primary study area into northern Monterey Bay. This survey was conducted along transect parallel to shore, approximately 400 m offshore.

In 2008 we conducted at-sea surveys for Marbled Murrelets (Brachyramphus marmoratus) in Conservation Zone 6 (central California) offshore of breeding habitat between Half Moon Bay and Santa Cruz. Using distance sampling estimation techniques, we estimated the central California population to be 122 (95% CL: 61-184) with surveys delineated from the north (n = 3), 225 (95% CL = 131-319) with surveys delineated from the south (n = 3), and 174 (95% CL: 91-256) with all surveys (n = 6). These estimates represent 54-55% declines since 2007 and 71-80% declines since 2003. No juveniles were detected and the date-corrected juvenile ratio, an estimate of productivity commonly used to index reproductive success in Marbled Murrelets, was therefore equal to zero for 2008. This was the first year since surveys for juvenile ratios started in 1996 that no juveniles were detected. Our results, in concert with previous and ongoing demographic and genetic work, indicate that Marbled Murrelets in central California will almost certainly become locally extirpated when the current cohort of adults dies.

In 2009 the surveys will be continued.

Monitoring Trends

  • The estimated abundance in 2008 represents a 71% decline from 2003.
  • No juveniles were detected during any of the four surveys conducted within the window used to estimate juvenile ratios (10 July to 23 August, 2008).


Our results suggest that the Marbled Murrelet population in central California underwent a significant and rapid decline between 2003 and 2008, recognizing that population estimates were not available from 2004-2006. Peery et al. (2006a) determined that, based on low levels of reproductive success, the central California population should show a consistent annual decline in the absence of immigration. However, abundance estimates based on at-sea surveys conducted between 1999 and 2003 showed no such decline; thus, Peery et al. (2006) suggested that immigration from northern California was supporting the central California population. Recent genetic analyses support the hypothesis that immigration without recruitment into the breeding population may have masked underlying deterministic declines in the population (M. Z. Peery L. A. Hall unpub data).

Whether immigration stopped or declined after 2003, making the local population decline evident in 2007 and 2008, or whether the remaining individuals in central California are largely immigrants from other populations is uncertain without more recent genetic, mark-recapture, and radio-telemetry work. The low abundance estimates in 2008, and to a lesser extent 2007, could also be due in part to increased dispersal out of the study area compared with previous years, as Marbled Murrelets sometimes disperse out of the central California study area during summer (Peery et al. 2008). However, given the low productivity estimates in all 10 years that juvenile-ratio surveys were conducted from 1996 through 2008, the observed population decline, as well as future declines, is virtually inevitable.

Indeed, our results indicate that current conservation projects in the Santa Cruz Mountains are insufficient to prevent the extirpation of Marbled Murrelets in central California when the current cohort of adults dies. Given the predicted and observed population decline, the genetic uniqueness of the population (Friesen et al. 2005, Piatt et al. 2007), and high probability of local extirpation, there is a clear need for immediate and stronger conservation actions in the region, and for annual monitoring of the success of these conservation efforts.

Study Parameters

  • Age & Growth
  • Reproduction
  • Abundance
  • Distribution
  • Age structure

Study Methods

We conducted six approximately 100 km long at-sea surveys between Half Moon Bay and Santa Cruz in 2008 from 16 June to 15 September that followed zig-zag transect routes consistent with surveys conducted from 1999 through 2003, and in 2007 (Peery et al. 2006a). Surveys were always initiated immediately outside of the Half Moon Bay Harbor a random distance (200-2500 m) from shore. Surveys included between 69.4 and 78.0 km of transect in a “nearshore” stratum (200-1350 m from shore) and between 15.1 and 26.6 km of transect in an “offshore” stratum (1350-2500 m from shore). In previous years an equal number of routes were drawn from starting points at the north and south ends of the survey area, and transects drawn from the south tend to yield a higher densities that transect delineated from the north. Therefore, three of the six 2008 transects were drawn from the south and three transects were drawn from the north.

For all surveys, line transect methods were used (Becker et al. 1997, Peery et al. 2006a). Two observers, standing on either side of a 6-m open skiff, recorded angle off the transect line and distance to all groups of Marbled Murrelets seen (prior to each survey, observers calibrated distance estimation using a laser rangefinder on buoys in the harbor). Birds in flight were counted if they crossed a line perpendicular to the track line, even with the observers. Counting flying birds (2% of sightings were of flying birds) may result in overestimation of abundance (Spear et al. 1992, Piatt et al. 2007), but this method was used for previous surveys in central California, and was used in 2008 for consistency. Sightings data were analyzed using DISTANCE v.5.0.

Estimating ESW (effective strip width) requires modeling the inevitable decline in detection probability as a function of distance from the sighting data. Due to the sharp decline in population size (see below), only 47 groups of Marbled Murrelets were detected during the six surveys conducted in 2008, a number that is insufficient to develop a robust detection function (Buckland et al. 2001). Therefore, we combined data from 2007 and 2008 (resulting in 131 detections) to estimate detection function parameters, but only estimated density and abundance for surveys conducted in 2008. All detections >120 m from the transect lines were discarded and the remaining detections were grouped into 7 20-m bins, similar to analyses conducted for previous years. A half-normal detection model with cosine adjustments (as used to model previous year’s data) did not fit the pooled 2007-2008 data well, largely because of distance data collected during the 12 September 2008 survey. Eliminating this surveys resulted in reasonable model fit (χ2 = 4.1, df = 4, P = 0.39) and an ESW of 61.4 m. To estimate abundance from density estimates, we multiplied survey-specific density estimates for the nearshore stratum generated by DISTANCE by the total area of the nearshore stratum (104.65 km2; no bird were detected in the offshore stratum in either 2007 or 2008). Confidence intervals for the mean 2008 abundance estimate were calculated based on the variance across surveys-specific abundance estimates.

Figures and Images

Figure 1. Abundance estimates for the central California population of Marbled Murrelets based on at-sea surveys, 1999-2008. Error bars are 95% confidence intervals. Because surveys before 2001 were conducted only on transects drawn from the north (Half Moon Bay), these survey data are presented separately. The south area is near Santa Cruz

Figure 2. The Marbled Murrelet, a small and easily spooked bird, is difficult to photograph in the field. This individual is in juvenile/winter plumage.


  • 2007 report
    Henkel and Peery (2008)
    Abundance and productivity of Marbled Murrelets off central California during the 2007 breeding season
    88 KB PDF
  • 2008 report
    Peery et al. (2008)
    Abundance and productivity of Marbled Murrelets off central California during the 2008 breeding season
    276 KB PDF