Abundance and productivity of marbled murrelets (Brachyramphus marmoratus) off central California during the 2019 breeding season

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.