Monitoring Kittlitz’s and marbled murrelets in Glacier Bay National Park and Preserve: 2021 annual report

Abstract

Since 2009, the Southeast Alaska Network (SEAN) has monitored the abundance and spatial distribution of Kittlitz’s (Brachyramphus brevirostris, “KIMU”) and marbled murrelets (B. marmoratus, “MAMU”) in Glacier Bay National Park, an important summer residence for both species. Species-specific, on-water density and abundance of murrelets is estimated from vessel-based line transect surveys, while accounting for detection probability and partial identification (identification only to genus). Design of the monitoring program focuses on KIMU, with secondary consideration of MAMU. Due to the global pandemic, surveys were not conducted in 2020. In 2021, we only surveyed permanent panel transects, which totaled 56% of the length of transects planned for 2021. Permanent panel transects are primarily located in the middle and upper areas of Glacier Bay and sample regions within the total survey area that have relatively high expected densities of KIMU. Inference was limited to the 339 km2 “high-density area” formed by these regions, which comprised 29% of the total survey area. To provide context for 2021 results, we also estimated species-specific densities in the high-density area from 2011 to 2019. We surveyed 25 transects totaling 130 km from 13 to 21 July 2021. Within the high-density area, we estimated an abundance of 2,921 (SE = 734) KIMU and 21,710 (2,421) MAMU and densities of 8.6 (2.2) and 64.0 (7.1) individuals/km2. Estimated densities of each species in the high-density area were similar between 2019 and 2021, with KIMU estimates near the lower end of the range from 2011 to 2021 and MAMU estimates near the upper end. Observations of KIMU were concentrated in smaller, glacially influenced middle and upper reaches of Glacier Bay, especially in Johns Hopkins Inlet in the upper West Arm (~40% of observations). MAMU were more evenly dispersed, with the densest aggregations located in the upper east side of the central bay and at the mouth of the East Arm. While estimated abundance of MAMU in the total survey area showed no clear trend from 2011 to 2019, the percent of MAMU occupying the high-density area steadily increased from ~20% to >40%, suggesting a northward shift in their spatial distribution. The estimated percent of the total KIMU population occupying the high-density area jumped from ~45% from 2011 to 2018 to 85% in 2019. This change coincided with exceptionally low estimated abundance in the total survey area, which resulted primarily from very low estimated density outside the high-density area. Resuming full survey effort should help clarify the nature and persistence of these changes and their implications for conservation and monitoring