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Trends in radar counts of Marbled Murrelets Brachyramphus marmoratus in British Columbia (1996-2018): Effects of 'The Blob' marine heatwave and prey fish abundance.



1Environment and Climate Change Canada, Pacific Wildlife Research Centre, 5421 Robertson Road, Delta, British Columbia V4K 3N2, Canada *(mark.drever@canada.ca)
2Institute for the Oceans and Fisheries, University of British Columbia, Vancouver, British Columbia V6T 1Z4, Canada
3Wildlife Research Division, Institute of Ocean Sciences, Environment and Climate Change Canada, Sidney, British Columbia V8L 4B2, Canada
4Bernard K. Schroeder Consulting, Nanaimo, British Columbia V9R 3R8, Canada

Received 17 April 2020, accepted 09 October 2020

Date Published: 2021/04/15
Date Online: 2021/02/02

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DREVER, M.C., MCALLISTER, M.K., BERTRAM, D.F., SCHROEDER, B.K. & WOO, K.J. 2021. Trends in radar counts of Marbled Murrelets Brachyramphus marmoratus in British Columbia (1996-2018): Effects of 'The Blob' marine heatwave and prey fish abundance.. Marine Ornithology 49: 37-49.

Key words: Brachyramphus marmoratus, radar counts, marine heatwave, Pacific Herring, sea surface temperature, trends


During 2014-2016, the northeast Pacific Ocean experienced a large-scale marine heatwave (nicknamed ‘The Blob'), an event that was associated with die-offs of several marine bird species. The Marbled Murrelet Brachyramphus marmoratus is a small seabird in this region for which we evaluated trends in abundance—for the purposes of conservation planning—during radar surveys at 58 sites in coastal British Columbia, from 1996 to 2018, and for which we determined whether trends may have been affected by ‘The Blob'. A hierarchical Bayesian model allowed us to separate long-term trends from short-term annual fluctuations (‘year effects') that might have resulted from changes in ocean conditions, and it also allowed us to test whether predicted regional counts were associated with two indices of marine conditions: (1) measured abundance of a prey species, Pacific Herring Clupea pallasii; and (2) sea surface temperature (SST). Province-wide mean annual rate of change in counts was significantly negative, with a posterior median of -0.023 (95% credible interval: -0.033, -0.014); mean rate of change in counts per year was negative for all six Marbled Murrelet Conservation Regions; and trends were significantly negative at the Central Mainland Coast, East Vancouver Island, Haida Gwaii, and the South Mainland Coast conservation regions. Year effects during or after ‘The Blob' at West and North Vancouver Island (2016) and East Vancouver Island (2018) conservation regions were not statistically significant. Mean predicted regional counts of murrelets showed weakly positive correlations with age two herring recruitment, and no consistent associations with SST. These results indicate that the marine heatwave did not strongly affect forest-bound murrelets, and are consistent with the hypothesis that ongoing loss of terrestrial nesting habitat is associated with population declines of Marbled Murrelets in British Columbia.


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