ESR 6:1-14 (2008)  -  DOI:

Population abundance and growth rate of western gray whales Eschrichtius robustus

Amanda L. Bradford1,*, David W. Weller2, Paul R. Wade3, Alexander M. Burdin4,5,6, Robert L. Brownell Jr.7

1School of Aquatic and Fishery Sciences, University of Washington, Box 355020, Seattle, Washington 98195-5020, USA
2Southwest Fisheries Science Center, NMFS, NOAA, 8604 La Jolla Shores Drive, La Jolla, California 92037-0271, USA
3National Marine Mammal Laboratory, Alaska Fisheries Science Center, NMFS, NOAA, 7600 Sand Point Way NE, Seattle, Washington 98115-6349, USA
4Kamchatka Branch of Pacific Institute of Geography, Far East Branch of the Russian Academy of Sciences, Pr. Rybakov,
19-a, Petropavlovsk-Kamchtsky 683024, Russia
5Alaska SeaLife Center, 301 Railway Avenue, Seward, Alaska 99664, USA
6University of Alaska Fairbanks, PO Box 757500, Fairbanks, Alaska 99775, USA
7Southwest Fisheries Science Center, NMFS, NOAA, 1352 Lighthouse Avenue, Pacific Grove, California 93950, USA

ABSTRACT: The western population of gray whales Eschrichtius robustus is one of the most endangered whale populations in the world. Recent studies of this population off the northeastern coast of Sakhalin Island, Russia, have produced a photographic dataset that was utilized for the first mark-recapture assessment of western gray whale abundance. Given encounter histories of 129 individually identified whales spanning 25 monthly capture occasions from 1997 to 2003, a closed capture estimator was employed to estimate the number of individuals using the study area in each year. Temporary emigration probabilities were then applied to the closed capture estimates to enumerate the total population size of whales off northeastern Sakhalin Island. Total abundances from 1997 to 2003 were estimated as 64 ± 5.1 (SE), 55 to 75 (95% CI); 75 ± 4.9, 66 to 85; 86 ± 3.1, 80 to 93; 77 ± 4.7, 68 to 87; 91 ± 3.4, 84 to 98; 98 ± 4.1, 90 to 106; and 99 ± 4.9, 90 to 109, respectively. These abundance estimates, particularly the last values in the series, most likely approximate the size of the entire western gray whale population. For comparison to the trend in the abundance estimates, life history data were used to estimate the growth rate of the population. Depending on the range of potential fecundity values incorporated, the resulting growth rate estimates indicate an annual population increase that is between 2.5 and 3.2%. The extremely small population size and slow rate of increase documented here further highlight concern about the viability of this critically endangered population.

KEY WORDS: Abundance · Mark-recapture · Temporary emigration · Population growth rate · Simulation approach · Photo-identification · Western gray whale · Sakhalin Island, Russia

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Cite this article as: Bradford AL, Weller DW, Wade PR, Burdin AM, Brownell RL Jr. (2008) Population abundance and growth rate of western gray whales Eschrichtius robustus. Endang Species Res 6:1-14.

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