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Length at sexual maturity and pregnancy rates of Bering-Chukchi-Beaufort Seas bowhead whales

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J.C. George, R. Suydam, G. Givens, L. Horstmann, R. Stimmelmayr, and G. Sheffield

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International Whaling Commission


Pregnancy rates were estimated from examinations of reproductively mature bowhead whales (n=208) landed during the Alaska Native subsistence harvest from 1976-2016. The estimated pregnancy rate was 0.317 (95% CI 0.251 to 0.385). This suggests an inter-birth interval of just over 3 years. Whales harvested in the fall at Utqiaġvik and Kaktovik comprise the most reliable pregnancy dataset because pregnancies are easier to detect and whales are more carefully examined. From this restricted dataset (n=33), the pregnancy rate is estimated to be 0.394 (95% CI 0.211 to 0.553). Logistic regression was used to estimate length at maturity from a separate dataset (n=150) that included whale lengths. We defined length at maturity, relative to an equally balanced set of mature and immature whales, as the length at which the estimated probability of maturity equals 0.5. Since the actual dataset is neither balanced nor representative, we introduced a correction calculation. The resulting length at maturity is estimated to be 13.65m (95% CI 13.29 to 13.94). We recognize that our data could be biased by sampling from harvested animals where hunter selectivity occurs and by the approximately 14-month gestation period of bowheads. The estimates reported here are consistent with past investigations and suggest a reproductively robust population. Our finding that pregnancy rates are stable or possibly increasing over the past 40 years is also consistent with the increase population abundance over the same time span. Finally, there is no evidence in the reproductive data of density-regulated reproduction or the population approaching carrying capacity.

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