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SC/69B/ASW/01
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Resource ID
22015
Access
Open
Document Number
SC/69B/ASW/01
Full Title
Trends in girth- and volume-based indices of body condition for Bering-Chukchi-Beaufort population bowhead whales (Balaena mysticetus), 1989?2023
Author
Citta, J.J., Givens, G.H., Scheimreif, K.A., Stimmelmayr, R., Von Duyke, A.L., Person, B.T., Sformo, T.L., And De Sousa, L.
Authors Summary
We examine temporal trends in body condition indices based on the measured girth and length of harvested bowhead whales (Balaena mysticetus) from the Bering-Chukchi-Beaufort (BCB) population during 1989?2023. Body Condition Indices (BCI) based on girth (BCIg; n=382; 1989?2023) and volume (BCIv; n=309; 1995?2023) were calculated as residuals from large linear regression models that accounted for a broad range of known or potential sources of variation in girth and volume. Models controlled for confounding effects of length, age class, season, sex, and their interactions. We then examined temporal trends in BCIg and BCIv for all whales pooled, and also for each age class separately, using generalized additive models. BCIg and BCIv were correlated (r=0.84) and yielded the same inferences. The most prominent effect occurred in the late 1990s, when body condition temporarily improved, and then returned to a roughly stable long-term average. For analyses stratified by age class, there is no significant evidence of long-term trends in body condition, except for a (downward) linear trend for the postweaning age class. However, it is debatable if a linear regression fit to the entire time series is appropriate since the overall significance may be due only to the prominent event in the late 1990s. To test for recent trend, we fit linear regressions to data collected since 2010 and found no significant declines for any age class. These analyses suggest that body condition indices for BCB bowheads are generally stable, but may have recently declined for postweaning whales. Continued monitoring is key for understanding how BCB bowhead whales may be responding to a changing environment. Given that abundance has approximately doubled during the study period, the relative stability of body condition indices is encouraging.