Abstract

The Eastern Pacific stock of northern fur seals (Callorhinus ursinus) is ‘depleted’ and the Pribilof Island population has been declining over the past two decades for unexplained reasons. To examine demographic factors associated with the decline, the Alaska Fisheries Science Center (AFSC) initiated a long-term mark-recapture study more than a decade ago to assess vital rates of adult female northern fur seals (AFNFS) as well as physiological and ecological correlates that may cause reproductive failures.¹ From 2010 to 2017, blood samples were collected from 536 AFNFS (n=207 from St. George Island, n=329 from St. Paul Island) during October, prior to weaning and departure of northern fur seals from rookeries. Tag resighting efforts and behavioral observations for those individuals occurred during subsequent pupping seasons. One component of this larger project entailed 1) examining hematology values of AFNFS with respect to their corresponding vital rates (i.e., survival and birth rates), 2) comparing AFNFS hematology values between islands after assessing the geographical structure of the dataset, and 3) establishing hematology reference ranges for AFNFS. A combination of binary logit and multinomial regression analyses indicated a subset of hematological variables (n=285) were not predictive of presumed survival of AFNFS, nor were they predictive of reproductive success (with the exception of mean cell hemoglobin concentration). Additionally, hematology value outliers were minimal and patterns with vital rates were not apparent. However, cluster analysis revealed two clusters in the full dataset; one cluster was primarily comprised of individuals from St. Paul Island, whereas the other cluster contained individuals from both islands. Bootstrap analyses indicated at least one threshold value for five (red blood cells, hemoglobin, mean cell volume, mean cell hemoglobin, and mean cell hemoglobin concentration) of 13 hematology variables differed significantly between AFNFS from the two islands. Because extreme hematology values indicative of diagnostic abnormalities were unsubstantial and AFNFS did not appear impacted by disease, inter-island differences may have been a result of foraging ecology or environmental effects. The assessment of additional health and condition indices coupled with ecological metrics will be useful for investigating possible mechanisms affecting the life history traits of northern fur seals.

Acknowledgements

The authors wish to thank the staff of the Alaska Ecosystems Program at the AFSC, notably J. Baker, J. Cutler, C. Kuhn, K. Luxa, D. Johnson, J. Sterling, J. Thomason, and R. Towell, countless contractors and volunteers for all of their resight efforts, NOAA’s Alaska Region (M. Williams), and others that assisted with sample collection (R. Beltran, S. Crawford, C. Duncan, and L. Rea). All data were collected under MMPA Scientific Research Permit #14327-01 issued to the MML and IACUC Assurances A/NW 2010-5, A/NW 2013-3, and A/NW 2016-2.

* Presenting author

Literature Cited

1.  Testa W, Ream R, Gelatt T. 2009. Alaska northern fur seal demographic research, 2009. AFSC Quarterly Report, 2.