High natality rates of endangered Steller sea lions in Kenai Fjords, Alaska and perceptions of population status in the Gulf of Alaska.
Steller sea lions experienced a dramatic population collapse of more than 80% in the late 1970s through the 1990s across their western range in Alaska. One of several competing hypotheses about the cause holds that reduced female reproductive rates (natality) substantively contributed to the decline and continue to limit recovery in the Gulf of Alaska despite the fact that there have been very few attempts to directly measure natality in this species. We conducted a longitudinal study of natality among individual Steller sea lions (n = 151) at a rookery and nearby haulouts in Kenai Fjords, Gulf of Alaska during 2003-2009. Multi-state models were built and tested in Program MARK to estimate survival, resighting, and state transition probabilities dependent on whether or not a female gave birth in the previous year. The models that most closely fit the data suggested that females which gave birth had a higher probability of surviving and giving birth in the following year compared to females that did not give birth, indicating some females are more fit than others. Natality, estimated at 69%, was similar to natality for Steller sea lions in the Gulf of Alaska prior to their decline (67%) and much greater than the published estimate for the 2000s (43%) which was hypothesized from an inferential population dynamic model. Reasons for the disparity are discussed, and could be resolved by additional longitudinal estimates of natality at this and other rookeries over changing ocean climate regimes. Such estimates would provide an appropriate assessment of a key parameter of population dynamics in this endangered species which has heretofore been lacking. Without support for depressed natality as the explanation for a lack of recovery of Steller sea lions in the Gulf of Alaska, alternative hypotheses must be more seriously considered.
Maniscalco
John M
JM
Department of Science, Alaska SeaLife Center, Seward, Alaska, United States of America. john_maniscalco@alaskasealife.org
Springer
Alan M
AM
Parker
Pamela
P
D000818:Animals; D001723:Birth Rate; D056727:Endangered Species / Q000639:trends*; D005260:Female; D008137:Longitudinal Studies; D008962:Models, Theoretical; D011157:Population Dynamics; D011336:Probability; D046070:Sea Lions; D015996:Survival Rate
Science. 2007 Feb 16;315(5814):960
17303746
Gen Comp Endocrinol. 2007 Jan 15;150(2):246-52
17052717
Nature. 2005 Jul 7;436(7047):14-6
16001032
Proc Natl Acad Sci U S A. 2003 Oct 14;100(21):12223-8
14526101
J Anim Ecol. 2007 May;76(3):448-58
17439462
Sci Total Environ. 2008 Jun 15;396(1):60-9
18384839
Ecol Appl. 2007 Dec;17(8):2214-32
18213964
J Wildl Dis. 2005 Jul;41(3):512-24
16244061
J Anim Ecol. 2009 Jan;78(1):143-51
18700872
Br J Nutr. 2003 Jan;89(1):71-82
12568666
Comp Biochem Physiol A Mol Integr Physiol. 1998 Aug;120(4):617-23
10400492
J Anim Ecol. 2006 Mar;75(2):358-67
16637989