Age Specific Survival Rates of Steller Sea Lions at Rookeries with Divergent Population Trends in the Russian Far East

Abstract

After a dramatic population decline, Steller sea lions have begun to recover throughout most of their range. However, Steller sea lions in the Western Aleutians and Commander Islands are continuing to decline. Comparing survival rates between regions with different population trends may provide insights into the factors driving the dynamics, but published data on vital rates have been extremely scarce, especially in regions where the populations are still declining. Fortunately, an unprecedented dataset of marked Steller sea lions at rookeries in the Russian Far East is available, allowing us to determine age and sex specific survival in sea lions up to 22 years old. We focused on survival rates in three areas in the Russian range with differing population trends: the Commander Islands (Medny Island rookery), Eastern Kamchatka (Kozlov Cape rookery) and the Kuril Islands (four rookeries). Survival rates differed between these three regions, though not necessarily as predicted by population trends. Pup survival was higher where the populations were declining (Medny Island) or not recovering (Kozlov Cape) than in all Kuril Island rookeries. The lowest adult (> 3 years old) female survival was found on Medny Island and this may be responsible for the continued population decline there. However, the highest adult survival was found at Kozlov Cape, not in the Kuril Islands where the population is increasing, so we suggest that differences in birth rates might be an important driver of these divergent population trends. High pup survival on the Commander Islands and Kamchatka Coast may be a consequence of less frequent (e.g. biennial) reproduction there, which may permit females that skip birth years to invest more in their offspring, leading to higher pup survival, but this hypothesis awaits measurement of birth rates in these areas.

Decline in abundance and apparent survival rates of fin whales (Balaenoptera physalus) in the northern Gulf of St. Lawrence

“…No transient effect (U‐CARE Test3.SR, χ 2 = 8.92, df = 17, p = 0.94) or over‐dispersion (overall χ 2 = 154.51, df = 152; = 1.01) was detected in the data. In addition to the strictly linear temporal trend, a more flexible temporal trend with multiple inflection points (knots) was explored using the bs() function in the R splines package (Altukhov et al, ). The optimal number of knots was chosen by comparing models with one to six knots based on the Akaike Information Criterion corrected for small sample size (AIC C , Anderson, Burnham, & White, ).…”

Population Trend and Elasticities of Vital Rates for Steller Sea Lions (Eumetopias jubatus) in the Eastern Gulf of Alaska: A New Life-History Table Analysis

“…Subadult and adult male survival estimates are currently unavailable for the EGOA. Therefore, survival of males at ages 5+ ( q x a ) was modeled with a beta distribution that had a peak at age 6 and dropped off more sharply after the males reached full maturity (10–11 yrs) as found by Altukhov et al [ 13 ] ( Fig 2 ; S1 Table ). Throughout the remainder of this document, further reference to adult survival without specification to sex refers to combined male and female adult survival.…”

“…However, such estimates can be highly imprecise because they do not account for, or poorly account for, detection probabilities, sampling variation, and biological process variation [ 11 , 12 ]. Estimates of Steller sea lion vital rates based on longitudinal studies, which do account for detection probabilities and some sources of sampling and process variation, are now widely available [ 13 – 17 ]. These data can be modeled in life history matrices to understand population trends and potential threats to population recovery that are, in some ways, more precise than using snapshot counts of population age structure [ 1 , 12 ].…”

Natal and breeding philopatry of female Steller sea lions in southeastern Alaska

“…Female age was structured five ways: constant across ages, <15 years versus 15+ years (two categories), a linear or quadratic trend in p with age, and a B-spline-fit of age to allow a complex pattern [46]. We then (step 2) fit 10 S models to the best p model, allowing S to vary among two rookery groups (F/H/B versus W/V, based on results in [36]) and with female age in the same manner as p .…”