SUMMARYAquatic animals use a variety of strategies to reduce the energetic cost of locomotion. Efficient locomotion is particularly important for breath-holding divers because high levels of exercise may quickly deplete oxygen reserves,leading to the termination of a dive. We investigated the swimming behavior of eight adult Weddell seals, which are proficient divers, in McMurdo Sound,Antarctica. A newly developed data logger was attached to free-ranging females at their own breeding sites to record swimming speed, depth, two-dimensional accelerations (stroke frequency and body angle) and temperature. All seals conducted multiple deep dives (the mean dive depth range for each animal was 223.3±66.5–297.9±164.7 m). Prolonged gliding while descending was observed with thinner females (N=5 seals). But the fatter females (N=3 seals) exhibited only swim-and-glide swimming,characterized by intermittent stroking and fluctuating swim speed, throughout their descent and ascent. The body angles of four of the seals were restricted to less than 30° by the location of breathing holes in the ice and the slope of local bathymetric features. Of these four, the three fatter seals adopted the stroke-and-glide method while the other thinner seal descended with prolonged periods of gliding. Prolonged gliding seems to be a more efficient method for locomotion because the surface time between dives of gliding seals was significantly less than that of stroking animals, despite their same stroke frequencies.

Since the 1960s, Weddell seals (Leptonychotes weddellii (Lesson, 1826)) have been tagged and surveyed annually in McMurdo Sound, Antarctica. Mark–recapture analyses and model selection trials using Akaike's Information Criterion indicate that sex, cohort, and year affect juvenile (ages 1 and 2) survival. In contrast, year and perhaps sex and cohort are less important factors for adult survival. Average annual survival is higher among adults (0.93) than juveniles (0.55–0.59) and there is little evidence for senescence to at least age 17. The oldest known-aged female and male in the study were 27 and 24 years old, respectively. Data suggest that the abundance of a resident population of Weddell seals remains relatively stable over time despite annual fluctuations in Jolly–Seber abundance estimates for the entire population. We argue that this annual variability is likely the result of temporary immigration of animals born outside the study area; mean rates are estimated from a simulation model and tagging data to be between 16.8% and 39.7% for females and between –13.1% and 31.6% for males. Sea ice extent appears to affect immigration where, during times of reduced fast-ice, immigrants are forced, or allowed easier access, into the ice-covered areas of Erebus Bay from surrounding locations. Our findings contradict previous studies reporting lower survival and higher immigration. Model choice is shown to be the most likely cause of these discrepancies and we provide evidence that our models are more appropriate than those used elsewhere.

Abstract:We consider how Antarctic seals may respond to changes in climate, realizing that anthropogenic alteration of food webs will influence these responses. The species considered include the ice-obligatecrabeater (Lobodon carcinophaga), Weddell (Leptonychotes weddellii), Ross (Ommataphoca rossii) and leopard (Hydrurga leptonyx) seal -and the ice-tolerant Antarctic fur seal (Arctocephalus gazella) and southern elephant seal (Mirounga leonina). The data analysed are from long-term censuses of Weddell seals in McMurdo Sound (1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006), and of Weddell, fur and elephant seals at Arthur Harbour, Antarctic Peninsula (1974Peninsula ( -2005. After considering their responses to recent changes in environmental features, as well as projected and current changes to their habitat our conclusions are that the distribution and abundance of 1) crabeater and Weddell seals will be negatively affected by changes in the extent, persistence and type of annual sea ice, 2) Ross and leopard seal will be the least negatively influenced by changes in pack ice characteristics, although, as may be the case for crabeater and Weddell, population size and distribution may be altered through changes in food web dynamics, and 3) southern elephant and fur seals will respond in ways opposite to the pack ice species, but could also be influenced most immediately by changes in their food resources due to factors other than climate.

A breeding population of the Weddell seal (Leptonychotes weddelli) was studied annually during the 2t2-mo pupping and breeding season in McMurdo Sound, Antarctica, from 1969 through 1974. Components of the population were estimated by direct counts of adult <;> <;> with pups, by capture-recapture studies of nonparous <;> <;> and adult o o, and by aerial counting. Total population size was estimated as being on the order of 2,500-3,000 seals. The Seber-Jolly method was adapted to circumstances of the present study.Reproductive rates (measured in terms of successful pupping) were =0.5 pups/'i' for the entire breeding colony, and =0. 7 pups/'i' for a subset of tagged <;> <;> observed for 4 yr in sequence. Agespecific reproductive data were also obtained, and it appears that full reproductive activity of <;> <;> is achieved at =age 7. The annual survival rate for adult <;> <;> is in the range of 0.80 to 0.85, as determined (by several methods) from tagging data. Survival rates for adult o o are lower (perhaps 0.50), but are not as well established. Little information was obtained on the subadult class (I to 3 yr of age).Underwater territories of adult o o on one breeding colony were studied by acoustic tagging and found to be steadily patrolled by dominant o o. Two experimental colonies having different densities of <;> <;> with pups demonstrated increased interactions at higher densities, but lower weight losses on the part of the <;> <;> (presumably due to isolated location of the colonies).We infer from the available data that the McMurdo Sound population is somehow regulated by a physiological or social connection between the number of adult <;> <;> at the pupping colonies and subsequent years' pup population.

Some criteria for appraising population level relative to the maximal or carrying capacity point are listed. Simple population-dynamics models are then used to explore some of the criteria. Age at first reproduction does not seem as important as in some more prolific and shorter-lived species, being at most equivalent to a few percentage points of adult survival. Age-specific reproductive rates are very much the same for a number of pinniped species. For most marine mammal species, survival through immature stages is an unknown quantity, but appears to be a factor of major importance in determining population trend. Data on the Pribilof fur seals (Callorhinus ursinus) lead to the speculative conclusion that the maximum sustained yield (MSY) point may be to the right of the median value frequently assumed, so that "optimal" population levels may be closer to the asymptotic or carrying capacity level. Such a view is proposed as a conservative management policy.

No abstract

No abstract

We contrasted body condition, and age‐specific reproduction and mortality between a growing population of sea otters (Enhydralutris) at Kodiak Island and a high‐density near‐equilibrium population at Amchitka Island, Alaska. We obtained data from marked individuals, population surveys, and collections of beach‐cast carcasses. Mass:length ratios indicated that females (but not males) captured in 1992 at Amchitka were in poorer condition than those captured at Kodiak in 1986–1987. In 1993, the condition of females at Amchitka improved in apparent response to two factors: (1) an episodic influx of Pacific smooth lumpsuckers, Aptocyclus ventricocus, from the epi‐pelagic zone, which otters consumed; and (2) an increase in the otters’ benthic invertebrate prey resulting from declining otter numbers. Reproductive rates varied with age (0.37 [CI=0.21 to 0.53] births female−1 yr−1 for 2–3‐yr‐olds, and 0.83 [CI=0.69 to 0.90] for females ≥4 yr old), and were similar at both areas. Weaning success (pups surviving to ≥120 d), in contrast, was almost 50% lower at Amchitka than at Kodiak and for females ≥4 yr of age was 0.52 (CI=0.38 to 0.66) vs 0.94 (CI=0.75 to 0.99), respectively. Sixty‐two percent of the preweaning pup losses at Amchitka occurred within a month of parturition and 79% within two months. Postweaning survival was also low at Amchitka as only 18% of instrumented pups were known to be alive one year after mother‐pup separation. Adult survival rates appeared similar at Amchitka and Kodiak. Factors affecting survival early in life thus are a primary demographic mechanism of population regulation in sea otters. By maintaining uniformly high reproductive rates over time and limiting investment in any particular reproductive event, sea otters can take advantage of unpredictable environmental changes favorable to pup survival. This strategy is consistent with predictions of “bet‐hedging” life history models.