Donald B. Siniff

Factors affecting stroking patterns and body angle in diving Weddell seals under natural conditions

SUMMARY Aquatic 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.

An Analysis of the Dynamics of a Weddell Seal Population

A breeding population of the Weddell seal (Leptonychotes weddelli) was studied annually during the 2 1/2—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 ♂ ♂, 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/♀ for the entire breeding colony, and ~ 0.7 pups/♀ for a subset of tagged ♀ ♀ observed for 4 yr in sequence. Age—specific 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 ♂ ♂ are lower (perhaps 0.50), but are no...

Opinion Projecting the effects of environmental change on Antarctic seals

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-obligate - crabeater (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 (1997–2006), and of Weddell, fur and elephant seals at Arthur Harbour, Antarctic Peninsula (1974–2005). 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.

POPULATION DYNAMICS OF WEDDELL SEALS (LEPTONYCHOTES WEDDELLI) IN McMURDO SOUND, ANTARCTICA'

Population dynamics of Weddell seals in McMurdo Sound, Antarctica, were studied from 1970 to 1984 using mark-recapture estimation, aerial surveys, age structure and magnitude of harvests, and direct counts of pup production. Similar data from earlier studies were used to reconstruct the history of the population during the period of human presence after 1956. Jolly-Seber estimates of population size indicated a general decline from 1970 to 1976, low numbers in 1976 and 1978, and relative stability from 1979 to 1984 at - 1500 adult seals. From 1970 to 1983, mean annual survival of adult female and male Weddell seals was estimated at 0.85 and 0.76. Counts made in aerial surveys in the summers of 1982/1983 and 1983/1984 were substantially below similar survey counts in the 1960s. Pup production in the breeding colonies also declined from 1967 to 1976, but has recovered and been stable since 1979. Collections spanning 28 yr indicated significant shifts in age structure characteristic of populations undergoing a rapid increase and then decline. The age structure of both sexes indicated a constant age of recruitment to the adult population at 5 yr, which corresponds closely with age of maturity in females. Generalized least squares and standard regression analyses failed to detect any correlation of adult survival or female reproductive estimates with population size or ice conditions, but there were significant trends in survival estimates from 1970 to 1983 that may be due to shifting age structure. The most plausible interpretation of these data is that heavy harvests of seals to feed dog teams in the mid-1950s severely depleted the resident population of adult Weddell seals. When population studies began in the early 1960s the population was expanding rapidly, probably as a result of immigration by juveniles. The population declined to low levels in 1976-1978 and has been fairly stable since 1979, probably at a level lower than before harvesting began. Survival and reproductive parameters of these Weddell seals are low relative to other pinnipeds, suggesting that the adult population is at an equilibrium with its environment.

The importance of Antarctic toothfish as prey of Weddell seals in the Ross Sea

Abstract Uncertainty exists over the importance of Antarctic toothfish (Dissostichus mawsoni) as prey of top predators in the Ross Sea. In this paper we assess relative weight given to direct, observational evidence of prey taken, as opposed to indirect evidence from scat and biochemical analysis, and conclude that toothfish are important to Weddell seals (Leptonychotes weddellii). The seals eat only the flesh of large toothfish and therefore they are not detected in scat or stomach samples; biochemical samples have been taken from seal sub-populations where toothfish seldom occur. Using direct observations of non-breeding seals away from breeding haulouts in McMurdo Sound, 0.8–1.3 toothfish were taken per day. Based on these and other data, the non-breeding portion of the McMurdo Sound seal population, during spring and summer, consume about 52 tonnes of toothfish. Too many unknowns exist to estimate the non-trivial amount consumed by breeders. We discuss why reduced toothfish availability to Weddell seals, for energetic reasons, cannot be compensated by a switch to silverfish (Pleuragramma antarcticum) or squid. The Ross Sea toothfish fishery should be reduced including greater spatial management, with monitoring of Weddell seal populations by CCAMLR. Otherwise, probable cascades will lead to dramatic changes in the populations of charismatic megafauna.

Inter-decadal patterns of population and dietary change in sea otters at Amchitka Island, Alaska

Abstract After having been hunted to near-extinction in the Pacific maritime fur trade, the sea otter population at Amchitka Island, Alaska increased from very low numbers in the early 1900s to near equilibrium density by the 1940s. The population persisted at or near equilibrium through the 1980s, but declined sharply in the 1990s in apparent response to increased killer whale predation. Sea otter diet and foraging behavior were studied at Amchitka from August 1992 to March 1994 and the data compared with similar information obtained during several earlier periods. In contrast with dietary patterns in the 1960s and 1970s, when the sea otter population was at or near equilibrium density and kelp-forest fishes were the dietary mainstay, these fishes were rarely eaten in the 1990s. Benthic invertebrates, particularly sea urchins, dominated the otter’s diet from early summer to mid-winter, then decreased in importance during late winter and spring when numerous Pacific smooth lumpsuckers (a large and easily captured oceanic fish) were eaten. The occurrence of spawning lumpsuckers in coastal waters apparently is episodic on a scale of years to decades. The otters’ recent dietary shift away from kelp-forest fishes is probably a response to the increased availability of lumpsuckers and sea urchins (both high-preference prey). Additionally, increased urchin densities have reduced kelp beds, thus further reducing the availability of kelp-forest fishes. Our findings suggest that dietary patterns reflect changes in population status and show how an ecosystem normally under top-down control and limited by coastal zone processes can be significantly perturbed by exogenous events.

Site fidelity of Weddell seals: the effects of sex and age

Abstract Site fidelity is believed to be an important life history strategy for Weddell seals (Leptonychotes weddellii), that return to traditional breeding colonies each spring. We examined four hypotheses concerning their fidelity to these colonies: 1) fidelity is stronger to natal sites (natal fidelity) than to other sites, 2) females exhibit greater site fidelity than males, 3) site fidelity for both sexes increases with age, 4) site fidelity in adult females is related to their reproductive status and their total number of offspring. Analysis of a long-term tagging database from McMurdo Sound did not support hypotheses 1 and 2. Although animals did express fidelity to specific sites over their lifetime (χ2 tests, P < 0.05), fidelity to natal colonies was lower than to other sites (χ2 test, P < 0.05). There were no differences in site fidelity between males and females (χ2 tests, P > 0.05). Hypothesis 3 was supported. Since the probability of a returning seal occupying the same colony as the previous year increased with age among both sexes to about age 12. Finally, in support of hypothesis 4, females with a higher degree of site fidelity were more likely to both have a higher reproductive rate and return to a site where they have previously given birth.

Onset and cessation of activity in cottontail rabbits and snowshoe hares in relation to sunset and sunrise

Summary Onset and cessation of activity were studied in cottontail rabbits (Sylvilagus floridanus) and snowshoe hares (Lepus americanus) through the use of an automatic radio-tracking system. These times varied greatly for the male cottontails studied in January but were more consistently correlated with sunrise-sunset times for the females in February. Contradictory conclusions of other workers are examined, and possible reasons for the contradictions given. Onset and cessation of activity in a female snowshoe hare studied in late winter and spring closely followed sunrise-sunset times, and there was a high correlation (r=0·95) between seasonal changes in periods of daily inactivity and seasonal changes in length of daylight. Data suggest that cessation of activity may be stimulated by a certain light intensity rather than actual sunrise.

Experiments on the response of sea otters Enhydra lutris to oil contamination

Abstract Two oiling experiments were conducted from 1977–1979 on sea otters Enhydra lutris in Prince William Sound, Alaska. In the first experiment, four captured otters were fitted with radio-transmitters and released following contamination of their pelage with 25 cc of Prudoe Bay crude oil; a fifth individual was oiled and cleaned with detergent prior to being released. For all experimental animals activity increased dramatically during the first week following treatment; this effect was accentuated in the otter cleaned with detergent. Most of the increase in activity corresponded to increased grooming, whereas the feeding pattern (as measured by dive time analysis) did not change. It appeared that all experimental otters survived the first experiment, probably because only a small portion of the pelage was treated, and because food resources in the study area were abundant. In the second experiment we observed behavioural reactions of two otters in an above-ground swimming pool which was partly covered with crude oil. Both otters spent very little time on the oiled side of the pool (less than 1 minute per hour), but occasionally surfaced in it for brief periods and eventually became contaminated. One of these individuals was not cleaned and died within 24 h of first encounter with the oil; the other was cleaned and released with a radio-transmitter that failed shortly afterwards.

Activity patterns and time budgets of the declining sea otter population at Amchitka Island, Alaska

Time budgets of predators may reflect population status if time spent foraging varies with local prey abundance. We assumed that the sea otter (Enhydra lutris) population at Amchitka Island, Alaska, USA, had been at equilibrium since the early 1960s and collected time budgets of otters to be used to represent future conditions of currently expanding sea otter populations. We used radiotelemetry to monitor activity-time budgets of otters from August 1992 to March 1994. Sea otter activity was directly linked to sex, age, weather condition, season, and time of day. Sea otters differed in percent time foraging among cohorts but not within cohorts. Percent time foraging ranged from 21% for females with very young (≤3 weeks of age) dependent pups to 52% for females with old (≥10 weeks of age) pups. Otters foraged more and hauled out more as local sea conditions worsened. Adult males spent less time foraging during winter and spring, consistent with seasonal changes in prey selection. Time spent foraging was similar to that reported for otters in California and an established population in Prince William Sound, Alaska, but greater than that of otters in recently established populations in Oregon and Alaska. Despite current evidence indicating that the population was in decline during our study, we were unable to recognize this change using time budgets. Our results illustrate the importance of stratifying analyses of activity patterns by age and sex cohorts and the complexity inherent in comparisons of behavioral data between different populations relying on distinct prey bases.