[Show abstract][Hide abstract] ABSTRACT: Most hypotheses on adaptation stem from comparisons among species. The comparative method offers a way to study questions of ecological mechanisms and evolutionary adaptations, particularly in situations where experimentation is near impossible. Comparisons among species and areas in itself can advance the perspective greatly, but this approach suffers from unknown and uncontrollable variation among species and regions. Adding another dimension of comparison, a large ecological disturbance, to the system is extremely illuminating because it acts independently of preexisting species level and spatial variation and, therefore, simplifies comparisons by eliminating confounding variables. Thus, the chapters in this book are of interest because they not only chronicle the impact of El Niño (EN) on eastern Pacific pinnipeds, but they also provide new perspectives on the ecology, life history, and evolution of these species which could not otherwise have been obtained.
[Show abstract][Hide abstract] ABSTRACT: During the 1983 El Niño event, California sea lions (Zalophus californianus) and Galapagos fur seals (Arctocephalus galapagoensis) responded to the reduction in food availability by extending their foraging trips to sea and delivering less milk to their pups (Trillmich and Limberger 1985; Ono et al. 1987). These observed changes likely reflected a change in foraging tactics as animals attempted to compensate for a reduced food supply. The manner in which these foraging patterns were altered, however, still remains unclear.
[Show abstract][Hide abstract] ABSTRACT: Adult female California sea lions were examined during the summer breeding season on San Miguel Island, California. During 17 feeding trips, representing 40.6 days at sea, animals made >8900 dives, the deepest of which was estimated at c274 m, the longest 9.9 min. Most dives, however, were <3 min in duration and 80 m in depth. From estimates of body oxygen stores, dives up to 5.8 min can be supported aerobically. Cost-benefit considerations based on prey availability and encounter rate may be more important than physiological limits in shaping the foraging patterns. Sea lions were active virtually throughout their time at sea, resting on the surface for only 3% of the average trip. Peak diving frequency occurred during twilight hours near sunrise and sunset. Dives were frequent, however, during all hours of the day and were typically clustered into bouts that lasted a mean (±SD) of 3.3 ± 1.5 h. These bouts probably represent active feeding on discrete prey patches. During short bouts (<3 h), dive depth was less variable than for dives occurring between bouts. During longer bouts, dive depth changed in a manner consistent with pursuit of vertically migrating prey. During the 1983 El Nino, sea lions compensated for a reduction in food availability by lengthening dive bouts. -from Authors
No preview · Article · Jan 1989 · Canadian Journal of Zoology
[Show abstract][Hide abstract] ABSTRACT: The energetic costs and behavioral changes associated with net entanglement were studied in three captive juvenile male northern fur seals, Callorhinus ursinus. Rates of energy expenditure were highly dependent upon swim velocity and size of the net fragment. At a speed of 1.1m/s, northern fur seals expended a mean (.plus-minus.SD) of 6.5 (.plus-minus.0.7) W/kg before entanglement, 9.7 (.plus-minus.3.8) W/kg when entangled in 100 g nets, and 13.8 W/kg with 200 g nets. These results showed that a free-ranging animal entangled in a net fragment of 200 g or larger will experience considerable difficulty swimming. The northern fur seals' average daily metabolic rates (ADMR) were measured with doubly labeled water over 6 day periods before and during entanglement in 225 g net fragments. Concurrent behavioral observations revealed a 75% reduction in time spent swimming and a 138% increase in time spent resting due to entanglement. Nevertheless, the northern fur seals' mean ADMR rose from 8.0 (.plus-minus.0.4) W/kg to 9.3 (.plus-minus.1.9) W/kg. While this increase was primarily due to one animal's performance, it suggests that entanglement may also elevate the costs of resting and grooming. At 17 months of age, the northern fur seals had averaged head diameters (.plus-minus.SD) of 14.7 (.plus-minus.0.2) cm, making them most susceptible to entanglement in nets with stretched mesh sizes of 23 cm or more. Observations showed that these juvenile fur seals were naturally inquisitive and rapidly became entangled upon their first encounter with a floating net. Subsequent entanglements depended more upon each animal's behavior than upon net fragment size. Captive aniamls were unable to free themselves from the entangling fragments.
[Show abstract][Hide abstract] ABSTRACT: The free-ranging dive pattern of seven adult female northern elephant seals (Mirounga angustirostris) was obtained with time-depth recorders during the first 14-27 days at sea following lactation. The instruments were recovered and mass gain at sea determined when the animals returned to the rookery at Ano Nuevo, California [USA] to molt. The seals gained a mean of 76.5 .plus-minus. 13.9 kg during a mean of 72.6 .plus-minus. 5.0 days at sea. The mean dive rate was 2.7 .plus-minus. 0.2 dives/h and diving was virtually continuous during the entire period at sea. Mean dive duration was 19.2 .plus-minus. 4.3 min with the longest submersion lasting 48 min. Mean surface interval between dives was 2.8 .plus-minus. 0.5 min, so that only 14.4% of the recorded time at sea was spent on the surface. Surface intervals did not vary with the duration of preceding or succeeding dives. Modal dive depth for each female was between 350 and 650 m. The maximum dive depth was estimated at 894 m, a depth record for pinnipeds and appears to serve in foraging, energy conservation, and predator avoidance.
Full-text · Article · Feb 1988 · Canadian Journal of Zoology
[Show abstract][Hide abstract] ABSTRACT: Adult female northern elephant seals were studied using time-depth recorders during the first 14-27 days at sea following lactation. When the animals returned to the rookery at Ano Nuevo, California, to molt, they had gained a mean of 76.5 ± 13.9 kg during a mean of 72.6 ± 5.0 days at sea. Mean dive rate was 2.7 ± 0.2 dives/h and diving was virtually continuous during the entire period at sea. Mean dive duration was 19.2 ± 4.3 min with the longest submersion lasting 48 min. Mean surface interval between dives was 2.8 ± 0.5 min, so that only 14.4% of the recorded time at sea was spent on the surface. Surface intervals did not vary with the duration of preceding or succeeding dives. Modal dive depth for each female was between 350-650 m. The maximum dive depth was estimated at 894 m, a depth record for pinnipeds. The deep, nearly continuous dive pattern of female northern elephant seals appears to serve in foraging, energy conservation and predator avoidance. -from Authors
Full-text · Article · Jan 1988 · Canadian Journal of Zoology