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In February 2015, we conducted a field study of causes of mortality of northern elephant seal (Mirounga angustirostris) pups on San Miguel Island, California. Autopsies were performed on 18 freshly dead pups. Ages of pups ranged from stillborn to 6–8 wk. Gross and histologic lesions included trauma (9 of 18 pups), multifocal necrotizing myopathy (8 of 18), starvation with emaciation (7 of 18), congenital anomalies (3 of 18), bacterial infections (3 of 18), and perinatal mortality (stillbirths and neonates; 2 of 18). Trauma and emaciation or starvation were the most significant contributors to death. Bacterial infections included hemolytic Escherichia coliisolated from the lungs of 2 pups with pneumonia. Additionally, non-hemolytic Streptococcussp. and hemolytic E. coliwere isolated from the liver of an emaciated pup that had mild multifocal suppurative hepatitis. Other lesions, including a previously described necrotizing myopathy, congenital anomalies, and bacterial infections, were detected concurrently in cases with starvation and/or emaciation or trauma.

The California sea lion (Zalophus californianus) population in the United States has increased steadily since the early 1970s. The Marine Mammal Protection Act of 1972 (MMPA) established criteria for management of marine mammals based on the concept of managing populations within the optimal sustainable population (OSP), defined as a range of abundance from the maximum net productivity level (MNPL) to carrying capacity (K). Recent declines in California sea lion pup production and survival suggest that the population may have stopped growing, but the status of the population relative to OSP and MNPL is unknown. We used a time series of pup counts from 1975 to 2014 and a time series of mark‐release‐resight‐recovery data from 1987 to 2015 for survival estimates to numerically reconstruct the population and evaluate the current population status relative to OSP using a generalized logistic model. We demonstrated that the population size in 2014 was above MNPL and within its OSP range. However, we also showed that population growth can be dramatically decreased by increasing sea surface temperature associated with El Niño events or similar regional ocean temperature anomalies. In this analysis we developed a critical tool for management of California sea lions that provides a better understanding of the population dynamics and a scientific foundation upon which to base management decisions related to complex resource issues involving this species. Published 2018. This article is a U.S. Government work and is in the public domain in the USA. We used a combination of 39 years of pup counts, mark‐resight survival estimates, and mortality associated with commercial fisheries to reconstruct the California sea lion population growth and current size after more than 40 years of protection under the Marine Mammal Protection Act. We conclude that the California sea lion population has achieved maximum net productivity and optimum sustainable population levels as defined in the law; however, the population growth is sensitive to changing environmental conditions that affect survival and birth rates and we stress the importance of continued monitoring of the population as it responds to environmental and anthropogenic changes that may alter its status.

A colony of northern fur seals has been found breeding on an island off California, and everyone hopes that this will not lead to its disappearance.

Hawaiian monk seals, found in the northwestern Hawaiian Islands, regularly haul out and give birth on the beaches of six atolls, regularly haul out but rarely give birth on three additional islands, and are seldom sighted elsewhere. Since 1958, counts of monk seals have indicated a decline in population of about 50%. The population of the westernmost atolls, Kure, Midway, Pearl and Hermes Reef, and Laysan and Lisianski islands, have all declined; the population at the only other major pupping atoll, French Frigate Shoals, increased dramatically from 1957 to 1975 and now appears to be stable. The primary cause of the decline, at least at some atolls, has been increased mortality in early life; however, only some of the factors causing the increase in mortality are known. Predation by sharks has been identified as a cause of mortality, and increased human activity was followed by a decline in seal population on two atolls. In 1978, high mortality, which may have resulted from ciguatera or gastrointestinal parasitism, occurred primarily at Laysan Island. Ciguatera is a disease caused by Ciguatoxin produced by a dinoflagellate; this toxin accumulates in prey species of monk seals, such as eels and other fishes. Even though it has been 15 or more years since the monk seal populations declined at Kure Atoll, Midway Island, and Pearl and Hermes Reef, there is no evidence of their recovery.

Adult northern elephant seals go to sea twice each year for periods of ≤8 months during which they range widely in the northern Pacific Ocean. Using new tracking technology, we showed that the species (and individuals) returned to the same foraging areas during postbreeding and postmolt movements, documenting the first double migration for any animal. We also showed segregation by sex during both migrations, the causes of which are unknown. Seals dove continually to depths of 250–550 m during both migrations and travelled linear distances of at least 18,000 (females)–21,000 km (males) during the 250 (males)–300 (females) days they were at sea. These are the longest annual migrations yet recorded for individual mammals. The double migrations apparently are modulated by the requirement for seals to return to land twice each year, to molt and to breed, although the reasons seals favor distant molting sites on the California Channel Islands over island and continental beaches nearer foraging areas are unknown.

Fur seals, Arctocephalus townsendi Merriam, 1897, at present land and breed regularly only along the precipitous, rocky east shore of Isla de Guadalupe, 256 km west of Baja California, Mexico. They seldom if ever land on open sandy beaches, and many haul out in caves and recesses along the narrow lava shoreline. The restricted range appears related to the near extermination of the species in the last century. Although congeneric with the fur seals of the Southern Hemisphere, which breed in the austral summer, A. townsendi breeds from May to July. Our censuses suggest that the population increased from a few individuals in 1954 to at least 500 in 1967. Form, pelage, vocalizations, certain activities, and habitat provide distinctive features for recognition of A. townsendi in the field.