SEX DIFFERENCES IN BODY SIZE AND BODY CONDITION OF CALIFORNIA SEA LION (ZALOPHUS CALIFORNIANUS) PUPS FROM THE GULF OF CALIFORNIA

The Gulf of California harbors about 15% of the total California sea lion ( Zalophus californianus californianus ) population. We studied pup morphometrics from eight Gulf of California rookeries during the 1996 and 1997 reproductive seasons to describe sex differences in body size and body condition indices. Newborn pup body size was not different from previous reports. Male pups were heavier and larger than female pups in terms of all linear dimensions. Morphometric relationships, however, showed that males were 3%-4% denser, and that after removing the effects of length, they were about 2% heavier than females. Sculp depth adjusted for length was 12% larger in female than in male pups. Our data provide further evidence that male otariid pups may allocate a larger fraction of milk energy to muscular and skeletal growth compared to female pups.     

INTRASPECIFIC GENETIC DIFFERENTIATION IN CALIFORNIA SEA LIONS (ZALOPHUS CALIFORNIANUS) FROM SOUTHERN CALIFORNIA AND THE GULF OF CALIFORNIA

Intraspecific patterns of mitochondrial DNA sequence variation were determined among California sea lions ( Zalophus califomianus californianus ) from three colonies along the Pacific coast of southern and Baja California and one colony in the Gulf of California. We found no variation in 368 base pairs (bp) of cytochrome b sequence among 40 sea lions from these localities, but analysis of 360 base pairs of control region revealed eleven genotypes. The four genotypes found in the Gulf of California population were unique and phylogenetically distinct from those found in sea lions along the Pacific coast. The average sequence divergence between Gulf and Southern California genotypes was 4.3%, suggesting a relatively long period of isolation. However, colonies along the Pacific coast, which are less than 200 km apart, shared mtDNA genotypes, indicating that recent genetic exchange has occurred between them. Therefore, we suggest that regional female philopatry exists in California sea lions. Regional boundaries may be related to oceanic currents or patchiness in the distribution of resources. Further research is needed to better understand the underlying causes of genetic differentiation in the California sea lion.     

COUNTS OF CALIFORNIA SEA LION (ZALOPHUS CALIFORNIANUS) PUPS FROM AERIAL COLOR PHOTOGRAPHS AND FROM THE GROUND: A COMPARISON OF TWO METHODS

Pups on San Nicolas Island were counted by two methods; counts by observers on the ground were compared to counts from aerial photographs taken with a 126-mm-format camera with image motion compensation. No difference was detected between photographic counts and ground counts (P= 0.367) when ground counters had unobstructed views. However, ground counts were significantly lower when areas with obstructed views were included in the analysis (P < 0.001). For areas with unobstructed viewing conditions, no difference was detected between counts by the two methods for rock substrates (P= 0.140), sand substrates (P= 0.468), or mixed rock-and-sand substrates (P= 0.968). No differences were found among three replicate aerial photographic censuses (P= 0.432), but a significant difference was found between two replicate ground censuses (P= 0.037). Total counts obtained from the aerial photographs were more precise (CV = 0.042) than counts obtained on the ground (CV = 0.078). Less variability in counts was found between photographic counters than for ground counters.     

ATTENDANCE PATTERNS OF CALIFORNIA SEA LION (ZALOPHUS CALIFORNIANUS) FEMALES AND PUPS DURING THE NON-BREEDING SEASON AT SAN MIGUEL ISLAND

The attendance patterns of California sea lions were studied during the non-breeding seasons from 1991 to 1994. Lactating females frequented the rookery to nurse their pups until weaning; most non-lactating females left the rookery for the season. Females spent over 70% of their time at sea except in 1993 when they spent 59% of their time at sea. The mean foraging trip length in the winter and spring ranged from 3.3 to 4.6 d; the mean nursing visit ranged from 1.2 to 1.4 d. The duration of foraging trips and nursing visits was variable over the season for individuals but no pattern of change was detected. Interannual and seasonal differences were not significant for time at sea, visits ashore, or foraging-trip duration before, during, or after the 1992-1993 El Niño event. Pups spent an average of 66.6% of their time ashore and up to three days away from the rookery during their mother's absence. Most females and pups stayed associated until April or May. The results suggest that seasonal movement of prey is more important in determining attendance patterns late in the lactation period than increasing energy demands of the pup.     

PHOTOPERIOD DEFINES THE PHENOLOGY OF BIRTH IN CAPTIVE CALIFORNIA SEA LIONS

A population-based analysis was used to evaluate the role of photoperiod in regulating the precise annual birth timing of captive California sea lions. Latitudinal variation of 463 birth dates was explained by a response to a common photoperiod, 11.48 h/d, occurring 242 d pre-partum and immediately before blastocyst implantation. The lengths of the pupping seasons at the 18 largest captive colonies were significantly correlated to those estimated by the photoperiod model. Hence, the phenology of birth in this species is defined by photoperiod.     

PRECISE BIRTH TIMING IN CAPTIVE HARBOR SEALS (PHOCA VITULINA) AND CALIFORNIA SEA LIONS (ZALOPHUS CALIFORNIANUS)

The absolute and relative precision of birth timing was assessed in 27 captive harbor seals and 30 captive California sea lions. Data were obtained from zoos and aquaria and only adult females with at least three live births were selected for analysis. Individuals of both species demonstrated very high absolute precision with narrow ranges of birth dates and small standard deviations of the individual mean birth dates. In addition, with the exception of one sea lion, no temporal trends for earlier or later birthing were found. Whereas marked differences in birth timing were found among individual harbor seals, these relative differences tended to disappear when subspecies affiliation and location were taken into account. Moderate inter-individual differences were also found among the California sea lions, but these differences persisted following the partitioning of adult females by location.
The absolute precision in birth timing strongly suggests that environmental regulation of birth timing occurs via a highly precise cue, such as photoperiod. The relative differences among individuals may indicate that birth timing in pinnipeds has a heritable component, and hence may evolve in response to natural selection, thus allowing adaptation of the pupping season to specific seasonal environments.     

Age-specific birth rates of California sea lions (Zalophus californianus) in the Gulf of California, Mexico

Estimates of demographic parameters are essential for assessing the status of populations and assigning conservation priority. In light of the difficulties associated with obtaining such estimates, vital rates are rarely available even for well-studied species. We present the first estimates of age-specific birth rates for female California sea lions ( Zalophus californianus ) >10 yr of age. These rates were estimated from the reproductive histories of five cohorts of animals branded as pups between 1980 and 1984 at Los Islotes colony in the Gulf of California, Mexico. Age-specific birth rates varied among age classes and ranged between 0.06 and 0.80. The highest birth rates were observed for females between 10 and 15 yr of age, with decreased birth rates among older females. The effect of age, year, and resighting effort were explored using logistic regression analysis. Based on Akaike Information Criteria, birth rates were best explained by female age, while year and resighting effort did not have a significant effect. The odds ratio of producing a pup decreased with age but did not change significantly for middle-aged females. Our estimates of age-specific birth rates are consistent with general patterns observed for other large vertebrates.     

Demographics and spatio‐temporal signature of the biotoxin domoic acid in California sea lion (Zalophus californianus) stranding records

California sea lions (Zalophus californianus) in otherwise good nutritional condition have been consistently affected by the marine biotoxin domoic acid since the late 1990s. In this study we evaluated the temporal and spatial stranding patterns of suspected and confirmed cases of domoic acid intoxicated sea lions from 1998 to 2006, using records of strandings along the California coast obtained from members of the California Marine Mammal Stranding Network. The majority of domoic acid cases were adult females (47%–82% of the total annual domoic acid cases), a contrast to strandings that were not related to domoic acid, which were generally dominated by juveniles and pups. Exposure to this biotoxin led to a 6.67‐fold increase in adult female strandings in 2000, and a 5.44‐fold increase in adult female deaths in 2006, relative to strandings and deaths of adult female not affected by domoic acid. Domoic acid cases have occurred annually since 1998 (except for 1999) between April and August, with clusters centered primarily at Pismo Beach (San Luis Obispo County), as well as at other beaches in San Luis Obispo, Monterey, Santa Cruz, Santa Barbara, Orange, and San Diego counties. The larger ecological and population level implications of increased domoic acid strandings and deaths, particularly among adult female sea lions, warrant further attention and need to be investigated.     

Trophic level and overlap of sea lions (Zalophus californianus) in the Gulf of California, Mexico

Stable isotope and scat analyses were used in concert to determine trophic level and dietary overlap among California sea lions from different rookeries in the Gulf of California. Isotopic analysis of the fur of sea lion pups revealed differences in δ¹⁵N and δ¹³C values among rookeries during the breeding season. Mean δ¹⁵N and δ¹³C values varied from 20.2‰ to 22.4‰ and from −15.4‰ to −14.0‰, respectively. The pattern of differences among rookeries was similar between years in most cases. Isotopic variations among rookeries were associated with differences in prey consumption. There was a significant correlation between δ¹⁵N value and trophic level, as determined by scat analysis. Joint application of isotopic and scat analyses allowed us to identify how the feeding habits of sea lions vary with location. Our results suggest the presence of spatial structure in available prey as well as the localized use of prey by sea lions across the Gulf of California.     

FIELD EXPERIMENTS SHOW THAT ACOUSTIC PINGERS REDUCE MARINE MAMMAL BYCATCH IN THE CALIFORNIA DRIFT GILL NET FISHERY

A controlled experiment was carried out in 1996–1997 to determine whether acoustic deterrent devices (pingers) reduce marine mammal bycatch in the California drift gill net fishery for swordfish and sharks. Using Fisher's exact test, bycatch rates with pingers were significantly less for all cetacean species combined ( P < 0.001) and for all pinniped species combined ( P = 0.003). For species tested separately with this test, bycatch reduction was statistically significant for short-beaked common dolphins ( P = 0.001) and California sea lions ( P = 0.02). Bycatch reduction is not statistically significant for the other species tested separately, but sample sizes and statistical power were low, and bycatch rates were lower in pingered nets for six of the eight other cetacean and pinniped species. A log-linear model relating the mean rate of entanglement to the number of pingers deployed was fit to the data for three groups: short-beaked common dolphins, other cetaceans, and pinnipeds. For a net with 40 pingers, the models predict approximately a 12-fold decrease in entanglement for short-beaked common dolphins, a 4-fold decrease for other cetaceans, and a 3-fold decrease for pinnipeds. No other variables were found that could explain this effect. The pinger experiment ended when regulations were enacted to make pingers mandatory in this fishery.     

BEHAVIORAL THERMOREGULATION OF THE NEW ZEALAND SEA LION (PHOCARCTOS HOOKERI)

Behavioral thermoregulation of New Zealand sea lions (Phocarctos hookeri) was studied at a male haul‐out ground at Papanui Beach, Otago Peninsula, New Zealand. The proportion of time spent by sea lions in each of five postures (prone, curled, oblique, ventral‐up, dorsal‐up) and also with the number of flippers exposed or tucked (hind and fore) at different black‐bulb temperature (T_bb°C) ranges was recorded. Use of prone and curled postures (0–1 flippers exposed) declined as T_bb increased, suggesting that these are adopted to conserve heat; oblique and dorsal‐up postures (3–4 flippers exposed) use increased with T_bb indicating a role in heat dissipation. The transition between heat conserving and heat dissipating postures occurred at about 14°–20°C (T_bb). Both foreflipper and hind flipper exposure increased with T_bb and the trends were similar, but overall hind flipper exposure was 89% of foreflipper exposure. The results show that surface area of flippers exposed to air is largely controlled by postural adjustment. The increase in flipper exposure with T_bb provides evidence of behavioral thermoregulation and that flippers are major sites for heat loss in the New Zealand sea lion, as observed for other otariid species. Nonpostural thermoregulatory behaviors such as flipper waving and sand flipping increased with T_bb, and may provide additional means of dissipating heat. Total body surface areas of six sea lions ranged from 1.72 to 3.39 m² (curvilinear length range from 1.60 to 2.35 m), and total flipper surface area averaged 22.7% of total body surface area. As otariids do not employ their hind limbs for aquatic propulsion, their role in behavioral thermoregulation may provide an explanation for the relatively large size of the hind flippers of the New Zealand sea lion.     

PROJECTING THE TREND OF STELLER SEA LION POPULATIONS IN WESTERN ALASKA

This paper attempts to project the trends of Steller sea lion ( Eumetopias jubatus ) populations in six subdivisions of the western Alaska population. The overall Western Alaska population has declined dramatically since the 1970s. Trends in half of the areas appear to have leveled-off and possibly to be on the increase. Bootstrapping has been used to provide confidence intervals on predictions for the 2004 counts. For the three areas in which we expect increases, the 95% confidence intervals on predictions were: Eastern Gulf (2,430–3,740), Central Gulf (3,260–3,660) and Central Aleutians (5,160–6,580). The Western Gulf counts have been somewhat erratic, with a gradual rate of decrease (about 2% per year) and wide confidence limits on a linear prediction (logarithmic scale) of 2,690–3,240. Trends in the Eastern Aleutians have been even more erratic, so that about all that can be inferred is that the population may be roughly stabilized. Only the Western Aleutians appear to be rapidly declining at about 10% per year, with a 95% confidence interval on a linear trend of 610–1,100. The predictions were made before the 2004 counts and are in reasonable accord with the 2004 counts. Age structure changes do not appeat to provide a viable explanation for the changing trends.     

Accuracy and precision of age determination using growth layer groups for California sea lions (Zalophus californianus) with known ages

Age determination from counts of growth layer groups (GLGs) in tooth dentine is a common method for aging marine mammals. Using known‐aged animals, we validated this method for acid etched teeth of California sea lions (CSLs), Zalophus californianus. Between 1991 and 2013, the upper left canine (n = 33) was collected opportunistically during necropsy from animals tagged or branded as pups that later died. Overall, 55%–61% of age estimates by GLG counting were within 1 yr of the known‐age in the sample of 1–30‐yr‐old CSLs. Accuracy of age estimates was found to be dependent on age of the CSLs, however. 71%–79% of age estimates were within 1 yr of the known‐age in CSLs <10 yr old. These findings support the validity of counting GLGs to estimate age for CSLs <10 yr old to within 1 yr of accuracy.     

THE RELIABILITY OF SKINFOLD-CALIPERS FOR MEASURING BLUBBER THICKNESS OF STELLER SEA LION PUPS (EUMETOPIAS JUBATUS)

Twelve dead Steller sea lion pups ( Eumetopias jubatus ) aged 3-14 d were recovered from rookeries in Southeast Alaska. They had a wide range of body sizes and conditions (small to large and fat to no fat). The ability of calipers to estimate the thickness of their blubber layer was assessed with a set of skinfold calipers. Average error of measurement for skin and blubber thickness was an acceptable 5.4%, but the skin and blubber of the pups were highly compressible. Skinfold thickness increased with body mass but did not necessarily reflect the development of blubber, given that pups with no blubber also showed an increase in skinfold thickness with increases in body mass. Skinfold thickness of sea lion pups appears to predict body size better than it predicts blubber thickness, making it difficult if not impossible to develop a simple index of body condition or a calculation of percent body fat for Steller sea lion pups from skinfold caliper measurements.     

ONTOGENY OF MOVEMENTS AND FORAGING RANGES IN THE AUSTRALIAN SEA LION

This study tracked the movements of Australian sea lion ( Neophoca cinerea ) pups, juveniles, and adult females to identify home ranges and determine if young sea lions accompanied their mothers at sea. Satellite tags were deployed on nine 15-mo-old pups, nine 23-mo-old juveniles, and twenty-nine adult female Australian sea lions at Seal Bay Conservation Park, Kangaroo Island, South Australia. Females did not travel with their offspring at sea, suggesting young Australian sea lions learn foraging behaviors independently. Although home ranges increased with age, 23-mo-old juveniles had not developed adult movement capacity and their range was only 40.6% of the adult range. Juveniles traveled shorter distances (34.8 ± 5.5 km) at slower speeds (2.0 ± 0.3 km/h) than adults (67.9 ± 3.5 km and 3.9 ± 0.3 km/h). Young sea lions also stayed in shallower waters; sea floor depths of mean locations were 48 ± 7 m for juveniles and 74 ± 2 m for females. Restricted to shallow coastal waters, pups and juveniles are more likely to be disproportionately impacted by human activities. With limited available foraging habitat, young Australian sea lions appear particularly vulnerable to environmental alterations resulting from fisheries or climate change.     

SEX RATIO SELECTION AND THE EVOLUTION OF ENVIRONMENTAL SEX DETERMINATION IN LABORATORY POPULATIONS OF MENIDIA MENIDIA

What happens when a population with environmental sex determination (ESD) experiences a change to an extreme environment that causes a highly unbalanced sex ratio? Theory predicts that frequency-dependent selection would increase the proportion of the minority sex and decrease the level of ESD in subsequent generations. We empirically modeled this process by maintaining five laboratory populations of a fish with temperature-dependent sex determination (the Atlantic silverside, Menidia menidia) in extreme constant temperature environments that caused highly skewed sex ratios to occur initially. Increases in the minority sex consistently occurred from one generation to the next across all five populations, first establishing and then maintaining a balanced sex ratio until termination of the experiment at 8 to 10 generations. The extent to which the level of ESD changed as balanced sex ratios evolved, however, was not consistent. Two populations that experienced high temperatures each generation displayed a loss of ESD, and in one of these ESD was virtually eliminated. This suggests that temperature-insensitive, sex-determining genes were being selected. In populations maintained in low temperature environments, however, the level of ESD did not decline. Instead, the response of sex ratio to temperature was adjusted upward or downward, perhaps by selection of sex-determining genes sensitive to higher (or lower) temperatures. The two different outcomes at low versus high temperatures occurred independent of the geographic origin of the founding population. Our results demonstrate that ESD is capable of evolving in response to selection.     

QUANTIFYING ERRORS ASSOCIATED WITH USING PREY SKELETAL STRUCTURES FROM FECAL SAMPLES TO DETERMINE THE DIET OF STELLER'S SEA LION (EUMETOPIAS JUBATUS)

We examined the digestion and passage times of bones and other hard parts from pollock, herring, salmon, and sandlance recovered from two juvenile captive Steller's sea lions ( Eumetopias jubatus ) subjected to varying activity levels. Key bones that could be identified to species were distributed over an average of 3.2 scats (range 1–6) following a single meal, with pollock remains occurring in significantly more scats than other species. Relying on otoliths alone to determine the presence of prey resulted in significantly fewer prey being identified than if other structures were also used (such as vertebrae, jaw bones, and teeth), particularly for salmon. Using either technique, there were significant differences in the likelihood that bones would be recovered from the series of scats produced following a meal, with pollock recovery exceeding herring (by three-fold) and sandlance (by eight-fold). Differences between species were reduced when recovery was calculated on a per scat basis rather than over multiple scats. Active animals passed greater numbers of bones, but the overall effect on prey recovery estimates was not significant. Defecation times of prey structures from a meal were variable and ranged from an initial 2–56 h to a final 28–148 h. The time interval to pass 95% of recovered structutes varied by a factor of two among prey species, and was highest for pollock due to retention beyond 65 h.