Claudia Janetl Hernández-Camacho

Survival Rates of the California Sea Lion, Zalophus californianus, in Mexico

Abstract California sea lions (Zalophus californianus) in the Gulf of California have declined by 20% over the past 2 decades. The lack of data on life-history parameters for this species has limited the development of demographic models to assess the status of this population. We estimated age- and sex-specific annual survival probabilities for California sea lions using resighting data on 5 pup cohorts from 1981 to 2006. We modeled apparent survival and resighting probability using age-class, sex, and time as potential explanatory variables. Apparent survival rates varied for different age- and sex-classes. Only survival of pups varied by year (from 0.556 to 0.998). Survival was the same for immature males and females (0.90), but differed by sex for young (males = 0.90, females = 0.97) and old (males = 0.75, females = 0.91) adults. Resighting probabilities varied by time, age-class, and sex. Resighting probabilities were higher for females than for males, and lowest for juveniles. The survival estimates presented here provide practical insight into understanding age- and sex-specific survival rates for California sea lions.

The Cost of Male Aggression and Polygyny in California Sea Lions ( Zalophus californianus )

In polygynous mating systems, males often increase their fecundity via aggressive defense of mates and/or resources necessary for successful mating. Here we show that both male and female reproductive behavior during the breeding season (June–August) affect female fecundity, a vital rate that is an important determinant of population growth rate and viability. By using 4 years of data on behavior and demography of California sea lions (Zalophus californianus), we found that male behavior and spatial dynamics—aggression and territory size—are significantly related to female fecundity. Higher rates of male aggression and larger territory sizes were associated with lower estimates of female fecundity within the same year. Female aggression was significantly and positively related to fecundity both within the same year as the behavior was measured and in the following year. These results indicate that while male aggression and defense of territories may increase male fecundity, such interactions may cause a reduction in the overall population growth rate by lowering female fecundity. Females may attempt to offset male-related reductions in female fecundity by increasing their own aggression—perhaps to defend pups from incidental injury or mortality. Thus in polygynous mating systems, male aggression may increase male fitness at the cost of female fitness and overall population viability.

Weak polygyny in California sea lions and the potential for alternative mating tactics

Female aggregation and male territoriality are considered to be hallmarks of polygynous mating systems. The development of genetic parentage assignment has called into question the accuracy of behavioral traits in predicting true mating systems. In this study we use 14 microsatellite markers to explore the mating system of one of the most behaviorally polygynous species, the California sea lion (Zalophus californianus). We sampled a total of 158 female-pup pairs and 99 territorial males across two breeding rookeries (San Jorge and Los Islotes) in the Gulf of California, Mexico. Fathers could be identified for 30% of pups sampled at San Jorge across three breeding seasons and 15% of sampled pups at Los Islotes across two breeding seasons. Analysis of paternal relatedness between the pups for which no fathers were identified (sampled over four breeding seasons at San Jorge and two at Los Islotes) revealed that few pups were likely to share a father. Thirty-one percent of the sampled males on San Jorge and 15% of the sampled males on Los Islotes were assigned at least one paternity. With one exception, no male was identified as the father of more than two pups. Furthermore, at Los Islotes rookery there were significantly fewer pups assigned paternity than expected given the pool of sampled males (p<0.0001). Overall, we found considerably lower variation in male reproductive success than expected in a species that exhibits behavior associated with strongly polygynous mating. Low variation in male reproductive success may result from heightened mobility among receptive females in the Gulf of California, which reduces the ability of males to monopolize groups of females. Our results raise important questions regarding the adaptive role of territoriality and the potential for alternative mating tactics in this species.

DETERMINANTS OF OUTCOMES OF AGONISTIC INTERACTIONS AMONG MALE CALIFORNIA SEA LIONS (ZALOPHUS CALIFORNIANUS)

Abstract Agonistic behavior in territorial male California sea lions (Zalophus californianus) may be influenced by multiple factors, including who initiates an interaction and who owns the territory on which an encounter takes place. We studied predictors of the outcomes of agonistic interactions between territorial male California sea lions on 3 islands (Los Islotes, Granito, and San Jorge) in the Gulf of California, Mexico, during the 2005 breeding season. We evaluated both displays and fights among males to examine the hypotheses that initiators of agonistic interactions and that territory owners were more likely to win disputes. The outcomes of agonistic encounters (win or lose) were independent of the types of interaction (displays or fights) by the initiator and, hence, all agonistic interactions were pooled for analyses. We used an overdispersed binomial logistic regression to determine if initiators and resident males were more likely to win an interaction than noninitiators and nonresidents, respectively. We found that initiators of agonistic interactions were more likely to win agonistic disputes. Resident and nonresident males were equally likely to initiate agonistic interactions and were also equally likely to win agonistic disputes. Our results suggest that agonistic interactions among male California sea lions are influenced by which individual initiates the encounter and not by territory ownership.

The Use of Surrogate Data in Demographic Population Viability Analysis: A Case Study of California Sea Lions

Reliable data necessary to parameterize population models are seldom available for imperiled species. As an alternative, data from populations of the same species or from ecologically similar species have been used to construct models. In this study, we evaluated the use of demographic data collected at one California sea lion colony (Los Islotes) to predict the population dynamics of the same species from two other colonies (San Jorge and Granito) in the Gulf of California, Mexico, for which demographic data are lacking. To do so, we developed a stochastic demographic age-structured matrix model and conducted a population viability analysis for each colony. For the Los Islotes colony we used site-specific pup, juvenile, and adult survival probabilities, as well as birth rates for older females. For the other colonies, we used site-specific pup and juvenile survival probabilities, but used surrogate data from Los Islotes for adult survival probabilities and birth rates. We assessed these models by comparing simulated retrospective population trajectories to observed population trends based on count data. The projected population trajectories approximated the observed trends when surrogate data were used for one colony but failed to match for a second colony. Our results indicate that species-specific and even region-specific surrogate data may lead to erroneous conservation decisions. These results highlight the importance of using population-specific demographic data in assessing extinction risk. When vital rates are not available and immediate management actions must be taken, in particular for imperiled species, we recommend the use of surrogate data only when the populations appear to have similar population trends.

Metal concentrations in demersal fish species from Santa Maria Bay, Baja California Sur, Mexico (Pacific coast).

Concentrations of 11 trace metals (Fe, Mn, Cr, Cu, Ni, Co, Pb, Zn, Cd, As, Hg) in 40 fish species from Santa Maria Bay, Baja California Sur, Mexico, the strategically important area for marine mammals and organisms were analyzed. Based on their concentrations the ranking of metals Fe>Zn>Ni>Cr>Mn>Pb>Cu>Co>As>Cd>Hg suggests that organism size, metabolism and feeding habits are correlated with metal concentrations. Local geological formations affect the concentrations of different metals in the aquatic environment and are subsequently transferred to fishes. The correlation analysis suggests that metabolism and nurturing habits impact the concentration of metals. Concentrations of Fe and Mn appear to be influenced by scavenging and absorption processes, which vary by species. The considerable variability in the metal concentrations obtained in different species underscores the importance of regular monitoring.

Purine metabolism in response to hypoxic conditions associated with breath-hold diving and exercise in erythrocytes and plasma from bottlenose dolphins (Tursiops truncatus)

In mammalian tissues under hypoxic conditions, ATP degradation results in accumulation of purine metabolites. During exercise, muscle energetic demand increases and oxygen consumption can exceed its supply. During breath-hold diving, oxygen supply is reduced and, although oxygen utilization is regulated by bradycardia (low heart rate) and peripheral vasoconstriction, tissues with low blood flow (ischemia) may become hypoxic. The goal of this study was to evaluate potential differences in the circulating levels of purine metabolism components between diving and exercise in bottlenose dolphins (Tursiops truncatus). Blood samples were taken from captive dolphins following a swimming routine (n=8) and after a 2min dive (n=8). Activity of enzymes involved in purine metabolism (hypoxanthine guanine phosphoribosyl transferase (HGPRT), inosine monophosphate deshydrogenase (IMPDH), xanthine oxidase (XO), purine nucleoside phosphorylase (PNP)), and purine metabolite (hypoxanthine (HX), xanthine (X), uric acid (UA), inosine monophosphate (IMP), inosine, nicotinamide adenine dinucleotide (NAD(+)), adenosine, adenosine monophosphate (AMP), adenosine diphosphate (ADP), ATP, guanosine diphosphate (GDP), guanosine triphosphate (GTP)) concentrations were quantified in erythrocyte and plasma samples. Enzymatic activity and purine metabolite concentrations involved in purine synthesis and degradation, were not significantly different between diving and exercise. Plasma adenosine concentration was higher after diving than exercise (p=0.03); this may be related to dive-induced ischemia. In erythrocytes, HGPRT activity was higher after diving than exercise (p=0.007), suggesting an increased capacity for purine recycling and ATP synthesis from IMP in ischemic tissues of bottlenose dolphins during diving. Purine recycling and physiological adaptations may maintain the ATP concentrations in bottlenose dolphins after diving and exercise.

Vibrissa growth rate in California sea lions based on environmental and isotopic oscillations

Pinniped vibrissae provide information on changes in diet at seasonal and annual scales; however, species-specific growth patterns must first be determined in order to interpret these data. In this study, a simple linear model was used to estimate the growth rate of vibrissae from adult female California sea lions (Zalophus californianus) from San Esteban Island in the Gulf of California, Mexico. The δ15N and δ13C values do not display a marked oscillatory pattern that would permit direct determination of the time period contained in each vibrissa; thus, time (age) was calculated in two ways: 1) based on the correlation between the observed number of peaks (Fourier series) in the δ15N profile and the length of each vibrissa, and 2) through direct comparison with the observed number of peaks in the δ15N profile. Cross-correlation confirmed that the two peaks in the δ15N profile reflected the two peaks in the chlorophyll-a concentration recorded annually around the island. The mean growth rate obtained from the correlation was 0.08 ± 0.01 mm d-1, while that calculated based on the observed number of peaks was 0.10 ± 0.05 mm d-1. Both are consistent with the rates reported for adult females of other otariid species (0.07 to 0.11 mm d-1). Vibrissa growth rates vary by individual, age, sex, and species; moreover, small differences in the growth rate can result in significant differences over the time periods represented by the isotopic signal. Thus, it is important to assess this parameter on a species-by-species basis.

Habitat partitioning mediates the coexistence of sympatric dolphins in a tropical fjord-like embayment

Bottlenose dolphins (Tursiops truncatus) and pantropical spotted dolphins (Stenella attenuata) co-occur in Golfo Dulce, a fjord-like embayment located in the southern Pacific coast of Costa Rica. We evaluated if spatial overlap by these dolphin species is associated with similar environmental requirements. Presence-only models based on niche theory were constructed by contrasting a set of spatial locations with the responses of the target species to abiotic ecogeographical variables (EGVs: depth, slope, distance to rivers, distance to 200 m isobath, mean sea surface temperature, and variation in sea surface temperature). Models were cross-validated with levels of discrimination that ranged from acceptable to excellent based on the area under the curve assessment (T. truncatus, rainy season: 0.76, dry season: 0.83; S. attenuata, rainy season: 0.84, dry season: 0.89). Both dolphin species occur in Golfo Dulce year-round; the lack of seasonality documented previously was supported by the models. Species distribution models showed no spatial overlap, with differences in EGVs affecting their distribution (T. truncatus: distance to river + distance to 200 m isobath, S. attenuata: depth + sea surface temperature). We argue that the coexistence of both predators in Golfo Dulce is linked to habitat heterogeneity, where critical habitats are spatially differentiated. The lack of fine-scale spatial overlap, along with influential abiotic variables, highlights a process of coexistence for dolphins that are sympatric at the scale of Golfo Dulce, but within the Gulf there is fine-scale allopatry.