Immunocontraception of captive exotic species. III. Contraception and population management of fallow deer (Cervus dama)

Immunocontraception has become an increasingly valuable tool in the population management of captive exotic ungulates. Although porcine zona pellucida vaccine (PZP) was used successfully in other cervids, a previous study with fallow deer (Cervus dama) suggested that the vaccine did not work in this species. In the current study, PZP was tested in two captive herds of fallow deer. Antibody titers were monitored over a 3‐year period to evaluate three different adjuvant protocols, and the vaccine was applied to an entire herd to determine the impact on fawning rates. In a semi‐free‐ranging herd, antibody titers rose from preimmunization levels of 2.6% of positive control serum to 56.5% 4 weeks after initial inoculations, to 65.1% at 1 year, and to 81.3% at 2 years, after a single annual booster was applied. Fawn production in this herd was reduced significantly over 3 years. The adjuvant protocol of Freund's Modified Adjuvant® (FMA) for the initial inoculation followed by a booster with Freund's Incomplete Adjuvant® (FIA), and the protocol of FMA for the initial inoculation followed in 3 weeks by a booster with FMA both produced significantly higher antibody titers than the 3× FIA (3 weeks apart) protocol after year 1. The FMA+FMA protocol produced significantly higher titers than the 3× FIA protocol at year 2, but was not different from the titers produced by the FMA+FIA protocol at year 2. Zoo Biol 22:261–268, 2003. © 2003 Wiley‐Liss, Inc.     

Immunocontraception in Wild Horses: One Inoculation Provides Two Years of Infertility

ABSTRACT ABSTRACT Previous studies reported one year of contraception associated with a 1-injection porcine zona pellucida (PZP) vaccine. We have subsequently determined contraceptive effectiveness of a presumptive 1-injection, 2-year-duration PZP vaccine in free-roaming wild horses (Equus caballus) in Nevada, USA. In January 2000, we captured, freeze-branded, treated, and subsequently released 96 adult females that received 1) a primary dose of vaccine emulsion consisting of aqueous PZP and Freund's Complete Adjuvant, and 2) booster doses of PZP and adjuvant in controlled-release polymer pellets. We determined PZP release characteristics of pellets in vitro, prior to field use. We determined reproductive success in treated and untreated females through October 2004 via measurement of estrone sulfate and progesterone metabolites in fresh feces collected from the ground and by twice-annual foal counts. Among treated females, annual reproductive success from 2001 though 2004 sequentially was 5.9%, 14.0%, 32.0%, and 47.5%. Untreated females showed average reproductive success of 53.8 ± 1.3% across this period. This study revealed that: 1) PZP acted as an effective contraceptive for 2 years posttreatment; 2) some residual contraceptive effect remained in year 3; and 3) fertility returned to control levels by year 4 posttreatment. It appears that controlled-release technology can replace both the second(1-month) and annual booster injection of PZP vaccine, thereby decreasing cost and increasing efficiency of use of this vaccine in wild horse management.     

Immunocontraception of captive exotic species. I. Przewalski's horses (Equus przewalskii) and banteng (Bos javanicus)

A contraceptive vaccine made of porcine zonae pellucidae (PZP) was tested in three Przewalski's mares and five banteng cows. The vaccine antigen consisted of the complete family of glycoproteins of the porcine zona pellucida, including the sperm receptor ZP3. All mares and three of five banteng were inoculated with 2 or 3 i.m. injections of approximately 65 μg of antigen given over a 6 week period. Two other banteng received inoculations of only 35 μg of antigen on the same schedule. Two of the three mares and three of five banteng cows were pregnant at the time of inoculation. No new pregnancies, as a result of postinoculation breedings, occurred among either the mares 36 months after 65 μg antigen inoculations or among the banteng for 24 months after 65 μg inoculations. One postinoculation pregnancy resulted among the two banteng receiving only 35 μg of antigen. Differences in fertility between treated and control mares and between preinoculation and postinoculation reproductive performance of the banteng were significant (P < 0.05). Urinary ovarian steroid metabolites and behavioral observations indicated follicular development and ovulations were occurring among treated mares during the year following PZP inoculations. PZP immunization produced progressively elevated anti-PZP antibodies in both species, which provided contraceptive protection. PZP immunization appears to be an effective form of contraception in both species. © 1995 Wiley-Liss, Inc.     

Correlates of seasonal change in the body condition of an Arctic top predator

Climate‐driven sea ice loss has led to changes in the timing of key biological events in the Arctic, however, the consequences and rate of these changes remain largely unknown. Polar bears (Ursus maritimus) undergo seasonal changes in energy stores in relation to foraging opportunities and habitat conditions. Declining sea ice has been linked to reduced body condition in some subpopulations, however, the specific timing and duration of the feeding period when bears acquire most of their energy stores and its relationship to the timing of ice break‐up is poorly understood. We used community‐based sampling to investigate seasonality in body condition (energy stores) of polar bears in Nunavut, Canada, and examined the influence of sea ice variables. We used adipose tissue lipid content as an index of body condition for 1,206 polar bears harvested from 2010–2017 across five subpopulations with varying seasonal ice conditions: Baffin Bay (October–August), Davis Strait and Foxe Basin (year‐round), Gulf of Boothia and Lancaster Sound (August–May). Similar seasonal patterns were found in body condition across subpopulations with bears at their nadir of condition in the spring, followed by fat accumulation past break‐up date and subsequent peak body condition in autumn, indicating that bears are actively foraging in late spring and early summer. Late season feeding implies that even minor advances in the timing of break‐up may have detrimental effects on foraging opportunities, body condition, and subsequent reproduction and survival. The magnitude of seasonal changes in body condition varied across the study area, presumably driven by local environmental conditions. Our results demonstrate how community‐based monitoring of polar bears can reveal population‐level responses to climate warming in advance of detectable demographic change. Our data on the seasonal timing of polar bear foraging and energy storage should inform predictive models of the effects of climate‐mediated sea ice loss.     

Preliminary evaluation of porcine zona pellucida (PZP) immunocontraception for behavioral effects in feral horses (Equus caballus)

Successful management of captive populations of wild animals requires effective control of reproduction. Contraception is one tool for controlling reproduction of animals in zoos; however, the options available to the animal manager are limited. Contraceptives vary in efficacy, reversibility, and side effects, and thus may not be suitable for widespread use. One consideration when selecting a contraceptive is its potential for side effects on behavior, especially given the fact that reproduction plays such a prominent role in the biology of any species. To date, there have been few evaluations of contraceptives for behavioral effects, and those that have been conducted have focused on hormone-based contraceptives. This study sought to evaluate a novel method of population control, immunocontraception, for behavioral effects in a population of feral horses. Porcine zona pellucida (PZP) immunocontraception prevents fertilization of ova and does not alter normal hormone secretion patterns. It therefore should leave the animal behaviorally intact in terms of reproductive behavior. The study examined the behavior of 43 sexually mature mares on Assateague Island during the 1997 breeding season and, with help from Earthwatch volunteers, collected observations over a 3-month period. The study found no significant differences between treated and untreated mares in general activity budget, aggression given or received, and spatial relationships relative to the stallion. These preliminary findings indicate that PZP contraception seems to have no acute behavioral effects on the behavior of individuals. The study findings also suggest that PZP could be a desirable and effective management tool for captive species in which social behavior plays an integral role in group dynamics. Analyses of group level effects and population level effects are continuing.     

The effects of age, season and geographic region on thyroid hormones in Steller sea lions (Eumetopias jubatus)

The purpose of this study was to investigate thyroid hormone concentrations, thyroxine (T4) and triiodothyronine (T3), in order to determine basal levels in Steller sea lions of different ages and over seasons. Serum concentrations of total T4 were highest in Steller sea lions followed by total T3 concentrations. Concentrations of free T4 and free T3 were three to four orders of magnitude lower. Concentrations for all four thyroid hormone measurements tended to a lower level as animals matured beyond the neonatal stage. When thyroid hormones from captive sea lions were evaluated across seasons, all thyroid hormones were highest in the July to September period. When compared across the geographic range, animals in southeast Alaska tended to have lower thyroid hormone levels, while the Steller sea lions west of Prince William Sound and animals from the Russian Far East had significantly higher concentrations. Significant inter-annual differences in concentrations were also observed across the geographic range. With an understanding of the basic changes in thyroid hormone concentrations, changes in plane of nutrition or life history states (i.e. fasting, pregnancy or lactation) can now be evaluated for their effect on the overall health of this endangered species.     

Strong Competition Does Not Always Predict Play Asymmetry: The Case of South American Sea Lions (Otaria flavescens)

Although play fighting has been studied for over a century in both human and non‐human animals, quantitative data on marine mammals are still scarce. Here, we investigated play fighting in South American sea lions (Otaria flavescens), one of the most sexually dimorphic species with an extreme polygynous mating system, high levels of both intra‐ and inter‐sexual competition. All these features make South American sea lions a good model species to test some predictions on play fighting. Our results indicate play is restricted to juveniles, being inhibited among adults, and as to be expected in a species that shows a high degree of sexual dimorphism, it is mainly expressed in males. Even though playful interactions were punctuated by competitive behaviours, animals played in a highly symmetric way and were able to adjust their competitive playful interactions in a flexible manner and so reduce the risk of escalation to a minimum level. They were highly selective in their choice of playmates by limiting the number of players per session and playing more with age‐matched companions and friends. All these factors taken together are probably at the basis of the low risk of escalation recorded during the study. This result is predictive of a high ability and motivation of these animals to engage in play behaviour which can have a possible role not only in the acquisition of dominance status, but also in establishing and maintaining social relationships, an unexpected role in a so highly competitive species.     

Aerobic dive limit: how often does it occur in nature?

Diving animals offer a unique opportunity to study the importance of physiological constraint in their everyday behaviors. An important component of the physiological capability of any diving animal is its aerobic dive limit (ADL). The ADL has only been measured in a few species. The goal of this study was to estimate the aerobic dive limit from measurements of body oxygen stores and at sea metabolism. This calculated ADL (cADL) was then compared to measurements of diving behavior of individual animals of three species of otariids, the Antarctic fur seal, Arctocephalus gazella, the Australian sea lion, Neophoca cinerea, and the New Zealand sea lion, Phocarctos hookeri. Antarctic fur seals dove well within the cADL. In contrast, many individuals of both sea lion species exceeded the cADL, some by significant amounts. Australian sea lions typically dove 1.4 times longer than the cADL, while New Zealand sea lions on average dove 1.5 times longer than the cADL. The tendency to exceed the cADL was correlated with the dive pattern of individual animals. In both Antarctic Fur Seals and Australian sea lions, deeper diving females made longer dives that approached or exceeded the cADL (P<0.01, r(2)=0.54). Australian and New Zealand sea lions with longer bottom times also exceeded the cADL to a greater degree. The two sea lions forage on the benthos while the fur seals feed shallow in the water column. It appears that benthic foraging requires these animals to reach or exceed their aerobic dive limit.     

Aerial hearing thresholds and detection of hearing loss in male California sea lions (Zalophus californianus) using auditory evoked potentials

Auditory evoked potential (AEP) measurements are useful for describing the variability of hearing among individuals in marine mammal populations, an important consideration in terms of basic biology and the design of noise mitigation criteria. In this study, hearing thresholds were measured for 16 male California sea lions at frequencies ranging from 0.5 to 32 kHz using the auditory steady state‐response (ASSR), a frequency‐specific AEP. Audiograms for most sea lions were grossly similar to previously reported psychophysical data in that hearing sensitivity increased with increasing frequency up to a steep reduction in sensitivity between 16 and 32 kHz. Average thresholds were not different from AEP thresholds previously reported for male and female California sea lions. Two sea lions from the current study exhibited abnormal audiograms: a 26‐yr‐old sea lion had impaired hearing with a high‐frequency hearing limit (HFHL) between 8 and 16 kHz, and an 8‐yr‐old sea lion displayed elevated thresholds across most tested frequencies. The auditory brainstem responses (ABRs) for these two individuals and an additional 26‐yr‐old sea lion were aberrant compared to those of other sea lions. Hearing loss may have fitness implications for sea lions that rely on sound during foraging and reproductive activities.     

Ancient DNA analysis of the Japanese sea lion (Zalophus californianus japonicus Peters, 1866): preliminary results using mitochondrial control-region sequences

In this study, we successfully extracted ancient DNA from skeletal remains of the Japanese sea lion-a species that is practically extinct-from archaeological sites and determined a partial sequence of its mitochondrial DNA control region. A molecular phylogenetic tree constructed by the neighbor-joining (NJ) method showed that the sequences from Japanese sea lions clustered together, with a high bootstrap value, and that this cluster was closest to the California sea lion cluster. The distinctly divergent cluster of Japanese sea lions reflected the morphological classification of these animals as a distinct species of the genus Zalophus; however, proximity to the California sea lion cluster simultaneously implied conformation with the traditional classification of these animals as a subspecies of Zalophus californianus. The average amount of nucleotide substitution between the Japanese and California sea lions was 7.02%. The Japanese and California sea lions were estimated to have diverged 2.2 million years ago, i.e., in the late Pliocene Epoch. This is the first report on a genetic analysis of the Japanese sea lion.     

Linking seasonal distribution patterns with prey availability in a central-place forager, the Steller sea lion

Aim We used a novel approach to infer foraging areas of a central‐place forager, the Steller sea lion (Eumetopias jubatus), by assessing changes in the temporal and spatial distribution patterns of sea lions at terrestrial sites. Specifically, our objectives were (1) to classify seasonal distribution patterns of Steller sea lions and (2) to determine to what extent the seasonal distribution of Steller sea lions is explained by seasonal concentrations of prey. Location Southeast Alaska, USA. Methods Steller sea lions of all age classes were counted monthly (2001–04) by aerial surveys at 28 terrestrial sites. Hierarchical cluster analysis and principal components analysis were used to classify seasonal distribution patterns of Steller sea lions at these terrestrial sites. We estimated the proportion of sea lions in the study area that were associated with each seasonal distribution pattern. Results Multivariate ordination techniques revealed four distinct seasonal distributional patterns. During December, 55% of the sea lions in the study area were found at Type 1 sites, located near over‐wintering herring aggregations. During May, 56% of sea lions were found at Type 2 sites, near aggregations of spring‐spawning forage fish. In July, 78% of sea lions were found at Type 3 sites, near summer migratory corridors of salmon. During September, 44% of sea lions were found at Type 4 sites, near autumn migratory corridors of salmon. Main conclusions Seasonal attendance patterns of sea lions were commonly associated with the seasonal availability of prey species near terrestrial sites and reflected seasonal foraging patterns of Steller sea lions in Southeast Alaska. A reasonable annual foraging strategy for Steller sea lions is to forage on herring (Clupea pallasii) aggregations in winter, spawning aggregations of forage fish in spring, salmon (Oncorhynchus spp.) in summer and autumn, and pollock (Theragra chalcogramma) and Pacific hake (Merluccius productus) throughout the year. The seasonal use of haulouts by sea lions and ultimately haulout‐specific foraging patterns of Steller sea lions depend in part upon seasonally available prey species in each region.     

Mercury Concentrations in Hair from Neonatal and Juvenile Steller Sea Lions (Eumetopias jubatus): Implications Based on Age and Region in this Northern Pacific Marine Sentinel Piscivore

Mercury is a global contaminant of concern for the fetus and the neonate of piscivores. Methylmercury, produced within marine ecosystems, is of particular concern as a readily absorbed neurotoxicant transported across the blood brain barrier and transplacentally. In the North Pacific Ocean, Steller sea lions are broadly distributed apex predators and, as such, integrate complex food webs and the associated exposure and possible adverse effects of toxic and infectious agents. Hair, including lanugo, was examined using regional and age groupings to assess mercury concentrations in young Alaskan Steller sea lions. The highest concentrations of mercury occurred in the youngest animals, likely via in utero exposure. Based on the adverse developmental outcomes of methylmercury toxicity this specific cohort is of concern. Regionally, higher concentrations of mercury were observed in the endangered western population of Steller sea lions and mirrored patterns observed in human biomonitoring studies of Alaskan coastal communities. These data have broader implications with respect to human and ecosystem health as Steller sea lions rely on similar prey species and foraging areas as those targeted by commercial fisheries and subsistence users and are therefore valuable sentinels of marine ecosystem health.     

Organochlorine exposure and health effects in stranded Steller sea lions (Eumetopias jubatus) and Pacific harbor seals (Phoca vitulina richardii) from Oregon and southern Washington coasts

Organochlorine (OC) pesticides and polychlorinated biphenyls (PCBs) have been detected in a variety of marine mammal species at levels associated with adverse health effects. Little is known about OC levels and impacts on health in pinnipeds with different life histories. We determined the health and levels of 18 OC pesticides and 16 PCB congeners in blubber samples from 20 Steller sea lions and 39 Pacific harbor seals stranded from Oregon and Southern Washington. The most commonly detected OC at the highest concentration was p,p′- dichlorodiphenyldichloroethylene (DDE). PCBs were detected in all samples as well. Hypothesis testing indicated that diseased Steller sea lions (males and females combined) had higher contaminant concentrations than healthy Steller sea lions, and diseased Pacific harbor seals had higher concentrations of total OCs than healthy animals. Differences were also noted between diseased and healthy animals when looking at individual sexes of each species. Diseased Steller sea lions had higher mean contaminant levels than diseased harbor seals and healthy Steller sea lions had higher mean contaminant concentrations than healthy Pacific harbor seals. These results show that species differences exist in both contaminant loads and sensitivity to contaminants, which may be due to differences in life histories and physiology.     

The energetic consequences of behavioral variation in a marine carnivore

Intraspecific variability in foraging behavior has been documented across a range of taxonomic groups, yet the energetic consequences of this variation are not well understood for many species. Understanding the effect of behavioral variation on energy expenditure and acquisition is particularly crucial for mammalian carnivores because they have high energy requirements that place considerable pressure on prey populations. To determine the influence of behavior on energy expenditure and balance, we combined simultaneous measurements of at‐sea field metabolic rate (FMR) and foraging behavior in a marine carnivore that exhibits intraspecific behavioral variation, the California sea lion (Zalophus californianus). Sea lions exhibited variability in at‐sea FMR, with some individuals expending energy at a maximum of twice the rate of others. This variation was in part attributable to differences in diving behavior that may have been reflective of diet; however, this was only true for sea lions using a foraging strategy consisting of epipelagic (<200 m within the water column) and benthic dives. In contrast, sea lions that used a deep‐diving foraging strategy all had similar values of at‐sea FMR that were unrelated to diving behavior. Energy intake did not differ between foraging strategies and was unrelated to energy expenditure. Our findings suggest that energy expenditure in California sea lions may be influenced by interactions between diet and oxygen conservation strategies. There were no apparent energetic trade‐offs between foraging strategies, although there was preliminary evidence that foraging strategies may differ in their variability in energy balance. The energetic consequences of behavioral variation may influence the reproductive success of female sea lions and result in differential impacts of individuals on prey populations. These findings highlight the importance of quantifying the relationships between energy expenditure and foraging behavior in other carnivores for studies addressing fundamental and applied physiological and ecological questions.     

Fecal steroid analysis for monitoring reproduction in the sun bear (Helarctos malayanus)

Fecal steroid analyses were conducted on captive (n = 10) and free-ranging (n = 2) sun bears (Helarctos malayanus) in order to establish a noninvasive technique for monitoring endocrine profiles during the estrous cycle and pregnancy. Secondly, the effect of the contraceptive porcine zona pellucida protein (PZP) on reproductive function was studied. Finally, we investigated whether the sun bear, naturally living in the aseasonal tropical forests of Southeast Asia, is a seasonal breeder. Fecal samples were collected over periods of 7-48 months in captive untreated (n = 8) and PZP-treated (n = 2) female sun bears. In addition samples were collected over a period of 12 months from radio-collared free-ranging females (n = 2) in their natural habitat in Indonesian Borneo. Androgens, precursors of estrogens, were found to be reliable indicators of the follicular phase, whereas estrogens were found unsuitable. Pregnanediol assay was found to be a reliable indicator of luteal function. Results indicate that sun bears are polyestrous, nonseasonal breeders. Interestrus intervals in nonpregnant animals (n = 2), which were monitored for 27 months, were between 140 and 216 days. Luteal phases (89.6 +/- 3.7 days; n = 9) were preceded by androgen peaks of 15.2 +/- 1.0 days (n = 10). Hormonal profiles of two females treated with PZP indicated missing ovarian activity in one, and persistent follicular and luteal activity in another animal. However, extended periods of missing ovarian, and persistent follicular and luteal activity were also observed in other sun bears studied.     

Immunocontraception of white-tailed deer using native and recombinant zona pellucida vaccines

We conducted a 2-year feasibility study with native porcine zona pellucida (PZP) vaccine and three recombinant rabbit zona pellucida vaccines (RC55, RC75a and a combination of RC55, RC75a and RC75b) as an initial phase of developing a recombinant immunocontraceptive vaccine to control reproduction in overpopulated herds of white-tailed deer (Odocoileus virginianus). Forty captive white-tailed does were divided into five groups (one sham and four treated), of eight each and injected with a 500microg prime dose of vaccine. Each prime dose was followed by a 300microg booster dose at 3-7 weeks post prime. The frequency and number of months of observed breeding were higher in PZP immunized does than in sham controls. Although the antibody titers of the three recombinant groups were 1000 or less, as compared with the PZP group with titers often over 128,000, the fawning rates of the two recombinants were significantly lower than that of the control group. The combined antigen group did not have a significantly lower fawning rate.     

Building Taxon Substitution Guidelines on a Biological Control Foundation

When a species becomes extinct, its ecological functions are lost as well. Taxon substitution is a controversial approach to restoring such functions via introduction of non‐native species known to serve similar functions elsewhere. Due to the possibility of nontarget effects from such introductions, taxon substitution has been proposed and implemented in only a few systems, but these attempts have successfully restored functions. As a conservation tool, taxon substitution bears similarity to biological control, wherein species are also introduced for their ecological function with consideration of potential nontarget effects. To improve both the safety and efficacy of taxon substitution, regulatory bodies that currently issue guidelines for biological control can do the same for taxon substitution. Indeed, many biological control guidelines would apply well to taxon substitution. We examine the standard practices followed by biological control programs and propose corresponding taxon substitution guidelines. Integration of taxon substitution into the existing national and international environmental management conversation will improve the tool and has the potential to enhance conservation efforts across a wide diversity of systems if appropriate and stringent precautions are taken.     

Prevalence and distribution of serum neutralizing antibodies to Tillamook (bovine) calicivirus in selected populations of marine mammals

Neutralizing antibodies to Tillamook calicivirus (TCV) were found in sera collected from California sea lions (Zalophus c. californianus Lesson) in 1983 and 1984 and in sera collected from Steller sea lions (Eumetopias jubatus Schreber) in 1976 and 1985. The combined prevalence of antibodies for these two species was 10/228 = 4.38%. Titers ranged from 1:20 (five animals), to 1:40 (four animals), to 1:80 (one animal) by standard microtiter neutralization assay. The seropositive pinnipeds were dispersed widely along the margins of the eastern Pacific rim, from the Bering Sea to the Santa Barbara Channel. Antibodies to TCV were not found in sera collected from northern fur seals (Callorhinus ursinus L.), Pacific walruses (Odobenus rosmarus divergens Illiger), seals of the family Phocidae, or several cetacean species. Tillamook calicivirus was isolated originally in 1981 from dairy calves in Oregon; the finding of neutralizing antibodies in two widely distributed species of sea lions suggests the possibility of a marine origin for this agent.     

Interannual variation in the at‐sea behavior of California sea lions (Zalophus californianus)

To be successful, marine predators must alter their foraging behavior in response to changes in their environment. To understand the impact and severity of environmental change on a population it is necessary to first describe typical foraging patterns and identify the underlying variability that exists in foraging behavior. Therefore, we characterized the at‐sea behavior of adult female California sea lions (n = 32) over three years (2003, 2004, and 2005) using satellite transmitters and time‐depth recorders and examined how foraging behavior varied among years. In all years, sea lions traveled on average 84.7 ± 11.1 km from the rookery during foraging trips that were 3.2 ± 0.3 d. Sea lions spent 42.7% ± 1.9% of their time at sea diving and displayed short (2.2 ± 0.2 min), shallow dives (58.5 ± 8.5 m). Among individuals, there was significant variation in both dive behavior and movement patterns, which was found in all years. Among years, differences were found in trip durations, distances traveled, and some dive variables (e.g., dive duration and bottom time) as sea lions faced moderate variability in their foraging habitat (increased sea‐surface temperatures, decreased upwelling, and potential decreased prey abundance). The flexibility we found in the foraging behavior of California sea lions may be a mechanism to cope with environmental variability among years and could be linked to the continuing growth of sea lion populations.     

Steller sea lion spatial‐use patterns derived from a Bayesian model of opportunistic observations

Despite acquisition of a substantial catalog of telemetry data from Steller sea lions (Eumetopias jubatus) over the past two decades, scientists still lack comprehensive regionally explicit knowledge about Steller sea lion habitat use. The Platforms of Opportunity data contain records of Steller sea lion sightings throughout the species’ entire range and have potential to fill gaps in knowledge about their spatial use; however, the data have not previously been used because effort (e.g., time spent surveying or area sampled) was not recorded when sightings were obtained. For this study a novel approach was used to overcome the lack of effort data through development of an effort index and a Bayesian negative binomial model. The model quantified Steller sea lion encounter rates and associated uncertainty within 15 × 15 km² grid cells across the species’ entire range. Year‐round, as well as breeding and nonbreeding season encounter rates were estimated. The results of this analysis identify several previously undocumented areas of high use by Steller sea lions, indicate that only 37% of Steller sea lion high‐use areas fall within designated critical habitat, and demonstrate that use of depth and distance from shore as indicators of Steller sea lion habitat is contraindicated.