[Show abstract][Hide abstract] ABSTRACT: The infection status of harbor seals Phoca vitulina in central California, USA, was evaluated through broad surveillance for pathogens in stranded and wild-caught animals from 2001 to 2008, with most samples collected in 2007 and 2008. Stranded animals from Mendocino County to San Luis Obispo County were sampled at a rehabilitation facility: The Marine Mammal Center (TMMC, n = 175); wild-caught animals were sampled at 2 locations: San Francisco Bay (SF, n = 78) and Tomales Bay (TB, n = 97), that differed in degree of urbanization. Low prevalences of Salmonella, Campylobacter, Giardia, and Cryptosporidium were detected in the feces of stranded and wild-caught seals. Clostridium perfringens and Escherichia coli were more prevalent in the feces of stranded (58% [78 out of 135] and 76% [102 out of 135]) than wild-caught (42% [45 out of 106] and 66% [68 out of 106]) seals, whereas Vibrio spp. were 16 times more likely to be cultured from the feces of seals from SF than TB or TMMC (p < 0.005). Brucella DNA was detected in 3.4% of dead stranded harbor seals (2 out of 58). Type A influenza was isolated from feces of 1 out of 96 wild-caught seals. Exposure to Toxoplasma gondii, Sarcocystis neurona, and type A influenza was only detected in the wild-caught harbor seals (post-weaning age classes), whereas antibody titers to Leptospira spp. were detected in stranded and wild-caught seals. No stranded (n = 109) or wild-caught (n = 217) harbor seals had antibodies to phocine distemper virus, although a single low titer to canine distemper virus was detected. These results highlight the role of harbor seals as sentinel species for zoonotic and terrestrial pathogens in the marine environment.
Diseases of Aquatic Organisms 09/2014; 111(2):93-106. · 1.73 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We measured total selenium and total mercury concentrations ([TSe] and [THg]) in hair (n=138) and blood (n=73) of harbor seals (Phoca vitulina) from California to assess variation by geography and sex, and inferred feeding relationships based on carbon, nitrogen, and sulfur stable isotopes. Harbor seals from Hg-contaminated sites had significantly greater [THg], and lesser [TSe] and TSe:THg molar ratios than seals from a relatively uncontaminated site. Males had significantly greater [THg] than females at all locations. Sulfur stable isotope values explained approximately 25% of the variability in [THg], indicating increased Hg exposure for seals with a greater use of estuarine prey species. Decreased [TSe] in harbor seals from Hg-contaminated regions may indicate a relative Se deficiency to mitigate the toxic effects of Hg. Further investigation into the Se status and the potential negative impact of Hg on harbor seals from Hg-contaminated sites is warranted.
[Show abstract][Hide abstract] ABSTRACT: On 16 June 1979, a herd of 41 sperm whales stranded near the mouth of the Siuslaw River in Florence, Oregon. The stomach contents from 32 whales were collected, identified to the lowest taxonomic level possible, enumerated, and measured. A total of 20,247 cephalopod lower beaks that represented 24 species from 14 different families were recovered. The most numerous species were Histioteuthis hoylei (25.9%), Taonius borealis (12.9%), Galiteuthis phyllura (11.2%), Gonatopsis/Berryteuthis type (10.9%), and Moroteuthis robusta (10.7%). Reconstructed estimates of mass indicated that M. robusta contributed almost 50% of the total mass of cephalopods consumed, followed by H. hoylei (19.3%), and T. borealis (7.0%). The most important species in the diet of stranded whales were M. robusta, H. hoylei, T. borealis, G. phyllura, Octopoteuthis deletron, and Gonatopsis/Berryteuthis type. There were significant differences in the diet of males and females, but no differences between sperm whales of different age groups. Overall, sperm whales primarily consumed small cephalopods that were likely eaten south of 45ºN in or near the California Current System. This study provides new estimates of the food habits of sperm whales in the northeast Pacific from one of the largest strandings of this species.
[Show abstract][Hide abstract] ABSTRACT: Marine plastic pollution affects seabirds, including Pacific Northern Fulmars (Fulmarus glacialis rodgersii), that feed at the surface and mistake plastic for prey or incidentally ingest it. Direct and indirect health issues can result, including satiety and possibly leading to inefficient foraging. Our objective was to examine fulmar body condition, identify cephalopod diet to species, enumerate and weigh ingested plastic, and determine if prey number and size were correlated with ingested plastics in beach-cast fulmars wintering in Monterey Bay California (2003, n = 178: 2007, n = 185). Fulmars consumed mostly Gonatus pyros, G. onyx, and G. californiensis of similar size for both years. We found a significant negative correlation between pectoral muscle index and average size of cephalopod beaks per stomach; a significant increase in plastic categories between 2003 and 2007; and no significant correlation between number and mass of plastic compared with number and size of prey for either year.
[Show abstract][Hide abstract] ABSTRACT: Skin biopsies were collected from free-ranging harbor seals (Phoca vitulina richardii) from central California (n = 53). Microscopic examination of hematoxylin and eosin-stained tissue sections revealed the presence of tightly coiled nematode larvae within the ostia of numerous hair follicles of four seals. Parasites were characterized by paired lateral alae, platymyarian musculature, and an indistinct, uninucleate digestive tract. Mild chronic superficial dermatitis and perifolliculitis were evident microscopically in association with the intrafollicular parasites. Histomorphologic features of the larvae and their presence within hair follicles are consistent with previous reports of the facultative nematode parasite Pelodera strongyloides. This is the first published report of P. strongyloides infection in any marine mammal. This parasite may be acquired by marine mammals through close contact with soil or decaying organic material and should be considered as a potential differential diagnosis for dermatitis in marine mammals that use terrestrial resting sites.
Journal of Zoo and Wildlife Medicine 09/2013; 44(3):799-802. · 0.43 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Given their coastal site fidelity and opportunistic foraging behavior, harbor seals (Phoca vitulina) may serve as sentinels for coastal ecosystem health. Seals using urbanized coastal habitat can acquire enteric bacteria, including Vibrio that may affect their health. To understand Vibrio dynamics in seals, demographic and environmental factors were tested for predicting potentially virulent Vibrio in free-ranging and stranded Pacific harbor seals (Phoca vitulina richardii) off California. Vibrio prevalence did not vary with season and was greater in free-ranging seals (29 %, n = 319) compared with stranded seals (17 %, n = 189). Of the factors tested, location, turbidity, and/or salinity best predicted Vibrio prevalence in free-ranging seals. The relationship of environmental factors with Vibrio prevalence differed by location and may be related to oceanographic or terrestrial contributions to water quality. Vibrio parahaemolyticus, Vibrio alginolyticus, and Vibrio cholerae were observed in seals, with V. cholerae found almost exclusively in stranded pups and yearlings. Additionally, virulence genes (trh and tdh) were detected in V. parahaemolyticus isolates. Vibrio cholerae isolates lacked targeted virulence genes, but were hemolytic. Three out of four stranded pups with V. parahaemolyticus (trh+ and/or tdh+) died in rehabilitation, but the role of Vibrio in causing mortality is unclear, and Vibrio expression of virulence genes should be investigated. Considering that humans share the environment and food resources with seals, potentially virulent Vibrio observed in seals also may be of concern to human health.
[Show abstract][Hide abstract] ABSTRACT: Domoic acid (DA) is a potent neurotoxin that has caused strandings and mortality of seabirds and marine mammals off the California coast. Pacific harbor seals (Phoca vitulina richardii) are an abundant, nearshore species in California; however, DA exposure and toxicosis have not been documented for harbor seals in this region. To investigate DA exposure in harbor seals, samples were collected from free-ranging and stranded seals off California to assess exposure, clinical signs of toxicosis, and brain lesions in harbor seals exposed to DA. Domoic acid was detected in 65% (17/26) of urine samples collected from apparently healthy free-ranging seals, with concentrations of 0.4–11.7 ng/ml. Domoic acid also was detected in feces (2.4–2887 ng/g), stomach contents (1.4 ng/g; stranded only), milk (2.2 ng/ml; stranded only), amniotic fluid (9.7 ng/ml; free-ranging only), fetal meconium (14.6–39.8 ng/g), and fetal urine (2.0–10.2 ng/ml). Clinical signs indicative of DA toxicosis were observed in two live-stranded seals, and included disorientation, seizures, and uncoordinated movements. Histopathology revealed the presence of brain lesions consistent with DA toxicosis in two live-stranded seals, and one free-ranging seal that died during capture. Results indicated that harbor seals were exposed to DA, exhibited clinical signs and histological lesions associated with DA exposure, and that pups were exposed to DA in utero and during lactation via milk. Future investigation is required to determine the magnitude of impact that DA has on the health and mortality of harbor seals.
[Show abstract][Hide abstract] ABSTRACT: We assessed temporal and spatial patterns of chronic oiling of seabirds in California during 2005–2010, using data on: (1) live oiled birds reported to the Oiled Wildlife Care Network (OWCN) from throughout the state, and (2) dead oiled birds found during systematic monthly beached-bird surveys in central California. A mean of 245 (±141 SD) live miscellaneous oiled birds (not associated with known oil spills) were reported to the OWCN per year, and 0.1 oiled dead birds km−1 per month were found on beach surveys in central California. Chemical fingerprinting of oiled feathers from a subset of these birds (n = 101) indicated that 89% of samples tested were likely from natural petroleum seeps off southern and central California. There was a pronounced peak during late winter in the number of oiled birds reported in southern California, which we theorize may be related to large storm waves disturbing underwater seeps.
[Show abstract][Hide abstract] ABSTRACT: Avian predation on juvenile salmonids is an important source of mortality in freshwater and estuarine habitats when birds and salmonids overlap spatially and temporally. We assessed the potential impact of avian predation upon juvenile steelhead Oncorhynchus mykiss in a coastal watershed in central California. We conducted stream surveys between 2008 and 2010 to determine the composition, distribution, and density of piscivorous birds in areas that provide rearing habitat for juvenile steelhead. The most commonly sighted bird species were common mergansers Mergus merganser and belted kingfishers Megacyrle alcyon. The density of avian predators varied spatially and temporally but was greatest in the estuary regardless of season and decreased with increasing distance from the estuary. In the absence of local predator diet data, we applied a bioenergetics model to estimate the potential predation on juvenile steelhead by mergansers and kingfishers in the Scott Creek estuary. Model parameters included (1) published values of bird energetic requirements and steelhead energy density, (2) the number of birds present in the estuary during the closure period (from stream surveys), and (3) the size frequency and abundance of steelhead present in the estuary during closure. We predicted the extent of predation for different values of steelhead in bird diets, accounting for uncertainty in the estimates using a Monte Carlo simulation approach. With the assumed contribution of steelhead to the diet ranging from 20% to 100%, the population of kingfishers foraging in the Scott Creek estuary had the potential to remove 3–17% of annual production, whereas mergansers had the potential to remove 5–54% of annual steelhead production. Our results suggest that predation by avian species, particularly mergansers, is an important source of mortality for threatened steelhead populations in central California and should be addressed in future salmonid research and recovery planning.Received February 13, 2013; accepted June 4, 2013
North American Journal of Fisheries Management 01/2013; 33(5):1024-1038. · 1.18 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Hair is used to determine trace elements exposure and status of pinnipeds because it is an excretory route for many elements and can be collected non-lethally. Despite increased use, there have been few studies on how sampling designs and procedures (e.g., hair type, collection site) affect results. The objective of this study was to determine whether concentrations of an essential (selenium; Se) and non-essential element (mercury; Hg) differed between hair samples collected from two body locations on harbor seals (Phoca vitulina). Concentrations of Se and total Hg (THg) differed between mid-dorsal midline and neck samples, and although the absolute differences were relatively small (Δ(absolute) Se=0.69μgg(-1), Δ(absolute) THg=2.86μgg(-1)), the relative differences were large (Δ(relative) Se=49%, Δ(relative) THg=17%). These differences highlight the need to standardize the collection site for trace element determination in pinnipeds.
[Show abstract][Hide abstract] ABSTRACT: Non-breeding sooty shearwaters are the most abundant seabird in the California Current Large Marine Ecosystem (CCLME) during boreal spring and summer months. This, combined with relatively great energy demands, reliance on patchy, shoaling prey (krill, squid, and forage fishes), and unconstrained mobility free from central-place-foraging demands—make shearwaters useful indicators of ecosystem variability. During 2008 and 2009, we used satellite telemetry to evaluate shearwater ranging patterns throughout the CCLME and specifically within the US Exclusive Economic Zone (EEZ) among birds captured at three locations: Columbia River Plume, WA; Monterey Bay, CA; and Santa Barbara Channel, CA. Shearwaters ranged throughout the entire CCLME from southeast Alaska to southern Baja California, Mexico. Within the EEZ during 2008 and 2009, shearwaters spent 68% and 46% of time over the shelf (<200 m), 27% and 43% of time over the slope (200–1000 m), and 5% and 11% of time over the continental rise and abyssal regions (>1000 m), respectively. In 2008 and 2009, shearwaters spent 22% and 25% of their time in the EEZ within the five west coast National Marine Sanctuaries, respectively; high utilization occurred in non-sanctuary waters of the EEZ. Shearwater utilization distribution (based on the Brownian-bridge movement model) among sanctuaries was disproportionate according to sanctuary availability (based on area) within the EEZ. Shearwaters utilized the Monterey Bay sanctuary (2008, 2009) and the Channel Islands sanctuary (2009) disproportionately more than other sanctuaries. Although all five sanctuaries were used by shearwaters, waters outside sanctuary zones appeared significantly more important and likely supported large aggregations of shearwaters. Utilization distributions among individual birds from three discrete capture locations were variable and revealed greater similarity in space-use sharing within capture-location groupings and during 2008 when shearwaters were more aggregated than in 2009. We identified several regional “habitat hotspot” areas, including the Columbia River Plume, Cape Blanco, Monterey Bay, Estero/San Luis Obispo Bays, and the eastern Santa Barbara Channel through the inner Southern California Bight.
[Show abstract][Hide abstract] ABSTRACT: Many populations of coho salmon Oncorhynchus kisutch and steelhead O. mykiss are listed under the U.S. Endangered Species Act. Until recently, the role of avian predation in limiting recovery of coho salmon and steelhead in central California coastal watersheds has been overlooked. We used recoveries of passive integrated transponder (PIT) tags from Año Nuevo Island (ANI), a breeding site for several species of piscivorous seabirds, to estimate predation rates on juvenile salmonids and identify susceptible life stages and species responsible for predation. A total of 34,485 PIT tags were deployed in coho salmon and steelhead in six watersheds in San Mateo and Santa Cruz counties. Tags were deposited on ANI by predators after ingestion of tagged fish. Because tags were not removed from the island and were detected on multiple sampling occasions, we were able to use mark–recapture models to generate a corrected minimum predation estimate. We used POPAN, a variation of the Jolly–Seber model, to generate an estimate of gross population abundance, which accounted for tags deposited on the island but not detected during surveys. Detections of 196 tags from surveys conducted between autumn 2006 and spring 2009 were incorporated into the model, producing a gross population estimate of 242 tags (SE = 9.8). Addition of tags detected between autumn 2009 and 2010 to the abundance estimate from POPAN produced a new minimum estimate of 362 tags on ANI. Western gulls Larus occidentalis probably were the primary predator depositing tags on ANI. Minimum predation estimates ranged from 0.1% (Soquel Creek) to 4.6% (Waddell Creek) of outmigrating coho salmon and steelhead smolts. Predation was potentially greater given still unquantified deposition of tags off-colony and destruction of tags during digestive processes of predators. Finally, avian predators targeted estuary-reared fish, which contributed disproportionately to adult populations, further impacting imperiled salmon populations.Received August 4, 2011; accepted August 27, 2012
North American Journal of Fisheries Management 01/2012; 32(6). · 1.18 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Persistent organic pollutants have been associated with disease susceptibility and decreased immunity in marine mammals. Concentrations of polychlorinated biphenyls (PCBs), dichlorodiphenyltrichloroethane and its metabolites (DDTs), polybrominated diphenyl ethers (PBDEs), chlordanes (CHLDs), and hexachlorocyclohexane isomers (HCHs) were evaluated in terms of stage of development and likely exposure routes (in utero, suckling, fasting) in the blubber of 202 stranded and wild-caught, primarily young of the year (n=177), harbor seals (Phoca vitulina) in the central California coast. This is the first report of HCH concentrations in the blubber of California seals. Lipid normalized concentrations ranged from 200 to 330,000 ng/g for sum PCBs, 320-1,500,000 ng/g for sum DDTs, 23-63,000 ng/g for sum PBDEs, 29-29,000 ng/g for sum CHLDs, and 2-780 ng/g for sum HCHs. The highest concentrations were observed in harbor seal pups that suckled in the wild and then lost mass during the post-weaning fast. Among the pups sampled in the wild and those released from rehabilitation, there were no differences in mass, blubber depth, or percent lipid although contaminant concentrations were significantly higher in the pups which nursed in the wild. When geographic differences were evaluated in a subset of newborn animals collected near their birth locations, the ratio of sum DDTs to sum PCBs was significantly greater in samples from an area with agricultural inputs (Monterey), than one with industrial inputs (San Francisco Bay). A principal components analysis distinguished between seals from San Francisco Bay and Monterey Bay based on specific PCB and PBDE congeners and DDT metabolites. These data illustrate the important influence of life stage, nutritional status, and location on blubber contaminant levels, and thus the need to consider these factors when interpreting single sample measurements in marine mammals.
Science of The Total Environment 06/2011; 409(18):3537-47. · 3.16 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Counts of pinnipeds provide a minimal estimate of population size because some unknown proportion of individuals is in the water during surveys. We determined a correction factor (CF) for Pacific harbor seals (Phoca vitulina richardii) by estimating the proportion ashore of 180 seals tagged with flipper-mounted radio tags throughout California. The mean proportions of tagged individuals ashore during four complete surveys in 2004 were not different between central and northern California (F= 1.85, P= 0.18) or between sexes (F= 0.57, P= 0.45), but a lesser proportion of weaners was ashore than subadults or adults (F= 7.97, P= 0.001), especially in northern California. The CF calculated for the statewide census of harbor seals was 1.65, using transmitters operating during the survey (n= 114). Using a mark-recapture estimator for tag survival (phi) and the four telemetry surveys the mean CF for central and northern California was 1.54 ± 0.38 (95% CI). A CF for southern California of 2.86 was based on a single survey. Using the mean CF of 1.54 and a statewide count in 2009 we estimated 30,196 (95% CI = 22,745–37,647) harbor seals in California.
[Show abstract][Hide abstract] ABSTRACT: Stable carbon and nitrogen isotope ratios (δ13C and δ15N) of serum, red blood cells (RBC), muscle, and blubber were measured in captive and wild northeast Pacific harbor seals (Phoca vitulina richardii) at three coastal California sites (San Francisco Bay, Tomales Bay, and Channel Islands). Trophic discrimination factors (ΔTissue-Diet) were calculated for captive seals and then applied in wild counterparts in each habitat to estimate trophic position and feeding behavior. Trophic discrimination factors for δ15N of serum (+3.8‰), lipid-extracted muscle (+1.6‰), and lipid-blubber (+6.5‰) are proposed to determine trophic position. An offset between RBC and serum of +0.3‰ for δ13C and −0.6‰ for δ15N was observed, which is consistent with previous research. Specifically, weaner seals (<1 yr) had large offsets, suggesting strong trophic position shifts during this life stage. Isotopic values indicated an average trophic position of 3.6 at both San Francisco Bay and Tomales Bay and 4.2 at Channel Islands. Isotopic means were strongly dependent on age class and also suggested that mean diet composition varies considerably between all locations. Together, these data indicate that isotopic composition of blood fractions can be an effective approach to estimate trophic position and dietary behavior in wild pinnipeds.
[Show abstract][Hide abstract] ABSTRACT: Nineteen occurrences of interspecific sexual behavior between male southern sea otters (Enhydra lutris nereis) and juvenile Pacific harbor seals (Phoca vitulina richardsi) were reported in Monterey Bay, California, between 2000 and 2002. At least three different male sea otters were observed harassing, dragging, guarding, and copu-lating with harbor seals for up to 7 d postmortem. Carcasses of 15 juvenile harbor seals were recov-ered, and seven were necropsied in detail by a vet-erinary pathologist. Necropsy findings from two female sea otters that were recovered dead from male sea otters exhibiting similar behavior are also presented to facilitate a comparison of lesions. The most frequent lesions included superficial skin lacerations; hemorrhage around the nose, eyes, flippers, and perineum; and traumatic corneal ero-sions or ulcers. The harbor seals sustained severe genital trauma, ranging from vaginal perforation to vagino-cervical transection, and colorectal per-forations as a result of penile penetration. One harbor seal developed severe pneumoperitoneum subsequent to vaginal perforation, which was also observed in both female sea otters and has been reported as a postcoital lesion in humans. This study represents the first description of lesions resulting from forced copulation of harbor seals by sea otters and is also the first report of pneu-moperitoneum secondary to forced copulation in a nonhuman animal. Possible explanations for this behavior are discussed in the context of sea otter biology and population demographics.
[Show abstract][Hide abstract] ABSTRACT: Body size is an important determinant of the diving and foraging ability in air-breathing marine vertebrate predators. Satellite-linked dive recorders were used during 2003-2004 to investigate the foraging behavior of 22 male California sea lions (Zalophus californianus, a large, sexually dimorphic otariid) and to evaluate the extent to which body size explained variation among individuals and foraging strategies. Multivariate analyses were used to reduce the number of behavioral variables used to characterize foraging strategies (principal component analysis, PCA), to identify individually based foraging strategies in multidimensional space (hierarchical cluster analysis), and to classify each individual into a cluster or foraging strategy (discriminant analysis). Approximately 81.1% of the variation in diving behavior among individuals was explained by three factors: diving patterns (PC1), foraging effort (PC2), and behavior at the surface (PC3). Individuals were classified into three distinct groups based on their diving behavior (shallow, mixed depth, and deeper divers), and jackknife resampling of the data resulted in correct group assignment 86% of the time. Body size as an independent variable was positively related to dive duration and time spent ashore and negatively related to time at sea, and it was a key parameter in PC2 used to classify the three distinct clusters. Differences among individual-based foraging strategies probably were driven by differences in body size, which enabled larger animals to dive deeper and forage more efficiently by targeting different and perhaps larger prey items. The occurrence of foraging specializations within a species and age class has implications for quantitative modeling of population-level predator-prey interactions and ecosystem structure.
[Show abstract][Hide abstract] ABSTRACT: Domoic acid (DA) is a neuroexcitatory toxin increasingly causing strandings and mortality of marine mammals. The hippocampus of mammalian brains, associated with learning, memory, and spatial navigation, is one of the predominant regions affected by DA exposure. California sea lions stranding from 2003 to 2006 as a result of DA toxicosis were classified as having acute (n= 12) or chronic neurologic (n= 22) clinical signs. Chronic neurologic cases were examined by magnetic resonance imaging to determine the extent of brain damage related to DA exposure. Brain damage included hippocampal and parahippocampal atrophy, temporal horn enlargement, and pathological T2 hyperintensity. Posttreatment, animals were fitted with satellite transmitters and their movement and dive behaviors compared with those of a control group. The only significant difference between acute and chronic animals was distance traveled per day. There were, however, significant differences between chronic neurologic cases and controls: chronic neurologic cases dove shallower for shorter durations, traveled further from shore, and spent less time hauled out and more time surface swimming than control animals. There was no relationship between severity of brain damage and behavioral patterns for chronic neurologic cases. Sea lions with chronic neurologic changes had a poor prognosis for survival following release.
[Show abstract][Hide abstract] ABSTRACT: Genetic assignment methods provide an appealing approach for characterizing dispersal patterns on ecological time scales, but require sufficient genetic differentiation to accurately identify migrants and a large enough sample size of migrants to, for example, compare dispersal between sexes or age classes. We demonstrate that assignment methods can be rigorously used to characterize dispersal patterns in a marbled murrelet (Brachyramphus marmoratus) population from central California that numbers approximately 600 individuals and is only moderately differentiated (F(ST) approximately 0.03) from larger populations to the north. We used coalescent simulations to select a significance level that resulted in a low and approximately equal expected number of type I and II errors and then used this significance level to identify a population of origin for 589 individuals genotyped at 13 microsatellite loci. The proportion of migrants in central California was greatest during winter when 83% of individuals were classified as migrants compared to lower proportions during the breeding (6%) and post-breeding (8%) seasons. Dispersal was also biased toward young and female individuals, as is typical in birds. Migrants were rarely members of parent-offspring pairs, suggesting that they contributed few young to the central California population. A greater number of migrants than expected under equilibrium conditions, a lack of individuals with mixed ancestry, and a small number of potential source populations (two), likely allowed us to use assignment methods to rigorously characterize dispersal patterns for a population that was larger and less differentiated than typically thought required for the identification of migrants.