Grey Seals Halichoerus Grypus Research Articles

Sex differences in grey seal diet reflect seasonal variation in foraging behaviour and reproductive expenditure: evidence from quantitative fatty acid signature analysis

1. Intraspecific variation in diet can be an important component of a species niche breadth. We tested the hypothesis that sex differences in seasonal foraging behaviour and energy storage of sexually size dimorphic grey seals Halichoerus grypus (Fabrisius 1971) are reflected in differences in the diet and niche breadth. Diet composition was estimated for 496 adult (226 males, 270 females) and 91 juvenile (46 males/45 females; all 6 months old) grey seals sampled between 1993 and 2000 using quantitative fatty acid signature analysis. Niche breadth and overlap were estimated using the Shannon-Weaver diversity index (H') and the Morisita-Horn index (C(H)), respectively. 2. Sand lance Ammodytes dubius (Reinhardt 1837) and redfish Sebastes sp. (Cuvier 1829) accounted for a high proportion of the diet in both sexes and age groups. However, the diets of adult males were significantly more diverse across all seasons (H': males 0.36 +/- 0.007 vs. females 0.28 +/- 0.007) and less energy dense in spring (male 5.3 +/- 0.07 kJ g(-1) vs. females 5.6 +/- 0.09 kJ g(-1)) than those of adult females. 3. Season and sex explained most of the observed variation in adult diets, but there were significant sex-season interactions. These differences were most evident during the post-breeding (spring) foraging period when energy acquisition is important to female recovery of nutrient stores needed to support pregnancy. Females selected fewer and higher quality prey species in spring than males. 4. There were no sex differences in the diets of juvenile grey seals. Although many of the species overlapped with those eaten by adults, juvenile niche breadth (H': 0.41 +/- 0.014, n = 91) was significantly broader than that of adults (H': 0.30 +/- 0.011, n = 115). Juvenile diets were also of lower energy density (5.3 +/- 0.04 kJ g(-1)) than those of adults (5.6 +/- 0.09 kJ g(-1)), suggesting less selectivity in these young and relatively naïve predators. 5. Sex-specific seasonal changes in diet correspond to seasonal changes in diving behaviour and rate of body energy accumulation of adult males and females. Sex-specific reproductive requirements appear to be a primary factor generating the intraspecific variation in the seasonal foraging ecology of this large marine carnivore. However, sex differences in the breadth and energy content of diets also suggest the influence of body-size dimorphism as a factor shaping the diet of this species.

Recoveries of grey seals (<i>Halichoerus grypus</i>) tagged on the Murman coast in Russia

A total of 612 grey seal (Halichoerus grypus) pups were tagged on the Murman coast, Russia, in the years 1991, 1992 and 1994. A majority of them (542) were tagged at the Ainov archipelago. Thirty-one (2.9%) of the tagged seals have been recovered, mainly (77%) as bycatches in fishing gear. The majority of recaptures (74%) were made during the first year after tagging, while the oldest recaptured seal was 10 years old. Only 1 recapture, at Ainov, was made in the area were the seals had been tagged, otherwise distances between tagging and recaptures varied from less than 100 km to more than 1,000 km. All recaptures were made north of 68°N.

Evidence that grey seals (<i>Halichoerus grypus</i>) use above-water vision to locate baited buoys

Fishing gear in the Baltic is often raided by grey seals (Halichoerus grypus). The seals remove the fish and damage the nets, or entangle themselves and drown. In order to develop ways of mitigating the seals-fisheries conflict, it is important to know exactly how the seals locate the fishing gear. A field experiment was conducted in order to clarify whether seals use their vision above water to do this. Bait (herring; Clupea harengus) was attached to the anchor lines of buoys of the type that is commonly used to mark the position of fishing gear. In all, 643 buoys were set. Some of the buoys (210) were also fitted with camera traps. Weather data were collected from official weather stations nearby. Bait loss (mean 18%) was significantly correlated with buoy size (P = 0.002) and wind speed (P = 0.04). There was a significant association between bait loss and seal observations near the buoys (P = 0.05). Five photos of grey seals were obtained from the camera traps. No fish-eating birds, such as cormorants or mergansers, were ever observed near the buoys or caught on camera. It was concluded that a main cause of missing bait was scavenging by grey seals, and that they did use above-water vision to locate the buoys. It was also concluded that wind strength (i.e. wave action) contributed tothe bait loss. The camera trap buoys had a somewhat lower bait loss than the other buoys (P = 0.054), which was attributed to a scaring effect. Neither the number of seal observations nor the bait loss differed significantly between the 2 study areas in the experiment (P = 0.43 and P = 0.83, respectively). Bait loss was not affected by the buoy colour (red, white, or grey; P = 0.87). We suggest that the findings of this experiment could be put into practice in a seal-disturbed area by deploying a number of decoy buoys, or by hiding live buoys below the surface of the water. This would increase the cost of foraging for the seals, and hence discourage them from exploiting fishing gear as a feeding place.

Grey seals on the Murman coast, Russia: status and present knowledge

Grey seals (Halichoerus grypus) are distributed along the entire northern Murman coast in Russia. Breeding sites are located mainly on the Ainov and Seven islands, which belong to the Kandalaksha Nature Reserve. The annual pup production was estimated to be around 800 pups in the early 1990s, and the pup mortality has been observed to be relatively high. The population was estimated to be approximately 3,500 individuals in 1994. Grey seals migrate in small numbers into the White Sea during summer. The grey seal is protected and registered in the Red Books of Russia, Murmansk region and Fennoscandia. The main results of grey seals investigations from 1986 to 2000 are briefly reviewed. There are no recent studies on abundance, seasonal distribution, growth, moulting and feeding of the species.

Present knowledge of grey seals (<i>Halichoerus grypus</i>) in Faroese waters

The grey seal (Halichoerus grypus) is the only pinniped species breeding in the Faroe Islands. They are present all year round, and can be observed throughout the archipelago, but prefer to reside in exposed coastal areas, away from human settlements. Grey seals breed primarily in caves, common along the Faroese coastline. Systematic scientific investigations have not been conducted on grey seals in the Faroes. Present population level is presumably around 1,000 to 2,000 animals. These seals have probably been hunted since the Norse settlement in the 8th century. Apparently, this removal has prevented the population from increasing above a certain threshold. Tagging studies have demonstrated a connection with seals from UK waters,but the magnitude and influence of this movement is not known. Present biological knowledge is very limited and insufficient; this is due largely to the inaccessible nature of these seals.

Seasonal and interannual variability in grey seal diets on Sable Island, eastern Scotian Shelf

We studied seasonal and interannual variability in the diet of grey seals (Halichoerus grypus) using faecal samples collected from Sable Island, Nova Scotia between 1991 and 1998. More than 28,000 prey from at least 28 taxa were identified from 1,245 faecal samples collect mainly in spring, fall and winter. Sand lance (Ammodytes dubius) dominated the diet in all seasons and years, but the importance of this and other species varied over time. There was also evidence of seasonal and interannual variation in the size of prey consumed both within and among species. We compared diet composition with estimates of prey numbers and biomass from annual researchtrawl surveys conducted in March and July. Species-specific numerical corrections were applied to otolith counts to account for the complete digestion of otoliths, and fish catchability correction factors applied to trawl survey catches to account for trawl selectivity. Based on an odds ratio index of prey selectivity, grey seals positively selected sand lance in both seasons. Other species were either relatively avoided or eaten roughly in proportion to their estimated abundance.

Abundance of grey seals in Icelandic waters, based on trends of pup-counts from aerial surveys

Grey seals (Halichoerus grypus Fabricius, 1791) are distributed all around the Icelandic coast. The majority of the population breeds on the west- and northwest shores, with a second high density in the breeding distribution on the southeast coast of Iceland. During the last 5 decades the Icelandic grey seals have dispersed from the west- to the northwest-, the north- and the northeast-coast. The breeding period occurs from the middle of September to early November, with a maximum in mid October. The time of peak pupping shows some variation, beginning earlier along the west coast and later in the north and southeast. Seven aerial surveys to estimate pup production in Iceland were flown during October to November during the period from 1980 to 2004. Pup counts of the Icelandic grey seal, at all breeding sites combined, have been decreasing annually by about 3% (±1% s.e.), during the period 1982-2002. During the period 1990-2002, this downward trend doubled to about 6% annually. The abundance of the grey seal around Iceland in the year 2002 was estimated to be 4,100 to 5,900 animals. This is higher than estimates of around 2,000 animals during the 1960s, but much less than the estimated population of 8,000 to 11,500 in 1982.

DNA‐based identification of salmonid prey species in seal faeces

Assessment of pinniped predation most often relies on analysis of the hard, undigested prey remains evident in faecal (scat) samples. For many prey species this method can yield valuable information on predator–prey interactions. For some genera, however, species diagnostic characteristics are lost during the process of prey digestion, thereby preventing morphological identification of fish prey species. Here, the feasibility of using faecal DNA to detect the presence of salmonids in pinniped scat samples and to distinguish reliably between sea trout Salmo trutta and Atlantic salmon S. salar was assessed. Novel salmonid mitochondrial DNA (mtDNA) primers were designed to amplify 162 bp of the 16S rDNA and a 327 bp section of the cytochrome b gene. Species-specific banding patterns were obtained by digestion of the cytochrome b PCR product with the restriction endonuclease AluI, and confirmed by the species-specific amplification of the 16S rDNA fragment from Atlantic salmon. Scats collected from captive grey seals Halichoerus grypus fed on known monospecific diets used to validate the PCR-RFLP assay indicated a probability of at least 95.8% (23 of 24 faecal extracts) of detecting salmonids using DNA extracted from the scat matrix. Implemented alongside conventional prey remains analyses, this technique presents a promising new method for examining prey composition and assessing pinniped predation on salmonids.

Foetal growth in north‐west Atlantic grey seals ( Halichoerus grypus )

The timing of implantation, duration of gestation and energetic costs associated with foetal development were examined in the north-west Atlantic grey seal Halichoerus grypus. Implantation occurs between 5 and 8 May. Active gestation lasts for 259–272 days and the duration of suspended development is 78–91 days. The duration of active gestation was slightly shorter among north-east Atlantic grey seals (238–255 days), while suspended development was longer (95–112 days). The rate of foetal growth increased in August, 3 months after attachment, until mid December when it began to slow once again, c. 1 month before parturition. During August, the placenta to foetus mass ratio was about 0.3, but declined to around 0.07 near birth. Foetal energy density increased rapidly reaching asymptotic levels of 22.6 kJ g−1 (dry weight) 3–4 months after implantation. Energy density of the placenta changed only slightly throughout gestation increasing from c. 21.7 to 23 kJ g−1 (dry weight) by the autumn. Energy costs associated with foetal growth are c. 648 MJ.

Photo-identification, Site Fidelity, and Movement of Female Gray Seals (Halichoerus grypus) Between Haul-outs in the Baltic Sea

The growing gray seal population in the Baltic Sea has led to increased conflicts with fisheries. Despite limited data on gray seal ecology, management measures, such as culling, have been implemented recently. We studied movements and site fidelity of Baltic gray seals using mark-recapture analysis based on photographic identification of individuals (photo-id). Seals were photographed at the major summer haul-out sites. Profile photographs of the head and neck were matched using purpose-written software to generate a database of capture histories from 1995-2000. The haul-outs were grouped into seven areas. Darroch's method (20) for a two-sample capture-recapture census was adapted to estimate rates of movement between the areas. The majority of seals were estimated to remain within the same area, suggesting that Baltic gray seals exhibit a high degree of site fidelity during the summer, and that fidelity to a site lasts for more than one season.

Morbillivirus neutralising antibodies in Scottish grey seals Halichoerus grypus: assessing the effects of the 1988 and 2002 PDV epizootics

Phocine distemper virus (PDV) may have killed a small number of grey seals Hali- choerus grypus in European waters in 2002, as it was thought to have done in 1988. PDV is known to affect harbour seal population and distribution patterns, but grey seal pup production trends did not correlate consistently with PDV outbreaks. Numbers of known mothers missing from study colonies do not increase in PDV years and pre-weaning pup mortality is similar to that in other years. Pup growth rates are similar in PDV and non-PDV years. Therefore, no direct evidence links PDV out- breaks to changes in grey seal reproductive parameters at the population, colony, or individual level. Investigation of exposure of grey seals to PDV used CDV virus-neutralisation tests on sera collected from breeding seals pre-1988, 1988, 2001 and 2002. No positive sera (≥titre 1:64) were detected prior to 1988. In 1988, 2001 and 2002 the prevalence was 96, 59 and 83%, respectively. In 2001, prevalence in 'old' mothers (of breeding age by 1988, ≥13 yr old) was 63% compared to 0% in 'young' mothers (born after 1989, <13 yr old). In 2002, prevalence was 88 and 93% in old and young mothers, respec- tively. Many pups were seropositive by the end of lactation (North Rona = 36%, Isle of May = 54%). Mean log10 antibody titres increased during lactation and were correlated with the mothers' titre. The high prevalence of seropositives in both adult age groups from geographically separated colonies suggests widespread exposure to a morbillivirus after 2001. This implies that PDV is highly infective and that contact was widespread before the 2002 breeding season.

Trace element bioaccumulation in grey seals Halichoerus grypus from the Faroe Islands

Grey seals Halichoerus grypus were sampled in the Faroe Islands archipelago during the summers of 1993 to 1995. Concentrations of Cd, Cu, Hg, Se and Zn were measured in the liver, kidney and muscle of 68 seals. All elements except Zn exhibited lowest concentrations in the muscle. The liver contained the highest concentrations of Cu, Hg, Se and Zn, while the kidney contained the highest Cd concentrations. However, trace element concentrations within the tissues were influenced by sex and age. Thus, females had clearly higher Cd concentrations than males. Age was the most important factor influencing the concentration of Cd, Hg and Se in the liver, and of Cd and Hg in the kidney. A strong positive correlation between Cd, Hg and Zn in the kidney suggests the presence of a detoxification process involving metallothionein proteins. Similarly, a strong positive correlation between Hg and Se and a molecular Hg:Se ratio close to 1 in the liver suggests a demethylation process leading to the formation of mercuric selenide granules. High Hg concentrations could be related to fish consumption by the grey seal, but a piscivorous diet could not be responsible for the very high Cd concentrations. This suggests that the seals¹ diet changes during the year, and may include a significant proportion of cephalopods in seasons other than summer. Finally very high Cd concentrations in grey seal tissues compared to other grey seal populations also suggest that the Faroe Islands are subject to sub-arctic influences.

Diet of harbour seals (Phoca vitulina) in Mull and Skye (Inner Hebrides, western Scotland)

Data on the diets of harbour seals (Phoca vitulina) from two islands in the Inner Hebrides (Scotland, UK) based on collections of faecal samples made in 1993 and 1994 are presented. The diet included a range of fish and cephalopod species, of which the most important were gadoids, particularly whiting (Merlangius merlangus), along with pelagic scad (Trachurus trachurus) and herring (Clupea harengus). There were significant temporal and spatial (between-island) differences in diet. The relatively high importance of pelagic species and the low importance of sandeels (Ammodytidae) is consistent with previous studies on grey seals (Halichoerus grypus) in the Inner Hebrides but differs from studies based in other parts of Scotland.

Marine mammals from northeast Atlantic: relationship between their trophic status as determined by delta13C and delta15N measurements and their trace metal concentrations.

The relationship between trophic position through delta13C and delta15N and trace metal concentrations (Zn, Cd, Cu and Hg) was investigated in the tissues of six marine mammal species from the Northeast Atlantic: striped dolphin Stenella coeruleoalba, common dolphin, Delphinus delphis, Atlantic white-sided dolphin Lagenorhynchus acutus, harbour porpoise Phocoena phocoena, white beaked-dolphin Lagenorhynchus albirostris, grey seal Halichoerus grypus stranded on French Channel and Irish coasts. White-beaked dolphins, harbour porpoises, white-sided dolphins, common and striped dolphins display the same relative and decreasing trophic position, as measured by delta15N values, along both the Irish and French channel coasts, reflecting conservative trophic habits between these two places. Hepatic and renal Cd concentrations were significantly correlated to muscle delta13C and delta15N values while Hg, Zn and Cu did not. These results suggest that Cd accumulation is partly linked to the diet while other factors such as age or body condition might explain Hg, Zn or Cu variability in marine mammals. Combined stable isotope and trace metal analyses appear to be useful tools for the study of marine mammal ecology.

Dynamics of PCB transfer from mother to pup during lactation in UK grey seals Halichoerus grypus: differences in PCB profile between compartments of transfer and changes during the lactation period

The composition of the PCB mixture which is transferred from mother to pup in grey seals Halichoerus grypus throughout lactation was measured in the successive compartments of transfer: maternal blubber, maternal serum, milk and pup serum. Mother-pup pairs were captured regularly between birth and weaning on the Isle of May, Scotland. The PCB profiles of the different compartments of transfer consisted mainly of congeners 153, 138, 180, 187, 170 and 101, which made up 85% of the total PCB burden. Outer blubber contained higher relative concentrations (R-153) Of hepta-, octa-, and nona-chlorinated congeners (higher chlorinated congeners) than inner blubber, at both early (less than or equal to 5 d) and late (greater than or equal to 11 d) lactation. There was no change in the composition of the outer blubber layer between early and late lactation, while in the inner blubber, there was a slight increase in the R153 of higher chlorinated congeners at late lactation. In maternal serum, profiles changed towards having a higher proportion of higher chlorinated congeners as lactation progressed, but in milk, the PCB pattern stayed constant throughout lactation. Surprisingly, in pup serum profiles also changed towards higher R153 of higher chlorinated congeners at late lactation compared to early lactation, which was different to the pattern observed in milk. In all body compartments, higher proportions of higher chlorinated congeners were associated with higher concentrations of total PCBs. Maternal blubber and pup serum contained a higher R153 of higher chlorinated congeners than milk and, to a lesser extent, maternal serum. Thus biomagnification of the higher chlorinated congeners was recorded in pup serum.

Marine mammals from the southern North Sea: feeding ecology data from δ13C and δ15N measurements

The harbour porpoise Phocoena phocoena, grey seal Halichoerus grypus, harbour seal Phoca vitulina and white-beaked dolphin Lagenorhynchus albirostris are regularly found stranded along southern North Sea coasts. Occasionally, offshore species such as the fin whale Balaenoptera physalus, the white-sided dolphin L. acutus and the sperm whale Physeter macrocephalus are also found stranded. In order to trace their diet, we measured δ 13 C and δ 15 N in their muscles as well as in 49 invertebrate and fish species collected from the southern North Sea. The δ 15 N data indicate that the harbour seal, grey seal and white-beaked dolphin occupy the highest trophic position, along with ichtyophageous fishes such as the cod Gadus morhua (mean muscle values of 18.7, 17.9, 18.8 and 19.2‰ respectively). The harbour porpoise occupies a slightly lower trophic position (mean δ 15 N value of 16.2 ‰), reflecting a higher amount of zooplanktivorous fishes in its diet (mean δ 15 N of 14.7‰); 2 suckling harbour porpoises displayed a significant δ 15 N enrichment of 2.2‰ compared to adult females. Adult females are δ 15 N-enriched compared to adult male harbour porpoises. Fin whales, sperm whales and white-sided dolphins are 13 C-depleted compared to southern North Sea particulate organic matter and species, suggesting that despite regular sightings, they do not feed within the southern North Sea area.

Quantitative dynamics of PCB transfer from mother to pup during lactation in UK grey seals Halichoerus grypus

PCB contamination was measured in the milk and serum of grey seal Halichoerus grypus mothers and in the serum of their pups sampled from 2 to 5 times between parturition and weaning on the Isle of May, Scotland, in 1998 and 2000. Blubber biopsies were also taken from the lactating females at early and late lactation in 2000. Concentrations of PCBs in milk stayed constant during the first part of lactation (0.31 +/- 0.17 mug g milk(-1)) and then increased at late lactation (0.67 +/- 0.42 mug g milk(-1)). Curiously, it did not follow the changes of milk lipid content, which increased at early lactation and then stayed constant until the end of the nursing period. As a result, even when expressed per unit of milk lipids, PCBs underwent a rise at the end of lactation. The changes in milk PCBs were accompanied by similar dynamics in maternal serum as well as in pup serum. Increased concentrations of PCBs at late lactation in serum and milk may be explained in part by the changes observed in maternal blubber. PCB levels increased significantly between early and late lactation in inner blubber, suggesting that PCBs are less easily mobilised from blubber than lipids. At late lactation, the retention capacity of the reduced blubber layer for PCBs might have reached its maximum. The mobilization of less polar lipids from blubber might also occur at this stage. In both cases, this could result in a higher mobilization of PCBs at this time. While inner blubber was significantly less concentrated than outer blubber at early lactation (1.26 +/- 0.72 mug g lipids(-1) in inner blubber vs 3.16 +/- 1.34 mug g lipids(-1) in outer blubber), these variations disappeared at late lactation (3.24 +/- 2.60 mug g lipids(-1) in inner blubber vs 3.59 +/- 1.46 mug g(-1) lipids in outer blubber). Newborn pups already had significantly higher serum levels of PCBs than their mothers, revealing an important placental transfer (11.9 +/- 7.0 ng ml serum(-1) in pups vs 6.7 +/- 3.5 ng ml serum(-1) in mothers). These differences were even greater in late lactation, due to the ingestion of milk (27.9 +/- 18.1 ng ml serum(-1) in pups vs 12.2 +/- 7.2 ng ml serum(-1) in mothers). As lactation progressed, PCB levels in pup serum increased exponentially as compared to the levels in the serum of their mothers.

Grey seal (Halichoerus grypus) pup mortality not explained by local breeding density on North Rona, Scotland

AbstractPre‐weaning pup mortality of grey seals Halichoerus grypus in a breeding colony (North Rona, Scotland, U.K.) was examined in relation to the density of local adult females over two breeding seasons. Accurate, fine‐scale maps of daily seal locations within a GIS database permitted the extraction of measures of female density determined at a range of spatial scales from 5 to 50 m around each pup on the date of death. Adult densities around live pups provided the null distribution of adult densities, which were tested against the distribution of densities where deaths occurred. Pre‐weaning pup mortality at this colony was not related to any of our density measures. The proportion of pups dying each day showed no seasonal trends. Analysis of topographic features from a sub‐meter digital terrain model for locations where deaths occurred suggested that there were fewer mortalities in prime pupping habitats. This could have been a direct effect of habitat or the relative quality of mothers in these habitats. The distance separating mothers and pups tended to be greater on the periphery of the colony compared to more central areas. Pups in the peripheral areas may be more exposed to predation by gulls.

Use of blubber fatty acid profiles to detect inter-annual variations in the diet of grey seals Halichoerus grypus

Diet, and how it varies in place and time, are important factors in understanding the interactions between seals and fisheries. The grey seal eats a wide variety of prey species and varia- tions in availability of prey can lead to changes in both foraging behaviour and diet that could affect the ecological impact of the seals. Since it is difficult to observe the feeding behaviour of seals directly, we have used changes in the fatty acid profiles of blubber to indicate changes in their diet. We studied inter-colony and inter-annual variations in the fatty acid profiles of female seals from 2 Scottish breeding colonies, North Rona (RON) and the Isle of May, over 3 consecutive years from 1996 to 1998. The fatty acid profiles obtained were compared and tested using several multivariate statis- tical methods, including a new inter-population measure Dfap (a distance measure based on fatty acid profiles). There were significant inter-colony differences in blubber fatty acids in all years of the study. There were significant within-colony variations among years for the seals at the Isle of May but not at RON. Individual seals sampled in more than 1 yr tended to change in a similar manner at the Isle of May but not at RON. Discriminant analysis of the fatty acid profiles provided classifications of the Isle of May seals according to year with 84 to 97% accuracy, but was less successful with the RON seals with 56 to 74% accuracy. Only 4 out of 166 seals were 'mis-classified' into the wrong Isle of May or RON grouping. This analysis suggested a shift in diet at the Isle of May that was supported by evidence from a separate study on the analysis of otoliths in seal faeces.

A high-performance-liquid-chromatography-based method for the determination of hydroxylated testosterone metabolites formed in vitro in liver microsomes from gray seal (Halichoerus grypus).

A reproducible and sensitive high-performance-liquid-chromatography (HPLC)-based method with UV-vis detection is developed and optimized for the determination of hydroxytestosterone compounds formed via the cytochrome P450 enzyme-mediated metabolism of testosterone. The method is used to characterize and quantitate hydroxytestosterone metabolites formed in vitro via testosterone incubation with hepatic microsomes from the liver of gray seals (Halichoerus grypus). The HPLC method employs a Zorbax Eclipse XDB-C18 column (5 microm, 250- x 4.6-mm i.d.) and a combination of step gradient and solvent systems of mixtures of acetonitrile, methanol, and water. Metabolites are detected at 254 nm. The eluted peaks of 10 testosterone metabolite standards are well-resolved and a flat baseline is maintained over the elution period of the entire chromatogram. The instrumental detection limits (signal-to-noise ratio = 3) of 6beta-, 16beta-, 16alpha-, and 7alpha-hydroxytestostone and androstenedione are 14, 3, 3, 14, and 3 pmol (20 microL injection), respectively. Eleven hydroxytestosterone metabolites are detected after in vitro testosterone incubation with hepatic microsomes of gray seals. Six are identified as 6beta-, 7alpha-, 16alpha-, 16beta-, and 2beta-hydroxytestosterone and androstenedione. In order of abundance, the formation rates are 2100, 39.6, 12.8, 26.2, and 132 pmol/mg protein/min for 6beta-, 7alpha-, 16alpha-, and 16beta-hydroxytestosterone and androstenedione, respectively. The within-day precision (relative standard deviation) is less than 3% for testosterone metabolites. Five relatively substantial peaks are detected but not identified.