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Despite 2 decades of very low fishing levels, numerous NW Atlantic demersal fish stocks have failed to recover from collapsed states or are presently collapsing. In the southern Gulf of St. Lawrence, adult natural mortality (M) appears to be the demographic rate that most limits population productivity in at least 3 species: Atlantic cod Gadus morhua, white hake Urophycis tenuis and winter skate Leucoraja ocellata. The causes of elevated M are not well understood, though there is indirect evidence consistent with an effect of predation by grey seals Halichoerus grypus. However, direct evidence is lacking due to uncertainty in the seal diet. Consequently, Monte Carlo simulations were undertaken using data on the spatial overlap between the seals and the fishes and a seal food-consumption model, to estimate the plausibility that different seal-diet compositions could explain observed M levels. Under the simulation assumptions, we find that predation could explain up to 20 to 50% of M in adult white hake and cod even if these species comprise a small percentage of grey seal diets (

The development of foraging strategies as animals mature depends upon experience, stage-specific physiology, the onset of reproductive maturity and the reproductive costs incurred by each sex. To understand the ontogeny of foraging behaviour, we compared movement behaviour of 24 young-of-year (YOY) juveniles (12 male, 12 female) with 6 subadult (4 male, 2 female) and 81 adult (43 male, 38 female) grey seals Halichoerus grypus. We used a behaviour discriminating state-space model followed by a series of mixed-effects models to examine trip structure and habitat use in these age classes. In foraging trips, tortuosity and speed of outbound travel were not different in YOY, subadults and adults, suggesting that YOY navigate as well as older animals. On average, however, YOY trips lasted 1.2 to 3.5 d longer and required up to twice as much transit time to reach foraging areas that were 1.5 to 3 times farther from haul-out sites than those of subadults and adults. This sug- gests an overall apparent higher foraging effort for YOY. Differences, however, were highly season- ally dependent. In all groups, apparent foraging effort decreased in the summer and increased in the winter, which is consistent with seasonal changes in prey distribution and energy content. Adult for- aging patterns showed complex seasonal patterns influenced by both reproductive cycles and sea- sonal environmental variation, whereas annual foraging patterns of YOY and subadults, neither of which invest in reproduction, were simpler and appeared more closely tied to seasonal changes in prey availability and condition. Although foraging in the same habitats, male and female YOY showed small but significant differences in movement behaviour and trip structure. These differences are unlikely to be due to size difference, which is minimal at this age, and suggest that sex-related differences in foraging develop early before sexual size dimorphism is significantly expressed.

Inter-specific competition for prey is thought to influence the structure of ecological communities and species niche breadth. Harp seals Pagophilus groenlandicus and hooded seals Cystophora cristata are geographically overlapping and highly migratory predators in the North Atlantic ocean. Hooded seals are known to dive deeper and longer than harp seals and are more closely associated with the continental shelf edge and deep ocean. Quantitative fatty acid (FA) signa- ture analysis (QFASA) was recently developed to estimate the species composition of diets by statis- tically comparing FA signatures of predator adipose tissue with that of potential prey. Using QFASA, we estimated diets for harp (adults, n = 294; juveniles, n = 232) and hooded (adults, n = 115; juveniles, n = 38) seals from the pre- and post-breeding periods between 1994 and 2004. We found evidence of inter- and intra-specific variation in diets, diet quality and breadth, reflecting different foraging tac- tics. Harp seal diets were comprised predominantly of amphipods, Arctic cod, capelin, herring, sand lance and redfish. Hooded seal diets were composed primarily of amphipods, Atlantic argentine, capelin, euphausiids and redfish. Relative to the other species, harp seals consumed twice the propor- tion of amphipods, while hooded seals consumed 3 times the proportion of redfish; percentages of capelin were similar. QFASA provided new evidence of the importance of amphipods in the diets of both species and of the pronounced differences in the proportions of pelagic forage fish between demographic groups.

What animals consume is perhaps one of the most fundamental questions in ecology. Limitations of stomach content and faecal analyses have led to the development of alternative bio- chemical methods, namely fatty acid (FA) signature analysis and stable isotope (SI) analysis. Propor- tional estimates of diet composition can be made at the level of the individual by statistically match- ing a consumer's FA signature to an inclusive prey database, after accounting for predator metabolism effects in a mixing model. Both of these approaches provide inferences about or quanti- tative estimates of diet that are time-integrated and readily applied to most taxa of interest, thereby alleviating many problems associated with conventional analysis. Moreover, analysis is done at the level of individual predators, thereby increasing our ability to detect differences amongst various demographic groups. However, it is difficult to validate diet estimates from either of these methods in the field, particularly for wide-ranging aquatic predators such as seals or seabirds consuming complex diets. Concurrent sampling of individual predators for both stable isotopes and fatty acids provide 1 such independent test. To that effect, we analyzed SI and FA for grey seals Halichoerus grypus (n = 75) and potential fish and invertebrate prey (n = 45). We show 2 lines of evidence for the convergence of stable isotope and fatty acid estimates of diet. First, in fish, invertebrates and grey seals, principal components derived from fatty acid signatures are correlated to the stable-carbon iso- tope (δ 13 C) signature of the same species. Second, both the estimated portion of pelagic prey and the trophic level of the diet derived from the quantitative analysis of fatty acid signatures of individual seals are positively correlated with the stable-carbon (δ 13 C) and stable-nitrogen (δ 15 N) signatures in those same individuals. The corroboration of independent measures of diet provides further validation of these biochemical methods to understand the foraging ecology of consumers.

Although many studies have documented diets for terrestrial mammals, much less is known about prey selection in marine mammals. Variation in body size of terrestrial carnivores is cor- related with mass of prey consumed, and predator size often determines dietary overlap. Grey seals Halichoerus grypus are a size-dimorphic phocid seal inhabiting the Northwest Atlantic, with males 1.5× larger than females. Stable isotopes of carbon (δ 13 C) and nitrogen (δ 15 N) provide useful tools for estimating trophic positions and carbon flow to consumers in food webs. δ 13 C and δ 15 N were analyzed for individual adult grey seals from Sable Island, Canada, between 1996 and 2001. Samples of skin were taken in May or September and the same animals were re-sampled when they returned to the breeding colony in January. In addition, samples were taken from juveniles (n = 29) in January 2004. δ 13 C and δ 15 N were also analyzed for 12 fish and 3 invertebrate species from the Scotian Shelf. Adult male (n = 36) and female (n = 36) grey seals exhibited significantly different δ 13 C and δ 15 N values (F1, 70 = 22.40; p < 0.001). On average, males were more enriched in both δ 13 C and δ 15 N than females. While there were no differences between sex in juveniles, signatures were significantly different from adults (F2,101 = 28.05; p < 0.001). Comparison of isotope values from potential prey suggests that adult males feed more heavily on benthic prey, whereas adult females appear to feed more on pelagic prey. Juveniles also appear to feed more on pelagic prey. These results are consistent with known dif- ferences in dive patterns and foraging behaviour in this population. In addition, there was a positive effect of body size on both δ 13 C (t = 2.54, p = 0.014) and δ 15 N (t = 2.48, p = 0.016) and a positive rela- tionship between δ 15 N and the rate of fat accumulation (kg d -1 ). These results suggest an underlying energetic basis to diet variation in grey seals. Finally, examination of the variance in stable isotope ratios suggested that while individual grey seals appear to be relative specialists in their diet, niche breadth was 1.5× greater in males than in females. On average, adult grey seals had a 2 × higher niche breadth than juveniles.