Accurate estimates of the diets of predators are required in many areas of ecology, but for many species current methods are imprecise, limited to the last meal, and often biased. The diversity of fatty acids and their patterns in organisms, coupled with the narrow limitations on their biosynthesis, properties of digestion in monogastric animals, and the prevalence of large storage reservoirs of lipid in many predators, led us to propose the use of quantitative fatty acid signature analysis (QFASA) to study predator diets. We present a statistical model that provides quantitative estimates of the proportions of prey species in the diets of individual predators using fatty acid signatures. We conducted simulation studies using a database of 28 prey species (n ϭ 954 individuals) from the Scotian Shelf off eastern Canada to investigate properties of the model and to evaluate the reliability with which prey could be distinguished in the model. We then conducted experiments on grey seals (Halichoerus grypus, n ϭ 25) and harp seals (Phoca groenlandica, n ϭ 5) to assess quantitative characteristics of fatty acid deposition and to develop calibration coefficients for individual fatty acids to account for predator lipid metabolism. We then tested the model and calibration coefficients by estimating the diets of experimentally fed captive grey seals (n ϭ 6, switched from herring to a mackerel/capelin diet) and mink kits (Mustela vison, n ϭ 46, switched from milk to one of three oil-supplemented diets). The diets of all experimentally fed animals were generally well estimated using QFASA and were consistent with qualitative and quantitative expectations, provided that appropriate calibration coefficients were used. In a final case, we compared video data of foraging by individual freeranging harbor seals (Phoca vitulina, n ϭ 23) fitted with Crittercams and QFASA estimates of the diet of those same seals using a complex ecosystem-wide prey database. Among the 28 prey species in the database, QFASA estimated sandlance to be the dominant prey species in the diet of all seals (averaging 62% of diet), followed primarily by flounders, but also capelin and minor amounts of other species, although there was also considerable individual variability among seals. These estimates were consistent with video data showing sandlance to be the predominant prey, followed by flatfish. We conclude that QFASA provides estimates of diets for individuals at time scales that are relevant to the ecological processes affecting survival, and can be used to study diet variability within individuals over time, which will provide important opportunities rarely possible with other indirect methods. We propose that the QFASA model we have set forth will be applicable to a wide range of predators and ecosystems.

No abstract

J. I. 2004. Intraspecific variation in movement patterns: modeling individual behaviour in a large marine predator. Á/ Oikos 105: 15 Á/ 30.In large marine predators, foraging entails movement. Quantitative models reveal how behaviours can mediate individual movement, such that deviations from a random pattern may reveal specific search tactics or behaviour. Using locations for 52 grey seals fitted with satellite-linked recorders on Sable Island; we modeled movement as a correlated random walk (CRW) for individual animals, at two temporal scales. Mean move length, turning angle, and net squared displacement (R 2 n : the rate of change in area over time) at successive moves over 3 to 10 months were calculated. The distribution of move lengths of individual animals was compared to a Lévy distribution to determine if grey seals use a Lévy flight search tactic. Grey seals exhibited three types of movement as determined by CRW model fit: directed movers Á/ animals displaying directed long distance travel that were significantly underpredicted by the CRW (23% of animals); residents Á/ animals remaining in the area surrounding Sable Island that were overpredicted by the model (29% of animals); and correlated random walkers Á/ those (48% of animals) in which movement was predicted by the CRW model. Kernel home range size differed significantly among all three movement types, as did travel speed, mean move length, mean R 2 n and total distance traveled. Sex and season of deployment were significant predictors of movement type, with directed movers more likely to be male and residents more likely to be female. Only 30% of grey seals fit a Lévy distribution, which suggests that food patches used by the majority of seals are not randomly distributed. Intraspecific variation in movement behaviour is an important characteristic in grey seal foraging ecology, underscoring the need to account for such variability in developing models of habitat use and predation.Most animals must move to locate and capture food. Thus, patterns of movement are considered a key factor in the survival of most organisms (Turchin 1998, Bergman et al. 2000. In any given environment, there is a range of behaviours (i.e. phenotypes) that can be considered successful. These can be learned behaviours, or alternatively, the products of longer-term selection for specific traits (Komers 1997). Intraspecific variation in movement behaviour reflects the different tactics used by individuals or sexes within a species or population to meet the demands of survival. We expect natural selection to favour those strategies that maximize fitness or some proxy of fitness, such as the rate of resource acquisition, or production of offspring. Given that natural selection operates at the level of the individual, ecological models that lump all individuals into the same behavioural category effectively disregard this variation (Judson 1994, Zollner andLima 1999). Consequently, examining average responses across populations obscures variability in behavioural ecology. Animal...

Many animal species segregate by sex. Such segregation may be social in nature, or ecological, or both. Grey seals (Halichoerus grypus), like many large mammals, are sexually size dimorphic. In size dimorphic species, allometric differences in morphology, metabolic rate and reproductive costs are likely. Such differences may require the sexes to use different foraging strategies or different habitats. To investigate sexual segregation of habitat in grey seals, we used satellite tracks from 95 (male 46; female 49) adults breeding at Sable Island, Nova Scotia (44 degrees N, 60 degrees W) collected from 1995 to 2005. Location estimates were made from satellite fixes using a state-space movement model to estimate true locations and regularize them in time. Location estimates were used to calculate home range kernels of male and female habitat use each month. Month by sex kernel home ranges revealed striking differences and dynamics in habitat use between males and females on spatial scales broader than most terrestrial examples and at temporal and spatial resolutions rarely available for marine species. Differences were most pronounced just before (October-December) and immediately after breeding (February-March). During both periods, males primarily used areas along the continental shelf break, while females mainly used mid-shelf regions. Coupled with previously identified sex-specific seasonal patterns of energy storage, diving and diet, our findings suggest that males and females differ profoundly in their spatial foraging strategies. These differences may serve to maximize fitness by reducing intersexual competition during key foraging periods.

ABSTRACT. Marine mammals are consumers of production at most trophic levels Because of their large body size and abundance, they are thought to have a major influence on the structure and function of some aquatic communities. However, there is relatively little empirical evidence of these roles. There are several reasons for this: research in marine ecosystems is expensive, man~pulatlve experiments are rarely possible, interactions occur at quite different spatial and temporal scales m a h n g measurement of system properties difficult, and there is an inherent indeterrmnacy in the behavior of these complex systems which makes simplifying deterministic explanations problematic. Nevertheless, experimental studies have demonstrated clearly that sea otters Enhydra lutris strongly affect kelp forest communities through predation on sea urchins, they also suggest that gray whale Eschrichtius robustus and walrus Odobenus rosmarus feeding can affect the structure of benthic invertebrate communities, and that dugongs Dugong dugon may cultivate the seagrass community upon which they feed. Changes in the abundance of many species following large-scale harvesting of whales in the Southern Ocean and perhaps also in the Bering Sea further suggest top-down effects of marine mammals. Nevertheless, the top-down effects of marine mammal predation in the open ocean remain poorly understood.

Summary1. 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 ShannonWeaver diversity index ( H ′ ) and the Morisita-Horn index ( C H ), respectively. 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.

Predators face decisions about which prey to include in their diet in order to maximize fitness. The foraging tactics used to capture prey and the resulting profitability of prey influence these decisions. We present the first evidence of prey-dependent foraging tactics and prey profitability in a free-ranging pinniped. We studied 39 adult male harbour seals Phoca vitulina at Sable Island, Nova Scotia using an animal-borne video system. Each male wore the camera system for 3 d during which 10 min video samples were recorded every 45 min from 06:00 h, resulting in approximately 3 h of videotape per male and a total of 1094 capture attempts of identified prey. Males foraged mainly on sand lance Ammodytes dubius and flounders (Pleuronectids), but salmonid and gadoid fishes were occasionally pursued. Foraging tactics differed among and within prey types based on differences in prey behaviour. Sand lance was both a cryptic prey, when in the bottom substrate, and a conspicuous schooling prey. Seal swimming speed, handling time and capture success differed between cryptic and conspicuous sand lance. The highest capture success and handling time was recorded for flounders. Estimated profitability, i.e. net energy intake per unit time, also differed with prey type and prey size. Our results suggest that diet selection may have important implications on the foraging energetics of pinnipeds.

Summary1. Sex differences in the timing and magnitude of energy expenditure for reproduction may result in sex-specific seasonal patterns of energy storage and utilization, particularly among capital-breeding species. We studied the seasonal patterns of energy storage in adult grey seals, Halichoerus grypus , a capital-breeding, marine carnivore, to test the prediction that males and females differ in their seasonal pattern of energy storage and expenditure. 2. We measured body mass and composition in 135 (67 males, 68 females) adult grey seals at five key points throughout the annual cycle. Longitudinal changes in body composition were also measured in 73 individuals (35 males, 38 females) to examine individual variability. 3. There were significant sex differences in seasonal patterns of total body energy, with females exhibiting greater energy content (after accounting for body mass) throughout the year. Females gained body energy following both the spring moult and the breeding season, whereas males only gained body energy following an extended period of loss that included both breeding and moulting, a period of some 6 months. 4. Mass loss of females during the breeding season was similar in magnitude and composition to that gained during the 7-month pre-breeding foraging period. In contrast, mass loss of males during the breeding season was more similar to that gained during the last three months of the pre-breeding foraging period. 5. Our results suggest that the balance of costs and benefits of storing body energy for reproduction differs between males and females. Females appear to be risk-averse, committing themselves to, and preparing for, reproduction at a much earlier stage in the annual cycle compared to males. As a result, females accumulate body energy stores earlier and carry a higher level of insurance against environmental uncertainty leading up to the breeding season relative to males.