Prey Resource Use Research Articles

Trait-based indicators of resource selection by albacore tuna in the California Current Large Marine Ecosystem

As global climate change reorganizes marine ecosystems, understanding how predators will respond to variable prey resources is critical to forecasting future community dynamics. Prey traits that affect the foraging process and recur across unrelated taxa offer a means to better anticipate predator resource use by simplifying complex foraging dynamics. Here we compare taxonomic and trait-based indicators of resource use and selection for albacore tuna (Thunnus alalunga), a commercially valuable pelagic predator undergoing climate-driven range shifts. We synthesized datasets from 2005 to 2019 to evaluate diets of albacore tuna in relation to prey availability estimates from shipboard surveys in the California Current Large Marine Ecosystem. Analyses with these data reveal that albacore and trawl surveys sample different aspects of the pelagic system, with albacore consuming a subset of taxa identified within trawls. Albacore consistently selected coastal prey that are schooling, undefended, silvered and countershaded, and have high energy density — suggesting that ecological mechanisms driving albacore foraging outcomes may be conserved across time and space. Ecological traits mediating predator-prey interactions consistently distinguished albacore diets from assemblages sampled by trawls across years and regions. We demonstrate that a traits-based approach simplifies taxonomically diverse predator-prey interactions and may be a valuable tool to facilitate predictions of prey resource use in changing environments.

Composition and temporal variation in the diet of intermediate horseshoe bat, Rhinolophus affinis, relative to insect abundance in a tropical rainforest environment of Southeast Asia

AbstractDietary studies on insectivorous bat species in Southeast Asia are relatively few. We studied diet of the intermediate horseshoe bat, Rhinolophus affinis, and the temporal variations in prey resource use. We also measured prey availability in order to explore whether this species is a generalist or specialist feeder. A total of 665 fecal samples were collected from 133 captured individuals from January to December 2019 in a tropical rainforest of Penang Island (Malaysia). A descriptive analysis of samples revealed a total of eight insect prey orders. The main taxon was Coleoptera (Percent Volume >50%), followed by Isoptera, Hemiptera, Lepidoptera, Hymenoptera and Diptera, respectively (5–15%). Neuroptera and Psocoptera were occasionally found in the samples (each <1%). This result exhibits a degree of dietary specialization on coleopterans although the diet composition was flexible and significantly varied over time (among months and within nights). Based on light trapping, Coleoptera, Hemiptera, Lepidoptera, Hymenoptera and Diptera were frequent insects in the study area, while Isoptera was seasonally abundant. The correlation between variation in the diet and light trap for each prey taxon was mostly not statistically significant, suggesting that R. affinis do not consume prey in proportion to abundance and alternatively may discriminate between prey items when foraging and take prey in rank order. Given this, we further investigated the heavy consumption of absolute abundance of Coleoptera (high‐ranking prey in diet). As a result, coleopterans were included in the diet regardless of the abundance of other prey showing a pattern of dietary specialization.

Energetic consequences of resource use diversity in a marine carnivore.

Understanding how intraspecific variation in the use of prey resources impacts energy metabolism has strong implications for predicting long-term fitness and is critical for predicting population-to-community level responses to environmental change. Here, we examine the energetic consequences of variable prey resource use in a widely distributed marine carnivore, juvenile sand tiger sharks (Carcharias taurus). We used carbon and nitrogen isotope analysis to identify three primary prey resource pools-demersal omnivores, pelagic forage, and benthic detritivores and estimated the proportional assimilation of each resource using Bayesian mixing models. We then quantified how the utilization of these resource pools impacted the concentrations of six plasma lipids and how this varied by ontogeny. Sharks exhibited variable reliance on two of three predominant prey resource pools: demersal omnivores and pelagic forage. Resource use variation was a strong predictor of energetic condition, whereby individuals more reliant upon pelagic forage exhibited higher blood plasma concentrations of very low-density lipoproteins, cholesterol, and triglycerides. These findings underscore how intraspecific variation inresource use may impact the energy metabolism of animals, and more broadly, that natural and anthropogenically driven fluctuations in prey resources could have longer term energetic consequences.

Ecological Traits and Trophic Plasticity in The Greater Pipefish Syngnathus acus in the NW Iberian Peninsula.

Simple SummaryThe population of the pipefish Syngnathus acus inhabiting Cíes Archipelago (NW Spain) was monitored in 2017–2018 for spatial and temporal changes in abundances, reproduction traits, trophic niche occupancy, and dietary regimes across reproduction states, through an isotopic (δ13C and δ15N) approach. The population consisted almost exclusively of large adults, whose abundances decreased significantly from mid-autumn after the breeding season. S. acus is a secondary consumer that prefers amphipods, but mature specimens were less selective than immature fish. The present study highlights the outstanding size of the fish and the exceptional occurrence of breeders on the studied area.The great pipefish Syngnathus acus is one of the most representative European syngnathids, being highly associated with seagrass and macroalgal beds. Surprisingly, the ecology of this large ovoviviparous marine fish has received scanty attention. The population inhabiting three sites on Cíes Archipelago (Atlantic Islands National Park, NW Spain) was monitored in 2017–2018 for spatial and temporal changes in abundances, reproduction traits, trophic niche occupancy, and dietary regimes across reproduction states, through an isotopic (δ13C and δ15N) approach. Abundances were highly variable across seasons and sites, decreasing significantly from mid-autumn. The population consisted almost exclusively of large adults that migrate by the end of the breeding season, which extended from mid-spring to summer. Operational sex ratios suggest that the species is sex-role reversed. S. acus is a secondary consumer (Trophic position = 3.36 ± 0.05), preferring amphipods but displaying annual and seasonal dietary plasticity. Mature fish were less selective than immatures (especially females), with a higher preference for amphipods (36–68%) in the former. The second most-preferred prey were carideans, copepods, or isopods, depending on the year and the reproduction state. Overall, the wider trophic niches in females and immature specimens compared to males and mature fish would indicate a higher variability in both the use of prey resources and/or their origin. The present study highlights the trophic plasticity and unique features of S. acus population in the Cíes Archipelago, especially regarding the outstanding size of the fish and the exceptional occurrence of breeders.

The role of behavioral type composition on resource use and growth of a juvenile predator

Abstract Juvenile largemouth bass have distinct behavioral types that separate along the exploring behavioral axis and differ in diet. We used a mesocosm experiment to test the hypothesis that groups composed of mixed behavioral types would have more efficient use of prey resources and reduced competition between individuals than experimental populations composed of similar behavioral types. Fish growth, diets, and prey composition were quantified over a 4-week period in mesocosms containing prey communities that were mixtures of zooplankton from the pond water used to fill them and natural colonization by terrestrial insects. Mesocosms contained juvenile largemouth bass of either all fast exploratory-type, all slow exploratory-type, or a 50/50 mix of the two behavioral types. Treatments with similar behavioral types had lower growth than treatments with mixed behavioral types. While evidence showed that slow explorers in homogeneous groups consumed fewer macroinvertebrates outside of refuge habitat, the same was not true of fast explorers. Results suggest that populations composed of different compositions of behavioral types may also differ in their food web interactions. Potential alternative explanations for the difference in growth between mixed versus homogeneous communities include higher activity, increased antagonistic interactions and the role of social cues that might signal when it is safe to begin foraging after exposure to a predator. The differences in growth suggest that individual performance can be higher in populations with a balanced mixture of behavioral types compared to more homogenous populations and adds to the growing knowledge that individual behavioral traits can have emergent population-level effects.

Seasonal hydromedusan feeding patterns in an Eastern Boundary Current show consistent predation on primary consumers

Abstract Cnidarian jellyfish can be dominant players in the food webs of highly productive Eastern Boundary Currents (EBC). However, the trophic role of inconspicuous hydromedusae in EBCs has traditionally been overlooked. We collected mesozooplankton from five stations along two cross-shelf transects in the Northern California Current (NCC) during winter and summer of 2018–2019. We analyzed gut contents of 11 hydromedusan species and the prey community to (i) determine prey resource use by hydromedusae and (ii) determine temporal shifts in the trophic niche of hydromedusae, focusing on the two most collected species (Clytia gregaria and Eutonina indicans). Hydromedusae in the NCC fed mostly on copepods, appendicularians and invertebrate larvae. Nonmetric multidimensional scaling of hydromedusan diets showed seasonal shifts in prey resource driven by the abundant C. gregaria, which fed mostly on copepod eggs during winter and fed mostly on appendicularians and copepods during summer. Prey selectivity for copepod eggs increased during winter for C. gregaria and E. indicans. Intriguingly, theoretical ingestion rates show that both species acquire similar amounts of carbon during upwelling and nonupwelling conditions. Hydromedusae’s consistent presence and predation impact across seasons may lead to significant effects in carbon and energy transfer through the NCC food web.

Laboratory constraints on feeding behaviours in polymorphic bluegill sunfish (Lepomis macrochirus)

Abstract Evaluating fish feeding behaviours is imperative to understanding prey resource use in the field. Previous work on fish feeding has taken place almost exclusively in a laboratory setting, which may impose artificial restrictions. Thus, we aimed to evaluate the constraints the captive setting places on fish feeding behaviours. We recorded polymorphic (littoral and pelagic) bluegill sunfish (Lepomis macrochirus) feeding in the laboratory and the field using a high‐speed camera and underwater cameras. Following successful strikes video events, were digitised using ImageJ (laboratory) and Argus (field). Gape velocity, ram velocity, and body deceleration were higher in the field than in captive fish. Significantly greater gape velocity in field fish suggests that these fish feed with greater suction pressure than captive fish. Prey effects were detected, as brine shrimp feeding events were characterised by slower gape and ram velocities and a smaller gape. Feeding events on brine shrimp in the field were similar to feeding events on worms in the laboratory suggesting an artefact of training. These results indicate that feeding behaviours measured in the laboratory may not be representative of feeding behaviours in the wild. Further consideration of organismal performance and laboratory constraints should be taken in future functional studies.

Finless porpoises (Neophocaena asiaeorientalis) in the East China Sea: insights into feeding habits using morphological, molecular, and stable isotopic techniques

Describing feeding habits of cetaceans is crucial to understanding their feeding strategies and conservation status. Here, both morphological and molecular techniques were employed to identify the stomach contents of 122 finless porpoises (Neophocaena spp.) in the East China Sea for insight into their short-term feeding habits, and stable isotopes of δ13C and δ15N were used to analyze prey resource use and trophic position as a manifestation of their long-term feeding habits. In total, 33 prey species consisting of 19 teleosts, seven crustaceans, five cephalopods, and two gastropods were identified. In both short- and long-term analyses, teleosts represented primary prey, cephalopods and crustaceans were secondary prey, and gastropods were occasional prey; but the primary prey species composition differs between the short- and long-term diets. The composition of stomach contents showed sexual and age-related variation. This finding is supported by stable isotopic analyses, which indicated the separation of trophic position of adult males, adult females, and young males. In general, finless porpoises prey on species that are primarily caught by fisheries.

Resource Partitioning in Food, Space and Time between Arctic Charr (Salvelinus alpinus), Brown Trout (Salmo trutta) and European Whitefish (Coregonus lavaretus) at the Southern Edge of Their Continuous Coexistence.

Arctic charr and European whitefish are considered to be strong competitors in lakes, with the latter usually being the superior species. However, high niche plasticity and lake morphometry may suggestively facilitate resource partitioning and coexistence between charr and whitefish. Here, we explore the trophic niche utilization (diet and habitat use) of charr and whitefish co-occurring with brown trout in the deep and oligotrophic Lake Fyresvatnet, southern Norway (59°05’N, 8°10’E). Using CPUE, stomach contents and stable isotope analyses, a distinct resource partitioning was revealed between brown trout and the other two species. Brown trout typically occupied the littoral zone, feeding on benthic invertebrates, surface insects and small-sized whitefish. In contrast, charr and whitefish were predominantly zooplanktivorous, but diverged somewhat in habitat utilization as charr shifted seasonally between the profundal and the littoral zone, whereas whitefish were found in the upper water layers (littoral and pelagic habitats). Accordingly, the stable isotope values of carbon (δ13C) reflected a pelagic orientated prey resource use for both charr and whitefish, whereas brown trout had elevated carbon and nitrogen (δ15N) signatures that reflected their benthivore and piscivore diet, respectively. The findings suggest that charr may not rely upon the profundal zone as a feeding habitat but as a refuge area, and may coexist with whitefish if a third competitive and predatory species like brown trout co-occur in the lake. The study indicates that a general high habitat plasticity of Arctic charr may be essential in the presently observed coexistence with a competitively superior fish species like whitefish, and that a third fish species like brown trout may facilitate this particular fish community structure.

Feeding habits of the alien brook trout Salvelinus fontinalis and the native brown trout Salmo trutta in Czech mountain streams

Quantifying patterns of prey resource use is fundamental to identify mechanisms enabling the coexistence of related fish species. Trophic interactions between the native brown trout, Salmo trutta , and the introduced brook trout, Salvelinus fontinalis , were studied monthly from May to October in three mountain streams in Central Europe (Czech Republic). To evaluate whether the feeding habits differ between separated and coexisting populations of these species, one locality where both species coexist, and two allopatric populations of either species were studied. Across the study period, the mean stomach fullness of fish varied, being highest in spring and declining through autumn. The diet overlap (Schoener's overlap index) between the species increased through the studied season (from 54.5% in July to 81.5% in October). In allopatry, both species had nearly the same feeding habits. However, in sympatry, brook trout consumed higher proportion of terrestrial invertebrates, while brown trout showed no changes either in the proportions of aquatic and terrestrial prey utilized or in the selectivity for prey categories in comparison to allopatric conditions. The dietary shift observed for brook trout, but not for brown trout, suggests that brown trout is a stronger competitor in the studied sympatric locality, leading the brook trout to change its feeding habits to reduce interspecific competition.

Evaluation of Trophic Niche Overlap between Native Fishes and Young-of-the-Year Common Carp

Common carp Cyprinus carpio is a ubiquitous invasive species that commonly imposes negative effects on aquatic ecosystems. However, research evaluating the effects of carp on native fishes is limited. Carp are highly fecund and larvae and juveniles can be abundant. If age-0 carp use similar prey resources as native fishes, they may compete if food becomes limited. We used traditional diet analysis for samples during Jul. and Aug. 2008 in Brant Lake. Stable isotopes were used for samples collected during Aug. and Sep. 2009 in Brant Lake and Lake Sinai to examine prey resource use by age-0 carp and four native fishes: bluegill Lepomis macrochirus, black crappie Pomoxis nigromaculatus, yellow perch Perca flavescens, and orangespotted sunfish Lepomis humilis. Age-0 carp were generally as or more abundant than native fishes. In Jul. Daphnia dominated the diets of all fishes sampled, resulting in high (>60%) dietary overlap ranging from 87–98%. In Aug. Daphnia density in Brant Lake declined and dominant diet prey types shifted for carp (to Trichoptera and Daphnia), yellow perch (to Amphipoda), and the three Centrarchids (to Cyclops and Diaptomus). This diet shift resulted in lower diet overlap between Centrarchids and either carp (<40%) or yellow perch (3–16%) but high diet overlap within Centrarchids (67–97%). Stable isotope analysis further indicated greater resource overlap when most fishes relied on zooplankton and lesser overlap as fishes shifted to benthic prey. Our results suggest that resource competition between age-0 carp and native fishes is most likely to occur during early summer if Daphnia availability becomes limited but becomes less likely as dominant prey in diets increasingly diverge among fish species over time.

Prey subsidy or predator cue? Direct and indirect effects of caged predators on aquatic consumers and resources

The non-consumptive effects of predators on prey can affect prey phenotypes, potentially having important consequences for communities due to trait-mediated indirect interactions. Predicting non-consumptive effects and their impacts on communities can be difficult because predators can affect resources directly through nutrient cycling and indirectly by altering prey resource use, which can lead to complex interactions among resources and consumers. In this study we examined the effects of caged dragonfly predators on aquatic resources in the presence and absence of two focal herbivores, the tadpoles of Neotropical tree frogs Agalychnis callidryas and Dendropsophus ebraccatus. We crossed the presence/absence of caged dragonflies with four tadpole treatments: no tadpoles, each tadpole species alone, and both species together to examine interactions among tadpole composition, predator presence, and time on tadpole growth, resources, and zooplankton abundances. Predator effects on growth changed through ontogeny and was species-dependent. Predators initially reduced then dramatically increased A. callidryas growth, but had no effect on D. ebraccatus. Predators also increased the abundances of both periphyton and phytoplankton. However, there was no evidence of a trait-mediated trophic cascade (i.e., tadpole by predator interaction). Instead, nutrients from prey carcass subsidies likely played an increasingly important role in facilitating resources, and shaping tadpole growth, competitive interactions, and zooplankton abundances through time. In nutrient-poor aquatic systems the release of nutrients via the consumption of terrestrially derived prey items by aquatic predators may have important impacts on food webs by facilitating resources independent of the role of trait-mediated trophic cascades.

Using stable isotopes to link breeding population trends to winter ecology in Willow Warblers, Phylloscopus trochilus

Capsule Stable isotope composition of feathers grown in Africa indicates that Willow Warblers, Phylloscopus trochilus, breeding in areas of Britain experiencing different population trends may vary in their distribution, timing of moult and/or use of prey resources during the non-breeding season. Aims To compare stable isotope ratios of feathers of Willow Warblers breeding in parts of Britain with differing rates of population change. Methods Feathers were collected from three regions with differing recent population trends; northern Scotland (population increased), western Wales (declined slightly) and eastern England (declined rapidly). Collection occurred at ringing sites run by British Trust for Ornithology (BTO) volunteers as part of the Constant Effort Sites (CES) scheme between April and July in 2008 and 2009. Results Willow Warblers from northern Scotland had significantly higher δ 15N and δ 13C values than those from eastern England, although the regional mean values did not differ greatly. There was no significant difference in δ 15N and δ 13C values between the sexes, but δ 15N values were significantly greater in individuals caught later in the breeding season. Conclusions During the premigratory moult period in Africa, Willow Warblers from different parts of Britain may vary in their distribution, timing of moult and/or use of prey resources.

Coexistence of Rhinolophus affinis and Rhinolophus pearsoni revisited

The investigation of mechanism of species coexistence promotes understanding of the mechanistic processes behind community ecology and ecosystem functions. Niche theory declares that species coexistence within a community must partition the resources of their environment. Two sympatric and morphologically similar bat species, Rhinolophus affinis and Rhinolophus pearsoni, provided a unique opportunity to test the causal mechanism of coexistence. Previous study showed that their coexistence was promoted not by the trophic and spatial niche differentiation but the relatively high abundance of prey resources, which was not in accord with the prediction of niche theory. Here, therefore, we reanalyzed the dietary composition by fecal analysis and surveyed the feeding time of both species. Our results showed that R. affinis and R. pearsoni hunt mainly mostly on Coleoptera and Lepidoptera, and there was a very high overlap (0.84) of trophic niche between the two species. However, significant difference in the duration of the activity period between both species was detected, which illustrated that temporal partitioning of prey resource use facilitated their coexistence. Additionally, our work highlighted the importance of integration of the traditional methods and next-generation sequencing methods for identifying dietary composition of carnivores, and suggested that ongoing studies of species coexistence must consider simultaneously multiple niche axes.

Snail responses to cues produced by an invasive decapod predator

Abstract. Cues released by predators and injured prey often induce shifts in prey behavior that allow prey to evade predators, but also affect prey resource use. I investigated the effects of chemical and mechanical signals produced by injured snails (Physella gyrina) and predatory crayfish (Procambarus clarkii) on microdistributions of P. gyrina. In an initial experiment, I observed snail responses to the presence of a caged crayfish predator, to injured conspecifics, or to both. There were significant effects of time and the treatment × time interaction on the proportion of snails moving above the water line, with greater proportions of snails above the water line at night than during the day and with weak snail crawl-out behavior being elicited by caged crayfish at night, but not during the day. In a second experiment, I examined snail microdistributions when exposed to crayfish confined to a small cage within each aquarium, crayfish confined to half of each aquarium, and crayfish ranging freely throughout each aquarium. Snails responded most strongly to free-ranging crayfish by moving above the water line, but also demonstrated significant, but reduced, crawl-out responses to crayfish confined to half of each aquarium; however, snails did not respond behaviorally to crayfish confined to small cages. In both experiments, there were marginally significant effects of unfed caged crayfish on the proportions of snail populations hiding under benthic shelters, with this response being the strongest at the start of the experiments but weak overall (with only 4–5% of P. gyrina responding in each experiment). These results indicate that cues (e.g., chemical or mechanical) produced by predators altered prey microdistributions, but that the exact prey responses (e.g., moving above the water line or into horizontal or benthic refugia) depended on the intensity and nature of cues.

Contrasting patterns of resource utilization between juvenile estuarine predators: the influence of relative prey size and foraging ability on the ontogeny of piscivory

In aquatic systems, ontogenetic patterns of resource utilization strongly influence growth and survival, particularly during early life stages. We compared prey resource use and evaluated potential factors affecting the timing of the shift to piscivorous feeding in two juvenile estuarine fish predators: striped bass ( Morone saxatilis ) and bluefish ( Pomatomus saltatrix ). In New York Bight estuaries, bluefish shift to piscivory earlier in ontogeny and consume larger relative prey fish sizes compared with striped bass. Predator gape allometries are similar and did not determine maximum prey sizes eaten. Experimental results revealed marked differences in foraging abilities between predators, with bluefish realizing much greater foraging efficiency compared with striped bass feeding on identical fish prey. Both predators demonstrated lower feeding efficiency and grew relatively poorly when feeding on invertebrates compared with fish prey. When held together under limited prey conditions, bluefish exploited a greater proportion of available prey at the expense of striped bass. Our findings highlight the importance of the availability of appropriately sized forage fishes to the ontogeny of piscivory and provide evidence that predator–prey size relationships and disparate foraging abilities can generate inter- and intra-specific variation in patterns of resource utilization and predator growth.

Prey resource use by bluegill and channel catfish in small impoundments

Abstract Channel catfish, Ictalurus punctatus (Rafinesque), are commonly stocked into small impoundments that contain bluegill, Lepomis macrochirus Rafinesque, and these species may compete for food resources because both prey on macroinvertebrates. Prey selectivity and diet overlap of sympatric bluegill and channel catfish were evaluated in two small impoundments. Both fish species fed primarily on macroinvertebrates; but diet overlap between the species generally was not extensive because channel catfish consumed a more diverse array of foods, including more crayfish, fish and plant material. The use of foods other than macroinvertebrates increased as channel catfish grew larger. Bluegill also ingested large quantities of plant material at times, and ingestion of plants increased with fish size. Results of this study suggest that food competition between bluegill and channel catfish may occur when small channel catfish are abundant because, like bluegill, they feed almost entirely on macroinvertebrates and may reduce macroinvertebrate abundance. Thus, overstocking channel catfish in small impoundments managed for bluegill should be avoided.

Prey resource use by coexistent hydromedusae from Friday Harbor, Washington

Prey selection patterns were quantified for a sympatric group of hydromedusae from Friday Harbor, WA. Selection patterns varied between species, but were largely replicable between sample dates and resembled dietary patterns found in similar studies from neighboring regions. Ambush‐foraging medusae (Aglantha digitale, Sarsia tubulosa, and Proboscidactyla flavicirrata) fed primarily on crustacean and ciliated prey but the dietary niches of these hydromedusan species centered on different fractions of the available plankton. Consequently, little dietary overlap occurred between the ambush foraging hydromedusae. In contrast, the dietary niches of cruising predators (Aequorea victoria, Mitrocoma cellularia, and Phialidium gregarium) overlapped substantially because those species all fed on similar soft‐bodied prey such as eggs and appendicularians. These results have two important implications for trophic patterns involving medusae. First, different mechanisms of prey encounter and capture used by hydromedusae (ambush vs. cruising patterns) result in important interspecific dietary differences and, hence, trophic roles of the medusae. Second, whereas cruising medusae may consume similar prey and hence form a feeding guild, ambush‐foraging medusae may experience substantially less prey overlap and, for the community examined here, do not experience potentially strong feeding competition from other medusan species.

Use of prey resources by the seastars Leptasterias polaris and Asterias vulgaris: a comparison between field observations and laboratory experiments

The subtidal predators Leptasterias polaris and Asterias vulgaris strongly overlap in their size, distribution and use of prey resources. We evaluate the factors determining their use of prey (feeding rate and prey selection) by comparing prey use in the laboratory and in the field. Both seastars have the same preferred prey, the mussel Mytilus edulis, select medium-sized mussels (1.5–3.0 cm), and show similar responses to increasing mussel density (functional response). Differences between the two seastars in their feeding abilities appear to affect both their size selection of mussels and use of alternative prey when mussels become rare or absent. A. vulgaris selects larger mussels than L. polaris when mussel abundance is low, and this may optimize its energetic intake. L. polaris is be better able to prey on the crevice-dwelling clam Hiatella arctica and A. vulgaris on the ophiuroid Ophiopholis aculeata. These differences in prey selection may reduce potential competitive interactions between the two seastars, as they cause prey partitioning when the preferred prey is rare or absent. The greater feeding rate of A. vulgaris on larger mussels likely provides it with a competitive advantage when the abundance of preferred prey is limited.

Diet, behaviour and reproduction of the whelk Buccinum undatum in the northern Gulf of St. Lawrence, eastern Canada

We examine the use of prey resources and seasonal variations in behaviour and reproduction for the whelk Buccinum undatum, the most abundant subtidal carnivore in the northern Gulf of St. Lawrence. Whelks used in this study were collected in the Mingan Islands in 1987 and 1988. The proportion of whelks with food in their stomachs varies seasonally and further with habitat, being greatest on sandy bottoms. It decreases at the onset of breeding in the spring and is generally low through the summer. Fragments of certain organisms (e.g. polychaetes, bivalves, urchins) in whelk stomachs suggest that they are active predators. However, predation is virtually never seen in the field. Other organisms in the stomachs, infrequent feeding, and their high mobility and capacity to detect and locate dead animals on the bottom suggest that whelks are carrion feeders. However, the amount of carrion available is probably insufficient to meet the needs of the biomass of whelks present in these waters. An interaction with seastars may contribute to the whelk's diet. They frequently approach seastars which are extracting bivalves from sediment bottoms and may benefit by feeding on prey remains left by seastars or by foraging in sediments recently disturbed by seastars.