Common Prey Types Research Articles

Foraging microhabitat selection of Spoon-billed Sandpiper in the Upper Gulf of Mottama, Myanmar

Spoon-billed Sandpiper ( Calidris pygmaea ) is Critically Endangered, declining 9% per year since 2009; although important wintering sites have been identified, little is known about why particular sites are selected. The Gulf of Mottama, Myanmar, supports perhaps 60% of the global population in winter. Our objective was to identify macrohabitat and microhabitat characteristics of Spoon-billed Sandpiper foraging locations. We considered 1) location (relative to distance to the main channel of the gulf), 2) temporal patterns (relative to the tide cycle), 3) physical substrate characteristics and 4) possible food sources. Spoon-billed Sandpipers were observed at 26 foraging locations during the study (November 2019-March 2020). The sandpipers were found more frequently on mixed sand-mud substrate, similar to over-wintering sites in Bangladesh, and while it is likely that the mixed sand-mud substrate was generally further from the main channel than the pure sand substrate, presumably due to tidal action, we found no evidence that Spoon-billed Sandpiper locations were associated with the tide cycle. Logistic regression models indicated that foraging locations were associated with shallow surface puddles, compared to random locations. The mixed sand-mud substrate supported more polychaetes relative to crabs or insect larvae, the three most common prey types in our 442 benthos samples used to assess prey availability. Our results reinforce the point that Mottama needs to be protected as an intact functioning estuary and key site for Spoon-billed Sandpiper and other threatened shorebirds. Although we have identified some characteristics of foraging sites associated with Spoon-billed Sandpiper at Mottama, we still lack understanding as to whether and how potential prey is driving site selection and how these characteristics vary among wintering sites.

The effect of aquatic and terrestrial prey availability on metal accumulation in pied flycatcher (Ficedula hypoleuca) nestlings

Ingestion of contaminated prey is a major route for metal exposure in terrestrial insectivores. In terrestrial ecosystems adjacent to lakes and streams, emerging aquatic insects can transport metals, accumulated during their larval stage, from aquatic to terrestrial ecosystems. However, contaminant exposure via aquatic insects has often been ignored in terrestrial environments, despite such insects representing a substantial part of the diet for terrestrial insectivores living close to lakes and streams. In this study, we investigated how dietary lead (Pb) and calcium (Ca) exposure from different aquatic and terrestrial prey types affects Pb accumulation in pied flycatcher (Ficedula hypoleuca) nestlings living close to a former Pb/zinc (Zn) mine in northern Sweden, which closed in 2001. Stable isotope analysis (δ 13C and δ15N) of nestling blood and different prey types was used to estimate nestlings' diet. Ants, Lepidoptera larvae and Trichoptera were the most common prey types in the nestlings’ diet, in which aquatic prey types (Trichoptera included) accounted for 2.0–96.4%. Ingestion of specific prey groups, such as aquatic insects and ants, were important for Pb accumulation in nestlings, and when access to aquatic prey was low, ants were the predominant source of Pb. The influence of dietary Ca on Pb accumulation was less consistent, but Ca availability was relatively high and often co-occurred with high Pb concentrations in invertebrates. The study shows that both the proportion of different prey and their individual metal concentrations need to be considered when estimating exposure risks for insectivores. Further, it highlights the need to account for metal exposure from emerging aquatic insects for terrestrial insectivores living close to lakes and streams.

Broadbill swordfish (Xiphias gladius) foraging and vertical movements in the north‐west Atlantic

AbstractThe northern edge of Georges Bank is an important seasonal foraging habitat for swordfish (Xiphias gladius) in the North Atlantic, where aggregations support commercial pelagic longline and harpoon fisheries. Following a period of overfishing during the 1990s, the North Atlantic X. gladius stock underwent a period of recovery during the early 2000s and was considered rebuilt in 2009. We analysed stomach contents from X. gladius (n = 39) harvested by the Canadian harpoon fishery on Georges Bank in 2007 to characterize diet in this important foraging habitat. We used electronic tagging data from X. gladius (n = 6) on Georges Bank in 2005–2007 to assess vertical habitat preferences and associated prey composition within those zones. We also used stable isotope analysis (δ13C and δ15N) of X. gladius liver (n = 2) and common prey types (Paralepididae, Myctophidae, Merluccidae, Ommastrephidae) as a longer‐term record of feeding. Stomach contents were co‐dominated by Paralepididae [31.9% weight (W)] and Ommastrephidae (36.8%W) with secondary contributions from hake (Merluccidae, 6.5%W), Myctophidae (2.9%W) and Sebastidae (2.1%W). X. gladius displayed diel vertical migrations, descending to depths of 300–400 m during daytime followed by residence in surface waters at night. X. gladius liver δ15N values were similar to or lower than values of primary stomach contents, likely due to bias of diet consumed in southerly waters with lower nitrogen isotope baselines prior to arrival on Georges Bank. Diet data are similar to results from historical studies from the late 1950s to the early 1980s. This apparent temporal stability to the underlying food web in this region may explain the high X. gladius site fidelity observed in electronic tagging studies and the consistent aggregation of these fish to this region.

The acoustic world of odontocete biosonar and technical sonar

Bats, toothed whales (odontocetes), and man-made sonar all use the same acoustical principles of echo-location. The fundamental components of these systems include transmitter frequency response, tailored waveforms, properties of a propagation medium, clutter, background noise, target strength, and receiver properties. Across all of these elements, there are very large differences (i.e., underwater versus in-air propagation). This gives rise to several interesting questions about the performance of these sonar systems. For example, given the bandwidth and duration of transmitted signals for bats and odontocetes, what are the relative target strengths/maximum imparted Doppler shifts for common prey types? How does the ambiguity function of the echo compare to these target sizes and relative Doppler shifts? What effect does the medium has and how do these systems adapt to clutter, interferers, and noise? This talk will focus on the underwater aspects of these systems which include (i) the properties of the underwater propagation medium, (ii) the geometry and material of the boundaries that limit the underwater propagation channel (this includes targets of interest and clutter), and (iii) the time-frequency and spatial properties of the underwater sources.Bats, toothed whales (odontocetes), and man-made sonar all use the same acoustical principles of echo-location. The fundamental components of these systems include transmitter frequency response, tailored waveforms, properties of a propagation medium, clutter, background noise, target strength, and receiver properties. Across all of these elements, there are very large differences (i.e., underwater versus in-air propagation). This gives rise to several interesting questions about the performance of these sonar systems. For example, given the bandwidth and duration of transmitted signals for bats and odontocetes, what are the relative target strengths/maximum imparted Doppler shifts for common prey types? How does the ambiguity function of the echo compare to these target sizes and relative Doppler shifts? What effect does the medium has and how do these systems adapt to clutter, interferers, and noise? This talk will focus on the underwater aspects of these systems which include (i) the properties of the ...

The influence of ontogenetic diet variation on consumption rate estimates: a marine example

Consumption rates are the foundation of trophic ecology, yet bioenergetics models used to estimate these rates can lack realism by not incorporating the ontogeny of diet. We constructed a bioenergetics model of a marine predatory fish (tailor, Pomatomus saltatrix) that incorporated high-resolution ontogenetic diet variation, and compared consumption estimates to those derived from typical bioenergetics models that do not consider ontogenetic diet variation. We found tailor consumption was over- or under-estimated by ~5–25% when only including the most common prey item. This error was due to a positive relationship between mean prey energy density and predator body size. Since high-resolution diet data isn’t always available, we also simulated how increasing dietary information progressively influenced consumption rate estimates. The greatest improvement in consumption rate estimates occurred when diet variation of 2–3 stanzas (1–2 juvenile stanzas, and adults) was included, with at least 5–6 most common prey types per stanza. We recommend increased emphasis on incorporating the ontogeny of diet and prey energy density in consumption rate estimates, especially for species with spatially segregated life stages or variable diets. A small-moderate increase in the resolution of dietary information can greatly benefit the accuracy of estimated consumption rates. We present a method of incorporating variable prey energy density into bioenergetics models.

Habitat-specific growth, survival and diet of late juvenile hatchery-reared spotted seatrout (Cynoscion nebulosus)

As a key recreational fishery species throughout the Gulf of Mexico, the spotted seatrout (Cynoscion nebulosus Cuvier) requires sound management practices (VanderKooy and Muller, 2003; Fulford and Hendon, 2010). Accordingly, it is imperative to fully understand the ecology of spotted seatrout (Lorio and Perret, 1978), especially factors affecting the successful recruitment of this estuarine-dependent carnivore. Considering the large gap in knowledge about the ecology of late juvenile stage spotted seatrout, as well as the need to evaluate current stock enhancement practices for this species, the objective of this study was to assess the growth, survival and diet of late juvenile hatchery-reared (HR) spotted seatrout within three prospective nursery habitats in a shallow bay system: submerged aquatic vegetation (SAV), non-vegetated shoreline (NVS), and non-vegetated open water (NVO). While caged under natural conditions for 4weeks, relative growth was significantly greater for fish caged in SAV and NVS habitats compared to NVO habitat. Mortality was relatively high in the first week of the study during acclimation. Stomachs of HR fish contained prey, and the diet composition of HR fish included common prey types consumed by comparable sizes of wild fish. Findings indicate that habitats within or in close proximity to SAV or marsh shoreline offer more favorable conditions than deeper open water habitat for late juvenile HR spotted seatrout. Moreover, HR fish can acclimate to natural conditions and successfully transition to a natural diet, in the absence of predators and competitors.

Krill feeding on sediment in the Gulf of Maine (North Atlantic)

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 455:157-172 (2012) - DOI: https://doi.org/10.3354/meps09632 Krill feeding on sediment in the Gulf of Maine (North Atlantic) Alison C. Cleary*, Edward G. Durbin, Tatiana A. Rynearson University of Rhode Island, Graduate School of Oceanography, Narragansett, Rhode Island, 02882, USA *Email: acleary@gso.uri.edu ABSTRACT: Krill are key members of many marine ecosystems, serving as a critical trophic link between microscopic organisms and large predators such as whales, fish, and seabirds. Krill feeding is thus important to ecosystem carbon cycling. Traditional approaches to determining in-situ krill feeding require a priori assumptions, and may have prey-type detection biases. We took a DNA-based approach to measuring in-situ feeding by northern krill Meganyctiphanes norvegica. The diversity of prey consumed by M. norvegica in situ was analyzed for 80 krill at 8 stations throughout the Gulf of Maine (North Atlantic) using peptide nucleic acid mediated polymerase chain reaction (PNA-PCR) clone library sequencing of 18S rDNA. Relative abundance of the 2 most common prey types was measured with quantitative PCR (qPCR) in the guts of 16 krill. The 245 prey sequences recovered from krill gut contents included copepods, salps, phytoplankton, and a poorly known organism found to be sediment associated. Calanus finmarchicus and the sediment-associated organism were found most commonly, at 7 and 8 stations, respectively, and their 18S rDNA was present in nearly equal quantities in individual krill guts. M. norvegica, like most krill, are typically considered planktivorous; thus krill feeding on sediment organisms represents an unrecognized pathway for carbon flow from the sediment to the pelagic. Calculations suggest that this unrecognized pathway could potentially bring over 100000 t of carbon annually back into the Gulf of Maine pelagic ecosystem, equivalent to 4% of annual primary production, or the energy demands of 80% of the region’s fin whale population. KEY WORDS: Meganyctiphanes norvegica · Gut contents · 18S rDNA · Benthic−pelagic coupling · PNA-PCR Full text in pdf format PreviousNextCite this article as: Cleary AC, Durbin EG, Rynearson TA (2012) Krill feeding on sediment in the Gulf of Maine (North Atlantic). Mar Ecol Prog Ser 455:157-172. https://doi.org/10.3354/meps09632 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 455. Online publication date: May 30, 2012 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2012 Inter-Research.

Latitudinal and taxonomic patterns in the feeding ecologies of fish larvae: A literature synthesis

The longtime focus on factors that influence the survival of marine fish larvae has yielded an extensive number of studies on larval fish diets and feeding success. In light of a recent increase in such studies within lower latitudes, results from the peer-reviewed literature were synthesized to examine both latitudinal and taxonomic differences in several trophic-related categories, including feeding incidence, trophic niche breadth, ontogenetic diet shifts, dominant prey types, diet broadness, and larval piscivory. A total of 204 investigations (taxon–article combinations) contained suitable results for at least one of these categories. Feeding incidences (proportions of larvae containing food) were significantly higher in lower latitudes with all taxa combined, as well as only within the order Perciformes. Feeding incidences also differed among orders, with Perciformes and Scorpaeniformes having the highest values. The number of larval taxa exhibiting a significantly increasing niche breadth (SD of the log of prey sizes) with larval size decreased toward lower latitudes, with some taxa in lower latitudes exhibiting a decrease in niche breadth with size. The frequency of exhibiting ontogenetic diets shifts decreased with decreasing latitude, as did relative diet broadness (a function of prey types). The most common dominant prey types in the diets of higher latitude larvae were nauplii and calanoid copepods, with cyclopoids being rare in higher latitudes. Dominant prey types in lower latitudes were more diverse, with nauplii, calanoids, and cyclopoids being equally important. Appendicularians increased in importance with decreasing latitude, and one of the clearest latitudinal distinctions was the display of larval piscivory (almost exclusively by scombroid taxa), which was highly concentrated in lower latitudes. Overall, the latitudinal differences observed for multiple trophic-related factors highlight inherent distinctions in larval fish feeding ecologies, likely reflecting differences in the overall structure of planktonic food webs over large latitudinal gradients.

Learning predator promotes coexistence of prey species in host-parasitoid systems.

Ecological theory suggests that frequency-dependent predation, in which more common prey types are disproportionately favored, promotes the coexistence of competing prey species. However, many of the earlier empirical studies that investigated the effect of frequency-dependent predation were short-term and ignored predator-prey dynamics and system persistence. Therefore, we used long-term observation of population dynamics to test how frequency-dependent predation influences the dynamics and coexistence of competing prey species using insect laboratory populations. We established two-host-one-parasitoid populations with two bruchid beetles, Callosobruchus chinensis and C. maculatus, as the hosts and the pteromalid wasp Anisopteromalus calandrae as their common parasitoid. When the parasitoid was absent, C. chinensis was competitively excluded in ∼20 wk. Introducing the parasitoid greatly enhanced the coexistence time to a maximum of 118 wk. In the replicates of long-lasting coexistence, the two host species C. maculatus and C. chinensis exhibited periodic antiphase oscillations. Behavioral experiments showed frequency-dependent predation of A. calandrae that was caused by learning. Females of A. calandrae learned host-related olfactory cues during oviposition and increased their preference for the common host species. Numerical simulations showed that parasitoid learning was the essential mechanism that promoted persistence in this host-parasitoid system. Our study is an empirical demonstration that frequency-dependent predation has an important role in greatly enhancing the coexistence of prey populations, suggesting that predator learning affects predator-prey population dynamics and shapes biological communities in nature.

Density‐dependent diet composition of juvenile Atlantic salmon (Salmo salar)

Abstract – Decreases in body growth with increasing population density may be caused by reduced prey delivery rate. However, changes in food quality because of an increasing inclusion of suboptimal prey in the diet may also contribute to such effects. Here, we test for density‐dependent diet composition by creating spatial variation in Atlantic salmon young‐of‐the‐year (YOY) density in three replicate streams and obtain detailed information on individual positions (±1 m) and diet. Diet breadth with respect to prey size increased with increasing local density for the two most common prey types (Chironomidae and Ephemeroptera). For the largest prey type (Ephemeroptera), there was also an increase in mean prey size with increasing density, suggesting that YOY salmon preferentially utilise the smaller prey among those available. According to optimal foraging theory, changes in diet with increasing local density are likely to come at an energetic cost and hence may contribute to the commonly observed density‐dependent growth of juvenile salmonids.

Adaptive significance of food income in European snakes: body size is related to prey energetics

307..317 Feeding strategies and diet patterns have been extensively investigated in vertebrates and, more specifically, in snakes. Although it has been hypothesized that prey species may differ in terms of energy content, almost no theoretical or practical study has been carried out to determine actual nutritional values of the common prey types of wild snakes. Our model taxa were a selection of widely distributed and well known European snake species, which have all been studied in depth: approximately 76% of their diet is composed of mammals, reptiles, and insects. We therefore selected a single model species for each of these categories and proceeded with the analyses. Nutritional values were determined using a standard procedure: lizards and mice were richer in proteins than insects (crickets); insects and mice were richer in lipids than lizards, and mice and crickets have a higher energy content than lizards; lizards were rich in ashes. We then applied our experimental results to a selected sample of European terrestrial snakes (11 populations, ten species, seven genera, two families) characterized by different body size (50‐160 cm total length) and reproductive strategies (oviparous versus viviparous), aiming to correlate these parameters with patterns of energy income. A direct relationship was found between body mass/body length ratio (BCI, body condition index) and meal energetics: the higher the BCI, the higher was the metabolic requirement, whereas BCI was independent of species or of reproductive system effect. Large-sized snakes thus need a highly diversified and more energy-rich diet than smaller snakes, supporting previous hypotheses. The simple applicability of this method could be of valuable support in further comparative research work, reducing experimental costs and stimulating further ecological, behavioural, and, possibly, phylogenetic comparisons. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 100, 307‐317. ADDITIONAL KEYWORDS: Colubridae ‐ comparative analysis ‐ diet ‐ energy balance ‐ Serpentes ‐ Viperidae.

The predation cost of female resistance

Mating costs to females can result in female reluctance to mate and the evolution of seemingly costly rejection strategies. The costs of mating have been widely studied; however, the costs of resistance have rarely been quantified. In the Lake Eyre dragon, Ctenophorus maculosus, gravid females flip over onto their backs to prevent superfluous matings. In doing so, they compromise their camouflage as females have bright orange ventral coloration during the breeding season. Visual models confirmed that resisting females are much more conspicuous to birds, their primary predator, than unresisting females. We assessed the predation risk of female resistance via a large-scale field experiment using model female lizards. Although the flipped over, orange models were more conspicuous to visual predators, they were attacked significantly less than cryptic models. It appears that predators avoid the bright females, possibly because they do not recognize orange individuals as food or avoid rare, conspicuously colored prey. Thus, conspicuous female rejection displays may be maintained in part by apostatic selection, in which predators form a search image for and preferentially attack more common prey types.

Diet of Feral Cats in Hawai?i Volcanoes National Park

We documented the diet of feral cats by analysing the contents of 42 digestive tracts from Kilauea and Mauna Loa In Hawai'i Volcanoes National Park. Small mammals, invertebrates, and birds were the most common prey types consumed by feral cats. Birds occurred in 27.8?29.2% of digestive tracts. The total number of bird, small mammal, and invertebrate prey differed between Kilauea and Mauna Loa. On Mauna Loa, significantly more (89%) feral cats consumed small mammals, primarily rodents, than on Kilauea Volcano (50%). Mice (Mus musculus) were the major component of the feral cat diet on Mauna Loa, whereas Orthoptera were the major component of the diet on Kilauea. We recovered a mandible set, feathers, and bones of an endangered Hawaiian Petrel (Plerodroma sandwichensis) from a digestive tract from Mauna Loa. This specimen represents the first well-documented endangered seabird to be recovered from the digestive tract of a feral cat in Hawai'i and suggests that feral cats prey on this species.

Sealing, whaling and caribou: the skeletal isotope chemistry of Eastern Arctic foragers

We obtained stable isotope signatures ( δ 13C, δ 15N) and AMS radiocarbon dates for a small set of Dorset remains, Thule-era burials from northwest Hudson Bay and proto-historic burials from Southampton Island to assess the importance of whaling in eastern Canadian Arctic economies. Classic Thule occupation of the Eastern Arctic (ca. AD 1000–1350) coincided with the Medieval Warm Period and was thought to have been facilitated by dog traction and open-sea hunting of bowhead whale. Despite the potential economic importance of whaling, dietary reconstructions for this period are based on the relative frequency of common prey types in midden faunal assemblages and rarely include bowhead whale skeletal elements, which often comprise the superstructure of Classic Thule residential dwellings. Although our findings are constrained by the paucity of Classic Thule burials at sites under study, they provide an empirically derived estimate of reliance on whaling for the Modified Thule, those who post-date AD 1350, indicating whaling accounted for approximately 12% of dietary intake. We also examine the relationship between Thule whaling and indicators of status and identify a Dorset-aged burial and three individuals with European diets among the proto-historic collection from Southampton Island.

Trophic Relationships of Three Sunfishes (Lepomis spp.) in an Estuarine Bayou

We examined ontogenetic, interspecific, and seasonal trophic patterns among sympatric sunfish species, redspotted sunfish,Lepomis miniatus; redear sunfish,Lepomis microlophus; and bluegill,Lepomis macrochirus, in an estuarine bayou. In particular we studied these feeding patterns in relation to relative abundances of prey from different benthic feeding habitats. All three sunfishes showed ontogenetic divergence in their trophic niches, reflecting different ecomorphological specializations. Small fishes depended on zooplankton, whereas larger fishes of all three species shifted their diets to benthic macrofauna. A potential for trophic resource partitioning was reflected by dietary differences among the three sunfish species. One impalied mechanism for resource partitioning was feeding habitat, as redear sunfish frequently used sediment-associated prey, while bluegill showed greater use of water-column-associated prey, and redspotted sunfish often used SAV-associated prey. However, all three sunfishes apparently used each feeding habitat to some degree; and, trophic differences were more clearly based on prey type than on feeding habitat. Redear sunfish, which can crush hard-shelled prey, exhibited the most distinctive diet. An apparent seasonal shift in feeding habitat occurred in autumn/winter, as indicated by increased overlap between diets and SAV. This shift was facilitated by changes in the relative abundances of several common prey types between benthic habitats. The relative abuandance and use of freshwater and estuarine-derived prey also varied seasonally, suggesting a possible trophic benefit of consistent prey availability in the estuarine bayou.

Ontogenic changes in prey consumption by the stoneflyParagnetina mediain relation to temporal variation in prey nutrient content

In Duffin Creek, Ontario, nymphs of the predatory stonefly Paragnetina media (Perlidae) commonly feed on detritus and three prey types: hydropsychid larvae (Hydropsyche sparna and Hydropsyche slossonae), nymphs of the mayfly Baetis tricaudatus, and chironomid larvae belonging to subfamilies Tanypodinae and Orthocladiinae. This study examined temporal changes in the nutrient (lipid and protein) content of these prey to determine if the predator tracks food resources by selecting prey on the basis of nutrient requirements at different stages in its own development. All three common prey types exhibited temporal variation in lipid levels, with peaks occurring at different times: June for the hydropsychids, August for the chironomids, and October for B. tricaudatus. Prey protein levels were less variable. The proportions of the prey types that were eaten varied throughout the year and according to predator size. For example, while small P. media ate mostly detritus, they also consumed early-instar hydropsychids during the larval recruitment period of the latter; mid-sized nymphs included more animal matter in their diet, primarily chironomid larvae; and larger nymphs primarily ate B. tricaudatus. Paragnetina media nymphs in all size categories showed an increase in body lipid level in the autumn, suggesting a general accumulation of lipid reserves in readiness for the winter, although dietary adjustment to accomplish this was detectable only in small P. media. There was no evidence to suggest that P. media selected prey on the basis of the latter's protein content. Male P. media nymphs preyed predominantly on chironomid larvae and included mites in their diet, whereas female nymphs preferred B. tricaudatus and hydropsychids. On a per milligram body mass basis, male nymphs had the higher nutrient gain, since, for both lipid and protein, intake by males was between two and three times that by females. However, male and female bodies had similar lipid contents. We conclude that whereas there is temporal variation both in the nutritional (lipid and protein) content of the common prey of P. media and in this predator's diet, there is only weak evidence for nutrient-resource tracking.

Ontogenic changes in prey consumption by the stonefly <i>Paragnetina media</i> in relation to temporal variation in prey nutrient content

In Duffin Creek, Ontario, nymphs of the predatory stonefly Paragnetina media (Perlidae) commonly feed on detritus and three prey types: hydropsychid larvae (Hydropsyche sparna and Hydropsyche slossonae), nymphs of the mayfly Baetis tricaudatus, and chironomid larvae belonging to subfamilies Tanypodinae and Orthocladiinae. This study examined temporal changes in the nutrient (lipid and protein) content of these prey to determine if the predator tracks food resources by selecting prey on the basis of nutrient requirements at different stages in its own development. All three common prey types exhibited temporal variation in lipid levels, with peaks occurring at different times: June for the hydropsychids, August for the chironomids, and October for B. tricaudatus. Prey protein levels were less variable. The proportions of the prey types that were eaten varied throughout the year and according to predator size. For example, while small P. media ate mostly detritus, they also consumed early-instar hydropsychi...

Diet and prey selection of six species of tuna baitfish in three coral reef lagoons in the Solomon Islands

The diets and prey selection of six species of tuna baitfish (the engraulids Stolephorus devisi and S. heterolobus, three dussumierids of genus Spratelloides, and the apogonid Archamiazosterophora. were examined at two heavily fished and one unfished site in the Solomon Islands. All species ate almost exclusively nektonic zooplankton, especially crustaceans. Calanoid copepods were the most importantpreyofallspeciesexcept A. zosterophora, whichateagreater biomassoflarvalfish. Baitfishcould be divided into two groups according to prey taxa: the apogonid A. zosterophora and the engraulids S. devisi and S. heterolobus ate similar prey in similar proportions; the three dussumierid species (Spratelloides) ate smaller zooplankton and a greater proportion of calanoids than the other species.The size of prey eaten differed greatly between baitfish species. A. zosterophora ate larger prey than other species. The two Stolephorus species ate similar‐size prey, which were larger than the prey of the three species of Spratelloides. Among the Spratelloides, S. delicatulus ate smaller prey than the other two species. All species differed in the size of calanoids, carids and brachyurans they ate. In all cases, prey differed more by the minimum dimension (width) than by volume. There were significant relationships between fish length and prey width and volume, except for Archamia. However, in terms ofvolume, the prey of Stolephorus increased with fish size at a much greater rate than in Spratelloides species.Baitfish diets were compared to the zooplankton available. Prey selection by the three Spratelloides species and two Sfolephorus species was density‐dependent for all common prey types, although Stolephorus positively selected hyperiid amphipods and brachyurans. Archamia showed strong negative selection for calanoids and selected larger zooplankters, including fish larvae and carids, avoiding smaller, more common, prey.Overall baitfish density and zooplankton density did not correlate at either of the heavily fished sites. Nor were there consistent differences for any species in diet or changes in prey selection between the heavily fished and unfished sites. This suggests that, in these lagoons, commercial baitfish catches are not directly influenced by the composition or density of prey.

Apostatic selection on prey that match the background

There is good experimental evidence that predators often remove disproportionately more of the common prey types. This ‘apostatic selection’ could maintain colour polymorphisms within prey species. In nature, morphs of many species appear to match components in the background, but most of the experiments that have tested apostatic selection have used prey that were conspicuous. In work described in this paper, wild birds at one site were presented a random order of 51 experiments. Seventeen frequencies of orange and grey pastry prey were presented on each of three types of background: a hessian sheet scattered with either orange and grey stones (the ‘matching’ background), or lilac and yellow stones, or green stones (two sorts of ‘control’ background). Each experiment consisted of four trials in succession and the numbers of the two colours eaten in each trial were recorded when about half the total prey had been eaten. Two methods of graphical analysis revealed that apostatic selection occurred on the prey on all three backgrounds, but was strongest in the grey/orange one. This last result must have been caused by some effect of the match between prey and background colour, and behavioural explanations are suggested. It is unclear whether the prey were exhibiting ‘crypsis’ or ‘masquerade’.

Selective predation and caloric consumption by the siphonophore Rosacea cymbiformis in nature

Individuals of the siphonophore Rosacea cymbiformis (Chiaje, 1822) were collected in surface waters of the Gulf of California during July/August 1978, off Southern California during May 1980, and in the Sargasso Sea during July 1979. Specimens were preserved within 30 min after capture, and the ingested prey in the gastrozooids were examined microscopically and identified to the closest practical taxon. Most gastrozooids (50 to 84%) contained recognizable prey organisms. The prey were primarily copepods, but crab zoeae, pelagic molluscs, juvenile shrimps and mysids were also commonly eaten. There were significant differences between the sizes and types of prey organisms that had been ingested and that were available in the environment of the siphonophores. Electivity indices for the prey demonstrated that the large and/or active prey were selected. The feeding selectivity exhibited by the siphonophores probably depends upon speed and diameter of the prey, which affect the frequency of contact with the siphonopore tentacles. Behavioral observations suggest that R. cymbiformis feeds primarily in the light. Digestion experiments indicate that most prey remain in the gastrozooids for 8 h or more before egestion. The caloric values of common prey types were derived from their length to dry weight regressions. The caloric consumption of R. cymbiformis during the 4 to 6 h feeding period following sunrise was estimated to be from 0.109 to 0.365 cal per gastrozooid; the daily caloric consumption was projected to be at least 2.4 to 8.2 times that required to balance metabolism.