Diet diversity among five co-existing fish species in a tropical river: integration of dietary and stable isotope data

Diets were estimated from stomach contents and the MixSIR model using stable isotope values for five co-existing and abundant benthic fishes in relation to potential prey from a riffle habitat in a tropical river in eastern Thailand. Collectively, aquatic insects were of greatest dietary importance based on stomach contents and, except for one fish species, predicted from the MixSIR. The most prominent functional feeding insect groups in fish diets were collector-filterers and scrapers and, to a lesser extent, predators. MixSIR predicted shrimp to be the most important single dietary constituent for all fishes in contrast to stomach contents, which indicated they are a major item for only one species. MixSIR predicted plant material to be more important in fish diets than stomach contents where the composition of detritus was a concern. Differences in temporal feeding schedules, prey availability, species adaptations and others are important in understanding diet and in the construction of food webs. Stable isotope and dietary analysis provides a more accurate assessment of the food web structure and dynamics of tropical river ecosystems than either method alone.     

A DNA-based method for identification of krill species and its application to analysing the diet of marine vertebrate predators

Accurate identification of species that are consumed by vertebrate predators is necessary for understanding marine food webs. Morphological methods for identifying prey components after consumption often fail to make accurate identifications of invertebrates because prey morphology becomes damaged during capture, ingestion and digestion. Another disadvantage of morphological methods for prey identification is that they often involve sampling procedures that are disruptive for the predator, such as stomach flushing or lethal collection. We have developed a DNA-based method for identifying species of krill (Crustacea: Malacostraca), an enormously abundant group of invertebrates that are directly consumed by many groups of marine vertebrates. The DNA-based approach allows identification of krill species present in samples of vertebrate stomach contents, vomit, and, more importantly, faeces. Utilizing samples of faeces from vertebrate predators minimizes the impact of dietary studies on the subject animals. We demonstrate our method first on samples of Adelie penguin (Pygoscelis adeliae) stomach contents, where DNA-based species identification can be confirmed by prey morphology. We then apply the method to faeces of Adelie penguins and to faeces of the endangered pygmy blue whale (Balaenoptera musculus brevicauda). In each of these cases, krill species consumed by the predators could be identified from their DNA present in faeces or stomach contents.     

Plant‐eating carnivores: Multispecies analysis on factors influencing the frequency of plant occurrence in obligate carnivores

Plant‐eating behavior is one of the greatest mysteries in obligate carnivores. Despite unsuitable morphological and physiological traits for plant consumption, the presence of plants in scat or stomach contents has been reported in various carnivorous species. However, researchers’ interpretations of this subject are varied, and knowledge about it is scarce, without any multispecies studies. This study assessed the extent of variation in the frequency of plant occurrence in scat and stomach contents, as well as its relationship with various factors in 24 felid species using data from 213 published articles. Since the frequency of plant occurrence has not always been reported, we created two‐part models and estimated parameters in a Bayesian framework. We found a significant negative relationship between the frequency of plant occurrence and body mass. This may be because plant‐eating behavior reduces the energy loss caused by parasites and increases the efficiency of energy intake, which has a greater importance in smaller animals that have relatively high metabolic rates. This exploratory study highlights the importance of considering plant consumption in dietary studies on carnivorous species to understand the adaptive significance of this behavior and the relationship between obligate carnivores and plants.     

A comparison of three methods to investigate the diet of breeding double-crested cormorants (Phalacrocorax auritus) in the Beaver Archipelago, northern Lake Michigan

In order to understand the role of waterbirds in aquatic food webs it is important to first get an accurate depiction of their diet. Three methods of dietary assessment (pellets, regurgitate and stomach contents) are compared here for breeding double-crested cormorants (Phalacrocorax auritus) of the Beaver Archipelago, northern Lake Michigan. By numerical frequency (percent number), each method yielded different depictions of the diet. However, in terms of presence and absence (percent frequency) of possible prey types, stomach content data did agree with both pellets and regurgitate data. However, differences were noted between regurgitate and pellets. In terms of biomass measured (percent biomass) in regurgitate and stomachs, data gathered agreed. In essence, pellets underestimate the importance of alewife (Alosa pseudoharengus) and overestimate the importance of crayfish (Orconectes sp.) in the diet when compared to both regurgitate and stomach analysis. The non-lethal method of regurgitate collection and analysis appears most practical in assessing cormorant diet in this system. In combination with information on avian foraging ecology and prey populations, these data may be used to investigate the relationships among cormorants and their prey, and lead to a better understanding of Great Lake food web dynamics.     

Two new indices for stomach content analysis of fishes

Two indices, Simple Resultant Index ( R _s) and Weighted Resultant Index ( R _H), are proposed for enabling the analysis of stomach contents of fishes. The indices are based on percentage occurrence and percentage volume, as in the case of the Index of Preponderance. Possible graphical representation of the indices helps towards understanding the relative representation of the two contributing parameters. The weighted index is, in particular, a means of comparison, grading food items in the order of importance.     

Stable isotope analysis as a tool to monitor dietary trends in little penguins Eudyptula minor

Long‐term dietary monitoring of seabirds can be used to relate population fluctuations to at‐sea events. Stomach flushing is a conventional dietary monitoring technique, but has a number of disadvantages. Stable isotope analysis (SIA) is a less invasive method that provides unbiased dietary information over a longer period. We evaluated stable isotope analysis as a potential tool for monitoring long‐term little penguin Eudyptula minor diet. We determined diet composition during the chick feeding stage using stomach flushing and SIA at three separate colonies, using spatial variation in diet as a surrogate for potential temporal variation. Bayesian isotopic mixing models were generated for blood and feathers to evaluate their ability to discriminate broad‐scale (fish, squid, crustaceans) and fine‐scale (individual prey species) diet composition. Differences in stable carbon and nitrogen isotope ratios were found between colonies: broad‐scale isotopic mixing models predicted different proportional contributions of broad taxa (fish, cephalopod, crustacean) to diet than was indicated by stomach samples, reflecting the bias incurred by one‐off stomach contents analysis. Fine‐scale isotopic mixing models predicted proportional contributions of prey items with less certainty. Blood isotopic mixing models had narrower confidence intervals than models for feathers, but trends in δ¹⁵N for feathers mirrored those for blood. Our results suggest that relying on stomach contents analysis to detect shifts in prey consumption in little penguins could be very misleading, resulting in a less‐than‐complete idea of total prey consumption. SIA of little penguin tissues could be used to monitor dietary shifts across dissimilar taxa that may affect population numbers, but would fail to detect shifts between fish species.     

The effects of alpha-lactabumin and whey protein concentrate on dry matter recovery, TCA soluble protein levels, and peptide distribution in the rat gastrointestinal tract

The effects of two dietary proteins on dry matter recovery, trichloroacetic acid (TCA) soluble protein concentration, and peptide distribution in gastrointestinal contents were investigated in rats trained to consume, in a single 2-hour daily meal, diets containing alpha-lactalbumin (alpha-LA) or whey protein concentrate (WPC) for two weeks. Compared with the WPC diet, the alpha-LA diet emptied faster from the stomach. Dry matter recovery was higher in the stomach contents of rats fed the WPC diet than in those given the alpha-LA diet, but dry matter content in the small intestine was comparable. TCA soluble protein levels in the stomach and the small intestinal contents were also significantly (P < 0.001) higher in rats fed the WPC diet. The concentration of peptides having molecular weights (MW) ranging from 12,500-30,000 daltons (Da) was higher in the stomach contents of rats fed the WPC diet. Conversely, the level of peptides ranging from 5000-12,500 Da was higher in the stomach contents of rats fed the alpha-LA diet. For both diets, the small intestinal contents were characterized by high levels of amino acids and small peptides. These results suggest that the hydrolysis and absorption of alpha-LA is faster than that of WPC.     

A non‐destructive sampling protocol for field studies of seed dispersal by fishes

This paper presents a standardized protocol for the non‐lethal capture of fishes, sampling of stomach contents and quantification of seed dispersal efficiency by frugivorous fishes. Neotropical pacu Piaractus mesopotamicus individuals were collected with fruit‐baited hooks. The diets of 110 fish were sampled using a lavage method, which retrieved >90% of stomach contents of both juveniles and adults and allowed individuals to recover within 5 min of treatment. The proportional volume of six food categories was similar for stomachs and whole digestive tracts retrieved by dissection. Fruit pulp was proportionally lower in the stomach. The abundance and species richness of intact seeds increased with fish size independent of whether only stomachs or whole digestive tracts were analysed. The analysis of stomach contents accounted for 62·5% of the total species richness of seeds dispersed by P. mesopotamicus and 96% of common seeds (seed species retrieved from more than one fish). Germination trials revealed that seed viability was similar for seeds collected from the stomach via lavage and seeds that passed through the entire digestive tract. Therefore, stomach contents provide an unbiased representation of the dietary patterns and seed dispersal of frugivorous fishes.     

Impact of diets varying in dietary fibre characteristics on gastric emptying in pregnant sows

The effects of feeding two fibre‐rich diets with contrasting solubility and a concentrated low dietary fibre on the rate of gastric emptying were examined in six gastric cannulated pregnant sows. Additionally, it was examined whether any effect could be related to the physico‐chemical properties of digesta, i.e. viscosity and/or water binding capacity.

The sows were fed each diet for one week in a 3 x 3 Latin Square design and the samples were taken in a randomised order 0.5, 1, 2, 3, 5 and 15.5 h after the morning meal. The stomach contents were evacuated through the gastric cannula once daily. The evacuated gastric digesta was quantified and a representative sample was taken to determine its viscosity, water binding capacity and its content of dry matter, dietary components and solid (Cr₂O₃) and liquid (polyethylene glycol) phase markers. The flow of liquid digesta was calculated as the difference between digesta and dry matter.

Increasing the content of dietary fibre in the diet led to higher recovery of liquid digesta but did not have any significant effect on the gastric emptying of dry matter and dietary components. The effect of dietary fibre could not be attributed to the viscosity of the liquid phase of digesta but might be related to the ability of the increased gastric dietary fibre content to hold water. The stomach selectively retained the insoluble dietary fibre components most noticeably seen with the bran‐supplemented diet where the concentration of insoluble NSP in digesta increased significantly from 2 hours and onward.     

Metabarcoding for stomach‐content analyses of Pygmy devil ray (Mobula kuhlii cf. eregoodootenkee): Comparing tissue and ethanol preservative‐derived DNA

The application of high‐throughput sequencing to retrieve multi‐taxon DNA from different substrates such as water, soil, and stomach contents has enabled species identification without prior knowledge of taxon compositions. Here we used three minibarcodes designed to target mitochondrial COI in plankton, 16S in fish, and 16S in crustaceans, to compare ethanol‐ and tissue‐derived DNA extraction methodologies for metabarcoding. The stomach contents of pygmy devilrays (Mobula kuhlii cf. eregoodootenkee) were used to test whether ethanol‐derived DNA would provide a suitable substrate for metabarcoding. The DNA barcoding assays indicated that tissue‐derived operational taxonomic units (OTUs) were greater compared to those from extractions performed directly on the ethanol preservative. Tissue‐derived DNA extraction is therefore recommended for broader taxonomic coverage. Metabarcoding applications should consider including the following: (i) multiple barcodes, both taxon specific (e.g., 12S or 16S) and more universal (e.g., COI or 18S) to overcome bias and taxon misidentification and (ii) PCR inhibitor removal steps that will likely enhance amplification yields. However, where tissue is limited or no longer available, but the ethanol‐preservative medium is still available, metabarcoding directly from ethanol does recover the majority of common OTUs, suggesting the ethanol‐retrieval method could be applicable for dietary studies. Metabarcoding directly from preservative ethanol may also be useful where tissue samples are limited or highly valued; bulk samples are collected, such as for rapid species inventories; or mixed‐voucher sampling is conducted (e.g., for plankton, insects, and crustaceans).     

Feeding Strategies in Marine Snakes: An Analysis of Evolutionary, Morphological, Behavioral and Ecological Relationships

Analysis of 1,063 stomach contents from 39 species of sea snakesindicates that about one-third of the shallow, warm, marine,Indo-Australian fish families are preyed upon by sea snakes.Families of eels and gobies are taken by the greatest numbersof snake species. Most species of sea snakes feed on fish familieswhose members are relatively sedentary, dwelling along the bottom,within burrows or reef crevices. With one exception, a fishegg-eating specialization found uniquely in the Aipysurus-Emydocephaluslineage, the dietary habits of sea snakes cannot be categorizedaccording to the snakes' three phylogenetic lineages. Eels,mullet-like, rabbitfish-like and goby-like fish forms are takenby all three lineages. Two or three snake species are generalists,and numerous ones specialize on eels, goby-like fish or catfish.There are differences among sea snake species in the relationshipbetween snake neck girth and the maximum diameter of the prey;in the relationships of both snake gape measurements and fanglength, to the type of prey taken; and in the relationship ofsnake shape and body proportions to the prey selected. Severalmodes of feeding have been observed among sea snakes: feedingin nooks and crannies in the bottom or in reefs, cruising nearthe bottom, and feeding in drift lines. Analysis of percentdigestion of stomach contents and projections backward to thetimes of prey capture provides evidence for feeding periodicity.The greatest amount of diet overlap is for two species of seasnakes which do not both occur at the same locality. Where speciesdo co-occur, diet overlap index values are lower. The numbersof species present as well as their relative abundances varyamong localities as does the relative importance of generalists,eel-eaters, egg-eaters and other specialized feeders.     

Stomach evacuation rate in the planktivorous Antarctic fish Pagothenia borchgrevinki

Summary Specimens of the Antarctic fish Pagothenia borchgrevinki were collected from just below the sea-ice in McMurdo Sound over the period 5–16 November 1987. The stomach contents of 54 freshly captured fish were analysed and showed very strong similarities with an earlier, more extensive, dietary analysis conducted at the same location (Foster et al. 1987). There was as much variation in stomach contents from fish within a school, as there was between schools, contrary to the expectation that fish within a school would have shared a more common feeding history. Fish were maintained in holding tanks for up to 48 h after capture; groups were sacrificed at 6 h intervals and the stomach contents analysed to determine evacuation rates. Evacuation rates of total stomach contents and individual prey species were fitted with an exponential model of decay. The exponential time constant for decay of stomach contents was 32 h (Table 2), while time constants for individual prey species ranged from 12 h for small prey species to 49 h for the larger crustacea. The feeding rate of P. borchgrevinki on important prey species was calculated from the above data (Table 2). Low feeding rates, and the low numbers of P. borchgrevinki argue against removal by predation as being the explanation for the observed reduction in plankton numbers seen in shallow waters (Foster 1989).     

The effects of fire on the frillneck lizard (Chlamydosaurus kingii) in northern Australia

Abstract A population of frillneck lizards, Chlamydosaurus kingii, was monitored by radio telemetry and mark-recapture techniques between April 1991 and April 1994, as part of a landscape-scale fire experiment, in Kakadu National Park, Northern Territory. The study aimed to investigate both the short- and longer-term effects of fire on a lizard species in a tropical savanna where fires are frequent and often annual. Frillneck lizards are able to survive fires that occur in the first few months of the dry season by remaining perched in trees. A high level of mortality (29%) occurred during late dry-season fires, along with changes in their behavioural response to fire: sheltering in either larger trees or hollow termite mounds. Food is more accessible after fires due to the removal of ground vegetation. This is reflected in greater volume and diversity of prey in stomach contents after fires. This increase is more pronounced after late dry-season fires, possibly due to increased accessibility of prey caused by more complete vegetation removal. Frillneck lizards show an overall preference for trees with a dense canopy cover located in an area with a low density of grass. Fire has an effect on this relationship. Frillneck lizards in habitat unburnt for a number of years tend to perch in trees with a smaller canopy, whereas lizards in annually burnt habitat perch in trees with a dense canopy. Volume and composition of lizard stomach contents was broadly similar among fire treatments over a 2 year period, although termites were more predominant in stomach contents of lizards in unburnt habitat. Wet-season body condition is lower in lizards from unburnt habitat, although the reason for this is unclear. These results demonstrate the importance of different fire intensities and regimes on the ecology of a lizard species in a tropical savanna.     

Diet and feeding of fish from Grande River, located below the Volta Grande Reservoir, MG-SP.

We compare the classic model of feeding of tropical fish by means of six bimonthly samplings using gillnets of varying mesh sizes that were inspected every twelve hours throughout a forty-eight hour period. The stomachs of the fish caught were classified in three categories according to quantity of food found. The amount of fat in the visceral cavity with respect to the energetic reserve deposition was also studied. The relative frequencies of the different categories of stomach repletion and fat deposition were examined for patterns of feeding seasonality. The stomachs considered full were examined to record diet composition. To assess the relative importance of the different food resources, we applied Feeding Importance Degree (FID), which is a useful index when difficulties exist in determining a common basis for volume, number, or weight of a given food item in different species, a common problem when dealing with fish species having different feeding habits. The fish species whose stomach contents were analyzed using the FID index were Serrasalmus spilopleura (Characidae), L. prolixa (Loricaridae), Schizodon nasutus (Anostomidae), and Pimelodus maculatus (Pimelodidae). Our findings indicate some contrasting elements, in dietary composition in relation to the classic model for tropical rivers. These factors include the importance of aquatic macrophytes, the lack of piscivorous species, and a lesser presence of allochthonous vegetation in the diet of the species studied.     

Dietary habits of the fanray Platyrhina tangi (Batoidea: Platyrhinidae) in Ariake Bay, Japan

The dietary habits of the fanray Platyrhina tangi were investigated by analyzing the stomach contents of specimens collected in Ariake Bay, Japan. Of 334 stomach specimens, 324 contained food and 10 (3.0%) were empty. The mean percentage weight of stomach contents per unit of body weight was 0.59%. Thirty-seven taxonomic levels of prey were identified. The most common prey was shrimp, followed by fish and mysids. There were no differences in the composition of the diet between sexes, but an ontogenetic dietary shift was observed. Shrimps were the most common prey in all size classes. In addition, smaller individuals frequently ate mysids, and larger individuals often consumed fish. Dietary breadth values increased with size. Trophic level analysis revealed that trophic level increased with size; however, this species is consistently a secondary consumer. Dental sexual dimorphism was also observed. Specifically, mature males had much longer and sharper cusps than females and immature males. Since males and females had similar diets, dental sexual dimorphism may be related to their reproductive behaviour.     

Altered trophic pathway and parasitism in a native predator (Lepomis gibbosus) feeding on introduced prey (Dreissena polymorpha)

Populations of invasive species tend to have fewer parasites in their introduced ranges than in their native ranges and are also thought to have fewer parasites than native prey. This ‘release’ from parasites has unstudied implications for native predators feeding on exotic prey. In particular, shifts from native to exotic prey should reduce levels of trophically transmitted parasites. We tested this hypothesis in native populations of pumpkinseed sunfish (Lepomis gibbosus) in Lake Opinicon, where fish stomach contents were studied intensively in the 1970s, prior to the appearance of exotic zebra mussels (Dreissena polymorpha) in the mid-1990s. Zebra mussels were common in stomachs of present-day pumpkinseeds, and stable isotopes of carbon and nitrogen confirmed their importance in long-term diets. Because historical parasite data were not available in Lake Opinicon, we also surveyed stomach contents and parasites in pumpkinseed in both Lake Opinicon and an ecologically similar, neighboring lake where zebra mussels were absent. Stomach contents of pumpkinseed in the companion lake did not differ from those of pre-invasion fish from Lake Opinicon. The companion lake, therefore, served as a surrogate “pre-invasion” reference to assess effects of zebra mussel consumption on parasites in pumpkinseed. Trophically transmitted parasites were less species-rich and abundant in Lake Opinicon, where fish fed on zebra mussels, although factors other than zebra mussel consumption may contribute to these differences. Predation on zebra mussels has clearly contributed to a novel trophic coupling between littoral and pelagic food webs in Lake Opinicon.     

A new approach to graphical analysis of feeding strategy from stomach contents data—modification of the Costello (1990) method

A modification of the graphical Costello method is proposed for the analysis of stomach contents data. The new method allows prey importance, feeding strategy and the interand intra-individual components of niche width to be explored using graphical presentation. The analysis is based on a two-dimensional representation of prey-specific abundance and frequency of occurrence of the different prey types in the diet. The paper describes the new method and the parameters therein, and also present some examples of the utilization of the method. The method may be particularly well-suited for the examination of predictions made from optimal foraging, competition and niche theories.     

Dietary daidzein decreases food intake accompanied with delayed gastric emptying in ovariectomized rats

ABSTRACT

We previously found that equol, a metabolite of intestinal bacterial conversion from soy isoflavone daidzein, has female-specific anorectic effects. In the present study, we used seven-week-old female ovariectomized (OVX) Sprague Dawley rats to test the hypothesis that the anorectic effect of dietary daidzein may be attributed to delayed gastric emptying. Results suggest that dietary daidzein delays gastric emptying and that it has an anorectic effect with residual gastric contents, but not without gastric contents. Dietary equol significantly decreased daily food intake in the OVX rats without sleeve gastrectomy, but not in those with sleeve gastrectomy, suggesting that the accumulation of food in the stomach is required for the anorectic effect of equol to occur. These results support the hypothesis that the anorectic effect of dietary daidzein is attributed to delayed gastric emptying.