Feeding habits of Schizothorax oconnori Lloyd, 1908 in the Yarlung Zangbo River,Tibet

Seasonal, ontogenetic and sexual feeding habits of Schizothorax oconnori in the Yarlung Zangbo River were studied using percentages by number, weight and index of relative importance (IRI). The Schoener overlap index was applied to compare diets related to season, fish size and sex. The feeding intensity exhibited seasonal trends, with a minimum food intake in summer. Overall, 103 prey taxa belonging to seven prey categories (diatoms, green algae, cyanophytes, other algae, small invertebrates, macroinvertebrates and remains) were identified in 136 guts. Highest feeding diversity and evenness were recorded in summer, while lowest values of both indices occurred in autumn. S. oconnori fed almost exclusively on periphyton. Diatoms were the most important prey in terms of number, weight and IRI, followed by macroinvertebrates; other prey were also occasionally ingested. S. oconnori is a generalized and opportunistic feeder relying upon a wide trophic spectrum, but mainly feeds on diatoms as they are the most prevalent periphyton. There were high diet overlaps among seasons, fish sizes and between sexes. S. oconnori showed no dietary preference for any of the other prey available in the periphyton community.     

Feeding strategy of the Deepwater Goby,Chasar bathybius,in the Southern Caspian Sea (Osteichthyes: Gobiidae)

Eleven animal taxa were identified as prey of the Deepwater Goby, Chasar bathybius (Kessler, 1877), by analysing stomach contents: these were some species of Gobiidae, Clupeidae, Atherina boyeri, Carassius gibelio, Liza sp., Cardium sp., and Rhithropanopeus sp. With a frequency of 66%, Neogobius fluviatilis was the most prevalent food item, and was the dominant prey taxon in all seasons and age groups. C. bathybius exhibited a specialist feeding strategy on this fish and the other prey types. The dietary breadth is highest in spring according to the Shannon index.     

Comparisons of the food niches of three native and two introduced fish species in an Australian river

Synopsis The breadth, correspondence and overlap of the diets of the small and large size classes of the three native species (Galaxias occidentalis, Bostockia porosa and Edelia vittata) and two introduced species (Gambusia holbrooki and Perca fluviatilis) of fishes found in the shallows of the main channel and in the tributary creeks of a south-western Australian river have been compared in each season. Classification and ordination were used to examine the overall interrelationships of the diets across species, size groups, seasons and the location where the fish were caught (channel or creek). The smaller fish had a narrower dietary breadth than larger fish in the spring and summer, presumably reflecting the size limit imposed on prey size by their possession of a relatively small mouth in these seasons. Intraspecific dietary overlap between large and small size classes was usually high in G. occidentalis, but generally low in G. holbrooki and P. fluviatilis, and also in B. porosa and E. vittata when the difference between the lengths of the two size groups was greatest. Dietary overlap was least in autumn when the main prey taxa were most abundant. During winter, the diets of the three native species in the tributaries converged, probably reflecting a relatively low faunal diversity in these highly seasonal water bodies. The only relatively consistent interspecific overlap in diet was between B. porosa and E. vittata. Classification and ordination of the dietary samples separated G. occidentalis (which fed extensively on terrestrial organisms from the water surface) from the smaller P. fluviatilis (that concentrated on copepods in the plankton) and from B. porosa and E. vittata (which ingested primarily benthic organisms). Furthermore, B. porosa tended to ingest larger prey taxa than E. vittata. The diet of Gambusia holbrooki is sometimes dominated by terrestrial insects and at other times by benthic organisms, demonstrating that this species is an opportunistic carnivore. It is concluded that food partitioning by the three native and two introduced fish species in the Collie River is likely to be one of the principal factors facilitating the coexistence of substantial populations of these species in this system.     

Patterns of prey use by lesser scaup <Emphasis Type="Italic">Aythya affinis</Emphasis> (Aves) and diet overlap with fishes during spring migration

Recent decline in the lesser scaup Aythya affinis population has been linked to changes in wetland conditions along their spring migration routes. In particular, the use of amphipod prey by lesser scaup has declined in many regions of the upper Midwest U.S.A. and has been linked to expanded fisheries, although empirical data on diet overlap are lacking. To explore patterns of prey use by lesser scaup and diet overlap with fishes, we quantified diets of scaup and fishes during the 2003 and 2004 spring migration in eastern South Dakota, U.S.A. We compared diet overlap between lesser scaup and fishes collected from Twin Lakes, South Dakota—an important stopover location for spring-migrating scaup. Plant seeds occurred in >95% of lesser scaup diets (n = 118) and represented an appreciable amount of consumed biomass (>70%). Gastropods, amphipods, and chironomids were the most abundant invertebrate prey taxa and occurred in 29–34% of lesser scaup diets. Although relatively frequent, these taxa each contributed only 4–27% of the diet by weight. Percent dry mass of amphipods, a preferred prey by lesser scaup, was low (4%) indicating that amphipod availability may be reduced during spring migration. Analysis of fish diets showed that black bullhead Ameiurus melas and yellow perch Perca flavescens had the highest diet overlap with lesser scaup at 94% and 92%, respectively. Moreover, mean size of amphipods and chironomids found in fish diets were significantly larger than that consumed by lesser scaup. Our findings support the notion that amphipod use by spring-migrating lesser scaup has declined and that size-selective predation by fishes may influence prey availability for lesser scaup. Handling editor: K. Martens     

Food and feeding habits of pearly razorfish, <Emphasis Type="Italic">Xyrichtys novacula </Emphasis>(Linnaeus, 1758), in the southern Tyrrhenian Sea: variation by sex and size

We collected pearly razorfish, Xyrichtys novacula (Labridae), from a sandy bottom of Capo dOrlando (Tyrrhenian Sea) to study their feeding habits. We caught fish by hand-lines and seine nets and identified, counted and weighed food items in their stomachs. We evaluated the importance of the different prey types by calculating the frequency of occurrence, abundance and mass. We used these values to calculate the index of relative importance (IRI) for each taxonomic category and a modified index (MI) which did not incorporate %N into the formula. Gammarideans were dominant food items in terms of %F but showed a very low MI value; the bivalve Acanthocardia tubercolata was the dominant species in terms of %W and MI value. The prey items were mostly benthic organisms belonging to the assemblage of fine, well-sorted sands showing that X. novacula is a benthic feeder. Despite the large number of prey taxa found, few species accounted for most of the prey consumed, indicating specialist feeding, as confirmed by the low value of the Levins standardised index. We found significant differences in prey distribution among size classes, with the highest prey diversity recorded in 120–140 mm TL size classes. Correspondence analysis showed a trend of increasing predator size classes among prey items, with small prey, such as copepods and amphipods, in the smallest predator size classes and bigger prey in the larger ones. Prey caught by larger predators differed significantly in weight from those taken by smaller specimens. There was a significant positive correlation of fish length with bivalves, decapods and echinoids, and a negative one with copepods, ostracods, caprellids, gammarideans. There were no significant differences between males and females of overlapping sizes. Juveniles are able to exploit only small, vulnerable prey, while adults take few, large prey, thereby avoiding competition with juveniles.     

Feeding ecology of a bagrid catfish,Mystus tengara (Hamilton, 1822) in the Tanore wetland of Rajshahi,Northwestern Bangladesh

The feeding ecology of Mystus tengara (Hamilton, 1822) was studied in the Tanore wetland of Rajshahi, northwestern Bangladesh from January 2011 to December 2011. In fish purchased from the local market, a total of 14 713 food items were identified in the stomach contents of 3191 M. tengara specimens, an omnivorous catfish with a diversified feeding strategy including cladocerans, copepods, rotifers, gastropods, green algae, diatoms, insects, ‘small fish remains, fish fry and eggs’ and ‘other’ food items. Insects were the most dominant group of prey throughout the year. Seasonal changes in diet composition were due to the availability of prey items and fish size. Ontogenetic changes were noted between the juveniles and adults. The gut contents of M. tengara showed that the food items were from different layers of the water bodies.     

Spatial and temporal variation in the diet of the Pacific sand lance Ammodytes hexapterus in waters off the coast of British Columbia,Canada

Variation in the diet of the Pacific sand lance Ammodytes hexapterus was examined in three years (2009–2011) at four sites in British Columbia, Canada. There were 12 major taxa of prey in diets, eight of which were Crustacea, with copepods being by far the dominant taxon in all 12 site‐years. Of the 22 copepod taxa recorded, only Calanus marshallae and Pseudocalanus spp. occurred in all collections, and these two calanoid species dominated diets in terms of frequency of occurrence and total numbers of prey (Pseudocalanus spp. in most collections), and total prey biomass (C. marshallae in all collections). Based on an index of relative importance, C. marshallae was the primary prey at the two southerly sampling sites (Pine and Triangle Islands) and Pseudocalanus spp. at the two northerly sites (Lucy Island and S'G ang Gwaay). Based on an index of dietary overlap, the species composition of the copepod component of A. hexapterus diets overlapped very strongly at the northerly and the southerly pairs of sites in both a cold‐water La Niña year (2009) and a warm‐water El Niño year (2010), but overall there was more homogeneity amongst all four sites in the La Niña year.     

First feeding diet of young brown trout fry in a temperate area: disentangling constraints and food selection

Trophic interactions are important factors structuring animal communities. We assessed the trophic relations of four fish species that live in sympatry in the River Ladra (NW Spain), and cluster analysis differentiated two feeding strategies: (1) species with omnivorous feeding habits, feeding mainly on detritus and plant material but with aquatic macroinvertebrates as an important complement (Achondrostoma arcasii and Pseudochondrostoma duriense) and (2) species feeding mainly (Salmo trutta) or exclusively (Gasterosteus gymnurus) on aquatic macroinvertebrates. Concerning ingested macroinvertebrates, the trophic overlap was quantified using Schoener’s index and the results obtained revealed a high diet overlap among the species (from 81.3 up to 99.2%). In order to get a deeper insight into mechanisms of fish species coexistence, we used ten biological and ecological traits of macroinvertebrate prey to discriminate feeding preferences. As a result, despite the high similarity among the diets, our analyses suggest that differences in diel activity patterns and drift behaviour of preys, as well as differences in the prey size, are important adaptive features that may reduce the inter-specific competition in the fish community and permit the partitioning of food that allows coexistence.     

Lunar influence on prey availability,diet shifts and niche overlap between Engraulidae larvae in tropical mangrove creeks

The influence of the lunar cycle on prey availability, diet shifts and overlap between larval Anchovia clupeoides and Cetengraulis edentulus was evaluated in mangrove creeks of the Goiana Estuary. Copepod eggs were highly abundant in the first and last quarter, at the full moon and zoea of Ucides cordatus (Ocypodidae) in the new moon. The Engraulidae larvae fed on microcrustaceans, algae and early planktonic stages of benthic organisms. The relative importance of prey varied according to prey availability in all moon phases. Larval diets were more even in the full and new moons, when the relative importance of calanoid copepods and zoeae of U. cordatus as food items increased (index of relative importance, >80% I_RI). Mangrove creeks were very important feeding grounds for engraulid larvae during spring tides. Larval diets were more diverse in the first and last‐quarter moon and included protozoeae of Caridean shrimp, larvae of Anomalocardia brasiliana (Veneridae), Isopoda, Gastropoda, ephippium of Daphnia sp. and nauplii of Cirripedia, Harpacticoidia and cyclopoid Copepoda. The last five items were not found in the creeks, suggesting feeding in the main channel. During neap tides, mangrove creeks were probably also used as refugia. These larvae are opportunistic and feed on highly available prey and both species feed on the same items, leading to high dietary overlap in all moon phases. The lunar cycle, which is related to the spring‐neap tidal cycle, was the major driver of quantitative and qualitative changes in feeding of engraulid larvae on a short time scale.     

Trophic behaviour of juvenile reef fishes inhabiting interlinked mangrove–seagrass habitats in offshore mangrove islets

Stable isotope (δ¹³C and δ¹⁵N) and gut content analyses were used to investigate size‐related feeding habits of four reef fishes (the beaugregory Stegastes leucostictus, the french grunt Haemulon flavolineatum, the schoolmaster snapper Lutjanus apodus and the yellowtail snapper Ocyurus chrysurus) inhabiting an offshore (non‐estuarine) mangrove islet off Belize, Central America. Comparisons of isotopic niche space and Schoener diet similarity index suggested a low to moderate degree of niche overlap between fish size groups. The δ¹³C gradient between mangrove and seagrass prey as well as results of Bayesian mixing models revealed that sampled fishes relied mostly on seagrass prey items. Only small and large juveniles of the carnivorous species L. apodus derived a part of their diet from mangroves by targeting mangrove‐associated Grapsidae crabs and fish prey, respectively. Isotopic niche shifts were particularly obvious for carnivorous fishes that ingested larger prey items (Xanthidae crabs and fishes) during their ontogeny. The utilization of mangrove food resources is less than expected and depends on the ecology and life history of the fish species considered. This research highlights that mangrove‐derived carbon contributed relatively little to the diets of four fish taxa from an offshore mangrove islet.     

Feeding strategy of silverside species in eutrophic Lake Pátzcuaro,Mexico

The trophic guild, niche breadth, and trophic position of two different ontogenetic phases were measured according to the size of Chirostoma spp. captured in different zones of Lake Pátzcuaro, Mexico. Analyses were carried out at the genus level due the difficulties to find diagnostic characters to separate the three small silverside species accurately, since even the molecular analyses of mitochondrial DNA for a subsample of the most morphologically differentiated specimens were conducted without success. Fish were captured with a seine net during the wet (September and November 2009) and dry (February and June 2010) seasons at four environmentally different sites. Gut contents of 480 Chirostoma specimens were divided into size‐class intervals of 38–51 mm and 66–71 mm for analyses. The contribution of each food item was quantified using the frequency of occurrence and area percentage. Fish were categorized by size, and diets were compared between fish sizes and sites. Dorsal muscle tissue and zooplankton were obtained for nitrogen isotope signature (δ¹⁵N) analyses. The trophic guild was determined using a modified quadrant method for gut content analysis; the importance of each prey item was estimated according to a modified index of relative importance. Niche breadth was calculated by the Levins’ Index. The trophic position was assessed using the TrophLab Program and stable isotope analysis. The index of relative importance among study sites and fish size classes was determined by a multivariate approach. Silversides represent a secondary and tertiary consumer in Lake Pátzcuaro that feed mainly on evasive prey, such as copepods (55%) and on non‐evasive prey, such as cladocerans (25%) at sites with the lowest transparency in the water column (from 350 mm to 150 mm). Fish scales (11%) and insects (6%) were secondary food items. Ingestion of juvenile silversides was rare (1%). Chirostoma spp. prey on insects on the lake bottom and on water hyacinth roots. Based on δ¹⁵N analysis, the trophic position of silversides changes between wet and dry seasons. Their opportunistic and specialized predatory feeding strategy indicates the capability of silversides to resist anthropic changes in the lake.     

Diet overlap among size classes of Colorado River cutthroat trout (Oncorhynchus clarki pleuriticus) in a high-elevation mountain stream

We examined the taxonomic composition, abundance, and size of food items consumed by young-of-year, juvenile, and adult Colorado River cutthroat trout (Oncorhynchus clarki pleuriticus) in order to determine the degree of diet overlap occurring in a relatively unproductive, high-elevation, mountain stream. Overall, we identified 49 Families of insects representing nine Orders, and 4 other Classes of organisms in the diets of the trout sampled and saw no evidence of piscivory. Each size class of fish consumed significantly different taxa and significantly different sizes of food items. However, despite these differences, the proportional similarity index (PSI) indicated that there was considerable overlap in taxa and sizes of organisms consumed by the three size classes. The greatest overlap occurred between young-of-year and juveniles, and between juveniles and adults. Both the relatively high proportion of small items in the adult diet and the slow growth rate of adults in these streams indicate that food may be limiting for adults and that intraspecific competition between adults and smaller size classes may be high.     

Prey selection by the larvae of three species of littoral fishes on natural zooplankton assemblages

The prey selection of larvae of three common littoral fish species (pike, Esox lucius; roach,Rutilus rutilus; and three-spined stickleback, Gasterosteus aculeatus) was studied experimentally in the laboratory by using natural zooplankton assemblages. Zooplankton prey was offered at four different concentrations to study the functional responses of the planktivores. The diets of pike and sticklebacks were formed mainly of copepod juveniles and adults, which dominated the prey communities, although sticklebacks ate also cladocerans. The diet of roach larvae consisted of rotifers, cladocerans and copepods, without prey selection, in equal proportions indicating a more omnivorous diet. All fish larvae were able to feed selectively although in sticklebacks prey selection was less pronounced. Pike and roach larvae preferred large prey to smaller prey types. Patterns of prey selection are discussed in the context of size-selection theory and apparent vs. true selection.     

Herbivory in omnivorous fishes: effect of plant secondary metabolites and prey stoichiometry

1. Many animals that consume freshwater macrophytes are omnivorous (i.e., they include both plant and animal matter in their diet). For invertebrate omnivorous consumers, selection of macrophyte species depends partly on the presence of secondary metabolites in plants, plant carbon/nutrient balances and/or physical structure of plants. However, little is known about the mechanisms influencing consumption of macrophytes in aquatic vertebrates. 2. For two fish species, the omnivorous rudd (Scardinius erythrophthalmus) and herbivorous grass carp (Ctenopharyngodon idella), feeding preferences were determined in three choice experiments. We tested (i) whether the presence of secondary metabolites and macrophyte stoichiometry affects macrophyte species selection by fish, (ii) the importance of macrophyte stoichiometry by manipulating the macrophytes experimentally and (iii) the rate of herbivory when the most palatable macrophyte is offered simultaneously with a common animal prey. 3. In a choice experiment with five species of submerged macrophytes (Callitriche sp., Chara globularis, Elodea nuttallii, Myriophyllum spicatum and Potamogeton pectinatus), Myriophyllum was clearly consumed least by both fishes, which strongly correlated with the highest phenolic concentration of this macrophyte. Additionally, a significant negative relationship was found between consumption and C : N ratio of the five macrophytes. The two most consumed macrophytes also had the lowest dry matter concentration (DMC). 4. In a second choice experiment, the C : N ratio of the least (Myriophyllum) and most (Potamogeton) palatable plants was manipulated by growing the macrophytes under fertilised and unfertilised conditions and subsequently feeding them to rudd. The avoidance of consumption of the chemically defended Myriophyllum by rudd was partly alleviated by the lowered C : N ratio. 5. The third choice experiment showed that both fishes preferred animal prey (the amphipod Gammarus pulex) over the most palatable macrophyte (Potamogeton) when offered simultaneously. The C : N ratio of the amphipods was about half that of the lowest C : N ratio measured in the macrophytes. Consumption by the fishes could not clearly be related to C : P or N : P ratios of prey items in any of the experiments. 6. We conclude that omnivorous fish avoid macrophytes that are chemically defended. However, when these defences are only minor, stoichiometry (C : N ratio) in combination with DMC may be a determining factor for consumption by vertebrate facultative herbivores.     

Feeding differences among common littoral mysids, <Emphasis Type="Italic">Neomysis integer,Praunus flexuosus</Emphasis> and <Emphasis Type="Italic">P. inermis</Emphasis>

The energy flow in aquatic food webs and their structures are largely determined by food utilisation of predators. Mysid shrimps are important predators in various aquatic ecosystems. We studied the stomach contents of three common littoral mysids from the Baltic Sea. The aim was to study whether the diets differ between species and size classes inhabiting shallow coastal areas. The effects of season (spring, summer, autumn) and habitat were also explored. Results showed that all species were highly omnivorous, utilising various phyto- and zooplankton prey, algal filaments and dead organic material through the growing season. No ontogenetic diet shifts were observed although different size classes preferred slightly different prey. The amount of detritus increased in the diets during growth. In addition, large mysids ate more macro- and less microzooplankton compared with the small ones. There were also species-specific differences in the food utilisation. Neomysis integer ate more benthic material, Praunus flexuosus more macrozooplankton and P. inermis more phytoplankton compared with the others. These differences reflect microhabitat differences and probably also size differences of the studied species. Seasonal variation was also observed in the diets. Food utilisation followed the changes in the food availability, e.g. phytoplankton spring bloom and zooplankton peak abundances in late summer. Results confirm the omnivorous nature of mysids showing the importance of a diversity of prey as energy sources during growth.     

Diet composition of two larval headwater stream salamanders and spatial distribution of prey

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The effect of zooplankton abundance on feeding behaviour and prey size selection in Atlantic salmon,Salmo salar,alevins1

In an experiment on the effect of zooplankton density on feeding behaviour and prey size selection in Atlantic salmon (Salmo salar) alevins, total behavioural activity (feeding, social, ambiguous) was positively related to prey abundance up to a density of 350 items 1^?1, after which activity peaked. Feeding error (missed attacks and/or rejected ingestions) increased with prey density. The likelihood that an alevin would attack an item upon which it had binocularly fixed (no. bites/no. visual fixes) peaked at densities of 270 items 1^?1 and then declined. Feeding success (no. ingestions per bite or per fixation) also peaked and then declined. Changes in success were reflected in total number of items found in the gut. At high prey abundance (608 items 1^?1) only 0.5 – 0.9 mm copepods were preferred components of alevin diets. Over all prey densities, preferred sizes of cladocerans and copepods did not overlap. These results may reflect a perceptual constraint (at high zooplankton densities) on alevin feeding behaviour.     

NESTLING DIETS OF PRAIRIE PASSERINE BIRDS AT MATADOR, SASKATCHEWAN, CANADA

The nestling diets of seven species of grassland passerines were studied from gullet samples collected in 1969–1971 at Matador, Saskatchewan, Canada. Relative population densities on census plots on grazed and ungrazed prairie showed that Horned Lark Eremophila alpestris, McCown's Longspur Calcarius mccownii and Chestnut-collared Longspur C. ornatus preferred grazed areas and Sprague's Pipit Anthus spragueii, Western Meadowlark, Sturnella neglecta, Savannah Sparrow Passerculus sandwichensis and Baird's Sparrow Ammodramus bairdii preferred ungrazed prairie. Fifteen invertebrate ordinal taxa, and seeds were represented to an important extent in combined gullet contents; but two taxa, Lepidoptera (77% larvae) and grasshoppers (all Acrididae), comprised the bulk of nestling diets. Four grasshopper species contributed 70–100% of grasshoppers eaten in June-August. Overlap at the family level was also high in several other important prey. Comparison with invertebrate populations of ungrazed prairie showed that the birds ignored some abundant taxa of very small invertebrates, and apparently preferred the two least abundant groups which offered large individual prey items. Average nestling food overlap indices for the season among all species except Savannah Sparrow and Baird's Sparrow, based on taxonomic composition, was 0.85. Overlap was moderate in May and June and became very high in July and August. The number of nestlings reached a peak in June and early July when nestling food overlap was much lower than later in the season. Horned Larks avoid competition by nesting earlier than do the other species, while McCown's Longspur, which depends on grasshoppers for nestling food more than do the other species, tends to breed later in the season than the other species. Finally, data suggest that nestling food may be superabundant.     

Variation of diet and food consumption of the scaldfish Arnoglossus laterna(Walbaum, 1792)

Although Arnoglossus laterna (Walbaum, 1792) is a common benthic fish in Portugal, several aspects of its feeding ecology remain incomplete. In this study, diet was examined and the food consumption estimated on inshore waters in the central coast of Portugal. The diet of the scaldfish included mainly mysids, amphipods and polychaetes, while the decapod Philocheras bispinosus Hailstone was the most frequent prey. Variation in the diet according to fish size (two size‐classes), sex and spawning/non‐spawning seasons was examined. Significant differences in diet composition were found between seasons, whereby the most frequently consumed prey in the non‐spawning season were crustaceans, while polychaetes and nemerteneans comprised most of its diet in the spawning season. The average value of the vacuity index was very similar between seasons (24.7% non‐spawning season and 25.0% spawning season). Diet overlap between size‐classes and sexes was high, but low between the two seasons. Food consumption of A. laterna was estimated based on diet characterization and evacuation rate. The estimate of the daily food consumption was 1.15 mg dry weight for the non‐spawning season and 1.67 mg dry weight for the spawning season. Significant differences in consumption rates were found between size‐classes and sexes. Larger individuals showed a higher consumption rate than smaller individuals. Females consumed more food than males in the non‐spawning season, while in the spawning season males consumed more than females.     

Diet, food resource partitioning and feeding periodicity of two riffle-dwelling fish species in a New Zealand river

The diets and feeding periodicities of two small, riffle-dwelling fish, the torrentfish, Cheimarrichthys fosteri , and the bluegilled bully, Gobiomorphus hubbsi , were investigated in an unstable, braided New Zealand river from May 1985 to April 1986. Aquatic insect larvae dominated diets of all fish and dietary overlap between species was high in 10 of 12 months. Nevertheless, the relative importance of prey items, as assessed by the index of relative importance (IRI), differed between species. Gobiomorphus hubbsi fed almost exclusively on larval Deleatidium (Ephemeroptera), whereas C. fosteri consumed a greater variety of prey items, with Deleatidium and trichopteran, elmid and chironomid larvae numerically dominant. A dietary switch from Deleatidium to chironomid larvae occurred in both species during December 1985, when total and relative abundances of mayfly and other benthic macroinvertebrate larvae were severely reduced by two consecutive floods. A diel sampling programme indicated that the feeding periodicity of the two species differed: C. fosteri was a nocturnal feeder, whereas G. hubbsi exhibited a strong crepuscular peak in feeding activity. Overall, the two fish partition their food resource weakly by consuming prey items in different proportions and sizes and more strongly by utilizing the food resource at different times.