Feeding ecology of Oreochromis niloticus (Linnaeus) & Tilapia Zillii (Gervias) in a Nile canal

Fish stomachs, 1149 for Tilapia zillii and 1698 for Oreochromis niloticus, were collected from April 1984 till April 1985 from a Nile canal in the Egyptian delta. Both species showed a significant overlap of diet (Schoener's index) among individuals ranging from 12 to 17.9 cm standard length. Outside this range differences in the importance of food of animal origin occurred. Macrophytes were the main food of O. niloticus and aquatic insects of the food of T. zillii. This initial diet overlap may have contributed to irregularity in the increase of full stomachs with increasing length, and to a decline of the stomach index (SI = stomach weight × 100/fish weight) with increase in length in O. niloticus. The stomach of either species correlated significantly with water temperature, but with daily photoperiod in O. niloticus only.     

The basis of food selection in flounders, Platichthys flesus (L.), in the Severn Estuary

The factors influencing the selection of food by flounders, Platichthys flesus (L.), have been investigated by analysing collections made in the Severn Estuary for 1 year and the results of experiments. Flounders measuring between 6–0 and 35 cm fed heavily on the polychaete Nereis diversicolor in February and on the amphipod Gammarus salinus between February and April. Thereafter these species were replaced by the mysid Neomysis integer with the decapod Crangon vulgaris. Flounders shorter than 6.0 cm, fed mainly on Neomysis integer regardless of month. Numerous factors were involved in the choice of the food including (1) the maximum and minimum length of prey, (2) its spatial distribution in the water column, (3) its degree of concealment, (4) its motility and ability to escape predation, (5) conditioning of the flounders for certain foods, (6) the fish's swimming speed and (7) the turbidity and temperature of the water.
Although the average length of ingested prey remained unchanged as the fish grew in size, an increase in the maximum and minimum lengths was observed. The percentage of stomachs that contained no identifiable remains in flounders greater than 6–0 cm averaged 80–95 % during part of the winter and 60% in the summer and between January and March. These changes partly reflected the influence of low water temperature on the metabolic rate and availability of prey. The dry weight of the stomach contents in flounders longer than 6–0 cm was lowest in winter but high values were recorded in the spring. This latter feature was probably because the rate of feeding was greater than that of digestion. During the summer, under more normal feeding conditions, the weight of the stomach contents remained relatively low compared to other fish populations. Flounders shorter than 6–0 cm always contained more food in their stomachs on a unit weight basis than larger individuals, reflecting metabolic rate and hunting efficiency.     

Pleuragramma antarcticum (Pisces,Nototheniidae) as food for other fishes in McMurdo Sound,Antarctica

Summary Nine spcies of notothenioid fishes were captured near the southern limit of their range in ice covered McMurdo Sound, Antarctica. Stomach contents were examined using an occurrence method. Fishes were present in the diets of 8 of 9 species;Dissostichus mawsoni andGymnodraco acuticeps were predominantly piscivorous.Pleuragramma antarcticum was the most common prey fish consumed, being present in stomachs of 4 of 8 species.Pleuragramma was also partially piscivorous (22%) as well as cannibalistic (13%). As evidenced by their presence in the stomach contents of other fishes, all life history stages from 20 mm SL postlarvae to 160 mm SL adultPleuragramma were represented under the ice of the Sound.Pleuragramma was also a major food item for Weddell seals, birds and possibly invertebrates. As a widely distributed species in the pelagic zone,Pleuragramma may be an ecological substitute for euphausiids in the food web of the Sound.     

Lipid‐rich zooplankton subsidise the winter diet of benthivorous Arctic charr (Salvelinus alpinus) in a subarctic lake

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The basis of food selection in some estuarine fishes. Eels, Anguilla anguilla (L.), whiting, Merlangius merlangus (L.), sprat, Sprattus sprattus (L.) and stickleback, Gasterosteus aculeatus L.

The factors influencing the selection of food by eels, Anguilla anguilla, whiting, Merlanglus merlangius, sprat, Sprattus sprattus, and stickleback, Gasterosteus aculeatus have been investigated by analysing collections made in the Severn Estuary for 1 year. Non–migratory (yellow) eels measuring from 19–5–56–5 cm in length fed mainly on either the decapod Crangon vulgaris or on the mysid Neomysis integer during the warmer months but ceased to feed in the winter. Whiting measuring between 2–5 and 15 cm fed almost exclusively on C. vulgaris, N. integer and the sand goby, Pomatoschistus minutus. Sprats fed chiefly on either the arnphipod Gammarus salinus or N. integer. Sticklebacks seldom contained any food in their stomachs despite the presence of large quantities of suitable prey and the high level of hunting efficiency. Gammarus salinus, although normally abundant in the environment, was usually ingested in disproportionately small quantities by all fishes except sprats reflecting its concealment among floating weeds and a selection against small (less than 1 cm) organisms. Because of the limitations of size and the unusually efficient escape reaction of larger animals, only young representatives of Crangon vulgaris were captured. The isopod Eurydice pulchra was rarely eaten by any of the fishes even though it was common in the environment. Burrowing polychaetes, mainly Nereis diversicolor, were never utilized possibly due to the difficulty involved in removing these organisms from the substrate. Neomysis integer was the most frequently consumed organism. Although the average length of prey ingested by the whitings increased with the size of fish, this was not the case for the other species. The proportion of non–feeding eels and the dry weight of stomach contents of this species depended on the temperature of the water but were not related to the availability of prey and tidal conditions whereas the feeding of whiting could not be related to any environmental parameter.     

Feeding habits of horse mackerel, Trachurus trachurus (Linneaus, 1758), from the central Adriatic Sea

The feeding habits of the horse mackerel, Trachurus trachurus, from the central Adriatic Sea were investigated with respect to fish size and season. Stomach contents of 1200 specimens 12.9–37.6 cm total length (TL) taken at monthly intervals (January–December 1995) were analyzed. Of the total number of stomachs examined, 597 were empty (49.7%). This percentage varied significantly with season; the maximum number of empty stomachs was observed during January (80%) and February (82%) and a minimum number observed during July (15%) and August (19%). A total of 30 different prey species belonging to five major groups: Crustacea (Euphausiacea, Mysidacea, Decapoda), Cephalopoda, and Teleostei was identified in the stomach contents. Euphausiids were the most important ingested prey in all seasons as well as in small‐ and medium‐size classes (<28 cm TL). Two species of euphausiids, Nyctiphanes couchii (%IRI = 36.0) and Euphausia krohni (%IRI = 12.9), were the most frequent prey. The proportion of teleosts in stomach contents increased with the increasing size class of horse mackerel and was 15.5% IRI in >30 cm TL individuals. The mean weight of stomach contents increased significantly for fish larger than 24.1 cm TL, while the mean number of prey items significantly declined in the large‐size classes.     

The food of Charr,*Salvelinus willughbii (Günther), in Windermere

The stomach contents of 1015 charr, (Salvelinus willughbii Günther)] from Windermere, of which 658 were feeding fish, were examined. The fish were caught over a period of several years and in every month of the year. The data were analysed by two methods, by a points system, which takes into account the abundance and volume of the food organisms, and by the frequency of occurrence of each organism. Charr of all sizes had fed mainly on planktonic Crustacea (particularly Cladocera); larval and pupal chironomids and Chaoborus were also important. From November to April most stomachs of charr netted on the spawning grounds contained charr eggs. The diet of charr is compared with that of seven other fish species in Windermere. There was almost no overlap between the food of charr, which was obtained in the pelagic region, and that of the seven other species, which was mostly bottom fauna, obtained from the littoral region. Thus by occupation of these two different niches the charr and the seven other fish in no way compete for food in Windermere. Some limited data from six other lakes in the English Lake District are presented, in these planktonic Crustacea are also found to be the main food.     

Body size, sex ratio and diet composition of Arctogadus glacialis (Peters, 1874) (Pisces: Gadidae) in the Northeast Water Polynya (Greenland)

Little is known about the biology and feeding ecology of Arctogadus glacialis (Peters, 1874), an endemic gadid fish species occurring circumpolarly in both ice-free and ice-covered Arctic seas. In this study we analysed specimens sampled from bottom trawl and Agassiz trawl catches conducted at eight stations in water depths from 115 to 490 m in the Northeast Water Polynya off Greenland in August 1990. Size composition and sex ratios were assessed for 585 fish with standard lengths ranging from 7.8 to 34.5 cm. Median fish sizes at the stations ranged between 8.6 and 18.8 cm. Quantitative stomach content analyses of a subsample of 175 specimens revealed that in the Northeast Water A. glacialis fed almost exclusively on pelagic prey. Truly benthic organisms were not found in the stomachs, and sympagic species were rarely recorded. Overall, calanoid copepods were the major prey in terms of numbers. However, stomach contents varied considerably between fish size groups. In terms of prey biomass, copepods dominated the diet of small fish (<12 cm), primarily occurring at shallow stations (115–250 m), whereas amphipods and mysids were relatively more important for larger fish (>16 cm), which dominated the catches at deep stations (360–480 m). The broad variety of diet composition indicated an opportunistic pelagic feeding pattern. Received: 6 November 1997 / Accepted: 11 May 1998     

The food and feeding habits of the silver pomfret, Pampus argenteus (Euphrasen), in Kuwait waters

The food habits of the silver pomfret, Pampus argenteus (Euphrasen 1788), in Kuwait waters were investigated by examining the stomach contents of 738 specimens collected from May 1996 to April 1997. Their diet consisted of a broad spectrum of food types, but Crustacea were dominant, with copepods and their eggs constituting 39 % other non-copepod Crustacea constituted 16 %. The next major food group was Bacillariophyta (21 %), followed by Mollusca (11 %), fish scales (10 %) and, finally, fish eggs and larvae (3 %). In summer the species fed on a wider variety of food items than in winter. Copepods, other non-copepod crustaceans, and molluscs tended to occur in the stomachs in higher frequencies with an increase in P. argenteus size (up to 18.5–20.4 cm), while the bacillariophytes tended to increase in stomachs at fish sizes between 22.5–24.5 cm. Analysis of monthly variations in stomach fullness indicated that feeding intensity fluctuated throughout the year, with a low during August and September, corresponding to the spawning period.     

Satiation meal and the effects of meal and body sizes on gastric evacuation rate in brook trout Salvelinus fontinalis fed commercial pellets

Gastric evacuation (GE) experiments were performed on brook trout Salvelinus fontinalis fed commercial food pellets. The experiments included small fish (36 g; 15 cm total length, L_T) fed meals of 0·2, 0·4 and 0·8 g and large fish (152 g; 23 cm) fed meals of 0·8, 2·0 and 4·0 g at temperatures ranging from 15·1 to 18·2° C. The stomach contents were thereafter sampled and weighed at 3 h intervals until the first empty stomach was observed. The course of GE was examined by use of a general power function of the data that revealed that the square‐root function described the GE rate (GER) by the current stomach content mass independently of original meal size. Using the square‐root function, the relationship between GER and fish size was described by a power function of fish length, whereas the effect of temperature was described by a simple exponential function. GER of the commercial pellets fed to S. fontinalis could thus be described by (g h^?1), where S_t is stomach mass (g) at time t (h), L is total fish length (cm) and T is temperature (° C). The result of this study should provide a useful tool for planning of feeding regimes in production of S. fontinalis by optimizing growth and minimizing food waste.     

Molecular identification of prey in the stomach contents of Harp Seals (Pagophilus groenlandicus) using species-specific oligonucleotides

All methods of diet analysis in marine mammals, including hard part analysis (HPA), have biases affecting the accuracy of prey-species identification and frequency in the estimated diet due to differential consumption, digestion and retention. Using PCR amplification of specific prey DNA with species-specific primers, we developed a DNA-based method that complements HPA and provides an alternative means to detect prey from stomach contents of Harp Seals (Pagophilus groenlandicus). The target size that could be reliably amplified was determined using a digestion time-series of Atlantic Cod (Gadus morhua) tissue in simulated seal stomachs. Various target lengths were trialed using general teleost primers; amplicons of approximately 800 bp or less were consistently obtained. Prey species-specific PCR primers for Atlantic Cod, Arctic Cod (Boreogadus saida) and Capelin (Mallotus villosus) were designed and tested with DNA from the stomach contents of 31 Harp Seals. Amplicons were obtained for all three species-specific primer sets. Amplification results compared with HPA revealed: (i) Atlantic Cod hard parts were found in five stomachs where no Atlantic Cod DNA amplified, suggesting that Atlantic Cod may be over-represented in the estimated diet, (ii) amplification of Arctic Cod DNA occurred for 17 stomachs, including all 12 stomachs with, and five stomachs without, Arctic Cod hard parts, and (iii) Capelin DNA amplified for four of five stomachs with Capelin hard parts and for one stomach without Capelin hard parts. We conclude that PCR amplification of specific prey DNA provides a viable means to complement Harp Seal diet analysis by HPA, but suggest that valuable information for quantitative diet analysis rests in a quantitative PCR approach.     

Use of the intertidal zone by fish in Nova Scotia

Synopsis In Nova Scotia the annual harvest of the intertidal seaweed Ascophyllum nodosum exceeds 20 000 tons. This study investigated the possible impact of removal of the seaweed on intertidal abundance of fishes and their gut contents. At sites near Lower Argyle, Nova Scotia, absolute abundance of fish in small areas with Ascophyllum removed and intact was determined with pop-up seines. Gut contents were investigated from fish collected with the seines and with trammel nets. Number and weight of fish were not significantly different between cleared and intact areas and averaged 4.6 individuals and 160 g per 400 m² during June to October. Cunner and sculpins had more food in their guts when leaving than when entering the intertidal, but only cunner had more food in their stomachs when leaving intact areas than when leaving cleared areas. Most fish captured in the intertidal were small (< 15 cm) and of no commercial value, and the numbers of commercial species were too low to be of much value. Numbers of fish in the intertidal was significantly lower than in the shallow subtidal. This study provided no evidence for adverse effects of the removal of patches of Ascophyllum on fishes.     

Occurrence of the wax cetyl palmitate in stomachs of the corallivorous butterfly fish Chaetodon trifascialis

Various organs of the butterfly fish Chaetodon trifascialis, which has been reported to feed exclusively on coral polyps, were analyzed for their content of lipid and of the wax cetyl palmitate which has been found to occur in various corals. Cetyl palmitate was isolated from stomachs containing food. The total lipids accounted for 12% of dry weight of stomachs and contents, with cetyl palmitate making up 30% of total lipids. Two stomachs contained a total of 315 mg of cetyl palmitate. No significant amounts of the wax were found in heads, ovaries, swim bladders and intestines of the fish, indicating an exogenous source of the cetyl palmitate present in the stomach. This finding supports the view that the butterfly fish feeds on corals, which contain cetyl palmitate. The butterfly fish does not appear to store wax.     

Feeding ecology of dominant groundfish in the northern Bering Sea

We investigated current diets of the six most abundant benthic fish in the northern Bering Sea. Our objective was to explore feeding strategies and potential competition with other top predators as ecosystem changes occur in the northern Bering Sea ecosystem. Our approach used stomach content data collected from field sampling during spring 2006 and 2007. Calanoid copepods and ampeliscid amphipods were important prey of Arctic cod (Boreogadus saida) but in different proportions depending upon fish size, feeding location, and local environmental conditions. Snailfish (Liparidae) occupied a broad niche and fed on a variety of benthic amphipods. Arctic alligatorfish (Ulcina olrikii) and Arctic staghorn sculpin (Gymnocanthus tricuspis) consumed ampeliscid amphipods predominantly. Shorthorn sculpin (Myoxocephalus scorpius) had a less-diverse diet, with snow crab (Chionoecetes opilio) most important by weight. Finally, all Bering flounder (Hippoglossoides robustus) sampled had empty stomachs. Our results indicate that ampeliscid amphipods, which have high biomass in the central region of the northern Bering Sea, are the most important prey for the dominant groundfish in the Chirikov Basin. Generally, all dominant benthic fish in the northern Bering Sea had narrow feeding niches, except snailfish. High diet overlap was found among many of the fish species, including Arctic cod and snailfish, snailfish and Arctic alligatorfish, and Arctic alligatorfish and Arctic staghorn sculpin. These findings are consistent with a relatively short food chain for benthic fish that are for the most part specialized feeders with narrow preferences for food and may be affected by changes in benthic prey distributions.     

Feeding Habits of Four Species of Istiophoridae (Pisces: Perciformes) from Northeastern Brazil

Billfishes from the Brazilian oceanic northeastern region feed on a large food spectrum composed mainly of epipelagic species of fish and cephalopods, with occasional occurrences of mesopelagic organisms and crustaceans. The oceanic pomfret, Brama brama, and the squid, Ornithoteuthis antillarum, were the main prey items in the diet of the four species, occurring in at least 50% of the stomachs containing food. There was no correlation between body length of prey and predator, or between mass of stomach content and body mass in the individuals from 100 to 330cm fork length. The feeding of the four species was characterized by constant ingestion of small quantities of food, and constant presence of food in the stomachs, with a predominance of epipelagic organisms of small sizes with an average of 8.0cm. The reduced stomach size forces the predators to feed constantly on several meals during the day, and consequently provide constant energy for migration.     

Feeding habits of megrim, Lepidorhombus whiffiagonis (Walbaum, 1792), from the central Adriatic Sea

The feeding habits of the megrim, Lepidorhombus whiffiagonis , from the central Adriatic Sea were investigated with respect to fish size and seasons. Stomach contents of 510 specimens, 9.0–28.9 cm total length (TL), collected by bottom-trawl from 2004 to 2006 were analyzed. The percentage of 97 empty stomachs varied significantly with season (maximum during the winter, 32.1%; minimum during summer, 7.4%). The food composition suggests that the megrim is a carnivorous fish. Prey items identified in the stomachs belong to four major groups: Teleostei, Decapoda, Mysidacea and Cephalopoda. Teleosts were the most important (%IRI = 82.2), especially in fish larger than 14 cm TL. Mysidacea constituted the main prey for fish less than 14 cm TL. At the species level, the teleost Antonogadus megalokynodon (%IRI = 1.5) and decapod Processa canaliculata (%IRI = 0.9) were the most frequent prey. The mean weight of stomach contents increased significantly for fish larger than 14 cm TL, while the mean number of prey items did not differ significantly among size classes. Diet composition showed little seasonal variation; teleosts were the most important prey in all seasons, especially during spring. The lowest intensity of feeding recorded in winter could be related to the lower sea temperature and to the spawning period.     

Some aspects of the biology of Petenia krausii Steindachner (Pisces: Cichlidae) in Lake Valencia, Venezuela

Petenia kraussii Steindachner (Pisces: Cichlidae) was collected from Lake Valencia, Venezuela from April 1973 to April 1974. A total of 457 specimens were examined for stomach contents, and 471 were analyzed for length-weight relationship, sexual condition including fecundity and parasitism. P. kraussii is primarily a plankton feeder when small and utilizes benthic invertebrates after attaining about 10 cm standard length. The larvae of a benthic ephemeropteran, Campsurus sp. (Hexigenidae) was the most important single food item with gastropods and fish eggs also being found consistently in stomachs. No mature male fish were found, and only 17 of 471 fish could be identified as males, while 157 were females. Gravid females contained an estimated mean of 6266 eggs and matured at about 18 cm standard length and 100 g live weight. It was not possible to determine age from either scale or otoliths.     

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.     

Gastric evacuation in endangered Russian sturgeon (Acipenser gueldenstaedtii): Effects of body and meal sizes

This study aimed to parameterize the effects of body and meal sizes on the gastric evacuation (GE) of farmed Russian sturgeon (Acipenser gueldenstaedtii) fed on live food (whiting fish) or commercial pellets. The radiography technique was used to monitor the evacuation of food from the stomach into the intestine at postprandial time. Acipenser gueldenstaedtii of different body sizes (total length ranged 82–105 cm) were fed on meal sizes ranged from 23 to 72 g composed of either live food or commercial pellets (had a 20% of BaSO₄ as a radiopaque material). The square root model was found to adequality describe the process of GE in A. gueldenstaedtii independently of meal type and size. The effects of fish size on GER was described by simple power model and can be summarized by , where ρ_L was 0.000053 for live food and 0.000062 for commercial pellets (estimated from combined GE data), S_t is stomach contents (g), L is fish total length (cm) and t is time (h). The results of this study will provide a useful tool to estimate the stomach fullness of A. gueldenstaedtii at postprandial time t. Such information should assess the feeding strategy to minimize feed wastage and optimize the fish growth.     

Experimental evidence of cannibalism and prey specialization in Arctic charr,Salvelinus alpinus

Synopsis Use of a radiographic technique enabled the study of prey selection and individual specialization in Arctic charr, Salvelinus alpinus, fed with small charr and dry pellets under laboratory conditions. Both naive and experienced fish (mean weight 475 g, mean length 34.9 cm), selected the smallest individuals when offered juvenile charr (6–16 cm) as prey. The selected prey were, on average, 22% of the predator length. Cannibalism appeared to involve individual specialization, since when groups of large charr were given the opportunity to feed on juveniles one day every two weeks, the same individuals were cannibalistic throughout the sampling period of two months. When large charr were presented with alternate cycles of different food types consisting of dry pellets and fish prey, the charr exhibited a strong and consistent feeding specialization with three distinct groups being recognised: cannibals, pellet eaters and non-feeding fish.