Resource seasonality and fish diets in an Illinois stream

Synopsis The purpose of this study was to evaluate the intensity of competition for food among 9 species of stream fishes that primarily eat aquatic invertebrates. The taxonomic and size composition, and numerical abundance of aquatic invertebrates were monitored for one year using drift and benthic samples. Diet data were obtained from stomachs of fishes captured at the same time and place that invertebrate sampling was done. Diet characteristics examined included taxonomic and size composition, number of prey per fish, and diet breadth. Drifting invertebrates were more abundant early in the year (March–June) than later (July–January). The summer-early fall scarcity of invertebrates was especially notable among those>3.6 mm long, which comprised the bulk of prey found in fish stomachs. Average prey size eaten by a fish species was positively correlated with fish mouth size, but interspecific overlap in prey size was extensive. Cyprinids as a group (5 species) ate proportionally fewer small (< 3.6 mm long) prey from July to January than did the centrarchids and stonecat. Taxonomic compositions of available invertebrates and fish diets varied markedly among sampling dates, but the use of prey taxa by fishes was not correlated with the availability of those taxa. Use of aquatic prey taxa was generally similar among fish species, but cyprinids as a group ate proportionally more terrestrial prey from July to January than did the centrarchids and stonecat. Diet breadths for all species increased as food levels declined, indicating that these fishes experienced resource depression. Food scarcity was evidently more severe for cyprinids since their stomachs contained few prey through the summer and fall relative to the centrarchids and stonecat. Though the fish species studied probably compete for food in the summer and fall, this competition did not account for the community structure observed.     

Diel variation in feeding rate and prey composition of herring and mackerel in the southern Gulf of St Lawrence

Diel feeding patterns of herring Clupea harengus and mackerel Scomber scombrus in the southern Gulf of St Lawrence were examined based on samples obtained by midwater trawling between 19 and 26 June 2001. Within 3 h time periods, stomach contents tended to be more similar between fish from the same tow than between fish from different tows. Thus, in contrast to previous diet studies, which have used individual fish stomachs as independent observations, tow was used as the experimental unit in statistical analyses in this study. Diel patterns in stomach fullness were identified using generalized additive models. Two peaks in stomach fullness occurred for herring, one in the morning and the other in the evening. Mackerel showed an increase in feeding intensity throughout the day with a peak in mid‐afternoon. The diel changes in stomach contents suggested rapid gastric evacuation rates for both species, especially for herring. The estimate of the instantaneous evacuation rate for herring was twice that for mackerel. Calanus copepods (mainly C. hyperboreus ), fishes (mainly capelin Mallotus villosus ) and euphausiids were the main prey found in the stomachs of both species. Calanus copepods dominated the diet of herring regardless of time period. They also dominated the diet of mackerel during the late afternoon, evening and night while fishes and euphausiids were dominant during the morning and early afternoon. These diel patterns emphasize the need for sampling throughout the day and night in order to estimate ration and diet composition for bioenergetic and ecosystem models.     

What do the empty stomachs of northern pike (<Emphasis Type="Italic">Esox lucius</Emphasis>) reveal? Insights from carbon (δ<Superscript>13</Superscript>C) and nitrogen (δ<Superscript>15</Superscript>N) stable isotopes

The frequency of individuals with empty stomachs (FES) can vary greatly among northern pike populations. However, the FES has only seldom been analyzed in this species and its meaning is still not fully understood. It has been suggested that a high FES may reflect a strongly piscivorous behaviour while low FES could reflect a higher utilization of invertebrates. We compared the stomach contents and the trophic position of northern pike in 16 populations of individuals feeding mainly on fish or benthic invertebrates. We tested the hypothesis that northern pike with empty stomachs or with fish in their stomachs have a higher trophic position than individuals feeding on invertebrates. Carbon (δ¹³C) and nitrogen (δ¹⁵N) stable isotope signatures were used to estimate the trophic position of individuals. We found no significant difference in the trophic position among piscivores, invertebrate feeders, and northern pike with empty stomachs. The average trophic position of northern pike was high (mean ± SD = 4.3 ± 0.4, n = 66) and was correlated with total length. These results indicate that, although invertebrates could be an important part of the diet of northern pike in Canadian Shield lakes, fish are still the dominant prey. Hence, feeding on invertebrates in our study lakes would reflect an opportunistic rather than a specialized feeding strategy.     

Feeding habits of ocellate spot skate,Okamejei kenojei (Müller & Henle, 1841), in coastal waters of Taean,Korea

The feeding habits of Okamejei kenojei were studied using 592 specimens collected in the coastal waters of Taean, Korea from April 2008 to March 2009. O. kenojei is a bottom‐feeding carnivore that consumes mainly shrimp, fishes, and crabs. Its diet also includes small quantities of amphipods, mysids, cephalopods, euphausiids, copepods, isopods, and polychaetes. The total length (TL) of individuals in this study ranged from 8.2 to 49.0 cm. Cluster analysis based on %IRI (index of relative importance) identified three size classes. Group A (< 20 cm TL) ate primarily caridean shrimp and amphipods; group B (20–30 cm TL) ate exclusively shrimp; and group C (> 30 cm TL) ate penaeoidean shrimp, fishes, and crabs. O. kenojei showed ontogenetic changes in feeding habits. Although shrimps were the primary food consumed by all size groups, the proportion of shrimp in the total diet decreased and the consumption of fishes and crabs gradually increased with the body size of O. kenojei. Size of the prey organisms also increased. Smaller individuals fed mainly on small prey, such as amphipods, mysids, and small shrimp, whereas larger individuals preferred larger prey, such as larger shrimp, fishes, and crabs. The size‐related diet breadth and the percentage of empty stomachs were significant; the diet breadth gradually increased with body size, whereas the percentage of empty stomachs decreased. Seasonal changes in the O. kenojei diet were not significant, but shrimp constituted 97.3% of the summer diet by %IRI. Seasonal changes in diet breadth and the percentage of empty stomachs were not significant.     

Studies on food organisms of pelagic fishes as revealed by the 1979 North Atlantic Eel Expedition

The extent to which pelagic fishes occurring in the Sargasso Sea and adjacent parts of the Atlantic prey on leptocephali (Anguilliformes) was investigated. Most of the fishes examined (c. 95%) were collected using a commercial pelagical trawl. The stomach contents of about 1000 fishes (25 species of 10 families), mostly belonging to the suborders Myctophoidei, Stomiatoidei and the order Anguilliformes, were examined. The remains of invertebrates, mainly crustaceans, molluscs, tunicates, chaetognaths, and siphonophores were found in 28.8 % of the stomachs. Fishes, mostly myctophids or fish remains, were observed in 11.2 % of the stomachs; 18.7 % contained unidentified items and 40.6 % were empty. Leptocephali (Ariosoma spp. andGnathophis sp.) were found in the alimentary tract of 0.5 % of the fishes examined, exclusively represented by the myctophid,Ceratoscopelus warmingii. This report indicates that the Sargasso Sea population ofAnguilla leptocephali, economically the most important eel, is not seriously affected by predation of oceanic fish species considered in this study.     

Feeding habits of the bluemouth, Helicolenus dactylopterus dactylopterus (Delaroche, 1809) (Pisces: Sebastidae) in the Portuguese coast

In order to investigate the feeding habits of Helicolenus dactylopterus dactylopterus along the continental Portuguese coast, a total of 619 individuals were sampled of which 60% contained food in their stomach and 35% had more than one prey item. Among the 81 prey items that were identified in the stomachs, benthic and benthopelagic prey prevail on this species diet. Acantephyra sp, Pasiphaea sp, mysidacea, and teleostei n.i. were the prey with the higher percent index of relative importance (%IRI) value. Three length groups (5?C20?cm, 21?C27?cm, and 28?C48?cm) were defined through cluster analysis of the mean abundance of prey items. A permutational MANOVA detected significant differences in the diet and stomach fullness index for TLG, season, and maturation stage. Smaller fishes had a generalized diet, feeding mainly on mysidacea changing their diet above 20?cm TL, where a major consumption of natantia was found. The larger individuals, >28?cm TL, present a less generalized diet with pisces as dominant prey group. Seasonally, natantia and pisces were the principal prey groups during spring and winter, respectively, while mysidacea and other crustaceans were predominant during the rest of the year. Mysidacea were also the main prey group for immature individuals while natantia and pisces were the principal prey groups to the other maturity stages. The results of this study indicate that H. d. dactylopterus has a diverse diet focused on small crustaceans such as misyds and as specimens grow shrimps and fishes become more consumed, with larger specimens having a more specialized diet. The different nutritional needs during spawning season also seemed to influence the feeding habits of H. d. dactylopterus.     

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.     

Temporal patterns of resource partitioning among Cichla species in a Venezuelan blackwater river

In channel and floodplain habitats of the Cinaruco River, Venezuela, Cichla temensis was more abundant and larger than C. intermedia and C. orinocensis . Seasonal variation in hydrology influenced habitat use, spawning, and predator-prey interactions. The three piscivores partitioned habitat, with C. intermedia showing a strong affinity for structured habitats in the main channel during all water level fluctuations. C. orinocensis was most abundant in shallow areas with submerged structure in lagoons and, to a lesser extent, in low velocity regions of the channel, and C. temensis occupied a wide range of lotic and lentic habitats. During the low-water period, the feeding frequency and body condition of all three species declined, and this was related, in part, to preparation for spawning near the end of the low-water season. The diet of C. intermedia was least similar to its two congeners during falling and rising water. C. orinocensis and C. temensis had lowest diet overlap during the low-water conditions, the period when many individuals of these two species move into lagoons for nesting. Prey in stomachs were significantly larger during the falling-water than the rising-water period, and predation by Cichla and other large piscivores during the falling-water period may have reduced the abundance of large prey, particularly Semaprochilodus kneri . These migratory detritivorous fish were important prey for C. temensis during the falling-water period and probably contributed a substantial fraction of the annual energy intake for this species. Together, the three Cichla species consume a wide spectrum of prey from a diverse fish assemblage, but prey are subdivided based on habitat, prey type, and season.     

The food and feeding habits of blackspot snapper, Lutjanus fulviflamma (Pisces: Lutjanidae) in shallow waters of Mafia Island, Tanzania

Natural diet and feeding habits of blackspot snapper, Lutjanus fulviflamma (Forsskål 1775) were investigated in Mafia Island Marine Park (MIMP) and intensively fished areas (IFA) between May 1999 and April 2001. An index of relative importance (IRI) was used to assess the food preference for L. fulviflamma. Feeding habits were studied with respect to sex and size of the predator, seasons and feeding periodicity. Of 4642 L. fulviflamma specimens sampled, 46% had empty stomachs. Brachyurans were the main prey of L. fulviflamma accounting for 48% IRI. Benthic invertebrates dominated by other crustaceans, gastropods, bivalves, cephalopods and polychaetes and demersal teleosts made up the remainder of the diet. Female and male L. fulviflamma were not effectively segregated by trophic dimension in the study areas. Specimens in the 90–120 mm total length (TL) fed exclusively on crustaceans, with shrimps accounting for 40% IRI. Lutjanus fulviflamma in the 270–300 mm TL preferred fish prey accounting for 50% IRI. The diet of fish species showed clear spatial differences, which were dependent on size distributions. Lutjanus fulviflamma fed mostly but not exclusively at night peaking at dusk and dawn. The results clearly demonstrate that L. fulviflamma is an opportunistic forager, which exhibits both diel and ontogenetic feeding habits.     

Long‐term changes in the feeding of Pollachius virens on the Scotian Shelf: responses to a dynamic ecosystem

The diet and feeding ecology of pollock Pollachius virens from the Scotian Shelf and Bay of Fundy in the north‐west Atlantic changed over the last few decades, which was associated with decreases in euphausiid abundance. Stomach contents data for 2078 pollock collected during the 1958–1967 period and for 1230 pollock collected during the 1996–2002 period indicated that pollock diet contained fewer euphausiids and feeding activity decreased. During the early period, euphausiids were present in 65% of the pollock stomachs that contained food and only 9% of these stomachs in the recent period. The decrease of euphausiids was not wholly offset by an increase in piscivory, since there was little increase in the frequency of fish prey in the diet or in the fullness index for this prey type. Empty stomachs were significantly more common in the recent period during both the winter and summer. The decreased occurrence of euphausiids in stomach samples coincided with a significantly decreased abundance of this prey, suggesting that the near‐absence of euphausiids in recently collected pollock stomachs reflected prey abundance. Concurrent with changes in diet and feeding intensity, the condition or 'plumpness' of pollock significantly declined from the early to the late sampling periods.     

Piscivory by the mummichog (Fundulus heteroclitus): Evidence from the laboratory and salt marshes

While piscivory is common in many fishes, there are few accounts for the fundulid, Fundulus heteroclitus (mummichog). We suspected that this species might be involved in several forms of piscivory including predation, cannibalism and scavenging. To evaluate these possibilities we conducted several laboratory experiments and field observations in its primary habitat, salt marshes. We found that digestion of larvae and small juveniles of F. heteroclitus was fast (< 1 h) and this makes detection of small fish prey difficult for any form of piscivory. In addition, laboratory experiments (one-on-one encounters, feeding on dead prey) indicated that both cannibalism and scavenging of conspecific prey were possible. Field observations (n = 2449 stomachs from fish 12-106 mm TL over four years at two salt marshes) also suggested that piscivory occurred frequently (4.3-24.7% of stomachs examined) based on the detection of numerous fish hard parts, especially pharyngeal jaws. These structures allowed us to determine that most prey were F. heteroclitus and to back-calculate the size of the prey and thus the piscivore/prey size relationship. As a result, it appears that both cannibalism (0.2-9.1% of stomachs) and scavenging (0.5-9.9%) are common feeding modes of F. heteroclitus in salt marshes and, subsequently, modes of energy transfer for salt marshes in general. We suggest that future studies of fish feeding consider that cannibalism and scavenging may be frequent possibilities when fish remains are detected in stomach contents.     

The stomach contents of Patagonian toothfish around South Georgia (South Atlantic)

The proportion of Patagonian toothfish Dissostichus eleginoides , caught around South Georgia in the south-west Atlantic, with empty stomachs was much lower in fish caught in pots compared to longlines (28 and 91%, respectively, of examined individuals). It is hypothesized that pots caused reduced levels of stress on capture. Stomach content data examined from pot-caught fish will probably therefore be more comprehensive than that from fish caught using longlines. A wide range of prey items was identified in the stomachs of D. eleginoides and stomach contents of individuals caught using the two fishing methods were significantly different. The most common prey item for D. eleginoides caught using pots was the decapod prawn Nauticaris sp., which was restricted in location and depth. However, prawns were not common in the stomachs of D. eleginoides caught from the same location using longlines. Stomach contents from the two fishing methods remained significantly different when Nauticaris sp. were eliminated from the assessment, although fishes then dominated the diet of D. eleginoides caught using either fishing gear. The study confirms that D. eleginoides is an opportunistic carnivore, and indicates that feeding habits depend on the local availability of food items, as well as factors such as depth and predator size. The potential ecological impacts of fishing for D. eleginoides on the South Atlantic ecosystem are discussed.     

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.     

Feeding of a benthic goby in a river where nektonic fishes are absent

The social structure and feeding behaviour of the benthic goby Rhinogobius sp. DA (dark type) were investigated in a small river in Shikoku, Japan, where nektonic fishes are absent. The gobies had overlapped home ranges but were often aggressive against conspecifics, especially of the same sex and similar body size. Their diet consisted mainly of aquatic invertebrates, but terrestrial invertebrates were also an important item. This contrasted with few terrestrial invertebrates in the diet of the conspecifics in an adjacent river harbouring a nektonic cyprinid fish. An increase in flow level greatly changed their feeding behaviour; on high flow days, a greater portion of the population appeared from the interstices of the bottom and more often rose into the water column to catch drift than on normal flow days. Plasticity of their feeding behaviour is discussed in terms of ecological release.     

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.     

Use of terrestrial arthropod prey by a stream-dwelling cyprinid fish

Synopsis The spatio-temporal availability to, and use of terrestrial (allochthonous) prey by, a subset of the fish assemblage of a Virginia piedmont stream were investigated during spring and summer. Terrestrial invertebrates were most abundant in drift nets, but least common in fish stomachs, during April. In contrast, during August, terrestrial prey dominated the diet of the major cyprinid fish,Notropis ardens, even though terrestrial prey availability in the stream declined by an order of magnitude. Several other co-occurring fishes only rarely consumed prey of terrestrial origin. Drifting terrestrial invertebrates, were concentrated at the surface near the thalweg, compared to stream margin or substrate locations. This study suggests that morphologial and behavioral constraints of individual fish taxa, including predator avoidance responses, regulate the use of terrestrial prey within a given fish assemblage.     

Food selection and growth of young snakehead Channa striatus

Food selection and growth of young snakehead Channa striatus were studied in the laboratory and in a field trial. In the laboratory, first-feeding snakehead larvae of 6–7 mm total length (TL) with a mouth opening of 0.55 mm selected for Artemia nauplii, and against formulated feed. Fish began feeding on formulated feed at 12mm TL when their mouth width reached 1.0mm. In both laboratory and field trials, snakehead diets changed as fish size increased. For fish 15–20 mm TL, cladocerans and copepods were 96.5% of their diet. With fish 30–40 mm TL, zooplankton consumption was greatly reduced while benthic organism consumption increased. Fish 45–50 mm TL fed exclusively on benthic invertebrates. Diet shift from zoo-plankton to benthic invertebrates was not due to reduced zoo-plankton availability, but was instead related to changes in gill raker structure. Low density of benthic invertebrates in the field trial caused reduced fish growth rates when fish switched diets from zooplankton to benthos. Our results indicate that snake-head can take Artemia nauplii as a larval starter food, then accept formulated feed at ≥12 mm TL. Zooplankton can serve as food for snakehead < 40 mm, but formulated feed should be provided for larger fish which are unable to catch zoo-plankton.     

Does Resource Availability Affect the Diet and Behavior of the Vulnerable Squirrel Monkey, <Emphasis Type="Italic">Saimiri vanzolinii</Emphasis>?

Seasonal changes in food availability influence the behavior and diet of primates, and an understanding of dietary changes is important for the development of informed conservation plans. Saimiri vanzolinii is a small-bodied, omnivorous squirrel monkey endemic to the floodplain forests in Central Amazonia. It has a restricted distribution of only ca. 870 km² and is classified as Vulnerable. We examined seasonal variation in this species’ diet. We collected phenological (presence and absence of ripe fruits from 205 tree species) and dietary and behavioral data for unhabituated groups of S. vanzolinii on nine 2-km transects in the floodplain forests of Central Amazonia for 2 years, encompassing the high-water and low-water periods. Fruit availability was higher, and the monkeys traveled less and fed more during the high-water than the low-water period. There were no seasonal differences between the high-water and low-water periods in the time spent feeding on fruits, flowers, or arthropods. S. vanzolinii spent more time consuming fruit than arthropods in the high-water period, but this was not the case in the low-water period. These findings suggest that food selection changed between seasons, and S. vanzolinii focused more on fruit in the low-water period, when fruit was less available, than in the high-water period, when fruit was more available. These results support the predictions of the ecological constraints model.     

Scavenging on discarded saury by demersal fishes off Sendai Bay, northern Japan

Pacific saury Cololabis saira were found in 24 out of 1314 stomachs of demersal fishes collected by bottom trawling at 245–393 m depths. The same area was occupied by the stick-held dipnet fishery for saury, in which a landing limitation was enforced to avoid overfishing. The fish species containing saury were: Pacific cod Gadus macrocephalus , walleye pollock Theragra chalcogramma , and oilfish Ruvettus pretiosus . These demersal species would not encounter living saury naturally, which normally are distributed above thermoclines. The ingested saury were smaller than market size which suggests that the saury from fish stomachs were discarded by fishermen because of small sizes to maximize profits under the landing limitation.
The scavenged saury made up 41.5, 24.6 and 77.7% of the diets of large-sized (>30cm)Pacific cod, walleye pollock and oilfish, respectively in terms of DW composition. The extent to which discarded saury contributed to the total diet in the demersal fish assemblage, calculated by considering species composition and diets of bottom fishes, was 21.8% of the total diet. The discarded saury seemed to compensate the less productive feeding environment during autumn for the bottom fishes.