Diet composition of bluefin tuna (Thunnus thynnus L. 1758) in the Eastern Mediterranean Sea,Turkey

This study gives relevant information on the diet composition of the bluefin tuna (Thunnus thynnus) during the spawning period in the eastern Mediterranean Sea. The stomach contents of 218 bluefin tuna were sampled from 2003 to 2006 during the fishing season (May–June) aboard purse seiners operating in the northern Levantine Sea off the coast of Turkey. Stomachs were removed from the fish soon after landing and kept frozen at ?18°C until analysis. Prey items were classified into large taxonomic categories and preserved in 70% ethanol. A total of 745 different prey specimens belonging to 47 taxa were identified, including 34 species of fish, 11 of squid, and two of crustaceans. The most important fish and cephalopod prey belonged to the families Myctophidae, Carangidae, Chauliodontidae, Paralepididae, and Octopoda. This study marks the observation of myctophid fish in the stomach contents of bluefin tuna from the Mediterranean Sea. The paper offers some new information of regional importance and compares the feeding habits of the species to other regions, bringing confirmation on the opportunistic feeding ecology of the species in the enclosed Mediterranean Sea, where bluefin tuna seasonally occur as a strong cohort. New information on the diet composition of T. thynnus in the eastern Mediterranean Sea is revealed; the findings indicate that, depending on the abundance of the different prey species in the habitat, the dominant prey species can be distinctive.     

The food of the Nile perch, Lates niloticus (L.), after the disappearance of the haplochromine cichlids in the Nyanza Gulf of Lake Victoria (Kenya)

After the disappearance of the haplochromine species in the Nyanza Gulf of Lake Victoria as a result of predation by Lates niloticus , the latter has turned its attention to aquatic invertebrates and other fish. Changes in the diet of the Nile perch with increase in its size have been observed: young L. niloticus preyed mostly on invertebrates, including crustaceans and various small aquatic insects; large, immature L. niloticus supplemented the invertebrate diet with both young and small fish; adults above 80 cm total length were mainly piscivorous. L. niloticus feeds on fish prey of about one third its own length.
The tendency of L. niloticus to switch from one prey item to another, depending on availability, is reported; e.g., in the Nyanza Gulf, the prey diet has shifted from the haplochromine to Caridina nilotica and L. niloticus juveniles.     

Diet and foraging opportunism of the Guiana dolphin (Sotalia guianensis) in the Abrolhos Bank,Brazil

Diet analysis allows exploring how coastal dolphins interact with the environment and their role in the marine food webs. We studied the diet and feeding ecology of the Guiana dolphin, Sotalia guianensis, through analysis of stomach content from 42 animals stranded on the eastern coast of Brazil. A total of 1,336 semidigested prey items (fish, otoliths, cephalopod beaks, and crustaceans) were identified. Teleost fish comprised the most frequent food item (92% of the total), followed by cephalopods, and crustaceans. Prey belonged to 34 taxa and richness in individual stomachs varied from 1 to 15 prey taxa. Prey were generally small, but showed a significant trend to increase in size with dolphin length. The main prey of Guiana dolphins were demersal, estuarine, and sound-making fish such as catfish and sciaenids. No sex-related differences in diet were found. Diet composition varied seasonally and occurrence of prey items was coherent with breeding or high abundance periods of some fish species and squids. Our study shows the importance of demersal prey from estuarine and soft-sediment habitats to Guiana dolphin in the Abrolhos Bank and reveals that feeding habits are generalist and opportunistic, with diet reflecting the seasonal abundance and availability of prey.     

Diet composition of smooth-hound shark, Mustelus mustelus (Linnaeus, 1758), in the Gulf of Gabès, southern Tunisia

The diet of the smooth-hound shark, Mustelus mustelus , from the Gulf of Gabès (southern Tunisia, central Mediterranean Sea) was investigated with respect to fish size and season. Stomach contents were analyzed from 540 specimens with total lengths ranging from 34 to 158.5 cm. Of the total number of stomachs examined, 63 were empty (11.67%). Smooth-hound shark fed mainly on crustaceans, fishes and cephalopods. Sipunculids, polychaetes and echinoderms were occasional preys. No differences were found between the diets of males and females. Ontogenetic changes in diet of M. mustelus were apparent, with crustaceans forming a greater proportion of the diet of smaller sharks. Both teleosts and molluscs increased in importance with increasing shark size. Consumptions of polycheates, sipunculids and echinoderms were not related to predator size. Prey diversity also increased with size, with large and mobile prey species found more commonly in the diet of larger sharks. The limited overlap in the dietary compositions of juveniles, subadults and adults suggests the possibility of resource partitioning. Seasonality in food habits was in accordance with the dynamics of the predator and the prey species.     

Feeding habits of <Emphasis Type="Italic">Bathydraco marri</Emphasis> (Pisces,Notothenioidei, Bathydraconidae) from the Ross Sea,Antarctica

In comparison with other bathydraconids, all species of the genus Bathydraco are poorly known from an ecological perspective. The diet of juvenile Bathydraco marri Norman, 1938 was studied for the first time in specimens collected in the southwestern Ross Sea during summer 1998. Fish were collected in a single otter trawl catch at 330–340 m depth. The stomach content analysis showed that this species fed exclusively on crustaceans. Overall, 20 prey taxa were identified to genus or species level. Mysids, amphipods and copepods were the most important prey in decreasing order of importance. Other prey, such as Euphausia superba, isopods and tanaids were eaten occasionally and in very small amounts. A multivariate analysis was applied to feeding data to assess ontogenetic or sex-related changes in diet. No difference was detected between sexes, whereas diet of small and large fish differed in some degree. An ontogenetic shift from small and pelagic crustaceans such as copepods to benthic–benthopelagic prey such as amphipods and mysids was observed. Relating present results with published data on physiological characteristics of B. marri, it was possible to infer their feeding behaviour and mode of life. Like other bathydraconids, this species appeared to be an inactive and sluggish fish, which relied on more or less motile benthic or epibenthic prey adopting a “sit and wait” feeding strategy. On the other hand, smaller fish seem to be more active, feeding also on pelagic prey such as copepods that can be seasonally abundant, thus reducing the intraspecific competition for food.     

Diet of young-of-the-year bluefin tuna, Thunnus thynnus (Linnaeus, 1758), in the southern Tyrrhenian (Mediterranean) Sea

The diet of juvenile bluefin tuna (Thunnus thynnus) caught 2 to 8 miles off NE Sicily was investigated in order to improve knowledge of the species’ early life history. From 1998 to 2000, 107 specimens ranging from 63 to 495 mm (total length) were fished between July and November. Fishes were caught by trolling line or purse seine in a Fish Aggregating Devices (FADs) area 2 to 8 miles offshore and as by‐catch of the purse‐seine sardine fishery 2 to 4 miles offshore. Most frequently found items in the T. thynnus stomachs were fishes (84.5%), crustaceans (54.6%) and cephalopods (50.5%). The largest contribution in weight was provided by cephalopods (47.3%) and fishes (46.5%), while the most abundant items were fishes (51%), cephalopods (27.2%) and crustaceans (21.1%). These results suggest that young‐of‐the‐year tuna have an essentially piscivorous diet, although invertebrate prey provide a substantial contribution to the food array. Prey show little relationship with FADs, although one prey species (blue runner, Caranx crysos) is associated with FADs in the Mediterranean.     

Diet of the Atlantic bonito Sarda sarda (Bloch, 1793) in the Northeast Aegean Sea

Seasonal and size‐related patterns in feeding habits of the Atlantic bonito (Sarda sarda) in the Northeast Aegean were investigated over a 2‐year period May 2007 – May 2009 based on analysis of stomach contents of 134 fish ranging from 266–655 mm fork length. The fish were caught by purse‐seine boats operating around the islands of Lesvos and Chios (Greece). Of the stomachs examined, 46% contained at least one prey item and significant seasonal and size‐related patterns were found in the occurrence of non‐empty stomachs. The overall diet composition in terms of numerical importance was 71% fish, 28% crustaceans and 1% cephalopods. Of the fish, Engraulidae (Engraulis encrasicolus) were the dominant prey taxon, followed by Centracanthidae (Spicara spp.) and then Clupeidae (Sardina pilchardus). Statistically significant monthly variation in diet was apparent, suggesting that S. sarda take advantage of seasonally abundant prey fish in the study area, but no size‐related variation in the importance of the main prey species was detected.     

Ontogenetic changes in the diet of the sandbar shark, <Emphasis Type="Italic">Carcharhinus plumbeus</Emphasis>, in lower Chesapeake Bay and Virginia (USA) coastal waters

This study describes the diet of the sandbar shark, Carcharhinus plumbeus, highlighting differences in diet within various regions of the Virginia (USA) nursery area, as well as ontogenetic changes in diet. Stomach samples were obtained in 2001 and 2002 from 232 sharks caught by gillnets or longlines. Historical data from the Virginia Institute of Marine Science (VIMS) Shark Ecology Program were also analyzed. Ontogenetic changes in diet were evident, with crustacean prey decreasing in frequency with increasing shark size, and elasmobranch prey importance increasing with increasing shark size. Whereas previous research in Chincoteague Bay, VA showed the blue crab, Callinectes sapidus, was the dominant crustacean in sandbar shark diet, the mantis shrimp, Squilla empusa, dominated the crustacean portion of the diet in this study. Differences in diet of sharks were observed among locations within the study area. Small juveniles (≤80 cm precaudal length) in the lower Chesapeake Bay ate more fishes, whereas Eastern Shore juveniles ate more crustaceans. Crustacean prey items varied among locations along the Eastern Shore, with more portunid crabs consumed in waters near Wachapreague and more mantis shrimp consumed near Sand Shoal Inlet. Our study showed that Carcharhinus plumbeus is a generalist predator and is thus unlikely to strongly impact the population of any particular prey species, and in turn is not likely to be strongly affected by fluctuations in abundance of a single prey species.     

Stomach content analysis of juvenile, scalloped hammerhead shark Sphyrna lewini captured off the coast of Mazatlán, Mexico

We quantified the diet of juvenile, scalloped hammerhead shark Sphyrna lewini in the area off Mazatlan, Sinaloa, Mexico, to understand their feeding ecology this shark. The prey species of Sphyrna lewini were identified and quantified from stomach content analysis. In addition, we determined the variations between genders. During two fishing seasons (2000–2001 and 2001–2002), we analyzed 232 stomachs, of which 85% contained food. The trophic spectrum was composed of three species of cephalopods, six of crustaceans and 19 species of fish from mainly pelagic and benthic habitats. According to the Index of Relative Importance (%IRI), the cephalopod Loliolopsis diomedeae with IRI = 18%, fish of the family Carangidae IRI = 25% and family Synodontidae IRI = 19% constituted the main prey in general. The trophic niche width was <0.4, which indicated that S. lewini juveniles in this area feed on a wide range of prey items, though they showed a preference for a few prey items.     

Inter-annual variation in the diet of female southern rockhopper penguin (Eudyptes chrysocome chrysocome) at Tierra del Fuego

Inter-annual variation in the diet of female southern rockhopper penguins (Eudyptes chrysocome chrysocome) at Staten Island was studied during the early chick-rearing period to investigate the components of the diet and highlight some points of the possible food web in the study area. Gregarious crustaceans, small juveniles of squid and octopus, fish larvae and juvenile fish dominated the diet. There was a high degree of variability in the relative contribution of the different prey taxa during the three seasons studied. Overall, crustaceans were by far the most abundant in terms of number. Cephalopods contributed less to numbers, while fish was the least represented of the prey item in 2 years. Inter-annual variation in the proportion of prey items consumed was apparent only for some prey species: Thysanoessa gregaria, Gonatus antarcticus, Themisto gaudichaudii, Harpagifer bispinis and Salilota australis. These data suggest variability in the prey resources at sea during the study period and a subsequent opportunism of this penguin species to exploit what is differentially available. In terms of the food web in the area, we suggest an apparent relationship in the availability between T. gaudichaudii and G. antarcticus, and between T. gaudichaudii and H bispinnis. These relationships emphasise the importance of understanding food web interactions, especially those involving multiple trophic levels, when determining the role of upper-trophic level predators in marine systems.     

The diet of king eiders wintering in Nuuk,Southwest Greenland,with reference to sympatric wintering common eiders

Coastal and offshore waters of Southwest Greenland are internationally important wintering areas for king eiders (Somateria spectabilis) breeding in eastern Canadian Arctic and in northwestern Greenland. This paper presents the first assessment of their winter diet. Based on esophageal-proventricular samples from 26 females (13 juveniles and 13 older birds) and 15 males (11 juveniles and four older birds) collected in 2000–2002 (November–May) in coastal waters of Nuuk, we identified 28 prey species. The diet consisted of almost equal proportions (aggregate fresh mass) of polychaetes, echinoderms, crustaceans and molluscs. The dominant prey species were Pectinaria spp. (26.8%), Strongylocentrotus droebachiensis (18.4%), Mya eideri (11.2%) and Hyas araneus (9.7%). The polychaetes have previously been identified as important prey for eiders in Greenland, but apparently not outside Greenland. Compared with a diet study of common eiders Somateria mollissima from the same wintering area, the king eiders consumed significantly less bivalves and significantly more echinoderms. This difference corresponded with observations that common eiders were feeding in shallow waters, while king eiders were feeding in deeper waters farther from the shore. Benthic surveys are needed to confirm that diet corresponds with prey availability.     

The diet of blue whiting, hake, horse mackerel and mackerel off Portugal

This paper deals with the diets of blue whiting Micromesistius poutassou (Risso 1810), hake Merluccius merluccius (L. 1758), horse mackerel Trachurus trachurus (L. 1758), and mackerel Scomber scombrus (L. 1758) off Portugal and explores variations in fish length, water depth, latitude and season. All four species feed on fish; however, hake and mackerel are the first and second most important predators, respectively, blue whiting being the most important fish prey for both species. The diets of blue whiting and horse mackerel are composed mainly of crustaceans. Diet variations according to predator fish size are more important than either latitude or depth. In the diets of blue whiting, hake and horse mackerel, prey importance increases with predator size. For blue whiting and horse mackerel, diet variations with fish length and water depth are correlated: small fish are closely associated with coastal areas where they feed on copepods and decapod larvae. Seasonality in the diet is apparent for blue whiting, hake and mackerel. For blue whiting, the decapod Pasiphaea sivado is the most important prey in summer and autumn, being replaced by the euphausid Meganyctiphanes norvegica in winter. In the diet of hake, seasonality was characterised by the major importance of Macroramphosus scolopax in autumn, whereas the diet of mackerel consisted of zooplankton in summer, fish and decapods in autumn and decapod larvae in winter. Seasonal changes in the diet of horse mackerel correspond to a higher diversity of prey in autumn compared to other seasons (although euphausids are the main prey in all seasons). Seasonality in feeding activity is not as marked for the other species as it is for horse mackerel; the percentage of empty stomachs of horse mackerel is greatest in winter, when spawning takes place at the Portuguese coast.     

Diet,intestinal morphology,and nitrogen assimilation efficiency in the damselfish,Stegastes lividus,in Guam

Synopsis Analysis of stomach contents of Stegastes lividus revealed a shift from omnivory in juveniles to herbivory in adults. Red algae, primarily Polysiphonia spp., Gelidiopsis intricata and Ceramium spp., formed the bulk of the diet in all size classes. Foraminifera and small crustaceans were of particular importance in the diet of juveniles. Net nitrogen assimilation efficiency of field-fed S. lividus was approximately 61%, and did not vary with fish size. The net assimilation efficiencies of fish fed on Enteromorpha in the laboratory ranged from size-class means of 36–79% for nitrogen and from 29–72% for total organic material, and increased from juveniles to adults. Apparently, juveniles compensate for a lower efficiency in assimilating plant food by including a higher percentage of animal material in their natural diets. The ratios of intestine length to standard length (IL/SL) and to intestine diameter (IL/ID) increased rapidly in juveniles and leveled off in adults. The retention time for ingested food items may only be about one-half as long in juveniles as in adults. The combination of observed IL/ID ratios and estimated retention times proved most valuable in the interpretation of ontogenetic changes in feeding habits and assimilation efficiency.     

Weight–length relationships for 20 fish species collected in the mangroves of Guadeloupe (Lesser Antilles)

Weight–length relationships (WLR) are reported for 20 fish species from the mangroves of Guadeloupe (Lesser Antilles). The fishes were collected using fishing hoop nets. In the present study, the regressions were calculated after separating juveniles and adults. The b coefficient in the WLR varied between 2.37 and 3.85, with a mean of 3.15 ± 0.15.     

Prey use of the fishing spider Dolomedes triton (Pisauridae,Araneae): an important predator of the neuston community

Summary Prey of feeding juvenile and adult Dolomedes triton (Walckenaer 1837) were sampled over two seasons on three small ponds in central Alberta, Canada. Prey were mainly insects active at the water surface with truly aquatic species making up about 14% of the diet. Throughout the season aquatic and semi-aquatic Heteroptera represented about 30% of the prey. Diptera and adult Odonata were also important prey items but their abundance in the diet was more variable seasonally. Of the 625 prey items recorded nearly 50% were represented by taxa taken no more than once by spiders in one of the five size classes (adult females, adult males, large, intermediate and small juveniles). Large spiders did not take the smallest prey available, although small and intermediate-sized spiders fed on nearly the full size range taken by larger spiders. Cannibalism was common, accounting for 5% of the observations, with females and large juveniles as the most frequently observed cannibals. We hypothesize that intraguild predation (including cannibalism) could be an important coevolutionary force structuring phenology, population dynamics and microhabitat use of the predatory guild of the neuston community.     

Feeding habits and trophic levels of bluefin tuna Thunnus thynnus of different size classes in the Mediterranean Sea

Possible changes in diet and trophic levels in relation to size of Mediterranean bluefin tuna, Thunnus thynnus, were investigated using labelled carbon (δ¹³C) and nitrogen (δ¹⁵N) stable isotopes. Samples were obtained from two locations in the southern Tyrrhenian Sea (Western Mediterranean Sea) in May and October 2004. The δ¹³C and δ¹⁵N analyses revealed at least three significant isotopic groups [small juveniles (0.7–2.2 kg), sub‐adults (15–50 kg) and adults (70 to 225 kg)]. δ¹³C was negatively dependent on weight, while δ¹⁵N was positively dependent on weight [TW = 8.2 (±0.16) + 0.03 (± 0.0) *δ¹⁵N (n = 49; r = 0.91; P < 0.001)]. Different prey contribution to the diet was highlighted for each class. The diet of juveniles comprised zooplankton, small pelagic fish and some coastal fish; sub‐adults relied on medium pelagic fish, shrimps and cephalopods, and adults relied mainly on cephalopods and larger fish. The trophic level (TL) of tunas belonging to each size class was closely correlated to weight, starting from ca 3.0 TL for Group I and reaching 4.4–4.8 TL for the giants. Bluefin tuna, from small juveniles to giants, showed a shift in feeding preferences due to different use of habitats and food items as a function of the life stage.     

The diet of common eiders wintering in Nuuk,Southwest Greenland

Southwest Greenland provides wintering grounds for 70% (∼460,000) of the northern common eider (Somateria mollissima borealis) population. From 241 gullet samples (esophagus and proventriculus content) collected over three winters (1999–2002) near Nuuk, we identified 39 species consumed by the eiders. In contrast to studies elsewhere, fresh mass of the diet was dominated by soft-bottom species: the bivalve Mya eideri (32.8%) and the polychaete Pectinaria spp. (24.2%). The hard-bottom blue mussel (Mytilus edulis), usually the dominant prey of common eiders, was only the fourth most important diet species (7.5%). Overall, bivalves accounted for 56% of the diet. Twenty-seven prey species were minor foods with aggregate fresh mass of only 5.5%. Diets of males and females were similar, whereas juveniles consumed greater mass of crustaceans and less of bivalves. Diet diversity was higher in mid-winter than late winter, and higher in coastal habitats than in fjords. Within one important wintering area the results indicate that optimal size of blue mussels may be depleted over winter.     

Feeding habits of grey gurnard,Eutrigla gurnardus (L., 1758), along the Catalan coast (northwestern Mediterranean)

The stomach content of 589 specimens of Eutrigla gurnardus (collected at quarterly intervals) have been analyzed to determine diet according to fish size and season. The results show that crustaceans (% No = 93%, %Weight = 81%) are the basic food and that teleostean fishes are quite important in the adult diet. The importance of nycthemeral migrators decreases significantly as size increases, whilst the importance of benthic prey increases inversely. In every size, the feeding intensity is greater at midday than in the morning. Larger prey are eaten in winter than in summer although feeding intensity is lower in winter.     

Feeding ecology of the Bay Goby (Pisces: Gobiidae). Effects of behavioral,ontogenetic, and temporal variation on diet

The ontogenetic, diel, seasonal, and yearly variations in gut fullness, diet, and prey diversity for a California estuarine gobiid (Lepidogobius lepidus Girard) were examined. Also the feeding behavior of this species was described.Small (<50 mm, SL) and large (? 50 mm, SL) gobies consumed similar prey types in different proportions. Major prey items were polychaetes, harpacticoid copepods, gammarid amphipods, molluscs, and other crustaceans. Diets of large and small gobies were not significantly correlated, and larger fish had a more diverse diet. Small fish fed at all times while larger gobies fed primarily at night. Changes in diet may be related to differential prey preferences, feeding chronologies, and increases in fish size.Both large and small gobies displayed seasonal differences in diet and prey diversity. Year-to-year changes in diet also were noted for both size classes. The bay goby uses different feeding behaviors to capture sedentary and motile prey and appears to forage opportunistically. This behavior is probably advantageous in an environment which fluctuates drastically.     

Diet and prey consumption of Antarctic petrels Thalassoica antarctica at Svarthamaren, Dronning Maud Land, and at sea outside the colony

The diet of the Antarctic petrel Thalassoica antarctica was studied during two seasons at Svarthamaren, an inland colony in Dronning Maud Land, Antarctica, and in the pack ice off the coast of Svarthamaren. The most important food (wet mass) at Svarthamaren was crustaceans (67%), fish (29%) and squid (5%); however, individuals collected in the pack ice took mostly fish (87%). The prey composition and lengths of prey are comparable to what has been documented in other studies on this species. Estimates of food consumption by birds breeding at Svarthamaren (ca. 250,000 pairs) suggest that approximately 6500 tonnes of crustaceans, 2800 tonnes of fish and 435 tonnes of squid are consumed during the breeding season. The annual consumptions of these birds are estimated to be 34,100 tonnes of crustaceans, 14,700 tonnes of fish, and 2300 tonnes of squid. Satellite telemetry data indicate that Antarctic petrels from Svarthamaren may fly more than 3000 km during one foraging trip, and thus may cover a huge ocean area to obtain their prey. Received: 1 September 1997 / Accepted: 3 February 1998