Trophic relations of the cephalopod Martialia hyadesi (Teuthoidea: Ommastrephidae) at the Antarctic Polar Front,Scotia Sea

Samples of the squid Martialia hyadesi were collected aboard two Japanese squid-jigging vessels carrying out commercial fishing trials at the Antarctic Polar Frontal Zone, north Scotia Sea, in February 1989. The dissected stomachs of 61 specimens were classified according to fullness and the contents were examined visually. Identifiable food items included fish sagittal otoliths, crustacean eyes, the lappets on euphausiid first antennule segments and cephalopod sucker rings. The most frequent items in the squid's diet were the myctophid fishes Krefftichthys anderssoni and Electrona carlsbergi, the euphausiid Euphausia superba and a hyperiid amphipod, probably Themisto gaudichaudi. A small proportion of the sample had been feeding cannibalistically. Total lengths of the fish prey were estimated from sagittal otolith size using published relationships. All fish were relatively small; 7 to 35% of squid mantle-length. However, it is possible that some heads of larger fish are discarded by the squid and so are not represented by otoliths in the stomach contents. Over the size range of squid in the sample there was no relationship between size of fish prey and size of squid. Similarly, when the squid sample was divided into groups according to prey categories: crustaceans, crustaceans+fish, fish, cephalopod, there was no evidence that dietary preference was related to squid size. The prevalence of copepod-feeding myctophids in the diet of this squid, which is itself a major prey item of some higher predators in the Scotia Sea, suggests that a previously unrecognised food chain: copepod-myctophid-M. hyadesi-higher predator, may be an important component of the Antarctic oceanic ecosystem.     

Optimal foraging: The influence of intraspecific competition on diet selection

Summary Three-spined sticklebacks (Gasterosteus aculeatus) showed a relative preference for a familiar prey size when hunting for two sizes of Daphnia magna in high density. This result is not compatible with the apparent size hypothesis. Ten groups were investigated, each consisting of two stickle-backs tested under three consecutive experimental conditions, to establish whether the function of the preference for a familiar prey size could be avoidance of competition.First, the relative competitive ability of each fish was determined by the proportion it consumed of three series of 60 medium-sized daphnia, offered pairwise. Second, during the consumption of five series of 30 pairs, each consisting of a large and a small daphnia, it was determined how many items each fish caught of each prey size. As handling times were equal for both prey types, the larger prey size was more profitable. There was a significant correlation between relative competitive ability and mean proportion of large daphnia in the diet. In the last series the less successful competitiors caught a higher proportion of small prey than in the first series. Finally, each fish was given the choice between large and small daphnia in the absence of its competitor. The sticklebacks chose a diet similar to the one they had been allowed to select previously with competition. The previously more successful competitors concentrated on large daphnia, whereas the poorer competitors fed as generalists but not unselectively.The fish probably learned the distance from which they had recently attacked familiar prey successfully. This sure attack distance depends on the fish's competitive ability.     

Feeding,starvation and metabolic rate in the shore crab Carcinus maenas

The relationship between food intake and metabolic rate (as measured by oxygen consumption) of Carcinus maenas (L.) was studied. The metabolic rate of feeding crabs which had been starved for a short period increased, and several days were required for it to return to its original level. During prolonged starvation, the metabolic rate of C. maenas fell in two stages. The first reduction was to about 60% of the feeding level; this occurred during the first week of starvation. There was then a further reduction to about 40% of the feeding level and, at this level of metabolism, the crabs were able to survive for 3 months, with only 50% mortality; most of this occurred in the last 2 weeks. Metabolic rate was found to affect food intake; crabs acclimated to 24°C took 2.4 times as much food as crabs acclimated to 10°C, although the metabolic levels of the two groups differed by a factor of only 1.4. From the results obtained when the crabs were starved and when starved crabs were fed, it is suggested that, during starvation, the metabolic rate of C. maenas first drops from the elevated feeding level to a level at which carbohydrate reserves are utilised, and subsequently to a minimum level at which lipid reserves and proteins are used.     

Age-related shifts in the diet composition of southern elephant seals expand overall foraging niche

Southern elephant seals are important apex predators in a highly variable and unpredictable marine environment. In the presence of resource limitation, foraging behaviours evolve to reduce intra-specific competition increasing a species’ overall probability of successful foraging. We examined the diet of 141 (aged 1–3 years) juvenile southern elephant seals to test the hypotheses that differences between ages, sexes and seasons in diet structure occur. We described prey species composition for common squid and fish species and the mean size of cephalopod prey items for these age groups. Three cephalopod species dominated the stomach samples, Alluroteuthis antarcticus, Histioteuthis eltaninae and Slosarczykovia circumantarcticus. We found age-related differences in both species composition and size of larger prey species that probably relate to ontogenetic changes in diving ability and haul-out behaviour and prey availability. These changes in foraging behaviour and diet are hypothesised to reduce intra-specific food competition concomitant with the increase in foraging niche of growing juveniles.     

Graded recruitment in a ponerine ant

Summary (1) The giant tropical ant, Paraponera clavata, exhibits graded recruitment responses, depending on the type, quantity, and quality of a food source. More ants are initially recruited to a large prey or scavenge item than to a large quantity of sugar water. (2) Individual ants encountering prey items gauge the size and/or unwieldiness of the item, regardless of the weight, when determining whether to recruit. (3) The trail pheromone of this species is often used as an orientation device by individual ants, independent of recruitment of nestmates. (4) It is proposed that the foraging behavior of P. clavata represents one of the evolutionary transitions from the independent foraging activities of the primitive ants to the highly coordinated cooperative foraging activities of many higher ants.     

Diet of Neotropic cormorant (Phalacrocorax brasilianus) in an estuarine environment

The diet of the Neotropic cormorant (Phalacrocorax brasilianus) was studied by analysing 289 regurgitated pellets collected from a roosting site at Lagoa dos Patos estuary, southern Brazil, between November 2001 and October 2002 (except April to June). In total, 5,584 remains of prey items from 20 food types were found. Fish composed the bulk of the diet representing 99.9% by mass and 99.7% by number. The main food items were White croaker (Micropogonias furnieri) (73.7% by frequency of occurrence, 48.9% by mass and 41.2% by number), followed by Catfish (Ariidae) and anchovies (Engraulididae). In Lagoa dos Patos estuary the generalist Neotropic cormorant fed mainly on the two most abundant demersal fishes (White croaker and Catfish), which accounted for the low niche breadth calculated. The total length of all fish preyed varied from 27.2 to 318.3 mm (113.5 ± 48.0 mm), and preyed White croakers’ size differed between months. Neotropic cormorants seem to prey on most abundant class sizes of White croaker instead of selecting similar prey size throughout the time. However, temporary changes in diet in terms of food items, abundance and prey size were detected, revealing a high ecological plasticity of the species. Individual daily food intake of Neotropic cormorants estimated by pellets and metabolic equations corresponded to 23.7 and 27.1% of their body mass, falling in the range of other cormorant species. Annual food consumption of the population estimated by both methods was 73.4 and 81.9 tonnes, comprising mainly immature and subadult White croaker and Catfish which are commercially important. Temporal variations in diet composition and fish size preyed by Neotropics cormorants, a widespread and generalist species, suggest shifts according to fluctuations in the abundance of prey. The plasticity of this cormorant is also revealed by their ability to adjust feeding behaviour in response to temporal or local changes in the environment, from a generalist at the species level to a specialist at the individual or local population level.     

An energy–distance trade-off in a central-place forager,the Antarctic fur seal (<Emphasis Type="Italic">Arctocephalus gazella</Emphasis>)

We tested the prediction that lactating fur seals (Arctocephalus gazella) at South Georgia will take prey of greater energy density with increasing distance of foraging from the colony. The study investigated the differences in diet of fur seals foraging within two regions, one near the breeding colony and the other at greater distance. Diet varied significantly in relation to foraging location. Dietary items of low quality were eaten in both regions but more food items with a high-energy content appeared in the diet of seals travelling to distant oceanic waters. We conclude that there is likely to be a trade-off between energy gain and distance travelled which enables female fur seals to maintain a relatively constant rate of energy delivery to their offspring irrespective of the distance travelled to find food.     

Distribution and diet of juvenile Patagonian toothfish on the South Georgia and Shag Rocks shelves (Southern Ocean)

The distribution and diet of juvenile (<750 mm) Patagonian toothfish are described from four annual trawl surveys (2003–2006) around the island of South Georgia in the Atlantic sector of the Southern Ocean. Recruitment of toothfish varies inter-annually, and a single large cohort dominated during the four years surveyed. Most juveniles were caught on the Shag Rocks shelf to the NW of South Georgia, with fish subsequently dispersing to deeper water around both the South Georgia and Shag Rocks shelves. Mean size of juvenile toothfish increased with depth of capture. Stomach contents analysis was conducted on 795 fish that contained food remains and revealed that juvenile toothfish are essentially piscivorous, with the diet dominated by notothenid fish. The yellow-finned notothen, Patagonotothen guntheri, was the dominant prey at Shag Rocks whilst at South Georgia, where P. guntheri is absent, the dominant prey were Antarctic krill and notothenid fish. The diet changed with size, with an increase in myctophid fish and krill as toothfish grow and disperse. The size of prey also increased with fish size, with a greater range of prey sizes consumed by larger fish.     

Feeding behaviour and prey size selection of gilthead seabream,Sparus aurata,larvae fed on inert and live food

Prey selection shortly after the onset of feeding by laboratory-reared gilthead seabream, Sparus aurata L., larvae was studied using larvae fed on two types of microcapsule (hard- and soft-walled) having diameters ranging from 25 to 300 m. Preferences between inert food and live prey (rotifers and Artemia sp. nauplii) were also studied. Seabream larvae were able to ingest inert food from first feeding. Larvae of all size classes ingested hard microcapsules with diameters in the range 25 to 250 m. However, larvae with a total length (TL) below 4 mm preferentially selected particles 25 to 50 m in diameter, larvae of TL 4 and 5 mm preferred particles 51 to 100 m in diameter, while larvae above 5 mm TL preferred particles 101 to 150 m in diameter. With soft microcapsules, larvae always preferred particles larger than in the previous case, and above 4.5 mm TL they preferentially selected particles 201 to 250 m in diameter. In addition, the gradual increase of preferred diameters with increasing TL was more pronounced when larvae were increasing TL was more pronounced when larvae were fed on soft particles. Mean values for prey width/mouth width ratios were approximately 0.24 and 0.30 when larvae were fed on hard-walled and soft-walled microcapsules, respectively, irrespective of the absolute value of larval length. When a mixed diet of live and inert food items was offered, live prey were always preferentially selected, even if the prey width/mouth width ratio was apparently not favourable. Only a physical constraint such as excessive prey width could counter this preference for living prey vs inert microcapsules. These results contribute to our knowledge in larval feeding behaviour, especially in the presence of inert food, and represent a fundamental step in developing prepared food for marine fish larvae.     

Feeding ecology of a size-structured predator population,the South American sun-star Heliaster helianthus

Patterns of feeding in a population of Heliaster helianthus (Lamarck), a common and dominant species of starfish indigenous to the Pacific South American coast, were investigated in an intertidal habitat in central Peru from October 1986 to April 1987. The H. helianthus population comprised individuals of 3.5 to 30.2 cm body size (diameter) with two modal size classes. The number of rays ranged between 18 and 40, and individuals with 31 to 33 rays accounted for ca. 42% of the total population. There was a higher rate of increase in ray number with body size amongst small individuals(<13.0 cm diam). H. helianthus is capable of feeding on more than one prey item at a time (average of 5.6 to 13.2 prey items handled, with several predators observed to hold >100), and both the number of prey individuals captured and the total prey biomass were significantly correlated with predator size. Amongst a total of 1132 feeding observations, the largest number of predators (an average of 85.4% of those feeding) were preying on the mussel Semimytilus algosus whilst another mussel, Perumytilus purpuratus, ranked second with 21,9% of predators feeding. The proportion of S. algosus in the diet increased from 65.4% in the smallest predator size-group (10.9 cm diam) to 91.2% in the largest (19.0 cm). In contrast, P. purpuratus and barnacles were more highly represented in the diet of small H. helianthus. The smallest size-group (10.9 cm) had low dietary overlap with larger sizes and less specialized prey utilization. Two geographically separated populations of H. helianthus in Peru and Chile showed contrasting patterns of prey utilization. S. algosus and P. purpuratus comprised 85.5 and 6.5% by number in the diet of the Peruvian population, respectively, whilst corresponding figures for the Chilean population were 8.3 and 60.5%, with barnacles attaining a higher share (22.6%). However, the total number of prey individuals per feeding predator was almost the same in Peru and Chile, with 10.0 and 10.7 individuals, respectively. H. helianthus individuals of different sizes occupy slightly different microhabitats within the intertidal area, which, coupled with differential spatial distribution of prey species, results in the predator population being able to utilize a wide range of resources.     

Foraging behavior in early Atlantic cod larvae (Gadus morhua) feeding on a protozoan (Balanionsp.) and a copepod nauplius (Pseudodiaptomussp.)

Food limitation is likely to be a source of mortality for fish larvae in the first few weeks after hatching. In the laboratory, we analyzed all aspects of foraging in cod larvae (Gadus morhua Linnaeus) from 5 to 20 d post-hatching using protozoa (Balanion sp.) and copepod nauplii (Pseudodiaptomus sp.) as prey. A camera acquisition system with two orthogonal cameras and a digital image analysis program was used to observe patterns of foraging. Digitization provided three-dimensional speeds, distances, and angles for each foraging event, and determined prey and fish larval head and tail positions. Larval cod swimming speeds, perception distances, angles, and volumes increased with larval fish size. Larval cod swam in a series of short intense bursts interspersed with slower gliding sequences. In 94% of all foraging events prey items were perceived during glides. Larval cod foraging has three possible outcomes: unsuccessful attacks, aborted attacks, and successful attacks. The percentage of successful attacks increased with fish size. In all larval fish size classes, successful attacks had smaller attack distances and faster attack speeds than unsuccessful attacks. Among prey items slowly swimming protozoans were the preferred food of first-feeding cod larvae; larger larvae had higher swimming speeds and captured larger, faster copepod nauplii. Protozoans may be an important prey item for first-feeding larvae providing essential resources for growth to a size at which copepod nauplii are captured. Received: 20 April 1999 / Accepted: 12 January 2000     

Daily ration and specific daily ration of the chaetognath Sagitta enflata

Digestion times, obtained directly from laboratory feeding, were used in conjunction with a day/night series of field samples to estimate the daily ration and specific daily ration of Sagitta enflata Grassi, the most abundant chaetognath in the Gulf Stream near Miami, Florida (USA), during winter and early spring. Feeding was independent of time of day and increased with increasing chaetognath size from 1.7 prey day^-1 for the 11.5 mm size class to 2.9 prey day^-1 for the 21.5 mm class. The diet of s. enflata consisted of 94.8% copepods having a mean weight of 21.8 g, and 5.2% prey chaetognaths estimated to be two-thirds the length of the predators. The daily ration of S. enflata from 12.5 to 20.5 mm in length increased from 48.3 to 143.3 g of prey dry weight and from 18.5 to 48.1 g of prey carbon. The specific daily ration declined from 0.12 to 0.08 dry weight basis, and from 0.26 to 0.14, carbon basis, over the same size range. Prey chaetognaths contributed as much as 51% of the ration on a carbon basis.     

Vertical distribution and feeding patterns of midwater fish in the central equatorial Atlantic II. Sternoptychidae

The vertical distribution of seven sternoptychid species was examined from RMT 1+8 samples collected aboard R. V. Meteor in March-April 1979 and from Royal Research Ship R.R.S. Discovery in July 1974 in the central equatorial Atlantic. During daytime sternoptychids occupied depths between 200 and 1250 m, with Sternoptyx pseudobscura living deepest, centering between 800 and 900 m, and Argyropelecus sladeni most shallow, aggregating predominantly at 300 and 400 m. They are all considered limited or partial migrants, ascending only some 100 and 200 m towards the surface at night. Only A. sladeni was observed to enter the epipelagic zone (0 and 200 m).-Feeding patterns were investigated from stomach content analyses of Sternoptyx diaphana, S. pseudobscura, Argyropelecus sladeni and A. affinis. Additional stomach contents were analysed from samples of S. diaphana, A. hemigymnus and A. olfersi collected in June 1985 from F.R.V. Walther Herwig in the temperate NE Atlantic at 46°N, 17°W by means of the Engel Trawl. The food spectrum of the six species is generally described, and additional dietary evidence regarding calanoid copepod prey is provided for four of these taxa. All sternoptychid species investigated were planktivorous, feeding predominantly on copepods and ostracods, except for the largest size class, which preyed heavily on euphausiids and amphipods. The relationship of predator size towards prey type and prey size is analysed for both Sternoptyx species. Of these, S. pseudobscura in particular exhibits taxonomic selectivity towards polychaete prey. The diet of both species of Sternoptyx included a number of epipelagic or even neustonic calanoid copepod species which contributed more than 50% of the total copepod population by numbers. So far it is not known how the predators find access to prey organisms of the upper 200 m, as netfeeding is considered unlikely. Cyclopoid copepods of the genus Sapphirina were observed as dietary component particular of S. diaphana.     

Population energetics of the intertidal isopod Cirolana harfordi

Laboratory experiments and field measurements generated a population energy-budget estimate for the isopod Cirolana harfordi (Lockington). Assimilation of food energy averaged 88% on a diet of fish. About 35% of assimilated energy is allocated for growth and reproduction, while the bulk of the remainder is used for maintenance and activity (respiration). The high growth efficiency of C. harfordi is discussed with respect to ecological efficiency and energy transfer in marine systems.     

Larval viability and heterozygote deficiency in populations of marine bivalves: evidence from pair matings of mussels

Using a pressurized temperature gradient, which rapidly equilibrates bacteria to a wide range of temperature and pressures, temperature ranges for growth and survival of Vibrio harveyi (strain B 392) were found to narrow with increasing pressures. Both temperature and pressure ranges were greater in an enriched seawater medium than in unenriched seawater. Using a kinetics apparatus, V. harveyi was also exposed to gradual changes in temperature and pressure, simulating a particle sinking at approximately 200 m d^-1 from surface waters (17 °C; 1 bar pressure) to 3 400 m (4°C; 340 bars pressure). With gradual temperature changes, cells were able to grow and survive at lower temperatures than they had in rapid equilibration experiments. Gradual pressurization, however, did not increase pressure tolerance. Under all experimental conditions, the combined effects of low temperatures and high pressures were lethal for V. harveyi. It is hypothesized that temperature and pressure stratify the ocean into regions of growth, survival, and death for V. harveyi, and perhaps other bacteria.     

Natural feeding activity of the crab Callinectes ornatus (Portunidae) in Ubatuba Bay (São Paulo, Brazil): influence of season, sex, size and molt stage

The natural diet of Callinectes ornatus in Ubatuba Bay, Ubatuba (SP), Brazil, was studied. Influence of season, sex, size and molt stage variation in the diet were also described. Specimens were collected monthly over a period of 1 year, in Ubatuba Bay, using a fishing boat equipped with a double rigged net. The relative degree of stomach fullness, frequency of occurrence, and quantitative scoring (percentage) were determined. Digested material, crustaceans, sediment and mollusks were the principal items of the diet. Echinoderms, annelids, plants, and fishes were also notable prey items, and other food categories (foraminiferans, cnidarians, worms and bryozoans) occurred in low percentages. A non-significant difference was found in many items during seasonal analysis, i.e. plants, foraminiferans, cnidarians, bryozoans and digested material were present at highest percentages during summer and autumn, while crustaceans and echinoderms were consumed in larger amounts during the spring and winter. Males and females consumed a similar diet, although ovigerous females showed a greater tendency to feed on items with soft tissues (fish, squid, shrimp). Juveniles fed more on sedentary and calcareous items than did adults, showing a significant tendency to reduce consumption of foraminiferans, annelids, bryozoans, echinoderms and sediment as the animal increased in size, accompanied by an elevation in the consumption of plants, crustaceans and fish. These crabs are opportunistic predators with a preference for crustaceans and mollusks that is correlated with the availability of prey and the ontogenetic phase of the predator. Received: 21 January 2000 / Accepted: 2 October 2000     

Fish and squid in the diet of king penguin chicks,Aptenodytes patagonicus,during winter at sub-antarctic Crozet Islands

The diet of king penguins, Aptenodytes patagonicus, rearing chicks was studied during three consecutive austral winters (1990, 1991 and 1992) at Crozet Islands. The mean stomach content mass of the 47 samples was 503 g. Percentages of wet and reconstituted masses showed that both fishes (66 and 36%, respectively) and squid (34 and 64%) are important components of the winter diet. Juveniles of the demersal onychoteuthid squid Moroteuthis ingens form the bulk of the cephalopod diet, and this was the main prey by reconstituted mass (57%). Myctophid fish (lantern-fishes) accounted for most of the fish diet, constituting together 32% by mass. The three main species of myctophids eaten in summer by king penguins were either very rare in winter (Electrona carlsbergi) or accounted for a smaller proportion of the diet (Krefftichthys anderssoni = 1.5% by mass and Protomyctophum tenisoni = 4.6%). Five other myctophids, which are rarely consumed in summer, contributed 24% of the diet by mass in winter (Gymnoscopelus piabilis = 18.1%, Lampichthys procerus = 2.4%, G. nicholsi = 1.3%, and Metelectrona ventralis and Electrona subaspera = 1.0%). The greater diversity of prey in winter suggests a more opportunistic feeding behaviour at a time probably marked by a change in prey availability. Both the known ecology of the fish and squid prey and the barely digested state of some items suggest that in winter breeding adults forage in the outer shelf, upper slope and oceanic areas in the close vicinity of the Crozet Islands to feed their chicks. Finally, using king penguins as biological samplers, the present work provides novel data on the previously unstudied mesopelagic/epibenthic marine community in waters surrounding the Crozet Islands. Seventeen myctophid fish have been identified to species level. These include several poorly known species in the southern Indian Ocean. The occurrence of small, nearly intact, cephalopods in the diet of king penguins suggests that spawning grounds of four squid species may be located near the Crozet Archipelago.     

Spatial and temporal variation in the diet of a high trophic level predator,the Australian fur seal (<Emphasis Type="Italic">Arctocephalus pusillus doriferus</Emphasis>)

This study quantifies the manner in which Australian fur seals, Arctocephalus pusillus doriferus, use their prey in a spatial and temporal context. We analysed 977 scat and 66 regurgitate samples collected from Tasmanian breeding colonies and haul-outs between 1994 and 2000. Diagnostic prey remains identified in the scats represented 35 fish taxa and 8 cephalopod taxa. The main taxa identified in scats, where frequency of occurrence was 10%, were leatherjacket species (family Monocanthidae), redbait (Emmelichthys nitidus), barracouta (Thyrsites atun), jack mackerel (Trachurus declivis) and red cod (Pseudophysis bachus). Regurgitates were dominated by cephalopods, primarily Goulds squid (Nototodarus gouldi), Octopus maorum, O. berrima/pallidus and Sepia apama. Discriminant function analyses indicated that there were generally no significant differences in the composition of the diet between colonies within a year, suggesting that prey distribution is fairly uniform throughout Bass Strait at those time scales. The diet at breeding colonies, however, exhibited significant inter- and intra-annual variation, determined by the presence of several key taxa, such as barracouta and a species of scorpionfish (family Scorpaenidae). The diet composition also varied regionally, between Bass Strait and southern Tasmania in spring 1999 and autumn 2000, with redbait, barracouta and a species of scorpionfish identified as the main taxa contributing to this difference. Redbait occurred in the diet only in southern Tasmania, whereas barracouta and scorpionfish occurred only in Bass Strait.Electronic Supplementary Material Supplementary material is available in the online version of this article at http://dx.doi.org/10.1007/s00227-003-1219-0.Communicated by G.F. Humphrey, Sydney     

Filtering rates of the juvenile Atlantic menhaden Brevoortia tyrannus (Pisces: Clupeidae), with consideration of the effects of detritus and swimming speed

From July to September 1982 feeding experiments were conducted with 138-mm fork length Atlantic menhaden Brevoortia tyrannus (Latrobe) (Pisces: Clupeidae) to determine their particle size-specific feeding abilities. Monoculture clearing-rate experiments showed that the minimum size of particles filtered, the minimum size threshold, for 138-mm fish is 7 to 9 m. Filtration efficiency for three species of phytoplankton below the minimum size threshold. Pseudoisochrysis paradoxa, Monochrysis lutheri, and Isochrysis galbana, averaged 1.0% (n=14). Tetraselmis suecica, Prorocentrum minimum, and 2-celled Skeleionema costatum, phytoplankton which are larger than the minimum size threshold and smaller than the 20-m upper limit for nanoplankton, were filtered at efficiencies averaging 21% (n=24). S. costatum chains of 3 to 6 cells, prey particles exceeding the size limits of nanoplankton, were filtered at average efficiencies ranging from 22 to 84%. The mean filtration efficiency for Artemia sp. nauplii (San Francisco Bay Brand) of 36% (n=7) was lower than for smaller phytoplankton prey. The presence of detritus at concentrations usually encountered in nature enhanced filtering efficiency and lowered minimum size thresholds at which phytoplankton were retained. For small food particles, filtering efficiency decreased as swimming speed of the menhaden increased. As menhaden grow, their feeding tepertoire shifts to larger planktonic organisms.Contribution No. 1201 Virginia Institute of Marine Science     

Ecological overlap between macrourids in the western mediterranean sea

Stomachs from over 323 specimens of Hymenocephalus italicus, 168 Nezumia aequalis, 160 Coelorhynchus coelorhynchus and 1670 Trachyrhynchus trachyrhynchus were collected from September 1976 to September 1978 from the West Mediterranean continental slope at depths between 200 and 800 m. Copepods, amphipods and other pelagic crustanceans form the main fraction of the diet of H. italicus. The diets of N. aequalis and C. coelorhynchus consist largely of polychaetes, isopods, amphipods, mysids and decapod crustaceans. T. trachyrhynchus feed heavily on decapods. Decreasing fractions of small crustaceans are found in diets of all species as fish size increases. The mean size of prey increases with the body size of the fish. A positive size-depth correlation has been observed in macrourids. Juveniles and intermediates of N. aequalis, C. coelorhynchus and T. trachyrhynchus were found in shallow-water zones (<400 m), while adults were more common in deeper areas. Niche breadth and niche overlap were calculated between size groups. Niches are relatively broader with respect to habitat and narrower for prey size and prey type. The food overlaps between N. aequalis C. coelorhynchus and between C. coelorhynchus and T. trachyrhynchus are notably the greatest. Overlap in relation to habitat is high, while the correlation between niche parameters is very poor, indicating a certain degree of independence between these factors. Alpha matrices were estimated using both multiplicative ( product) and additive ( summation) multidimensional estimates for niche overlaps. Rates of competitive exclusion are low.