Selective feeding and grazing rates of oyster (Crassostrea virginica) larvae on natural phytoplankton assemblages

Results of laboratory feeding experiments suggest that oyster (Crassostrea virginica) larvae select small phytoplanktonic organisms (<10 μm) over larger-celled forms from natural estuarine assemblages, but that little selection occurs within the small phytoplankton fraction. Laboratory-reared larvae grazed each of five small-celled phytoplankton groups enumerated (coccoids, centrate diatoms, pennate diatoms, dinoflagellates, and flagellates) at rates proportional to phytoplankton group densities in controls (no larvae added). Larval grazing rates (number of cells removed per larva per hour) were inversely proportional to both larval density and experimental duration.     

Spatio-temporal Variability in Larval-Stage Feeding and Nutritional Sources as Factors Influencing Striped Bass (Morone saxatilis) Recruitment Success

Prey availability and feeding success affect survival of larval striped bass (Morone saxatilis) in Chesapeake Bay and contribute to the >30-fold interannual recruitment variability. Gut contents and stable isotope analyses (δ¹⁵N and δ¹³C) were conducted on striped bass larvae to evaluate sources of nutrition in 2007 and 2008, years of high and poor recruitment, respectively. Ichthyoplankton and zooplankton were surveyed in the upper Chesapeake Bay, in proximity to the estuarine turbidity maximum and associated salt front. Feeding incidence and numbers of prey per gut were similar in both years and varied in relation to the salt front. The primary prey in each year was the estuarine copepod Eurytemora affinis. Substantial consumption of the freshwater cladoceran Bosmina spp. also occurred, especially up-estuary of the salt front in 2007, demonstrating that secondary prey are important to larval diets in some years. Stable isotope analysis of yolk sac and feeding-stage larvae of striped bass revealed an ontogenetic shift from maternal stable isotope signatures to those indicative of prey source. Feeding-stage larvae from up-estuary locations had the most negative δ¹³C values, indicating a relatively high terrestrial carbon source in prey. Spatio-temporal variability in δ¹⁵N signatures of larvae followed similar trends of δ¹⁵N variability in zooplankton prey with the highest δ¹⁵N values up-estuary of the salt front and estuarine turbidity maximum. A stable isotope analysis on archived striped bass larvae collected in 1998 and 2003, years of moderate and high recruitment, respectively, expanded the documented range of isotope signatures but did not clearly distinguish effects of nutritional sources on recruitment.     

Macrophyte detritus in the winter diet of the estuarine mysid,Neomysis americana

Neomysis americana, a common prey item in the diet of estuarine fish, is seasonally abundant in saltmarsh tidal creeks of North Inlet, South Carolina. Visual examination of mysids collected in winter revealed only diatom tests in their guts, with nearly all of the ingesta unidentifiable. Immunological diet analysis provided strong evidence that proteins fromSpartina alterniflora detritus were present in the guts of field-collected mysids. Laboratory determinations of weight-specific ingestion rates for male and female mysids fed particulateS. alterniflora were best described by a curvilinear quadratic function. Ingestion rates ranged from zero to 0.116 mg C ingested per mg C body weight per hr, or greater than 200% of body carbon per day if continuous feeding is assumed. These high ingestion rates may indicate low assimilation efficiency, but ifN. americana’s assimilation efficiency is similar to that of other mysid species, then this mysid may represent an efficient trophic link between saltmarsh macrophyte production and higher trophic levels. The occurrence ofSpartina detritus in the guts of winter mysids suggests that detritivory may be important during periods of scarce food.     

Trophic ecology of the dominant fishes in Elkhorn Slough,California, 1974–1980

Food habits of the dominant fishes collected from 1974 to 1980 at eight locations in Elkhorn Slough, California, and the adjacent ocean were investigated. Epifaunal crustacea was the major prey group identified from stomach contents of more than 2,000 fishes, followed by epifaunal and infaunal worms, and molluscs. Overall, 18 fish species consumed 263 different prey taxa, ranging from 10 taxa to 125 taxa per fish species and including 99 crustacean, 56 polychaete, and 39 molluscan taxa. Mean prey richness was greatest at stations near the ocean and lowest at inshore stations. Detailed dietary data for all prey taxa were summarized as trophic spectra for each fish species. Trophic spectra represented functional groups of prey and were used for comparisons of dietary similarity. Cluster analyses, based on trophic spectra, resulted in four feeding guilds of fishes. Of 18 fish species, seven (Amphistichus argenteus, Leptocottus armatus, Embiotoca jacksoni, Clevelandia ios, Gillichthys mirabilis, Cymatogaster aggregata, andCitharichthys stimaeus) fed principally on epifaunal crustacea. Four species (Pleuronectes vetulus, Platichthys stellatus, Phanerodon furcatus, andMyliobatus californica) consumed mostly molluscs and infaunal worms. Two species (Psettichthys melanostictus andTriakis semifasciata) fed on mobile crustacea, and five species (Hyperprosopon anale, Engraulis mordax, Clupea pallasi, Atherinopsis californiensis, andAtherinops affinis) fed largely on zooplankton and plant material. Our results suggest that high food availability enhances the nursery function of imshore habitats, and emphasize the importance of invertebrate prey populations and the indirect linkage of plant production to the ichthyofaunal assemblarly marine immigrant species that are likely ‘estuarine dependent’.     

Trophic relationships of three sunfishes (Lepomis spp.) in an estuarine bayou

We examined ontogenetic, interspecific, and seasonal trophic patterns among sympatric sunfish species, redspotted sunfish,Lepomis miniatus; redear sunfish,Lepomis microlophus; and bluegill,Lepomis macrochirus, in an estuarine bayou. In particular we studied these feeding patterns in relation to relative abundances of prey from different benthic feeding habitats. All three sunfishes showed ontogenetic divergence in their trophic niches, reflecting different ecomorphological specializations. Small fishes depended on zooplankton, whereas larger fishes of all three species shifted their diets to benthic macrofauna. A potential for trophic resource partitioning was reflected by dietary differences among the three sunfish species. One impalied mechanism for resource partitioning was feeding habitat, as redear sunfish frequently used sediment-associated prey, while bluegill showed greater use of water-column-associated prey, and redspotted sunfish often used SAV-associated prey. However, all three sunfishes apparently used each feeding habitat to some degree; and, trophic differences were more clearly based on prey type than on feeding habitat. Redear sunfish, which can crush hard-shelled prey, exhibited the most distinctive diet. An apparent seasonal shift in feeding habitat occurred in autumn/winter, as indicated by increased overlap between diets and SAV. This shift was facilitated by changes in the relative abundances of several common prey types between benthic habitats. The relative abuandance and use of freshwater and estuarine-derived prey also varied seasonally, suggesting a possible trophic benefit of consistent prey availability in the estuarine bayou.     

Filtration and oxygen consumption in mussels,Mytilus edulis,with and without pea crabs,Pinnotheres maculatus

Filtration rates and oxygen consumption rates were measured in mussels (Mytilus edulis) with and without pea crabs (Pinnotheres maculatus). Noninfested mussels had a significantly higher rate of oxygen consumption per hour (0.578 ml±0.012) than did infested mussels (0.352 ml±0.012). There was no significant effect of pea crab size on mussel respiration. Filtration rates of infested mussels were significantly lower than those of uninfested mussels. Assimilation efficiency was not significantly affected by pea crab infestation. The relationship between body size and oxygen consumption inP. maculatus is given by the following equation: {ie264-1} W^0.626, where {ie264-2} is oxygen uptake (ml h^?1), and W is dry weight (g). There was no difference between the sexes. It is concluded that the decreased oxygen consumption observed in infested mussels is not due to limitation of oxygen availability, but rather reflects a real metabolic response to the presence of the symbiont and the concomittant deprivation of food to the host. The effect is probably reversible, that is, damage can be compensated for after the symbiont has vacated the mussel, depending upon the period of infestation. Our results indicate that the mussels infested by pea crabs may be at an energetic disadvantage relative to mussels without pea crabs.     

Growth rates of American alligators in estuarine and palustrine wetlands in Louisiana

A comparative study of American alligator (Alligator mississippiensis) growth rates was made in estuarine and palustrine wetlands in southwestern Louisiana. In the estuarine wetlands, where characteristic salinity levels were ≤5%, alligators grew faster and therefore reached sexual maturity earlier than did those in palustrine wetlands, which are characterized by shallow, freshwater marsh vegetation. Slower growth rates in palustrine wetlands appeared to be related to prey density, indicated by previous studies to be lower than in estuarine wetlands. Males grew faster than females and therefore reached sexual maturity at an earlier age in both habitats. This study revealed a major limitation in using total lengths as an index upon which population age structure can be based even when alligators are in the same geographic region.     

The feeding and growth of the sea nettle,Chrysaora quinquecirrha (Desor), in the laboratory

The sea nettle,Chrysaora quinquecirrha, can capture ctenophore prey on its exumbrella, tentacles, and oral arms. Once attached to the medusa, the prey is transferred to the oral arms, where it becomes enveloped by the lip-like folds of the oral arms. The oral arms move the prey as far proximally as possible by ciliary creeping and muscular contractions. If the prey is too large to enter the stomach, it is digested within the oral arms by extracellular, proteolytic enzymes that probably originate from the gastrodermis of the oral arms and/or from the gastric cirri. Several large ctenophores can be digested simultaneously, owing to the extensive surface areas of the oral arms. Of the threeC. quinquecirrha medusae maintained in the laboratory for up to 60 d, the smaller two specimens (25–150 g wet weight) showed a biomass doubling in ～14 d; the largest specimen (300 g), a sexually mature female, showed little growth but did release >100 ova 24 h^?1. Growth efficiencies were estimated to be ～2% for the largest specimen (not including biomass of gametes) and ～10% for the smaller two specimens. These growth efficiencies are comparable to those of other gelatinous predators.     

Early life history and biology of the common fish parasite,Lironeca ovalis (Say) (Isopoda,Cymothoidae)

The first post-marsupial stage of the common fish parasite, Lironeca ovalis, is described and found to be identical to Aegathoa medialis Richardson. It is suggested that A. oculata (Say) may also be a young stage of L. ovalis. The juvenile isopods were observed to attach readily to potential host fish, juvenile striped bass and white perch, in the laboratory. However, the parasites could be dislodged easily, and there was no indication that they were actually feeding on the host fish. The isopods also did not feed on newly hatched Artemia nauplii or unicellular algae (Monochrysis lutheri and Phaeodactylum tricornutum) in culture trials. It appears that the young isopods can survive free-swimming for only 1–2 weeks without a host, but the animals may be able to maintain a more-or-less free existence for an extended period by moving from host to host via plankton.     

Food of juvenile American shad,Alosa sapidissima,juvenile blueback herring,Alosa aestivalis,and pumpkinseed,Lepomis gibbosus,in the Connecticut River below Holyoke Dam,Massachusetts

Despite·a paucity of available prey, little feeding competition occurred between juvenile American shad,Alosa sapidissima, juvenile blueback herring,Alosa aestivalis, and pumpkinseed,Lepomis gibbosus, in the Connecticut River below Holyoke Dam. Pumpkinseed differed from clupeids in pronounced selection for chironomid larvae, and bottom feeding. Feeding conflicts between the twoAlosa species were reduced by: 1) more opportunistic feeding by shad; 2) differential selection for cladoceran prey; 3) higher utilization of copepods by herring.     

Comparative growth of juvenile white sturgeon and striped bass: Effects of temperature and hypoxia

The influences of temperature and environmental hypoxia on the growth rates of two California anadromous fishes, white sturgeon (Acipenser transmontanus) and striped bass (Morone saxatilis) were examined. Fish (0.5–0.6 g initial weight) were fedad libitum rations ofArtemia in flow-through aquaria regulated for temperature (15, 20, and 25°C) and oxygen tension (130 and 90 torr P_{o 2}). Growth of sturgeon was significantly greater at 20 °C compared with 15 °C, but there was no difference between 20 and 25 °C. Striped bass growth increased with each 5° increment of temeprature elevation to 3.2% body weight per d at 25 °C, the fastest growth rate measured. The temperature of maximum growth reflected the temperature of the native estuarine rearing area. Environmental hypoxia (90 torr P_{o 2}) reduced growth of sturgeon within each temperature level, whereas striped bass growth was reduced by hypoxia only at the upper two temperatures. Sturgeon were much more active in the growth chambers than striped bass. Sturgeon activity increased with each 5 °C temperature increase under normoxia and hypoxia, except at 25 °C (hypoxia) where activity was insignificantly different from that at 20 °C (hypoxia).     

Enclosure experiments on survival and growth of black drum eggs and larvae in lower Chesapeake Bay

Experiments in 2.2 m³, in situ mesocosm enclosures indicate that black drum, Pogonias cromis, eggs and larvae potentially can survive in the lower Chesapeake Bay at ambient microzooplankton prey levels (≈200 prey 1^?1) in the absence of predators. In growth experiments, larva mean growth rates to 10 d posthatch were similar (0.17 mm d^?1 and 0.18 mm d^?1) when fed at prey levels of 50 prey 1^?1 and 200 prey 1^?1. Individual growth rates, however, were more variable at 50 prey 1^?1. Mortality rates also were comparable in 50 (27% d^?1) and 200 (23% d^?1) prey 1^?1 enclosures. In a second experiment, the predation potentials of the hydromedusa Nemopsis bachei and the lobate ctenophore Mnemiopsis leidyi were estimated in relation to initial black drum egg prey density, presence of alternative <1 mm zooplankton prey, and estimated daily abundance of the jellyfish on the black drum spawning grounds. Mortality rates per medusa and ctenophore were similar (0.02–0.03 d^?1), were not affected by presence of alternative prey, and were directly related to initial egg density. Results suggest that the gelatinous predators, especially the hydromedusa, could have cleared a high (≈38%) but variable fraction of the water column daily of fish eggs and yolk-sac larvae during the black drum spawning season. We hypothesize that the poor or episodic recruitment success of black drum in Chesapeake Bay results from a short spawning season that often coincides with abundance peaks of gelatinous predators and that predation on eggs and yolk-sac larvae may control recruitment.     

Predation by the ctenophoreMnemiopsis leidyi in Narragansett Bay,Rhode Island

The impact ofMnemiopsis leidyi as a predator was studied quantitatively to determine their feeding rates on mixed natural zooplankton. These studies showed that feeding rate was independent of food concentration, but a function of both temperature and size of ctenophore. The feeding rate (liters cleared per mg dry weight per day) ranged from about 0.01 for larger ctenophores at lower temperatures (10–15°C) to about 0.1 for smaller ctenophores at higher temperatures (20–25°C). Combining these results with ctenophore biomass estimates from three years of sampling, numerical estimates were made of zooplankton mortality due to ctenophore predation. The maximum summer cropping byM. leidyi was calculated to be an average of 5–10% per day for the bay as a whole. Although substantial, this predation pressure alone does not account for the observed summer decline of zooplankton.     

The effect of Southwestern Atlantic burrowing crabs on habitat use and foraging activity of migratory shorebirds

Southwestern Atlantic estuaries (Southern Brazil to Northern Patagonia, Argentina) are characterized by the presence of an intertidal burrowing crab Chasmagnathus granulata. This crab species is an important bioturbator which lives in large assemblages and excavates semi-permanent burrows that affect sediment characteristics. Our observations showed that distribution of the crabs in the Mar Chiquita coastal lagoon, Argentina (37°45′S, 57°26′W) affected habitat use and feeding success of migratory shorebirds. During the migratory season the two-banded plover (Charadrius falklandicus) foraged more frequently inside crab beds, and yellowlegs (Tringa flavipes and T. melanoleuca) fed more freqeuntly outside crab beds. Focal observations on the feeding behavior of the white-rumped sandpiper (Calidris fuscicollis) and the two-banded plover inside and outside crab beds showed that the plover was a visual searcher and captured more prey inside crab beds, and the white-rumped sandpiper was a tactile feeder. Although consumption rates (prey min^?1) did not differ between sites, their efficiency (prey probe^?1) inside crab beds was less. These differences were probably related to changes in sediment characteristics and prey behavior, which vary with crab activity. Burrowing crabs alter the suitability of intertidal habitats used by shorebirds in southwestern Atlantic estuaries. We believe that the same process could be occurring with other burrowing curstaceans such as thalassinidean shrimps in other estuaries of the world and could have important implications for management of flats for shorebirds. *** DIRECT SUPPORT *** A01BY090 00007     

Prey density requirements of the striped bass,Morone saxatilis (Walbaum), larvae

Striped bass,Morone saxatilis, larvae of 9 DAH (days after hatching) were fed for 16 d with five densities of adults (5–100 l^?1), copepodites (10–200 l^?1), and nauplii (50–1,000^?1) of the copepodEurytemora affinis, and 5-DAH and 10-DAH larvae were fed at seven densities (5–500 l^?1) of brine shrimp,Artemia salina, nauplii. The study determined the effects of prey types and densities on the survival and growth of the fish larvae, and the effects of delaying first feeding from 5 DAH to 10 DAH. The survival and grwoth of the larvae fedE. affinis adults was higher than those fedE. affinis copepodites.E. affinis nauplii were the poorest prey. OneE. affinis adult provided nutrition equivalent to 1.45 copepoditer or 11.12 nauplii. The percentage survival of the larvae was higher for those fedA. salina nauplii thanE. affinis adults at the densities below 67.25 l^?1, but there was no difference at this density or higher. Delaying first feeding from 5 DAH to 10 DAH did not affect percentage survival and size of larvae at the end of the experiments. Because the survival and growth of the larvae increased as the prey density increased, it was difficult to set a point along the regression line as the minimum density requirement level. It may be said that for striped bass larvae, the higher the prey density, the higher the survival and growth, and thus the greater the recruitment success.     

Seasonal variations in chlorophyll and nutrients in a Canadian arctic estuary

The Eskimo Lakes and Liverpool Bay constitute a series of estuarine waters to the Beaufort Sea in arctic Canada. Salinity ranges in summer from 20‰ at the mouth to less than 1‰ at the head of the system. Arctic features include an ice cover lasting for about 8 months annually and water temperatures which fluctuate from ?1°C in winter to as high as 12°C in late summer. Subsurface light is severely attenuated. Reactive phosphate varies from a spring high of 0.3 μg-at P per 1 to undetectable levels during summer. Nitrate is more abundant, and silicate is consistently plentiful. Chlorophyll a reaches a maximum only occasionally higher than 3 mg per m³ in June and July, rising from undetectable levels in winter. Photosynthetic rates are low by all standards, and have not been measured at greater than 6.4 mg C per m² per hour in summer. Low levels of subsurface light and reactive phosphate and nitrate characterize this exceptionally oligotrophic arctic estuary.     

Size selection of Atlantic rangia clams,Rangia cuneata,by blue crabs,Callinectes sapidus

Male blue crabsCallinectes sapidus Rathbun were allowed to forage on six size classes of the Atlantic rangia clam,Rangia cuneata (Sowerby), in laboratory squaria under conditions of restricted or unrestricted prey size availability. Prey profitability (energy gained divided by handling time) decreased as prey size increased. Crabs preferred smaller clams (1–2 cm long, then 2–3 cm long), as predicted by an energy maximization model. Under conditions of restricted prey sizes (nonreplacement experiments), when the smaller clams were no longer available, crabs tended to choose progressively larger clams. Crabs did not exhibit selective feeding behavior among clams larger than 3 cm long. For the range of crab sizes tested (71–167 mm carapace width), there was no correlation between crab carapace width and the mean clam size that was eaten.     

A comparative assessment of a natural and created estuarine slough as rearing habitat for juvenile chinook and coho salmon

Short-term otolith growth rates, residence times, and forage of two species of juvenile salmon (Oncorhynchus spp.) were compared in a created and natural estuarine slough on the Chehalis River in Washington to assess the functional equivalency of the created slough in providing suitable rearing habitat. Otolith microstructure, mark-recapture data, and forage of sub-yearling chinook, O. tshawytscha, and coho, O. kisutch, residing in both sloughs during the spring of 1991 and 1992 served as indicators of rearing habitat quality. No significant differences in the number or width of coho daily otolith increments were detected between the sloughs. Juvenile chinook residence and emigration times were also comparable. Aquatic and terrestrial insects composed the majority of all chinook and coho diets; however, the order of importance of main prey items did differ between sloughs, and salmon in the created slough had emptier stomachs, possibly because of reduced prey availability and/or foraging efficiency.     

Seasonal Diet Shifts and Overlap Between Two Sympatric Catfishes in an Estuarine Nursery

This study describes the seasonal feeding habits of different size classes of Cathorops spixii and Cathorops agassizii along an estuarine ecocline and the food overlap when different size classes occur together. These species were essentially zoobenthivorous, feeding mainly on Polychaeta Nereis sp., Copepoda Pseudodiaptomus acutus, Ostracoda Asterope sp., Gastropoda: Littorinidae, and Bivalvia Mytilus sp. and Anomalocardia brasiliana. However, during their life cycle and between different habitats and seasons, their trophic guild can change to zooplanktivore. The competition for resources was observed among C. spixii and C. agassizii, but was significantly reduced due to the seasonal diference in habitat use by different ontogenetic phases in the main channel of the estuary. The food niche separation was strongly influenced by environmental fluctuations, principally of salinity, resulting from rainfall and river inflow. High abundance of some preys, such as P. acutus (all estuary), Asterope sp. (upper and middle estuary), and Littorinidae (upper and lower estuary), could influence the significant diet overlap, principally during the rainy season, and call for more detailed studies of the benthic community structure. Moreover, dietary overlap was observed mainly between smaller sizes (intraspecific and/or interspecific) or between corresponding ontogenetic phases (interspecific), suggesting some differentiation in the diet in relation to the size class. Differences in prey type and size between the different ontogenetic phases of these ariids, principally among juveniles and adults, could be related to the size of the mouth, since adults are able to successfully capture larger preys or larger quantities of particular items.     

Comparison of spawning seasons,age, growth rates,and food of two sympatric species of searobins,Prionotus carolinus and Prionotus evolans,from Long Island Sound

Prionotus carolinus and Prionotus evolans were collected from many locations within Long Island Sound in 1971–1973, and in 1976–1977. Data from earlier collections in Block Island Sound (1943–1945) were also included. A total of 1751 specimens, 960 P. carolinus and 791 P. evolans was examined within these two time periods. Both species entered the Sound in April and spawned during June and July. P. evolans appeared to spawn slightly earlier in summer than P. carolinus. Adults began to leave the Sound after spawning and were usually absent after November. Young-of-the-year were taken regularly from August to November and, occasionally in water over 20 m deep, into February when the bottom water temperature was 1.4°C. At the end of the first growing seasons both species exhibited large variations in standard lengths. Back-calculations from scale annuli measurements indicated that linear growth rates during the juvenile years were similar in both species. However, P. evolans was considerably heavier than P. carolinus. During adulthood P. evolans was not only longer and heavier than P. carolinus, but lived longer. Growth rates are described by the following equations: P. carolinus L_t+1=9.60+0.68 (L_t), and p. evolans L_t+1=7.70+0.80 (L_t). Both species were opportunistic feeders, and crustaceans were clearly the dominant group of prey. Young-of-the-year, between 3–6 cm, ate copepods. As they grew they ate larger prey, such as Neomysis americana, Diastylis quadrispinosus, various species of amphipods of small sizes, and juvenile Crangon septemspinosus. Older fish ate larger sizes of these same prey, a number of species of crabs, juvenile Homarus americanus, and Squilla empusa. Occasionally they ate polychaetes, molluscs, and juvenile fish. Partitioning of the resources of Long Island Sound by these two species appeared to be by prey size. P. evolans ate prey that, on the average, were slightly larger than those eaten by P. carolinus. Furthermore, P. evolans ate a greater amount of nektonic species than P. carolinus, which appeared to prefer benthonic invertebrates.