Relationship between zooplankton distribution,geographic characteristics and hydrographic patterns off the Catalan coast (Western Mediterranean)

Astract Northern anchovy larvae, Engraulis mordax, were collected on five cruises covering the Southern California Bight, one cruise in April 1984, four cruises in January, February and May 1986. The amounts of triacylglycerol, cholesterol and polar lipid were measured in individual anchovy larvae as indicators of their nutritional condition. There was a significant difference in each of the size-specific lipid components between the stations for four of the five cruises. This indicates that all the ocean habitats within a cruise were not equal in promoting growth (and presumably survival) in the anchovy larvae. There were also differences between cruises, where the percentage of the larvae judged to be in poor condition varied from 8 to 27%. Canonical correlation analyses revealed a significant relationship in most cases between the larval fish parameters and the physical and biological data collected from each occupied station. Cholesterol and triacylglycerol were the most important larval components, and the short-term energy storage component in the copepod Calanus pacificus was the most important station component. The data, however, are not sufficient to identify and characterize stations in the field which produce anchovy larvae in good condition.     

Calculating carbon biomass ofPhaeocystis sp. from microscopic observations

Conversion factors for calculating carbon biomass ofPhaeocystis sp. colonies and free-living cells were determined from microscopic observations and chemical analysis conducted on cultured and naturalPhaeocystis sp. populations originating from the Southern Bight of the North Sea in 1986 and 1987. They allow calculation, in terms of carbon biomass, of the different forms ofPhaeocystis sp. that succeed each other when the population is growing, on the basis of microscopic observations. The latter include enumerations of free-living cells (flagellated and non-motile) and colonies, as well as colonial biovolume measurement. Specific application to natural populations from Dutch coastal waters during spring 1986 shows that more than 90% ofPhaeocystis sp. carbon biomass is under colonial form, most of it exceeding the grazing characteristics of current zooplankton at this period of the year. Detailed analysis of seasonal changes shows in addition that the size of the colonies greatly increases during the course ofPhaeocystis sp. flowering, reaching sizes as high as 1 mm diameter at the top of the bloom when nutrients are depleted. Physiologically this corresponds to an enhanced synthesis of mucilaginous substances, with the decrease of available nutrients leading to an increasing contribution of the matrix to the total colonial carbon during the course of the bloom. Carbon content ofPhaeocystis sp. colonies therefore greatly varies with their size, ranging from 0.3 to 1430 ngC colony^–1.     

Lipid components as a measure of nutritional condition in fish larvae (Pleuragramma antarcticum) in East Antarctica

Pleuragramma antarcticum is a key component of the neritic assemblages in the Antarctic coastal waters. Larvae of this species were sampled from 2008 to 2011 in the Dumont d’Urville Sea (East Antarctica). The lipid class composition [triacylglycerols (TAG), cholesterol (Chol) and polar lipids (PL)] of larvae was measured to assess the larval condition. The total amount of lipids was linearly related to the quantity of structural polar lipids, suggesting that growth is favored over lipid storage. The TAG:Chol ratio showed interannual variability in the condition of fish larvae, probably related to prey availability. Nevertheless, the essential fatty acids composition of polar lipids illustrates that larvae with low levels of TAG:Chol could be either growing or under starvation. Only the combination of a low TAG:Chol ratio and low polar lipids content, which can also be mobilized during starvation periods, allowed identification of larvae in poor condition. This lipid condition index should be of great assistance to evaluate the probability of survival of P. antarcticum larvae in long-term monitoring. It has widespread applicability and should also be useful in the diagnosis of nutritional condition in other species.     

Food availability and physiological state of sea urchin larvae (<Emphasis Type="Italic">Strongylocentrotus purpuratus</Emphasis>)

Food availability is highly variable in the ocean. Many species of marine invertebrates have a larval form that depends upon exogenous nutrients for growth, yet there are few biochemical and physiological indices for determining changes in the nutritional status of larvae. In this study, the effects of food availability on biochemical compositions and metabolic processes of larvae of the sea urchin, Strongylocentrotus purpuratus, were determined. Larvae were cultured under different food concentrations (fed-to-excess and unfed) and a suite of biological processes assayed, ranging from measurements at the level of the whole organism to that of specific molecules. These data were normalized to DNA content (an index of cell number) to allow comparisons of physiological rates in larvae of different sizes. Changes in the following were measured during larval growth: free amino acid pool, protein, lipid classes (cholesterol, free fatty acids, hydrocarbons, phospholipids, triacylglycerol), enzyme activities (Na⁺, K⁺-ATPase and citrate synthase), and respiration rates. In growing larvae, the two key components that showed differential cell-specific content relative to unfed larvae were glycine in the free amino acid pool and phospholipids. Additionally, several lipid classes were detectable only in fed larvae (cholesterols, free fatty acids, and hydrocarbons). While triacylglycerols were present in eggs and utilized during pre-feeding development, they were not re-accumulated at detectable levels in feeding larvae. Respiration rates, protein content, and enzyme activities were all similar on a cell-specific basis, showing that these variables did not provide useful indices of differences in physiological state between fed and unfed larvae. In contrast, measurements of the cell-specific content of glycine and certain lipid classes did provide useful indices of physiological state of larvae. Application of these indices could potentially allow for determinations of nutritional state of larvae in the ocean.     

Symbiosis between bacteria and oceanic clonal sea star larvae in the western North Atlantic Ocean

Bacteria were present in the majority of clonal sea star bipinnariae (Luidia sp.) collected between June 1987 and August 1990 from oceanic populations in the subtropical western North Atlantic Ocean. Light and electron microscopy revealed dense aggregations of bacteria within lobes of the epidermal cuticle in the gastric region of larvae. Gram-negative rods and less-common elongate spiral bacteria were observed, always in close association with branching epidermal microvilli. Intact and partially digested bacteria occurred in vesicles and phagosomes within epidermal cells, a probable indication of phagocytotic activity by the host. The association of larvae of Luidia sp. and bacteria ranged widely on both geographical and temporal scales. During July and August 1988, larvae were abundant (1.6 to 11.1 m³) in the mixed surface layer (<10 to 100 m) of stations in the Gulf Stream, the Sargasso Sea and the North Equatorial Drift, and 96% (mean of 5 stations, n=8 to 10 larvae in each station) of the individuals surveyed harbored bacteria beneath their cuticle. In a single station on the Florida Current sampled during winter, spring, and summer months between 1989 and 1991, 79 to 90% of the larvae harbored bacteria. The incidence of symbiosis was higher in actively cloning larvae than in non-cloning larvae. This is the first documentation of a symbiosis in field populations of echinoderm larvae.     

Recruitment,growth, and diel vertical migration of Euphausia pacifica in a temperate fjord

The pelagic crustacean Euphausia pacifica Hansen was sampled with a multiple-sample 1.0 m² Tucker trawl and a multiple-sample 1.0 m² vertical net in Dabob Bay, Washington on 17 dates between May 1985 and October 1987. Size (stage) structure and abundance of the population were determined for each date, while vertical distribution and diel migration were determined for 13 dates. Although internannual variability in both timing and magnitude of events occurred, consistent patterns were discernable. The population produced a large pulse of larvae (2 to 5 mm) in late spring of each year, apparently in response to the vernal phytoplankton bloom. Much lower abundances of larvae occurred during summer and autumn of each year, and larvae were completely absent during winter. Recruitment to the juvenile (6 to 9 mm) and adult (10 mm) stages was strongest during the summer, with abundances of these individuals peaking in summer and autumn. Individual growth rates, determined by modal progression analysis, were calculated for E. pacifica. Rates ranged from zero for some adult cohorts during the winter to 0.12 mm d^-1 for larvae during spring. The latter are among the highest ever reported for this species in the field. The vertical distributions and diel vertical migrations (DVM) of E. pacifica varied seasonally and between size (stage) classes. At night, all size classes were distributed in the surface layer (upper 25 m) irrespective of season or year. During the day, the larger/older stages were always distributed at middepths (50 to 125 m). In contrast, the daytime distribution of the larvae was more variable, being concentrated at the surface during spring and early summer of 1985, and at increasing depths later in the summer and autumn of 1985 and again in spring of 1986. This resulted in invariant DVM in the juveniles and adults, but variable DVM in the larvae, the latter of which is hypothesized to be a response to variable abundances of zooplanktivorous fish.     

Lipid-class composition of organs and tissues of the tiger prawnPanaeus esculentus during the moulting cycle and during starvation

Neutral lipid and phospholipid fractions and their component lipid classes in the digestive gland, abdominal muscle, epidermis and cuticle ofPenaeus esculentus Haswell were analysed and compared during the moulting cycle and during starvation. The prawns were collected from Moreton Bay, Queensland, Australia, by trawling during 1985–1987, and were fed on a standard, semi-purified diet. The digestive gland appears to be a major site of lipid synthesis, storage and mobilisation in preparation for moulting. Neutral lipid, 59 to 80% of which was triacylglycerol, was the larger fraction. It accumulated during early premoult, mainly due to the increase in triacylglycerol. The digestive gland contained only 18% of the total body lipid, or 8% of body lipid as triacylglycerol. Thus, the reserve lipid available for energy production is very small. Digestive gland triacylglycerol was markedly depleted after 4 d starvation and was almost completely absent after 8 d. In the other tissues, the major fraction was phospholipid, of which over 50% was phosphatidylcholine and up to 20% phoshatidylethanolamine; cholesterol was the major class in the neutral lipid fraction and appeared to be very stable. Most of this lipid was probably a component of cellular membranes. The lipid composition of muscle changed very little during the moulting cycle: total lipid levels in the epidermis were high in late premoult and early postmoult, when new cuticle is being secreted, but the proportions of the component lipids were closely similar. Cuticle lipid, together with other major components, was resorbed from the old cuticle prior to ecdysis, but the cuticle phospholipids appeared to be labile at all moult stages. The total of all lipids in fedP. esculentus was about 3.6% dry weight, of which about 70% was phospholipid. Earlier research had shown that when digestive gland lipid is exhausted after a short period of starvation, muscle is metabolised for energy. The present research showed that in the remaining muscle only about 13% of lipid was lost after 21 d starvation, mostly as phosphatidylcholine. This is in keeping with the need to maintain this tissue in a functional state. In contrast, epidermal lipid levels were markedly reduced after only 4 d starvation and the proportions of phospholipids changed significantly. This sensitivity of the cuticle lipids to starvation may be the cause of delayed moulting, which is characteristic of poor nutrition.     

Changes in lipid class and fatty acid contents in the ovary and midgut gland of the female fiddler crab Uca tangeri (Decapoda,Ocypodiadae) during maturation

Changes in biochemical composition, lipid class and fatty acid contents were studied in the ovaries and midgut glands of the fiddler crabs Uca tangeri Eydoux during maturation. Wild females were caught during spring and early summer of 1992 in the Bay of Cádiz (southwest Spain), near the mouth of the San Pedro river. Protein and total lipid contents in the ovaries increased significantly from Stages III to IV, at the expense of total carbohydrate, which showed a large decrease during the same period. In the midgut gland, the protein content did not present any significant variation, whereas total lipids and total carbohydrates presented opposite up and down trends during maturation. In the ovary, total polar lipids increased significantly during the final phase of maturation (Stages III to IV), mainly due to the significant contribution of the phosphatidylcholine and phosphatidylethanolamine fractions. In contrast, total neutral lipids showed an upward trend throughout the whole maturation period, mainly due to significant increases of the triacylglycerol fraction. In the midgut gland, total polar lipids (mainly phosphatidylcholine) and total neutral lipids (mainly triacylglycerol) presented significant decreases from Stages II to III, the phase which preceded major increases in both polar and neutral lipids in the ovaries. Cholesterol content did not vary during maturation in either organ, in the ovary or midgut gland. Major fatty acids in the ovaries [16:0, 16:1 (n-7), 18:1 (n-9), 18:1 (n-7), 18:2 (n-6), 18:3 (n-3), 20:4 (n-6), 20:5 (n-3) and 22:6 (n-3)] did, however, accumulate significantly at later stages of maturation. It is noteworthy that arachidonic acid [20:4 (n-6)] content remained constant during all stages of maturation but decreased significantly in total polar lipids in the later phases of maturation. In contrast, eicosapentaenoic acid [20:5 (n-3)] increased significantly in all lipid fractions in the later stages, and docosahexaenoic acid [22:6 (n-3)] remained constant in the polar lipids and increased during later stages in the triacylglycerol fraction. Major fatty acids in the midgut gland lipids showed significant decreases from Stages II to III, just before the final period of maturation.     

Diel vertical migration and timing of metamorphosis of larvae of the Dungeness crab Cancer magister

Joint USA/USSR ichthyoplankton surveys off the coasts of Washington, Oregon and northern California during the years 1981 to 1985 sampled more than 120 stations each year, from 5 to 360 km offshore and between Latitude 40° and 48° N, providing information on ontogeny and diel migration of larvae of the Dungeness crab Cancer magister on a scale not studied previously. We developed a maximum likelihood method for estimating abundance and fraction in the neuston at each station from a neuston tow and an oblique bongo tow. Latestage megalopae migrate vertically on a diel basis, with the fraction in the neuston being (on average) 62% at night (19.00 to 08.00 hrs Pacific Standard Time, PST) and 8% during the day (08.00 to 19.00 hrs PST). The hourly pattern of this migration includes a peak in the early evening, possibly another in the early morning, and an intermediate level in the late afternoon. We detected no dependence of vertical migration on cloud cover or sea state. Early-stage megalopae were present in much lower fractions in the neuston, but weakly displayed the same diel pattern of migration. Zoeae appeared to be below the neuston at all times, except for 2 or 3 h in the evening. From an abrupt change in larval stage in samples from a north-south cruise, we concluded that the majority of the larvae metamorphose from zoeae to megalopae over a fairly short time span (2 to 4 wk) at a given latitude. In later cruises, 95% of the larvae were megalopae, indicating that metamorphosis over the study area either occurs at the same time or proceeds from south to north over a time span of less than a month in early spring.     

Changes in lipid content,lipid class composition and fatty acid composition of developing eggs and unfed larvae of cod (Gadus morhua)

Changes in the lipid class and fatty acid composition of developing eggs and unfed larvae of cod (Gadus morhua L.) were studied with the objective of determining probable requirements of cod larvae for dietary lipid. The eggs were collected on 24 March 1985 from holding tanks containing cod which had been caught off the northwest coast of Scotland. Phosphatidylcholine (PC) was the only lipid class to decline in absolute terms during embryogenesis. Catabolism of neutral lipid was initiated during the first week after hatching, and the rate of neutral lipid utilisation increased after the larvae had completely absorbed their yolk sacs. The quantity of triacylglycerol (TAG) remained constant during embryogenesis, but the percentage of 22:6(n-3) int TAG increased substantially during this period. It was calculated that ca. 33% of the 22:6(m-3) released during the process of PC catabolism was incorporated into TAG and sterol ester. The results suggest that PC, replete in appropriate essential fatty acids, should represent a major proportion of the lipid in artificial diets for fish and crustacean larvae.     

Degradation of mangrove leaf litter by the tropical sesarmid crab Chiromanthes onychophorum

The food and feeding habits of 3 species of gadoid larvae — the cod Gadus morhua Linnaeus, 1758, the whiting Merlangius merlangus (Linnaeus, 1758), and the bib Trisopterus luscus (Linnaeus, 1758), collected in the eastern English Channel and Southern Bight during the spring of 1971 are described. All 3 species began to feed in the yolk-sac stage on diatoms, dinoflagellates and tintinnids, but the principal food was the nauplii and copepodites of calanoid copepods, particularly of Pseudocalanus minutes, but also of Paracalanus parvus, Temora longicornis and Acartia clausii. Pseudocalanus minutus and Paracalanus parvus were eaten mainly early in the season and T. longicornis later when it became more abundant. The larvae discriminated for prey size as growth proceeded. They sometimes took the largest prey available to them, but in general the size of the prey was considerably less than the maximum size which could have been swallowed. Feeding larvae were found at all times of the day, but the incidence of feeding was lowest before dawn. Feeding increased at sunrise, declined until late in the morning, and then increased again to a maximum around sunset. There was evidence of feeding by moonlight, particularly by whiting and bib larvae. There was little difference between the English Channel and Southern Bight in regard to the food eaten.     

Transfer of the chlorinated hydrocarbon PCB in a laboratory marine food chain

The transfer of chlorinated hydrocarbons (CHC) in a laboratory simulation of a three trophic level marine food chain was studied. The food chain consisted of the algal flagellate Dunaliella sp., the rotifer Brachionus plicatilis, and the larva of the Northern anchovy Engraulis mordax. CHC were introduced into the seawater at concentrations representative of near-shore conditions off southern California without the use of dispersing agents. Each trophic level appeared to be in a steady-state at the time of first sampling, 5 days after inoculation. Apparent partition coefficients were calculated for each trophic level. The CHC contamination in the diet of the rotifers and anchovy larvae was also calculated. Unfed anchovy larvae accumulated the same amount of CHC as fed larvae and the final concentration appeared to be dependent on the CHC concentration in the seawater. The data in this report suggest that CHC accumulation is not a food-chain phenomenon but rather the result of direct partitioning of the compounds between the seawater and the test organisms.     

Variation in pelagic larval growth of Atlantic billfishes: the role of prey composition and selective mortality

Atlantic blue marlin (Makaira nigricans) and sailfish (Istiophorus platypterus) larvae were collected from 10 monthly cruises (June–October 2003 and 2004) across the Straits of Florida to test (1) whether growth differed between the more productive western region near the Florida shelf, and the less productive eastern region toward the Bahamas, and (2) whether growth was related to prey consumption. Examination of larval sagittal otoliths revealed that instantaneous growth and daily growth during the first 2–3 weeks of life did not vary significantly between the two regions for either species. However, recent growth during the last two full days prior to collection was greater in the west for blue marlin larvae. Recent growth of blue marlin larvae <9 mm SL (primarily zooplanktivorous) was significantly related to prey composition (faster growth when higher proportions of Farranula copepods were consumed). Western larvae grew faster and had higher proportions of Farranula in their guts. Trends for sailfish larvae were not significant. In both species, comparison of early growth between <9 and ≥9 mm SL size groups indicated that growth trajectories diverged around 5–8 mm SL, the time when billfish larvae become capable of piscivory. Significantly faster growth of larger (older) larvae suggests that mortality was selective for fast growers and that the transition to piscivory may be a critical point in the early life of billfish.     

Lipids and fatty acids as indicators of egg condition,larval feeding and maternal effects in Cape hakes (<Emphasis Type="Italic">Merluccius paradoxus</Emphasis> and <Emphasis Type="Italic">M. capensis</Emphasis>)

Cape hakes, Merluccius paradoxus and M. capensis, are important gadoid fish that are commercially harvested in the Benguela Current system off Namibia and South Africa. The aim of this study was to elucidate the nutritional condition and feeding preferences of their larvae. Hake eggs and larvae were sampled in austral spring of two consecutive years, 2007 and 2008, off the west coast of South Africa. They were identified to species using genetics, and total lipid content and fatty acid (FA) composition were analysed for each individual egg and larva to compare the condition of different early life stages of both hake species. Higher abundances of M. paradoxus eggs and larvae were consistently found compared to M. capensis. In both species, eggs contained wax esters (WE) and had significantly higher lipid content per dry mass than larvae. Lipid content as well as FA composition changed with the developmental stage of larvae. Quantities of essential fatty acid (EFA) increased with feeding of larvae due to dietary lipid incorporation. In 2007, yolk-sac larvae contained significantly lower total lipids than in 2008. It is argued that this was due to reduced lipid transfer by the spawning females to the eggs. These findings indicate that maternal effects are important in determining condition of hake larvae and that this may have an effect on their survival and subsequent recruitment.     

Interspecific comparisons of growth and diet among late larvae of three co-occurring clupeoid species in the Kii Channel,Japan

Growth and diet were compared among larvae of Japanese anchovy Engraulis japonicus, Japanese sardine Sardinops melanostictus and Pacific round herring Etrumeus teres. Compositions of prey items of the three species in the same month showed greater similarity than for the same species in different months. Prey size as well as prey taxa of the three species overlapped considerably with one another. Therefore, interspecific prey competition is likely in the case of limited food availability. The most abundant species tended to change from anchovy to round herring in early winter, from round herring to sardine in late winter and from sardine to anchovy in early spring, indicating a temporal segregation in use of the nursery grounds. Similar seasonal changes in growth rates were observed for the three species. Although interspecific prey competition is likely, the temporal segregation and similar temporal changes in growth rates could favor their coexistence.     

Physiological energetics of developing embryos and yolk-sac larvae of Atlantic cod (Gadus morhua). I. Respiration and nitrogen metabolism

This paper provides the basis for a general model of catabolic metabolism for developing embryos and yolk-sac larvae of Atlantic cod (Gadus morhua L.). Yolk-dependent routine rates of oxygen consumption, ammonia excretion, and accumulation of ammonium ions were related to quantitative changes in contents of glucose, glycogen, lactate, free amino acids, proteins and lipid classes (lipid classes published separately) in order to determine the rate and sequence of catabolic substrate oxidation that occurs with development at 6.0°C, 34.5 S. The stoichiometric relation of the oxygen consumption and total ammonia production to substrate utilisation indicated that during the first 2 to 3 d of development, glycogen was the sole substrate of oxidative metabolism. After formation of the syncytium, free amino acids (75%) together with polar lipids (13%, mainly phosphatidyl choline) and neutral lipids (9%, mainly triacylglycerol) comprised the metabolic fuels of embryonic development. Following hatch (Day 16 post fertilisation), the fuels were free amino acids (32%), polar lipids (20%, mainly phosphatidyl choline), neutral lipids (17%, mainly triacylglycerol) and proteins (31%). Thus, the catabolic metabolism of endogenously feeding Atlantic cod larvae was predominantly fuelled by amino acids (67%) and lipids (32%), while glycogen only accounted for 1% of the total enthalpy dissipated. It is proposed that the above sequence of catabolic substrate oxidation is also generally applicable to other cold-water fishes which spawn eggs that do not contain oil glubules.     

Vertical distribution of eggs and larvae ofEngraulis encrasicolus in stratified waters of the western Mediterranean

The vertical distribution of the eggs and larvae of the European anchovy (Engraulis encrasicolus) in the western Mediterranean Sea in June 1984 and Agust 1985 was analyzed based on multiple plankton tows carried out at varying depths and using nets equipped with opening and closing mechanisms. Hydrographic parameters such as temperature, salinity, and chlorophylla were recorded simultaneously. Maximum abundance of anchovy eggs and larvae always occurred above the thermocline, even when maximum chlorophylla concentrations were located below the thermocline. Larval distribution appeared to be associated with the availability of suitable food organisms. As in other clupeoid species, the anchovy larvae carried out vertical migrations related to the photoperiod.     

Seasonal changes in biochemical composition of the bivalve Lima hians from the Clyde Sea Area

Seasonal changes in the tissue weight and biochemical composition of the bivalve Lima hians (Gmelin) from a station in the Skelmorlie Bank, Clyde Sea Area, were studied during 1969–1971. The composition of an animal of standard size was calculated for each sampling date to give information on seasonal changes independent of shell growth. Increases in tissue weight took place between April and July as the gonad developed and reserves were built up. Between July and March tissue weight fell, at first rapidly as a result of spawning in the population, and later more slowly as reserves were utilized. Differentiation of the gonad began during the winter, at the expense of stored reserves, but was greatly accelerated during the spring due to increased food from the plankton. The lipid and nitrogen content of a standard animal rose to a pre-spawning maximum in July and then fell. Carbohydrate content also rose during this period, but continued to rise to reach maximum values in September. Carbohydrate and nitrogen both declined during the winter, but there was little indication of any net utilization of lipid during the winter months. The tissues of the ripe female contained more lipid than the male, and the tissues of the ripe male contained more nitrogen than those of the female. The total calorific content of the tissues of the male was greater than that of the female.     

Diet of young Atlantic bluefin tuna (<Emphasis Type="Italic">Thunnus thynnus</Emphasis>) in eastern and western Atlantic foraging grounds

Atlantic bluefin tuna (Thunnus thynnus) are highly migratory predators whose abundance, distribution, and somatic condition have changed over the past decades. Prey community composition and abundance have also varied in several foraging grounds. To better understand underlying food webs and regional energy sources, we performed stomach content and stable isotope analyses on mainly juvenile (60–150 cm curved fork length) bluefin tuna captured in foraging grounds in the western (Mid-Atlantic Bight) and eastern (Bay of Biscay) Atlantic Ocean. In the Mid-Atlantic Bight, bluefin tuna diet was mainly sand lance (Ammodytes spp., 29% prey weight), consistent with historic findings. In the Bay of Biscay, krill (Meganyctiphanes norvegica) and anchovy (Engraulis encrasicolus) made up 39% prey weight, with relative consumption of each reflecting annual changes in prey abundance. Consumption of anchovies apparently declined after the local collapse of this prey resource. In both regions, stable isotope analysis results showed that juvenile bluefin tuna fed at a lower trophic position than indicated by stomach content analysis. In the Mid-Atlantic Bight, stable isotope analyses suggested that >30% of the diet was prey from lower trophic levels that composed <10% of the prey weights based upon traditional stomach content analyses. Trophic position was similar to juvenile fish sampled in the NW Atlantic but lower than juveniles sampled in the Mediterranean Sea in previous studies. Our findings indicate that juvenile bluefin tuna targeted a relatively small range of prey species and regional foraging patterns remained consistent over time in the Mid-Atlantic Bight but changed in relation to local prey availability in the Bay of Biscay.     

Biological and oceanographic insights from larval labrid (Pisces: Labridae) identification using mtDNA sequences

Polymerase chain reaction and direct comparison of mitochondrial DNA sequences from a cytochrome b gene fragment were used to identify two morphologically distinct larval types ofXyrichtys, a genus of tropical wrasse (Pisces: Labridae). Both larval types were collected during ichthyoplankton surveys on the Middle Atlantic Bight shelf in the summer of 1988. DNA sequence comparisons indicated that both types were larvae ofXyrichtys novacula (Linnaeus). Back-calculated birthdate distributions for those larvae collected on the Middle Atlantic Bight shelf demonstrated that the two larval types formed two distinct cohorts indicating a biological difference. The two distinct larval types may be a consequence of an ecophenotypic effect, or they may represent offspring from genetically distinct populations. These results emphasize that important biological and oceanographic information can be gained through the use of the polymerase chain reaction and DNA sequencing for larval identificaiton.