Variability of albacore (<Emphasis Type="Italic">Thunnus alalunga</Emphasis>) diet in the Northeast Atlantic and Mediterranean Sea

This study aims to describe the variability of albacore (Thunnus alalunga) diet in the Northeast Atlantic and Mediterranean Sea and to identify possible relationships between this variability and the features of different feeding areas, the behavior, and the energetic needs of albacore. Stomach contents from albacore caught in five zones of the Bay of Biscay and surrounding waters (n = 654) and three zones of the Mediterranean Sea (n = 152) were analyzed in terms of diet composition and stomach fullness. Carbon and nitrogen stable isotope and C/N ratios were measured for white muscle and liver from albacore in the Bay of Biscay (n = 41) and Mediterranean Sea (n = 60). Our results showed a spatial, seasonal, inter-annual, and size-related variability in the diet of albacore. Albacore diet varied by location in the Mediterranean Sea, with a particularly high proportion of cephalopods, and low δ¹⁵N values in the Tyrrhenian Sea. In the Northeast Atlantic, albacore consumed a higher proportion of crustaceans and a lower proportion of fishes in the most offshore sampling zone than inshore. The digestion states of the major prey reflected a diurnal feeding activity, indicative of feeding in deeper waters offshore, whereas on the continental slope, feeding probably occurred in surface waters at night. Important seasonal and inter-annual diet variability was observed in the southeast of the Bay of Biscay, where preferred albacore prey appeared to be anchovy (Engraulis encrasicolus). Stomach fullness was inversely related to body size, probably reflecting higher energetic needs for smaller individuals. Albacore from the Bay of Biscay had significantly lower δ¹³C and higher δ¹⁵N values compared with albacore from the Mediterranean Sea, indicative of regional baseline shifts, and trophic position and muscle lipid stores in albacore increased with body size.     

Extrinsic and intrinsic determinants of winter foraging and breeding phenology in a temperate seabird

In temperate regions, winter presents animals with a number of challenges including depressed food abundance, increased daily energy requirements, higher frequency of extreme weather events and shortened day length. Overcoming these constraints is critical for overwintering survival and scheduling of future breeding of long-lived species and is likely to be state dependent, associated with intrinsic abilities such as food acquisition rates. We examined the relationship between environmental and intrinsic factors on overwintering foraging and subsequent breeding phenology of the European shag Phalacrocorax aristotelis, a diurnal marine predator. We tested a range of hypotheses relating to overwintering foraging time and location. We found that individuals greatly increased their foraging time in winter to a peak of more than 90% of available daylight at the winter solstice. The seasonal patterns of foraging time appear to be driven by a combination of light levels and weather conditions and may be linked to the availability of the shag's principal prey, the lesser sandeel Ammodytes marinus. There was no evidence that shags dispersed south in winter to increase potential foraging time. Foraging time decreased after the winter solstice and, crucially, was correlated with subsequent breeding phenology, such that individuals that spent less time foraging in February bred earlier. The relationship was much stronger in females than males, in line with their more direct control of timing of breeding. Our results demonstrate that pre-breeding intrinsic foraging ability is critical in determining breeding phenology.     

Physiological and behavioral aspects of Calanus euxinus females (Copepoda: Calanoida) during vertical migration across temperature and oxygen gradients

 In the Black Sea, during summer stratification, Calanus euxinus (Hulsemann) undertakes diel vertical migrations with an amplitude of about 117 m from oxygenated, warm (18 °C) surface layers to hypoxic (∼0.8 mg O₂ l⁻¹) zones with lower temperature (7.9 °C). When such changes in temperature and oxygen concentration are reproduced in the laboratory, total metabolism, basal metabolism and scope of activity of copepods decrease 7.2, 7.8 and 6.7 times, respectively, while the frequency of locomotory acts and mechanical power decline 3.4- and 9.5-fold, respectively. These changes allowed the copepods to conserve a significant portion of food consumed near the surface for transformation to lipid reserves. Diel respiratory oxygen consumption of migrating individuals, calculated so as to include actual duration of residence in layers with different temperature and oxygen concentrations, is estimated at 17.87 μg O₂ ind⁻¹. The net energy cost of vertical migration made up only 11.6% of the total. Copepods expend 78.6% of diel energy losses during approximately 10 h in the surface layers, while about 5.4% is required during about 9 h at depth. Hypoxia is shown to have a significant metabolic advantage during diel vertical migrations of C. euxinus in the Black Sea. Received: 1 October 1999 / Accepted: 11 July 2000     

Energy budget of the predatory marine gastropod Polinices duplicatus

Based on direct monitoring of feeding and growth under field conditions and frequent measurements of respiration and energy content in the laboratory, annual energy budgets were derived for Polinices duplicatus feeding on Mya arenaria at Barnstable Harbor, Massachusetts. The gross growth efficiency (P_g/C) of P. duplicatus was high (35%) and varied inversely with snail size. The proportion of ingested energy expended on respiration (R/C) was modest (44%) and varied directly with snail size and inversely with temperature. The energetic efficiencies of P. duplicatus agreed with the few results available for othe predatory molluscs and differed from most results available for herbivorous and detritivorous molluscs.     

Ontogenetic change in the diet ofAplodactylus punctatus (Pisces: Aplodactylidae): an ecophysiological explanation

Aplodactylus punctatus is a temperate berbivorous fish that changes from an omnivorous to a herbivorous diet and increases its ability ot assimilate algae as it grows. To investigate whether this dietary shift is related to size-specific differences in energetic demands imposed by metabolism and the amount of assimilated energy, oxygen consumption ( ) was determined experimentally in 12 specimens ranging in size from 62 to 545 g. increased allometrically with body size from 8.41 to 55.95 mg O₂ individual^-1 h^-1. Individual energetic requirements were 2.8 to 33.7 kJ d^-1. The assimilated energy was estimated, taking into consideration: (1) the energetic value of the most important alga in the diet (Lessonia trabeculata); (2) size-specific differences in assimilation rates for fish fed on this alga; (3) size-specific differences in throughput time and in the amount of food in a full gut. Comparison of the energy required and the assimilated energy revealed that fishes of < 22 to 29 cm total length had a negative energetic balance when consuming algae exclusively. This may explain the reliance of smallA. punctatus on more easily-digested invertebrates. The largest individuals can meet their energetic demands by consuming algae alone, apparently because of their higher assimilation capability. InA. punctatus, changing energetic requirements and capacities for algal assimilation may be responsible for the observed ontogenetic change in diet.     

Energy storage and dynamics in bay anchovy Anchoa mitchilli

The energy of the body components and the energy costs of spawning and overwintering in the bay anchovy Anchoa mitchilli, the most abundant fish in Chesapeake Bay, were studied to determine seasonal variability during the different stages of its life cycle. Bimonthly samples were collected by trawl from April 1990 through October 1991. Fish condition and body energy levels fluctuated seasonally, and were related to anchovy size. Energy equivalents (cal g^-1 dry wt) was highest in December, before the overwintering period. The somatic weight component increased by 32 to 33% and total body weight by 26% during the spawning season, indicating that feeding not only met energy requirements of daily spawning but also provided surplus energy for growth. The overwintering loss of energy was 33 to 35% of total body calories, and was primarily derived from deposit fat in somatic and visceral tissues.     

Reproductive investment reduces recuperation from exhaustive escape activity in the tropical scallop Euvola zizac

In scallops, mobilization of reserves from the adductor muscle to support maintenance and reproductive activity may impinge upon a major role of the adductor muscle, the movement of the valves during swimming and escape responses. The tropical scallop Euvola ziczac (Linnaeus 1758) invests energy reserves to different degrees during its two periods of reproduction each year. We evaluated the impact of reproductive investment on the escape responses (clapping capacity and recovery after exhaustive exercise) of E. ziczac sampled at different reproductive stages (immature, mature, spawned) during the two reproductive periods in 1997. We compared the escape response capacities with the levels of muscle energetic reserves (glycogen, proteins, and arginine phosphate) and muscle metabolic capacities [activity of the glycolytic enzymes: glycogen phosphorylase (GP), pyruvate kinase (PK), phosphofructokinase (PFK), octopine dehydrogenase (ODH), arginine kinase (AK), and the mitochondrial enzyme, citrate synthase (CS)]. Gonad size, gamete volume fraction, and levels of gonadal protein and lipid were greater for mature scallops during the first than during the second reproduction. Numbers of claps during escape responses (49–57) and levels of muscle arginine phosphate remained similar throughout the different reproductive stages in both reproductive periods. In contrast, recovery was slowed during gonadal maturation in both reproductive periods and during spawning in the first reproduction. Scallops generally took more time to regain their initial clapping capacity during the first (25–40 min) than during the second reproduction (20–30 min). Muscle glycogen decreased markedly during both gonadal maturation and spawning in both reproductions; whereas, muscle proteins decreased only in the first reproduction. The levels of most enzymes decreased during gonadal maturation in both reproductions, and also after spawning, particularly during the first reproduction. We concluded that gonadal maturation and spawning did not decrease clapping capacity of E. ziczac, but decreased its capacity to recover from exhaustive exercise most likely due to decreased levels of energetic reserves and a reduced metabolic capacity of the adductor muscle. Moreover, these effects were probably stronger during the first cycle because of the greater reproductive investment coincident with decreased food availability. Received: 28 April 2000 / Accepted: 21 September 2000     

Growth,survivorship, life-span,and sex change in the hermaphroditic shrimp<Emphasis Type="Italic"> Lysmata wurdemanni</Emphasis> (Decapoda: Caridea: Hippolytidae)

Lysmata wurdemanni (Gibbes) is a protandric simultaneous hermaphrodite. All individuals first mature as a male-phase individual (MP) and then later change to a female-phase individual (FP) that spawns and broods embryos but can also mate as a male. A Gulf of Mexico population was sampled monthly for 1 year and bimonthly the next. Estimates of basic population parameters were obtained from cohort analysis to reveal possible factors explaining the unusual sexual biology of L. wurdemanni as well as the broad variation in the size (age) of change from MP to FP. Growth rates of individuals from cohorts varied from 4–7 mm carapace length year ^-1. Growth of small MPs in the laboratory was somewhat faster but concordant with growth rates estimated from field samples. The period from recruitment to >50% sex change in cohorts varied from 3 months to 1 year. In the laboratory, the size and interval to sex change was similar to that of the most rapidly changing cohort observed. Survivorship of cohorts was high until later in life; life-span was estimated to be 12–18 months. Rates of sex change were highest from late winter through spring, in time for the spring–summer breeding season. The size and age of sex change in cohorts were related to the season of recruitment. MPs recruited from late winter to mid-spring rapidly changed to FPs at a relatively small size. A majority of MPs recruited in the summer and autumn did not change to FPs until the following late winter to spring, and they did so at a larger size. Rates of sex change were not correlated with the sexual composition of the population. We conclude that seasonal factors related to female breeding greatly influence sex change in L. wurdemanni. We found no evidence to support demographically influenced and socially mediated environmental sex determination, which has been suggested for L. wurdemanni and other sex-changing caridean shrimps.     

Overwintering strategies of dominant calanoid copepods in the German Bight, southern North Sea

Abundance, stage composition and reproductive parameters (egg production, egg viability, proportion of spawning females) of the four copepod species Acartia clausi, Centropages hamatus, C. typicus and Temora longicornis were measured at the long term sampling station Helgoland Roads (German Bight, southern North Sea) from September 2003 to May 2004 to study their overwintering strategies. A. clausi was overwintering as females with arrested reproduction from November to January. T. longicornis, which is known to produce resting eggs in the North Sea, had a pelagic population with all developmental stages present during winter and reproductive rates closely related to food concentrations. Although their females produced eggs in response to ambient food conditions, both C. hamatus and C. typicus were rare in the pelagic. The C. hamatus population returned in May, probably from resting eggs, whereas C. typicus depended on advection. The Centropages species seemed to be less adapted to pelagic life in winter than A. clausi and T. longicornis. Sporadic occurrence of large numbers of nauplii and young copepodids of A. clausi and Centropages spp. pointed to different overwintering strategies or more successful survival in adjacent regions and advection of them into the waters around Helgoland island. While A. clausi was decoupled from environmental conditions in late autumn and winter, the other species were able to respond to variations in the food environment. Thus, egg production of T. longicornis increased during an unusual autumn diatom bloom.     

The effect of ration size, temperature and body weight on specific dynamic action of the common cuttlefish Sepia officinalis

The effect of meal size (shrimp Crangon crangon) [0.83–18.82% dry body weight (Dw)] on specific dynamic action (SDA) was assessed in cuttlefish Sepia officinalis (1.03–6.25 g Dw) held at 15 and 20°C. Cuttlefish <2 g significantly expended less energy in feeding and digesting their meal than cuttlefish >2 g when given the same quantity of food. Handling, eating and digesting a shrimp meal was temperature dependent with cuttlefish processing and digesting a similar sized shrimp meal faster at 20°C than at 15°C. The proportional increase in oxygen consumption (2.07 ± 0.02) was not correlated with feeding rate (FR) and was independent of temperature and cuttlefish size. The SDA peak was not correlated with FRs, and increased as cuttlefish size and temperature increased. The mean SDA coefficient was 0.87 ± 0.07% of the ingested energy; one of the lowest SDA values recorded amongst vertebrates and invertebrates. Daily energy requirements (KJ day⁻¹) for S. officinalis were calculated from laboratory estimates of energy losses due to standard (MO_{2 Standard}), routine (MO_{2 Routine}) and feeding (MO_{2 SDA}) oxygen consumption. Laboratory estimates of daily metabolic expenditures were combined with results from previous investigations to construct an energy budget for 1 and 5 g cuttlefish consuming a meal of 5 and 15% Dw at 20°C and the amount of energy available for growth was estimated to be between 35 and 80.3% of the ingested energy.     

Seasonal lipid dynamics of the copepods <Emphasis Type="Italic">Pseudocalanus minutus</Emphasis> (Calanoida) and <Emphasis Type="Italic">Oithona similis</Emphasis> (Cyclopoida) in the Arctic Kongsfjorden (Svalbard)

Seasonal lipid dynamics of various developmental stages were investigated in Pseudocalanus minutus and Oithona similis. For P. minutus, the dominance of 16:1(n−7), 16:4(n−3) and 20:5(n−3) fatty acids indicated a diatom-based nutrition in spring, whereas 22:6(n−3), 16:0, 18:2(n−6) and 18:1(n−9) pointed to a flagellate-based diet during the rest of the year as well as omnivorous/carnivorous low-level feeding during winter. The shorter-chain fatty alcohols 14:0 and 16:0 prevailed, also reflecting biosynthetic processes typical of omnivores or carnivores. Altogether, the lipid signatures characterized P. minutus as an opportunistic feeder. In contrast, O. similis had consistently high amounts of the 18:1(n−9) fatty acid in all stages and during all seasons pointing to a generally omnivorous/carnivorous/detritivorous diet. Furthermore, the fatty alcohol 20:1(n−9) reached high percentages especially in adult females and males, and feeding on Calanus faecal pellets is suggested. Fatty alcohols, as wax ester moieties, revealed significant seasonal variations in O. similis and a seasonal trend towards wax ester accumulation in autumn in P. minutus. P. minutus utilized its lipid deposits for development in the copepodite stages III and IV and for gonad maturation in CV and females during the dark season. However, CVs and females depended on the spring phytoplankton bloom for final maturation processes and reproduction. O. similis fueled gonad maturation and egg production for reproduction in June by wax esters, whereas reproduction in August/September co-occurred with the accumulation of new depot lipids. Both species revealed significantly higher wax ester levels in deeper (>50 m) as compared to surface (0–50 m) dwelling individuals related to a descent prior to overwintering.     

Feeding preferences,seasonal gut repletion indices,and diel feeding patterns of the sea urchin<Emphasis Type="Italic"> Tripneustes gratilla</Emphasis> (Echinodermata: Echinoidea) on a coastal habitat off Toliara (Madagascar)

The feeding biology of the echinoid Tripneustes gratilla from a seagrass bed off Toliara, on the south-west coast of Madagascar, was investigated during the summer and winter seasons. Repletion indices of the gut of T. gratilla varied seasonally, in such a way that higher amounts of food occurred in the gut in winter than in other seasons. Conversely, a more detailed analysis of one summer and one winter sample of the gut tissue dry weights revealed higher weights in summer (February) than in winter (August). This was interpreted as a varying capability of nutrient storage and closely related to the feeding activity of the echinoid. This inverse relationship between feeding activity and nutrient storage was suggested to be dependent on gonadal growth. Indeed, higher food consumption was observed during the post-spawning period (August), that is, when most of the energy accumulated was used up for gonadal growth. Low feeding activity occurred when a sufficient amount of energy was accumulated in gut tissues (February), the time corresponding to the initiation of gonadal growth. Investigations on the feeding habits, using Ivlev's electivity index, revealed preferential feeding of T. gratilla on Syringodium isoetifolium, the dominant seagrass species found in the gut. This was partly related to the higher availability of that particular seagrass species in the field. However, its soft and terete leaf blade's morphology also makes it easier to manipulate and ingest. Studies on the feeding behaviour indicated the presence of a diel feeding cycle in T. gratilla individuals in both summer and winter seasons. Yet, its pattern varied between the two studied months (one summer and one winter) as the cycle was less contrasted during period of active feeding, i.e. in winter compared to summer. This study provides novel information on how intrinsic factors, such as physiological state, can govern the feeding activity of T. gratilla living in a seagrass habitat.     

Age and growth of the naticid gastropod <Emphasis Type="Italic">Polinices pulchellus</Emphasis> (Gastropoda: Naticidae) based on length frequency analysis and statolith growth rings

In Red Wharf Bay, UK the naticid gastropod, Polinices pulchellus, was more abundant and more highly aggregated during the summer months (June–August 2001) than during the winter (December 2000). Whilst small numbers of juvenile P. pulchellus (4–6 mm shell length) were present throughout the year the population consisted mainly of individuals of 12–14 mm shell length. Juvenile snails grew rapidly in size during the winter and early spring; growth then virtually ceased between May and June, following which there was a further period of rapid growth between August and February. Densities ranged between 57 and 4,073 ha⁻¹ and the largest individual collected during this investigation measured 16.2 mm in shell length. Statoliths from adult P. pulchellus revealed the presence of a settlement ring and two prominent growth rings (rings 1 and 2). A curvilinear relationship exists between statolith diameter and shell length in snails up to 16 mm in length. Settlement rings ranged in diameter from 19.7 to 45.2 μm (mean 29.8 μm; SE=0.41) giving an estimated shell length of the settled juvenile of 1.1 mm. The diameter of ring 1 and ring 2 were significantly correlated indicating that rapid growth during the first year is maintained during year 2. Shell lengths estimated from the diameters of the prominent statolith rings and those obtained from length frequency data analysis (LFDA), were broadly congruent strongly suggesting an annual periodicity to the statolith rings. The largest snails (>15 mm) present within this population were estimated to be between 2 and 3 years old. Von Bertallanfy seasonal growth curves obtained from the LFDA predicted values of L∞, K and t ₀ of 14.32 mm, 1.54 and −0.14 years, respectively, suggesting that P. pulchellus rapidly attains its maximum asymptotic size.     

Population biology and life strategies of Chlorophthalmus agassizii Bonaparte, 1840 (Pisces: Osteichthyes) in the Mediterranean Sea

The population biology and life strategies of Chlorophthalmus agassizii were studied in the Ionian Sea (eastern–central Mediterranean) using the data collected during the experimental trawl surveys carried out from 1995 to 2000. Depth-related trends of both density and size were found. With depth the former decreased while the latter increased. A typical bigger–deeper phenomenon was detected: young-of-the-year individuals occur on the shelf during autumn–winter months and move towards bathyal bottoms as they grow. The sampled population was made up of several size–age groups. The maximum age of 8 years was identified by means of otolith readings. The Von Bertalanffy growth parameters were estimated from the age–length key (L _∞=189.04±5.401 mm; k=0.24±0.021; t _o=−1.20±0.132; φ′=3.94) and modal progression analysis (L _∞=218.33±18.397 mm; k=0.164±0.028; t _o=−1.694±0.171; φ′=3.89). Reproduction of this monoecious fish was observed during summer–early autumn. Considering the female portion of the gonad, the size at attainment of 50% maturity was 115 mm TL. The corresponding age is within the third year of life. The simultaneous occurrence of oocytes in different development stages was shown in the ovary. Both the asynchronous ovary and oocyte size distribution indicate that C. agassizii spawns more than once during a reproductive season (batch spawner). Functional fecundity (on average 3,018 hydrated oocytes) was between 37 and 69% of the absolute fecundity and increased significantly with the individual size. Since adult specimens are mostly distributed on the slope, eggs and larvae develop in epipelagic waters and migrate in-shore where juvenile forms recruit on the shelf. Juveniles migrate ontogenetically towards deeper bottoms and after 2–3 years start to reproduce annually within a life span greater than 10 years.     

Scope for growth of <Emphasis Type="Italic">Mytilus galloprovincialis</Emphasis> (Lmk., 1819) in oligotrophic coastal waters (Southern Tyrrhenian Sea,Italy)

The ‘scope for growth’ (SFG) tool was used to study the growth performance of cultivated populations of Mytilus galloprovincialis (Lmk., 1819) in an oligotrophic area of the Southern Mediterranean Sea. The study was carried out between 1993 and 1996 by using data from four seasonal oceanographic cruises and from growth experiments. Water samples were collected and analysed for total suspended matter (TSM), particulate organic carbon (POC) and nitrogen (PON), particulate lipids, proteins and carbohydrates and chloropigments. The sum of the carbon equivalents of carbohydrates, proteins and lipids is indicated as the total biopolymeric particulate organic carbon (BPC) and was converted into a unit of energy in order to calculate the SFG of a theoretical mussel of 5 cm length. In order to test the performance of mussel growth at two depths (5 and 15 m water depth), mussel body size [as ash free dry weight (AFDW)] and the actual concentrations of BPC were used to calculate the monthly SFG using the physiological energetic relationships suggested in the current literature. Data from the field cruises led us to characterise the study site as ultra-oligotrophic (annual average of chloropigment concentration approximately 0.5 μg L⁻¹). SFG calculations allowed us to identify a site where mussels grown successively were found to reach a commercial size in approximately 12 months. The good agreement obtained between energetic response and subsequent production response suggests that the available energy from particulate food could be fully available for organic production for maintaining “proportionate” growth trajectories, even in a ultra-oligotrophic system.     

Feeding energetics and metabolism in demersal fish species from Antarctic,temperate and tropical environments

The energetics of feeding has been investigated in demersal fish with similar sedentary lifestyles from the Antarctic (Notothenia neglecta Nybelin), North Sea (Myoxocephalus scorpius L.) and Indian Ocean (Cirrhitichys bleekeri Bleeker). In general, the metabolic rates of fasting individuals were positively correlated with adaptation temperature: values for a standard 100 g fish (mg O₂/h) were 3.3 for N. neglecta at around 0 °C, 2.7 for winter-acclimatized M. scorpius at 5 °C, 4.3 for summer-acclimatized M. scorpius at 15 °C, and 7.0 for C. bleekeri at 25 °C. In all species, following a single satiating meal, oxygen consumption increased to a peak of 2 to 3.5 times the fasting values. Maximum rates of oxygen consumption after feeding were several-fold higher in the warm-than in the cold-water species. After controlling for the effects of body mass and energy intake by analysis of covariance, the duration of the increase in metabolic rate, referred to as specific dynamic action (SDA), was found to be 3 to 4 times shorter in the warm- than in the cold-water fish, ranging from 57 h in C. bleekeri to 208 h in N. neglecta. In contrast, the SDA was not significantly different in the various species, corresponding to 15 to 23% of the energy ingested. Seasonal influences on metabolism and feeding were also studied in N. neglecta acclimated to simulated winter (-1.0 to-0.5 °C; 3 h light:21 h dark) or summer (0 to 0.9 °C; 21 h light:3 h dark) conditions. The metabolic rates of fasting and fed individuals, and the characteristics of the SDA were found to be independent of acclimation conditions. This suggests that N. neglecta is capable of processing food at similar rates throughout the year. Energy stores and enzyme activities were measured in the swimming muscles and liver of fish fed ad libitum. Summer-acclimated fish had higher concentrations of liver triglyceride stores and elevated activities of some enzymes of intermediary metabolism relative to winter-acclimated fish. The observed changes in intermdiary metabolism are probably related to annual cycles of growth and reproduction. It is suggested that the low aerobic scope for physiological performance in Antarctic fish may necessitate the seasonal switching of energy allocation between growth and reproduction.     

Feeding habits of the albacore tuna <Emphasis Type="Italic">Thunnus alalunga</Emphasis> (Perciformes,Scombridae) from central Mediterranean Sea

In order to assess feeding habits and trophic level of this species, we analysed the stomach content of 189 specimens collected monthly in the southern Tyrrhenian Sea (central Mediterranean Sea). The results showed that this species is a pelagic predator focusing on paralepidid fish and, more in detail, on two species such as Paralepis speciosa and Paralepis coregonoides. Among cephalopods, Heteroteuthis dispar was the most abundant prey item in the stomach contents, whereas among crustaceans the hyperidean amphipods Phrosina semilunata and Brachyscelus crusculum were the most representative species. In the present study, the diet of the albacore did not vary significantly throughout the period of study focusing, in the different season of catch, on the same prey species. Not even a size-related diet shift was observed, with a partitioning of the albacore sample into five size groups. Levin’s standardized index showed a low value indicating a limited trophic niche width, with a diet dominated by a limited number of taxa and then a specialist behaviour for this predator. The estimated TROPH value for T. alalunga resulted very similar to that calculated in Mediterranean Sea for other species as well as Xiphias gladius, Thunnus thynnus, Euthynnus alletteratus and Sarda sarda. Then, the albacore tuna can be considered as a top carnivore.     

Vertical distribution and mortality of <Emphasis Type="Italic">Calanus finmarchicus</Emphasis> during overwintering in oceanic waters southwest of Iceland

The seasonal vertical distribution and the predatory regime encountered by Calanus finmarchicus were studied along a transect across the Reykjanes Ridge in the oceanic area southwest of Iceland from data collected during four cruises between November 1996 and June 1997. The mortality for the overwintering period was estimated using linear regressions of density estimates from November 1996 to April 1997. In addition, we also estimated the mortality of the oldest population stages (C4 and older) in April and June by applying the vertical table method. During winter (November/December–January/February), the animals mainly resided at a depth of ∼300–1,500 m in the water of Atlantic origin. Ascent to upper layers took place mainly during March and April, and continued until May. During all cruises, continuous deep-scattering layers were observed, mainly within the range of 400–500 m to 700–800 m depth. Based on sampling with a Harstad pelagic trawl in April, the scattering was mainly ascribed to jellyfish (mainly Periphylla periphylla), small mesopelagic oceanic fishes (several species but Benthosema glaciale and Maurolicus muelleri were most abundant), euphausiids (mainly Meganyctiphanes norvegica) and shrimps (mainly Sergestes arcticus). These species may represent a predatory threat to overwintering C. finmarchicus. From November to April, daily per capita mortality rates were estimated to be (mean ± 95% CL) 0.004 ± 0.0028 for the total data set, and 0.004 ± 0.0033 and 0.004 ± 0.0023 (day^-1) for the Iceland Basin and Irminger Basin, respectively. Mortality rates were higher later in life (mean ± 95% CL) for C5/females (0.13 ± 0.044) and C5/males (0.19 ± 0.051) than for C4/C5 (0.00 ± 0.035) when averaged over all samples taken in April and June 1997. We discuss how the observed distribution and mortality rates of overwintering C. finmarchicus might be related to predatory regime.     

Seasonality in gonads,fat deposits and condition of tropical surgeonfishes (Teleostei: Acanthuridae)

Lipids are stored in mesenteries surrouding the gut and in paired, retroperitoneal fat bodies dorsal to the anal fin in at least 24 surgeonfishes representing 5 genera (Acanthurus, Ctenochaetus, Naso, Prionurus, Zebrasoma) from Western Mexico, Hawaii, French Polynesia, Great Barrier Reef and the Red Sea. In A. nigrofuscus from the Red Sea and C. striatus from French Polynesia, fat stores and condition index (K=100 W/L ³) increased during cool seasons, several months before gonads developed, and declined with the onset of gonadal development. Fat bodies and mesenteric fat deposits developed in all sizes of A. nigrofuscus and C. striatus, and condition paralleled seasonal changes in fat deposits. Gonads of A. nigrofuscus developed in specimens of 76 to 159 mm standard length (SL), while they developed in C. striatus only at lengths>170 mm total length (TL) for females and 195 mm TL for males. Mesenteric fats and fat bodies exhibited similar changes in size, and did not differ in occurrence of major groups of lipids or their relative prevalence. The relative proportions of the major lipid classes differed, however, between these storage sites and other tissues. Muscle, liver and gonads contained less triacylglycerol relative to cholesterol and polar lipid than did mesenteric or fat body lipids. Energy and nutrients acquired during cool saasons with short feeding days appear to support reproductive events during warm seasons, when feeding during longer days fails to meet demands. Fat stores and condition continue to decline beyond the reproductive season until winter.     

Lipid metabolism of the Antarctic krill Euphausia superba and its ecological implications

 Various developmental stages (early larvae to adults) of Euphausia superba have been collected in different seasons in the Weddell Sea, the Lazarev Sea and off the Antarctic Peninsula to investigate the role of lipids and fatty acids in the life cycle of the Antarctic krill. The total-lipid data for E. superba exhibited seasonal variations, with low lipid levels in late winter/early spring and the highest levels in autumn. Seasonal changes were most pronounced in the immature and adult specimens, increasing from about 10% lipid of dry mass to more than 40%. The fatty-acid compositions of the younger stages were dominated by 20:5(n-3), 22:6(n-3) and 16:0. These are typical phospholipid fatty acids, which are major biomembrane constituents. The phospholipid composition was similar in the older stages. With increasing storage of triacylglycerols in the lipid-rich immature and adult stages, the fatty acids 14:0, 16:0 and 18:1(n-9) prevailed, comprising about 70% of total triacylglycerol fatty acids. The trophic-marker fatty acids 16:1(n-7) and 18:4(n-3), indicating phytoplankton ingestion, were less abundant. They reflected, however, the dependence of the larvae on phytoplankton as well as the seasonal changes in algal composition. The generally close linear relationships between fatty acids and lipid suggest that the fatty-acid compositions of the collected specimens were largely independent of the respective developmental stage, season and region. The linear fit indicates that triacylglycerol accumulation started at a level of about 5% of total lipid. Considering the various overwintering scenarios under discussion, the life cycle and reproductive strategies of krill are discussed in the context of the lipid metabolism and fatty-acid composition of E. superba. Lipid production is effective enough to accumulate large energy reserves for the dark season, but E. superba does not exhibit the sophisticated biosynthetic pathways known from other Antarctic euphausiids and copepods. Although important, lipid utilisation appears to be just one of several strategies of E. superba to thrive under the extreme Antarctic conditions, and this pronounced versatility may explain the success of this species in the Southern Ocean. Received: 16 June 2000 / Accepted: 18 December 2000