Krill Diet Research Articles

Investigation of mercury concentrations in fur of phocid seals using stable isotopes as tracers of trophic levels and geographical regions

Recent studies have shown that the complementary analysis of mercury (Hg) concentrations and stable isotopic ratios of nitrogen (δ15N) and carbon (δ13C) can be useful for investigating the trophic influence on the Hg exposure and accumulation in marine top predators. In this study, we propose to evaluate the interspecies variability of Hg concentrations in phocids from polar areas and to compare Hg bioaccumulation between both hemispheres. Mercury concentrations, δ15N and δ13C were measured in fur from 85 individuals representing 7 phocidae species, a Ross seal (Ommatophoca rossii), Weddell seals (Leptonychotes weddellii), crabeater seals (Lobodon carcinophagus), harbour seals (Phoca vitulina), grey seals (Halichoerus grypus), ringed seals (Pusa hispida) and a bearded seal (Erignathus barbatus), from Greenland, Denmark and Antarctica. Our results showed a positive correlation between Hg concentrations and δ15N values among all individuals. Seals from the Northern ecosystems displayed greater Hg concentrations, δ15N and δ13C values than those from the Southern waters. Those geographical differences in Hg and stable isotopes values were likely due to higher environmental Hg concentrations and somewhat greater number of steps in Arctic food webs. Moreover, dissimilarities in feeding habits among species were shown through δ15N and δ13C analysis, resulting in an important interspecific variation in fur Hg concentrations. A trophic segregation was observed between crabeater seals and the other species, resulting from the very specific diet of krill of this species and leading to the lowest observed Hg concentrations.

Aspects of krill growth and condition during late winter-early spring off East Antarctica (110–130°E)

Antarctic krill, Euphausia superba, is a keystone species in the Southern Ocean. However, information on growth, diet and condition during winter and early spring is limited, hampering our understanding of these fundamental biological parameters at this important period in their lifecycle. Our study assessed diet and condition of larval and postlarval krill collected from open water and below the ice off East Antarctica (110–130ºE) in September/October 2007. Condition was assessed using lipid content, growth rates and digestive gland size; feeding history was assessed using fatty acid profiles and stomach content analysis; and a 207-day starvation study investigated the response of krill to long-term food deprivation. Potential food items ( Calanus propinquus and sea-ice biota) were analysed for lipid and fatty acid composition to compare with krill samples. Krill were found to be in good condition, with mean growth rate of 0.95% per moult for postlarvae and 14.79% for larvae, and mean lipid content of 24.1% for postlarvae and 6.6% for larvae. Fatty acid profiles and stomach content analysis revealed two main feeding strategies - krill below the ice were feeding mostly on sea-ice diatoms, while those in open water were ingesting copepods and detritus. Krill below the ice had larger digestive glands than those in open water. Furciliae fatty acid profiles indicated a diet of heterotrophic flagellates and/or detritus. Postlarval krill survived 207 days of food deprivation by using body protein and lipid reserves for energy. In contrast, krill furciliae were severely depleted after just 5 days of food deprivation, indicating that they must feed continually at this time of year. Krill, copepods and sea-ice biota were all low in polyunsaturated fatty acids, indicating that krill must rely on later spring phytoplankton blooms to obtain these essential nutrients required for reproduction.

Stability of Formulated Diets and Feeding Response of Stage I Western Spiny Lobster, Panulirus cygnus, Phyllosomata

Abstract This study is a preliminary assessment of suitable binders and formats of potential formulated diets for hatchery growout of early‐stage western spiny lobster, Panulirus cygnus, phyllosomata. Six dry meal (each bound with a different binder) and three gelatinous (all bound using gelatine) diets were tested for stability in water over increasing periods of time (4, 8, and 12 h), and the feeding response to each was observed by video analysis. The gelatinous krill diet appears most suited to the larval feeding biology of P. cygnus phyllosomata as it elicited a heightened feeding response immediately following immersion, was most stable in water up to 4 h, and did not foul the maxillae during ingestion. The dry meal diets were not as stable in water and contributed to excessive fouling of the maxillae during ingestion. Of the dry meal diets, the most suitable was bound with 5% k‐carrageenan, presumably because of high moisture content and softer texture.

The effect of dietary krill supplementation on epithelium-associated bacteria in the hindgut of Atlantic salmon (Salmo salar L.): a microbial and electron microscopical study

Atlantic salmon (Salmo salar L.) were fed fishmeal protein for 46 days, and 500 g kg−1 of fishmeal protein substituted with meal from Northern krill (Meganyctiphanes norvegica). No differences were observed in weight gain, length gain, feed conversion or specific growth rate between the groups that could be attributed to dietary manipulation. The adherent microbiota in the hindgut of the two rearing groups were further investigated. By substituting fishmeal with krillmeal, the total viable counts of aerobic and facultative aerobic bacteria colonizing the hindgut of Atlantic salmon increased from 8.5 × 104 to 2.2 × 106. Furthermore, dietary krillmeal affected the adherent hindgut microbiota. The Gram-positive bacteria Carnobacteria piscicola, Microbacterium oxydans, Microbacterium luteolum and Staphylococcus equorum spp. linens and the Gram-negatives Psychrobacter spp. and Psychrobacter glacincola were not isolated from hindgut of fish fed the krill diet. On the other hand, Pseudomonas fulgida, Pseudomonas reactans and Stenotrophomonas maltophila were not isolated from the control group fed fishmeal. Acinetobacter lwoffi, which is not normally found in the fish gut, was isolated from both feeding groups. Transmission electron microscopy showed bacteria-like profiles between the hindgut microvilli in both feeding groups indicating autochthonous microbiota. When fish were fed the krill diet, hindgut enterocytes were replete with numerous irregular vacuoles. These vacuoles were not observed in fish fed the fishmeal protein.

Protozoans as a food source for Antarctic krill, Euphausia superba: Complementary insights from stomach content, fatty acids, and stable isotopes

We studied the diet of Antarctic krill, Euphausia superba, at five stations across the southwest Atlantic sector in summer 2003 by analyzing stomach content, fatty acids, and stable isotopes on the same individuals. Our aim was to examine what each method could contribute to our understanding of krill nutrition and whether differences seen in growth rates were linked to their food. All three methods indicated clear regional differences in diet, but small ontogenetic and sex-related differences. Overall, diatoms were the most abundant item in the stomach, but at three of the stations, tintinnids, large dinoflagellates, and other armored flagellates dominated the identifiable biomass. Copepod remains were rare. Fatty acids profiles gave additional information about feeding on weakly silicified diatoms and athecate heterotrophic dinoflagellates, with the latter being the main food source at one of the stations. Two independent indices of carnivory, Δ15N and the fatty acid ratio 18:1(n-9)/18:1(n-7), were correlated among krill from the same swarm, suggesting consistent differences in diet between individuals. An internal index of trophic position, (i.e., Δ15Nglutamic acid-Δ15Nphenylalanine) underlined the importance of heterotrophic food for the nutrition of krill, even in summer. Highest growth rates of krill were found during a diatom bloom and coincided with a mixed diet, large digestive gland, and fast stomach passage. However, even in a nonbloom, flagellate-dominated system, krill were able to sustain medium growth rates when feeding on heterotrophic dinoflagellates. Each method supplied specific information on krill nutrition, and the true picture is only revealed when the various methods are used together.

DNA as a Dietary Biomarker in Antarctic Krill, Euphausia superba

The diet of Antarctic krill (Euphausia superba) has been studied using a variety of techniques, but current methods still suffer from problems that are difficult to solve. This study examined an alternative approach utilizing DNA as a prey biomarker. Methods were developed for the preservation, extraction, and identification of prey DNA from krill collected in the field. Group-specific polymerase chain reaction (PCR) was used to amplify diatom prey (Phylum: Bacillariophyta) and the results from DNA clone libraries were compared with microscopic diet analysis. DNA analysis was superior to microscopy for prey detection. However, differences in prey relative abundance estimates between the two techniques suggested some bias in the DNA-based estimates. Quantification showed that large amounts of prey DNA had been successfully preserved and extracted. Overall the results suggest that the application of DNA-based diet analysis to krill warrants further investigation, particularly for prey that are difficult to study using other methods.

Molecular approach (PCR-DGGE) to diet analysis in young Antarctic krill Euphausia superba

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 319:155-165 (2006) - doi:10.3354/meps319155 Molecular approach (PCR-DGGE) to diet analysis in young Antarctic krill Euphausia superba Daniel L. Martin1,3, Robin M. Ross1, Langdon B. Quetin1, Alison E. Murray2,* 1Marine Science Institute, University of California, Santa Barbara, California 93106, USA 2Earth and Ecosystem Sciences, Desert Research Institute, 2215 Raggio Parkway, Reno, Nevada 89512, USA 3Present address: Department of Biological Sciences, LSCB-124, University of South Alabama, Mobile, Alabama 36688, USA *Corresponding author. Email: alison@dri.edu ABSTRACT: Antarctic krill Euphausia superba Dana comprise a key component of the Southern Ocean food web, yet despite decades of research, questions concerning the regional, seasonal and ontogenetic differences in their diet remain. All current methods used to characterize krill diet have limitations for identifying the full complement of the diet. Using DNA as a marker molecule, our goal in this study has been to evaluate the efficacy of a PCR-DGGE (denaturing gradient gel electrophoresis) approach targeting the 18S rDNA gene to discriminate among diet constituents in gut and fecal pellet samples from young Antarctic krill relative to their feeding environment — the seawater and sea ice microbial community. We conducted 2 laboratory-based feeding experiments with known food items and 3 field samplings of both the krill and their feeding environment. Sequenced PCR-DGGE phylotypes from laboratory trials clearly distinguished diatom and copepod prey, while in situ feeding analyses revealed that a broad diversity of taxa were ingested, including diatoms (Bacillariophyta, the most prevalent group detected), dinoflagellates, cryptomonads, prasinophytes, ciliates, cercozoans, choanoflagellates, turbellarians and (possibly) sponge larvae. Band image analyses allowed environmental and diet phylotypes to be matched. On average, 32% of those from the environment were present in the diet; conversely, of the phylotypes detected in the diet, an average of 59% were in common with the environment. Changes in environmental phylotypes among sampling dates were reflected by similar changes in the krill diet as potential prey diversity (richness) decreased during a phytoplankton bloom. KEY WORDS: Antarctic krill · Euphausia superba · Diet · DGGE · 18S rDNA · Sea ice microbial community · Omnivory Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 319. Online publication date: August 18, 2006 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2006 Inter-Research.

The replacement of fish meal with Antarctic krill, Euphausia superba in diets for Atlantic salmon, Salmo salar

A total of six isoprotein and isolipid diets for Atlantic salmon, Salmo salar L., were prepared substituting from 0 to 100% of fish meal protein (0–68% of diet by dry weight) with meal from Antarctic krill (Euphausia superba). The feed produced from high inclusion levels of krill meal had lower ability to absorb lipid during vacuum coating than fish meal. Both amino acid and fatty acid compositions of the diets were fairly similar. The experiment commenced using salmon averaging 500 g and ended at a mean weight of 1500–1800 g (140 days of feeding). Moderate amounts of krill meal (20–60% of krill protein) in the diets increased growth during the first 71 days of feeding compared with the fish meal control, while no growth difference was observed during the last 69 days of feeding. This may, at least in parts, be explained by a feed-attractant function of the krill meal. Muscle dry weight and lipid concentrations were unaffected by the diet. Feed conversion rate increased with high levels of krill meal in the diets (e.g. for the last period from 0.94 in the 0% diet to 1.26 in the 100% diet). This indicates that the fish were able to compensate by eating more to maintain growth. The apparent digestibility coefficients of dry matter and protein were not influenced by diet, but both faecal moisture and lipid had a tendency to increase at the highest inclusion level (all protein from krill meal). This may be related to chitin in the krill diet that is known to decrease lipid absorption and induce diarrhoea (increased water content in faeces). Chitin was not utilized to any major extent. Welfare parameters such as blood haemoglobin, red blood cell counts, plasma protein, cholesterol, triacylglycerols and glucose levels were unaffected by diets. Clinical indicators of cellular damage (alanine aminotransferase and aspartate aminotransferase) were similar indicating no diet-induced tissue damage during the trial.

Lipids as trophic markers in Antarctic krill. III. Temporal changes in digestive gland lipid composition of Euphausia superba in controlled conditions

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 296:81-91 (2005) - doi:10.3354/meps296081 Lipids as trophic markers in Antarctic krill. III. Temporal changes in digestive gland lipid composition of Euphausia superba in controlled conditions Frédéric Alonzo1,2,3,5,*, Patti Virtue1, Stephen Nicol2, Peter D. Nichols3,4 1Institute of Antarctic and Southern Ocean Studies, University of Tasmania, Private Bag 77, Hobart, Tasmania 7001, Australia2Australian Antarctic Division, Channel Highway, Kingston, Tasmania 7050, Australia3CSIRO Marine Research, Castray Esplanade, GPO Box 1538, Hobart, Tasmania 7001, Australia4Antarctic Cooperative Research Centre, University of Tasmania, GPO Box 252-77, Hobart, Tasmania 7001, Australia5Present address: Institut de Radioprotection et de Sûreté Nucléaire, Centre de Cadarache, BP 3, 13115 Saint-Paul-Lez-Durance cedex, France *Email: fred.alonzo@wanadoo.fr ABSTRACT: Two phytoplankton diets with markedly different fatty acid (FA) profiles (thraustochytrid and diatom over a range of concentrations) were fed to juvenile Euphausia superba Dana to evaluate the time lapse required for the development of trophic lipid signatures in the digestive gland (DG). Krill were collected after 5, 10 and 20 d of feeding on thraustochytrids. Krill fed on thraustochytrids for 20 d were also collected after a further 5, 10 and 20 d of feeding on diatoms. An accumulation of polar lipid (0.4 mg per DG) was observed after 5 d of feeding on thraustochytrids at every concentration of food. The deposition of triacylglycerol (0.25 mg per DG) required 10 d of feeding and only occurred at the 2 highest food concentrations. Amounts of thraustochytrid-derived FA, 14:0, 16:0 22:6ω3 and 22:5ω6, increased after 10 d of feeding. The final concentration of 22:6ω3 was dependent on food concentration. Though it was absent in the diet, the essential FA 20:5ω3 accumulated from the retroconversion of 22:6ω3 after 20 d. Changing the krill diet to diatoms resulted in a marked decline in thraustochytrid FA, particularly 14:0 and 22:6ω3, after 5 d, whilst diatom FA, 16:1ω7 and 20:5ω3, remained constant. Thraustochytrid signatures were completely erased from the FA profiles of DG after 20 d feeding on diatoms. Slow accumulation of storage lipid and short residence time of dietary FA suggest that lipid turnover rates are high in Antarctic krill DG. KEY WORDS: Euphausia superba · Antarctic krill · Lipid class · Fatty acid · Digestive gland · Trophic signature · Temporal scale Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 296. Online publication date: July 12, 2005 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2005 Inter-Research.

Diet of the Antarctic krill ( Euphausia superba Dana) : I. Comparisons of grazing on Phaeocystis antarctica (Karsten) and Thalassiosira antarctica (Comber)

Diatoms and prymnesiophytes are major constituents of spring phytoplankton blooms in the Antarctic. While diatoms are known to be well grazed by krill, the role of prymnesiophytes in the krill's diet is unknown. Clearance and ingestion rates of Euphausia superba grazing on three size classes of Phaeocystis antarctica were compared to rates on the diatom Thalassiosira antarctica in parallel, unialgal experiments. Clearance rates were calculated from disappearance of chlorophyll a during grazing, and converted to carbon and nitrogen ingestion rates based on elemental (CHN) analysis of phytoplankton cultures. Clearance and ingestion rates of krill on small P. antarctica colonies (50–100 μm) were similar to rates on T. antarctica, whereas rates on medium P. antarctica colonies (100–500 μm) and single cell P. antarctica were significantly lower than for T. antarctica. Overall, clearance rates on P. antarctica decreased with increasing colony size. Clearance rates on T. antarctica were variable, and correlated with degree of chain formation. These results suggest that E. superba effectively grazes small colonies of P. antarctica, but not larger colonies or single cells.

Diet of the Antarctic krill ( Euphausia superba Dana): II. Selective grazing in mixed phytoplankton assemblages

Diatoms, prymnesiophytes, and cryptophytes are the major taxonomic classes of phytoplankton available to Antarctic krill as primary food resources. However, the relative contribution of each class to the diet of krill is unknown. In this study, selectivity for different phytoplankton taxa by Euphausia superba was examined during laboratory experiments with E. superba grazing on mixed phytoplankton assemblages from the wild and from laboratory cultures. Clearance rates and changes in relative concentrations of diatoms, prymnesiophytes and cryptophytes were measured by analysis of taxon-specific accessory photopigments with high performance liquid chromatography (HPLC). E. superba grazed diatoms at higher rates than Phaeocystis and cryptophytes. Grazing was negligible in cryptophyte-dominated assemblages. Increases of up to 369% in 19′-hexanoyloxyfucoxanthin (HF)/fucoxanthin (FUCO) (prymnesiophyte/diatom) ratios indicated a high level of selectivity for diatoms over Phaeocystis. This selectivity occurred even for Phaeocystis colonies of similar size to diatoms, and thus cannot be entirely attributed to differential sieving efficiency based on particle size. The results suggest that krill actively select diatoms in phytoplankton mixtures.

Radiocarbon dating of mumiyo from the Vestfold Hills, East Antarctica

Accelerator mass spectrometer (AMS) radiocarbon dating of mumiyo deposited by snow petrels at nesting sites in the Vestfold Hills indicates the colonies were established after 4800 yr BP (uncorrected for marine reservoir effect). Sites on the highest hills were probably established first, but colonisation took place long after initial deglaciation, which was well advanced by 12.5k yr BP. The reason for late colonisation is not known. Perhaps older sites exist, but current evidence suggests the main colonisation occurred after the mid-Holocene re-advance of the Sorsdal Glacier. (il3C values indicate a diet of krill.

Feather mercury levels in seabirds at South Georgia: influence of trophic position, sex and age

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 243:261-269 (2002) - doi:10.3354/meps243261 Feather mercury levels in seabirds at South Georgia: influence of trophic position, sex and age Peter H. Becker1,*, Jacob González-Solís2, Brigitte Behrends1, John Croxall3 1Institut für Vogelforschung ŒVogelwarte Helgoland¹, An der Vogelwarte 21, 26386 Wilhelmshaven, Germany 2Departamento Biología Animal (Vertebrats), Universitat de Barcelona, Avenida Diagonal 645, 08028 Barcelona, Spain 3British Antarctic Survey, Natural Environment Research Council, High Cross, Madingley Road, Cambridge CB3 0ET, United Kingdom *Email: peter.becker@ifv.terramare.de ABSTRACT: We studied the mercury contamination of 13 species of seabirds breeding on Bird Island, South Georgia, in 1998. Total mercury concentrations in body feather samples of birds caught at their breeding colonies were determined. Among the species, grey-headed albatross (8933 ng g-1) and southern giant petrel (7774 ng g-1) showed the highest, and gentoo penguin (948 ng g-1) the lowest body feather mercury concentrations. Mercury levels were negatively correlated with the proportion of crustaceans (mainly krill) in the species¹ diets, suggesting that the trophic level is the most important factor in explaining the variation of mercury concentrations in Antarctic seabirds. In 4 species studied for age effects among adult birds (grey-headed and black-browed albatross, northern and southern giant petrel), no age-dependent variation in mercury levels was found. Sex differences were also assessed: female gentoo penguins had lower mercury levels than males, which may be related to the elimination of part of the mercury body burden by females into eggs. In contrast, northern giant petrel males had lower levels than females, which may be related to a higher consumption by males of carrion from Antarctic fur seals. In grey-headed albatrosses, mercury levels were 113% higher than in 1989, when this species was investigated at the same site, indicating a possible increase in mercury pollution of the Southern Ocean during the last decade. KEY WORDS: Interspecific variability · Intraspecific variability · Trophic position · Krill diet · Foraging · Temporal trend · Total mercury · Body feathers Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 243. Online publication date: November 13, 2002 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2002 Inter-Research.

Contribution of heterotrophic material to diet and energy budget of Antarctic krill, Euphausia superba

A novel approach was used to estimate the heterotrophic carbon component in the diet of the Antarctic krill Euphausia superba. Over 200 specimens from seven samples collected in the Lazarev Sea (January 1993 and 1995), at the Antarctic Polar Front (January 1993), and around South Georgia (February/March 1994) were dissected, and the total carbon content of their stomachs was estimated with a CHN-analyser. Gut-pigment contents were also measured by the gut-fluorescence technique in specimens collected at the same time, and the equivalent amount of their gut carbon was then subtracted from the total organic carbon content of guts from the same samples. The remaining carbon was assumed to originate entirely from heterotrophic food sources. This heterotrophic component accounted for a substantial proportion of the total food consumed by Antarctic krill, ranging from 17.4 to 98.9% of the mass of the gut contents (mean = 78.8% ± 21.2 SD). The results make an important contribution to the elucidation of the energy budget of krill and its daily carbon ration. With a few exceptions, previous estimates were largely calculated from a solely autotrophic carbon source, and were unable to account for the metabolic requirements of E. superba. Krill plays an important role in Antarctic food webs, as it often constitutes ≃50% of the total biomass of the zooplankton, and produces fast-sinking, dense faecal pellets which are important in the vertical transport of organic carbon from the euphotic layer to the deep ocean. High consumption rates of smaller heterotrophic organisms by krill suggest that this large microphage may be more important than previously believed in re-packaging micro- and mesozooplankton into a longer-lasting and more easily sequestered carbon pool.

Dietary lipid composition affects blood leucocyte fatty acid compositions and plasma eicosanoid concentrations in European sea bass ( Dicentrarchus labrax)

Broodstock European sea bass were fed three dry pelleted feeds for 12 months prior to the onset of spawning. The diets contained (a) Northern hemisphere fish meal and oil (control), (b) krill meal and Northern hemisphere fish oil (krill), or (c) Northern hemisphere fish meal and tuna orbital oil (RoPUFA). The RoPUFA diet contained more docosahexaenoic acid (22:6 n−3; DHA) (60%), less eicosapentaenoic acid (20:5 n−3; EPA) (20%) and more arachidonic acid (20:4 n−6; AA) (two-fold) compared to the other two diets. The total lipid fatty acid composition of purified blood leucocytes broadly reflected the dietary fatty acid input, with respect to the three long-chain highly unsaturated fatty acids (HUFA) described above, although DHA levels were only slightly elevated in the fish fed RoPUFA. The fatty acid compositions of the major phospholipid classes of blood leucocytes, phosphatidylcholine (PC), phosphatidylethanolamine (PE), phosphatidylserine (PS) and phosphatidylinositol (PI), were measured. In all phospholipid classes, the 22:6 n−3 level was relatively constant for all dietary treatments suggesting that dietary 22:6 n−3 concentrations were adequate in all diets. By contrast, the apparent demand for AA was not met by the control and krill diets while the moderately increased AA present in the RoPUFA diet was rapidly and efficiently incorporated into cellular lipids. The percentages of 20:4 n−6 varied in accordance with dietary input with highest levels in leucocytes from fish fed RoPUFA followed by those fed the krill diet and lowest values in control fish. Percentages of 20:5 n−3 were also influenced by diet with the highest levels found in fish fed the krill diet and the lowest in fish fed RoPUFA. Thromboxane B 2 (TXB 2) and TXB 3 derived from AA and EPA, respectively, were measured in plasma. In all dietary treatments, more TXB 2 than TXB 3 were produced with the highest values of the former found in fish fed RoPUFA. Leukotriene B 4 (LTB 4) and LTB 5 derived from AA and EPA, respectively, were also measured in plasma but no significant differences were found between dietary treatments.

The effect of diet on dorsal fin erosion in steelhead trout ( Oncorhynchus mykiss)

A 2 × 2 factorial experiment of diet type (krill vs. fish meal) and steroid supplementation (0 vs. 30 μg 17α-methyltestosterone kg −) was conducted to determine the effects on dorsal fin erosion in steelhead trout. Triplicate tanks of 250 fry were fed one of the four diets at a rate calculated to produce 115 g fish in 34 weeks. Fish were transferred to larger tanks when mean density index reached 0.40. Dorsal fin index (DFI, measured as mean dorsal fin height × 100/total fish length) was greater ( P < 0.001) among fish fed krill-based diets than for fish fed fish-based diets at weeks 12, 22, and 34 of the trial. Added testosterone decreased ( P = 0.04) DPI among fish fed the krill diet at week 12 but otherwise had no effect on fin condition. Addition of testosterone to either diet type decreased ( P = 0.02) critical thermal maximum, which is a measure of fish resistance to thermal stress. The results suggest that diet composition can influence the rate of dorsal fin erosion in steelhead trout through a metabolic, behavioral, or combined change.

Krill-copepod interactions at South Georgia, Antarctica, I. Omnivory by Euphausia superba

Feeding by juvenile Antarctic krill Euphausia superba near South Georgia was assessed during the austral summer of 1995/1996. Gut fluorescence results were compared with those from incubations in natural seawater and seawater enriched with phytoplankton and zooplankton. In natural seawater, with typically low food concentrations (median 56 mg C m-3) the median ration was 0.68% of krill carbon d-1. Phytoplankton dominated carbon in the natural incubation water but dinoflagellates, ciliates and small calanoid copepods dominated the carbon intake of krill. In both natural and enriched water, maximum clearance rates were on 1 to 3 mm calanoid copepods. Copepods larger than this (e.g. late copepodite stages of Calanoides acutus and Rhincalanus gigas) were cleared more slowly despite dominating the carbon in the enriched incubations. Oithona spp. were cleared more slowly than calanoids of similar size, despite their greater abundance and their similar contributions to available carbon. These trends could reflect detection/escape interactions between krill and copepods. With enriched food, copepods dominated krill diet, krill rations exceeded 10% of body carbon d-1 and rations did not appear to reach a plateau even at food concentrations of ~1 g C m-3. This suggests that krill could feed rapidly during periodic encounters with layers or patches of zooplankton. Gut fluorescence revealed gut passage times of 3.7 to 6.3 h and an algal carbon ration of 0.43% d-1, thus supporting the low algal carbon rations derived from the incubations. Published acoustic values of mean krill biomass north of South Georgia that summer of 8.3 g dry mass m-2 were combined with their clearance rates to give estimates of krill removing daily 0.2% of phytoplankton standing stocks, 0.6% of protozoans and 1.6% of small calanoid copepods. This impact on copepods is much higher than previous estimates from Antarctic amphipods and chaetognaths. The long generation times of Antarctic copepods mean that krill were potentially important predators of small copepods during our study.

Do minke whales (Balaenoptera acutorostrata) digest wax esters?

Mammals are known to utilize wax esters with an efficiency of less than 50%. The purpose of the present study was to examine whether or not minke whales (Balaenoptera acutorostrata), which at times may eat considerable amounts of wax-ester-rich krill, represent an exception to this general pattern. Samples of fresh undigested forestomach, as well as colon, contents were obtained from minke whales (n 5) that had been feeding on krill (Thysanoessa inermis) for some time. The samples were analysed for dry mass, energy density, lipid content and the major lipid classes, including wax esters. The concentrations of wax esters were compared with previous estimates of dry-matter disappearance of the same type of prey using an in vitro technique, to calculate the dry-matter digestibility of wax esters (DMDwax). Wax esters contributed 21% of the energy and 47% of total lipids in the krill diet. The energy density of gut contents decreased by 50% after their passage from forestomach to the end of the colon. The DMDwax was 94.1 (SD 2.8)% (n 5). This high DMDwax and the occurrence of fatty alcohols, one of the products of wax-ester hydrolysis, in faeces show that minke whales are very efficient digesters of wax esters and absorb most of the energy-rich products of this process.

Fluoride turnover in ad�lie penguins (Pygoscelis adeliae) and other brid species

Fluoride concentrations found in soft tissues of free ranging birds are below 10 mg/kg d.w. In ducks (Anas platyrhynchos) 99.5% of all fluoride is bound in the skeleton. Krill-eating adelie penguins (Pygoscelis adeliae) reached equilibrium concentrations in the order of 10000 mg F/kg d.w. in femur. Fluoride uptake with the diet is buffered by the skeleton where it has a half-life of 3–4 weeks. Fluoride from fresh krill does not reach the organs of adelie penguins to the same extent as NaF. Most fluoride remains bound to the krill cuticle and is not absorbed by the penguin gut during digestion. The availability of krill fluoride was in the range of 21%–30% for muscle and 48%–68% for femur of adelie penguins as compared to NaF (100%). The role of the salt glands in fluoride excretion is neglegible. The uptake of approx. 240 mg F daily with the krill diet is balanced by excretion via the cloaca.

Fat content, fatty acid composition and estimates of energy metabolism of adélie penguins ( Pygoscelis adeliae) during the early breeding season fast

1. 1. Adélie penguins ( Pygoscelis adeliae) breeding at Cape Crozier, Antarctica, arrive in the colony from the sea in October at the beginning of the austral summer. Both sexes fast while on the breeding colony for 3–6 weeks before returning to the sea to feed. 2. 2. Six individuals collected during the first fast period showed decreased blubber thickness and a linearly corresponding decrease in ether extractable fat with time after arrival in the breeding area. Birds contained about 45 per cent of dry weight as fat upon arrival, and in a typical incubation, males decreased to about 20 per cent after day 27. 3. 3. The decrease in fat represents approximately 56 g of fat used per day by fasting male Adélies for the 27-day period. On the basis of this value, it has been estimated that 490 kcal/bird per day approximates energy demands for these fasting birds during the early part of the breeding season. 4. 4. Fatty acid compositions of total ether extractable lipids, subcutaneous fat and abdominal depot fat did not differ significantly except in a few of the long-chain acids. Depot fat fatty acids of normal breeding birds did not change significantly from arrival on the colony to day 27. 5. 5. Proportions of fatty acids in Adélie penguin depot fat correspond with the proportions of fatty acids in their normal diet of krill ( Euphausia superba).