Underwater video observation of the Antarctic toothfish Dissostichus mawsoni (Perciformes: Nototheniidae) in the Ross Sea, Antarctica

A towed benthic camera system captured images of a ca. 66-cm-long Antarctic toothfish partially under a boulder at 76°30'S, 174°59'E (depth 454 m) in the southern Ross Sea. This is noteworthy because, excluding McMurdo Sound, there are only two published records for this species in the Ross Sea and none in an offshore locality. Adult Dissostichus mawsoni are neutrally buoyant and live and feed in the lower reaches of the water column. Benthic perching is unexpected, suggesting that this subadult is not neutrally buoyant.     

Age estimation and lead�Cradium dating of Antarctic toothfish (Dissostichus mawsoni) in the Ross Sea

Antarctic toothfish (Dissostichus mawsoni) are the target of an important commercial fishery in the Southern Ocean, yet age data used for management have not been comprehensively tested for accuracy. In this study, Antarctic toothfish were aged using counts of otolith growth zones based on criteria established for Patagonian toothfish, D. eleginoides, a closely related species. To validate these ages, the radioactive disequilibrium of lead-210 and radium-226 in otolith cores was measured and used as an independent chronometer to accurately determine age across the range of fish caught in large numbers by the fishery. Growth-zone counts indicated Antarctic toothfish live to at least 39 years of age, and were in close agreement with the chronometer, validating the age estimation criteria and the accuracy of age estimates. Von Bertalanffy growth function parameters indicated Antarctic toothfish were relatively slow-growing (k = 0.111), especially in relation to their maximum size (L _∞ = 158.9 cm).     

Assembly of the antifreeze glycoprotein/trypsinogen-like protease genomic locus in the Antarctic toothfish Dissostichus mawsoni (Norman)

To investigate the genomic architecture underlying the quintessential adaptive phenotype, antifreeze glycoprotein (AFGP) that enables Antarctic notothenioid survival in the frigid Southern Ocean, we isolated the AFGP genomic locus from a bacterial artificial chromosome library for Dissostichus mawsoni. Through extensive shotgun sequencing of pertinent clones and sequence assembly verifications, we reconstructed the highly repetitive AFGP genomic locus. The locus comprises two haplotypes of different lengths (363.6 kbp and 467.4 kbp) containing tandem AFGP, two TLP (trypsinogen-like protease), and surprisingly three chimeric AFGP/TLP, one of which was previously hypothesized to be a TLP-to-AFGP evolutionary intermediate. The ~ 100 kbp haplotype length variation results from different AFGP copy number, suggesting substantial dynamism existed in the evolutionary history of the AFGP gene family. This study provided the data for fine resolution sequence analyses that would yield insight into the molecular mechanisms of notothenioid AFGP gene family evolution driven by Southern Ocean glaciation.     

The diet of toothfish species Dissostichus eleginoides and Dissostichus mawsoni with overlapping distributions

The diets of Antarctic toothfish Dissostichus mawsoni and Patagonian toothfish Dissostichus eleginoides were examined around the South Sandwich Islands in the Southern Ocean, one of few regions with overlapping populations of the two species. Despite large differences in the proportion of stomachs containing prey (76·2% of D. mawsoni compared to 7·2% of D. eleginoides), diet composition was broadly similar (Schoener overlap index of 74·4% based on prey mass) with finfishes (particularly macrourids and muraenolepidids) and cephalopods (mainly Kondakovia longimana) comprising >90% of the prey mass of both species. Predation rates of the main fish prey, as mean counts per stomach sampled, were spatially correlated with their relative abundance around the islands derived from fishery by-catch data, suggesting a general lack of prey selectivity. This study supports the view that bathyal Dissostichus are opportunistic carnivores and finds that D. mawsoni and D. eleginoides occupy a similar trophic niche and are likely to compete for prey in regions where both are distributed. The large increase in rate of prey occurrence and size of prey in D. mawsoni stomachs relative to D. eleginoides suggests, however, species differences in feeding behaviour, which may reflect the increased metabolic demands of a cold-water adapted physiology. [Correction added after online publication 13 June 2011: spelling of species name corrected].     

The Antarctic toothfish (Dissostichus mawsoni) lacks plasma albumin and utilises high density lipoprotein as its major palmitate binding protein

Plasma from the Antarctic toothfish, Dissostichus mawsoni, a member of the advanced teleost Nototheniidae family, was analysed. Agarose gel electrophoresis showed a major diffuse anionic protein that bound [14C]palmitic acid but not 63Ni2+, and two more cationic proteins that bound 63Ni2+ but not palmitate. Oil Red O staining following cellulose acetate electrophoresis indicated that the palmitate binding protein was a lipoprotein. Two-dimensional electrophoresis showed that this palmitate binding band was composed of three proteins with M(r) of 11, 30, and 42 kDa, without any trace of material at approximately 65 kDa, the mass of albumin. N-terminal sequencing of the palmitate binding band gave a major sequence of DAAQPSQELR-, indicating a high degree of homology to apolipoprotein A-I (apo-AI), the major apolipoprotein of high density lipoprotein (HDL). N-terminal sequencing of the major nickel binding band produced a sequence with no homology to albumin. When ultracentrifugation was used to isolate the lipoproteins from Antarctic toothfish plasma, the palmitate binding protein was found solely in the lipoprotein fraction. In competitive binding experiments, added human albumin did not prevent palmitate binding to toothfish HDL. In conclusion, there is no evidence for albumin in Antarctic toothfish plasma and HDL assumes the role of fatty acid transport.     

Aspects of body size and gonadal histology in the Antarctic toothfish, Dissostichus mawsoni, from McMurdo Sound, Antarctica

Twenty-eight specimens of the large notothenioid Dissostichus mawsoni were dissected after capture on a set line near the southern limit of its range in McMurdo Sound, Antarctica. Total length (L _T) averaged 127.3 cm (range 90–162 cm) and weight (W ) was 26.7 kg (range 10.4–60.3 kg). The length-weight relationship was W=3.44×10⁻⁵ L _T ^2.85 (n=28, r ²=0.96). Subcutaneous lipid thickness averaged 2.6 mm and showed no difference due to sex, but a significant weak relationship to W and L _T. Hepatosomatic index (I _H) was 1.6% for females and 1.7% for males; gonadosomatic index (I _G) was 0.9% for females and 0.2% for males. Although specimens were large and sexually mature, the histology indicated that the gonads of this November sample were in a resting stage. Testes lacked spermatids and spermatozoa. Oocytes were in the previtellogenic stage (91.2%) or in the first stage of vitellogenesis (8.8%). A few atretic oocytes and empty follicles indicated that some females in this sample had spawned previously. A summary of the life-cycle is also presented. Accepted: 27 September 1999     

Ontogenetic shift in buoyancy and habitat in the Antarctic toothfish, <Emphasis Type="Italic">Dissostichus mawsoni</Emphasis> (Perciformes: Nototheniidae)

Buoyancy measurements and depth of capture were taken on 70 individuals of Dissostichus mawsoni collected from the Southern Scotia Arc and McMurdo Sound, Antarctica, to examine the effect of age on buoyancy and habitat use. Standard lengths (SL) ranged from 10.4 to 138.0 cm. Juveniles were not buoyant (heavy in water), whereas adults were neutrally buoyant. The slope of the relationship between buoyancy and SL was significantly negative for juveniles (individuals less than 81 cm SL), but there was no significant relationship for adults (individuals greater than 81 cm SL). These results demonstrate an ontogenetic shift in buoyancy. For juveniles, depth of capture and SL had a significantly positive relationship. As individuals reach adulthood they achieve neutral buoyancy and appear to use deeper water habitats. This interpretation is supported by a significant positive correlation between buoyancy and depth of capture for juveniles. Changes in buoyancy with maturation of juveniles may also be associated with a shift in habitat use. Juveniles appear to exploit benthic habitats, whereas adults use the entire water column over deeper water. Given the differences in prey species available in these habitats and based on our results, we predict that diets of juveniles and adults should differ significantly. We also hypothesize that accumulation of lipid deposits from the diet during maturation of juveniles may account for the ontogenetic shift in buoyancy and allow neutral buoyancy to be achieved in adulthood.     

Isolation and characterization of insulin from the Brockmann body of Dissostichus mawsoni, an Antarctic teleost fish.

The Brockmann body of fish synthesizes and secretes insulin. The Brockmann body of Antarctic fish has been described anatomically and shown to contain insulin immunoreactive sites, however, the primary structure of an Antarctic fish insulin has yet to be reported. Insulin was isolated from the Brockmann bodies of the Antarctic perciform teleost, Dissostichus mawsoni. The peptide was purified to homogeneity by gel filtration and reversed-phase HPLC. Insulin-containing fractions were identified by radioimmunoassay using antisera raised against porcine insulin. Electrospray ionization-mass spectrometry determined the mass of the isolated product to be 5725.27 a.m.u. The amino acid composition and primary structure were determined for the pyridylethylated A- and B-chains. The amino acid sequences of the A chain and B chain were H-Gly-lle-Val-Glu-Gln-Cys-Cys-His-Gln-Pro10-Cys-Asn-Ile-Phe- Asp-Leu-Gln-Asn-Tyr-Cys20-Asn-OH and H-Ala-Pro-Gly-Pro-GIn-His-Leu-Cys-Gly-Ser10-His-Leu-Val-Asp-Ala-Le u-Tyr-Leu-Val-Cys20-Gly-Glu-Arg-Gly-Phe-Phe-Tyr-Asn-Pro-Lys30++ +-OH, respectively. The primary structure of insulin from Antarctic fish is compared with known structures of insulin from other vertebrates.     

Ontogeny of vision in the Antarctic fish Pagothenia borchgrevinki (Nototheniidae)

Summary Pagothenia borchgrevinki ranging in size from 63 to 245 mm were captured from beneath sea ice in McMurdo Sound by fishing and diver collection. Changes in ocular morphology with increasing body size were measured, and assessed in relation to ingested prey. Relative eye size was highest among smaller fish (<100mm total length), and declined with increasing fish size. This was accompanied by a decrease in cone density in the retina from a maximum of 14,200 mm^–2 in the smallest fish examined (63 mm), to 1000 mm^–2 in a 220 mm long fish. Theoretical acuity was lowest among fish at either end of the size range examined (minimum separable angle 40–50) but approximately constant over the remainder of the size range (25–40). Rod density also decreased with increasing body size but rod numbers per unit visual arc were relatively constant, except in the smallest fish, where angular rod density was low. The same prey taxa occurred in fish of all sizes; however, prey items smaller than about 1.5 mm were not taken by fish of any body size.     

Genetic markers for identification of Patagonian toothfish and Antarctic toothfish

Fillet samples of the toothfish Dissostichus eleginoides and D. mawsoni can be distinguished readily by muscle proteins revealed by isoelectric focusing and mitochondrial DNA markers. The proteins also distinguish toothfish from other species marketed under similar trade names.     

Interannual variability in reproductive traits of the Patagonian toothfish Dissostichus eleginoides around the sub‐Antarctic island of South Georgia

Commercial fisheries data, collected as part of an observer programme and covering the period 1997–2014, were utilized in order to define key reproductive traits and spawning dynamics of the Patagonian toothfish Dissostichus eleginoides at South Georgia. Multi‐year spawning site fidelity of D. eleginoides was revealed through the identification of previously unknown spawning hotspots. Timing of female spawning was shown to have shifted later, leading to a shorter spawning duration. A decrease in length and mass of female and male spawning fish and a reduced number of large spawning fish was found, evidence of a change in size structure of spawning D. eleginoides. During the study period fewer later maturity stage females (including spawning stage) were observed in conjunction with increased proportions of early stage female D. eleginoides. The findings are discussed in the context of reproductive success, with consideration of the possible effects such spawning characteristics and behaviours may have on egg and larval survival. This work presents the first long‐term assessment of D. eleginoides spawning dynamics at South Georgia and provides valuable knowledge for both the ecology of the species and for future fisheries management of this commercially important species.     

Distribution of cephalopods recorded in the diet of the Patagonian toothfish (Dissostichus eleginoides) around South Georgia

The cephalopod component of the diet of Patagonian toothfish, Dissostichus eleginoides, around South Georgia was analysed from stomach contents collected between March and May 2000. Cephalopods occurred in 7% of D. eleginoides stomachs. A total of 363 cephalopod beaks were found, comprising 16 cephalopod species, of which 15 had not been previously recorded in the diet. Octopodid A (probably Pareledone turqueti) was the most important cephalopod species by number of lower beaks (36 beaks; 20.2% of the lower beaks) and Kondakovia longimana was the most important in terms of estimated mass (76% of the cephalopod component). When the cephalopod component of D. eleginoides was compared with other predators between March and May 2000, D. eleginoides fed more on octopods (25% of the lower beaks) than black-browed and grey-headed albatrosses (<1% of the lower beaks). The low frequency of the squid Martialia hyadesi in the diet of D. eleginoides around South Georgia was also noticed in the diet of albatrosses, and suggests that M. hyadesi was not present in these waters in 2000 (probably due to migratory movements or reproduction failure), despite being a candidate for commercial exploitation. The presence of the squid Mesonychoteuthis hamiltoni in the diet of D. eleginoides and being caught by a longline hook whilst presumably feeding on D. eleginoides, may indicate that juveniles of Mesonychoteuthis hamiltoni are prey of D. eleginoides adults, and when Mesonychoteuthis hamiltoni reach a large size as adults, they become the predator.     

Calorimetric analysis of antifreeze glycoproteins of the polar fish, Dissostichus mawsoni

Solutions of antifreeze glycoproteins 1 through 5 and 8 were analyzed for activity by differential scanning calorimetry. With a scan rate of 1 degree C min-1, antifreeze glycoproteins 1-5 (20 mg/ml) revealed antifreeze activity with a delay in the freeze exotherm during cooling in the presence of ice. Antifreeze glycoprotein 8 (60 mg/ml), however, did not reveal antifreeze activity. When a 0.1 degree C min-1 scan rate was used, glycoproteins 1-5 again yielded a delay in the freeze onset, but the exotherm consisted of multiple events. At the slower scan glycoprotein 8 revealed an initial freeze followed by multiple exothermic events resembling those of glycoproteins 1-5. Thermograms exhibiting antifreeze activity had an initial shoulder in the exotherm direction upon cooling followed by a delay before the exotherm. The shoulders were correlated with c-axis ice growth observed in visual methods. The glycoprotein antifreezes had a linear increase in activity with decreased ice content.     

Dietary composition of juvenile Dissostichus eleginoides (Pisces, Nototheniidae) around Shag Rocks and South Georgia, Antarctica

A diet analysis of the Patagonian toothfish Dissostichus eleginoides, trawled in the South Georgia Islands area in March–April 1996, was carried out by frequency of occurrence (F%) and coefficient Q (%) methods. The samples consisted chiefly of immature specimens, with predominant length ranges of 30–70 cm (TL). Fish was by far the main food on the shelves of Shag Rocks and South Georgia, accounting for about 70% of prey. Krill appeared as secondary food, although its importance was overestimated by the frequency of occurrence method. Cephalopods and mysids were infrequent in the stomachs, and only at Shag Rocks and South Georgia, respectively. Lepidonotothen kempi, Champsocephalus gunnari and Chaenocephalus aceratus constituted the main fish prey and their variability between Shag Rocks and South Georgia depended on their local abundance. The large proportion of fish exhibiting stomachs full or close to fullness (together 62%) suggests that feeding intensity of the species was high.     

Spatio‐temporal dynamics in maturation and spawning of Patagonian toothfish Dissostichus eleginoides on the sub‐Antarctic Kerguelen Plateau

This study investigated maturation and spawning of Patagonian toothfish Dissostichus eleginoides in the Heard Island and McDonald Islands (HIMI) fishery on the Kerguelen Plateau in the Indian Sector of the Southern Ocean based on gonads and otoliths collected between 2004 and 2015 and using histological analyses and calibration of macroscopic staging criteria. Dissostichus eleginoides at HIMI spawn throughout the austral late autumn and winter months of May–August and spawning activity is concentrated on slopes along the west and south of the plateau around HIMI at depths of 1500–1900 m. Comparison between histological analyses and macroscopic gonad staging indicated that many fish that had spawned, as indicated by the presence of post‐ovulatory follicles, returned to a resting stage which was macroscopically indistinguishable from maturing fish. Furthermore, the occurrence of females of all size classes with low gonado‐somatic index and low macroscopic gonad stage during the spawning season suggested that a proportion of mature females did not spawn every year. Age‐at‐maturity estimates, based on the assumption that fish of macroscopic stages ≥2 were mature, decreased between the 2004–2009 and 2010–2015 periods for both sexes. The magnitude of this temporal variation in age at maturity, however, varied between gear types and fishing depths and variable sampling regimes probably influenced these variations. This study highlights the importance of correct interpretation of macroscopic gonad stages and understanding the influence of fishery operations on estimations of life‐history parameters.