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The genus Pogonophryne is the most species-rich genus of barbeled plunderfishes (Artedidraconidae) and includes more than 25 poorly known species endemic to the Southern Ocean. In this study, we provide new data on the age and reproductive traits of some species of Pogonophryne from the southern Weddell Sea, inferred through otolith reading and histological analyses of gonads. Individual age estimates ranged between 16 and 18 years for Pogonophryne barsukovi and Pogonophryne immaculata and between 10 and 22 years for Pogonophryne scotti. As is commonly found in notothenioids, P. barsukovi followed a group-synchronous type of ovarian development, with pre-vitellogenic and vitellogenic oocytes forming two well-separated egg-size groups. A single spawning female in the sample produced ~1097 eggs and 7.9 eggs g-1. The sample of P. immaculata consisted exclusively of developing males, with testes composed of cysts of spermatogonia, spermatocytes and spermatids. Pogonophryne scotti was the most abundant species, including relatively small males at immature or developing stages of gonad development. Larger females were regressing, being characterized by ovaries with postovulatory follicles and atretic oocytes. Based on the macroscopic and histological analyses of gonads, the spawning season would take place in autumn for P. barsukovi and P. immaculata and in spring–early summer for P. scotti.
A survey of Antarctic toothfish (Dissostichus mawsoni) was conducted in the northern Ross Sea region during the winter of 2016 to document the timing and location of spawning activity, to collect biological information about reproductive status during the spawning season and to look for temporal signals in biological data from D. mawsoni that may indicate a spawning migration of mature toothfish from the continental slope region to the northern Ross Sea region. The 58 day survey showed that spawning of D. mawsoni began on some seamounts by early July. No changes were detected between winter and summer in length, age, sex ratio or condition factor distributions for D. mawsoni in the northern Ross Sea as hypothesized following a spawning migration from the slope to the northern Ross Sea region. These results suggest that the distribution of D. mawsoni in the Ross Sea is mainly accomplished through ontogenetic migration and not annual return spawning migrations.
The life history traits of bathydraconids, deep-living fishes distributed all around the Antarctic continent, are poorly known. In particular, very few data are available on the relatively rare genera Akarotaxis and Bathydraco. With the aim to fill this gap, sagittal otoliths and gonads were analysed to assess individual age and reproductive features of Akarotaxis nudiceps (Waite, 1916), Bathydraco macrolepis Boulenger 1907 and Bathydraco marri Norman, 1938 collected in the Weddell Sea. Based on the annual growth increment patterns, age estimates ranged between 6–11, 5–11 and 8–11 years for A. nudiceps, B. macrolepis and B. marri, respectively. Most of the gametogenetic processes could be described based on gonad histology for both sexes. Females shared the reproductive features commonly reported in notothenioids, such as group-synchronous ovary development and prolonged gametogenesis. Total fecundity estimates were comparable between the two species of Bathydraco (1500–2500 eggs/female), whereas that of Akarotaxis was one order of magnitude smaller (200–250 eggs/female). Consistently, the mean size of late vitellogenic oocytes showed an opposite trend, being 1.6–1.8 mm in Bathydraco and 2.2 mm in Akarotaxis.
The geographical distribution of the two species of the genus Parachaenichthys is allopatric and restricted to the inner shelves of South Georgia–South Sandwich Islands (P. georgianus) and South Orkney Islands–South Shetland Islands (P. charcoti). To evaluate the consistency between the geographical patterns of adult distribution and early life history traits of P. charcoti, sagittal otoliths were used to estimate growth rate and pelagic duration in larvae and juveniles of this species collected in the Bransfield Strait in winter and summer, respectively. Individual age was determined through microincrement counts, assuming they were daily increments. The Gompertz model was fitted to age–length estimates, providing a mean growth rate of 0.22 mm day-1 estimated for 28–204-day-old individuals. Larval hatching was spread over a relatively wide period, lasting from July throughout September. The pelagic larval duration of P. charcoti was about six months based on ageing data of larvae and juveniles, as reported for P. georgianus from South Georgia. The strong dependence of larvae on the inshore habitat may hamper their dispersal at large spatial scale limiting the connectivity among distant populations, providing clues to interpret the present geographical distribution of the two species.
Early life history traits of the blackfin notothen, Trematomus scotti, were investigated through otolith microincrement pattern and stomach content analyses. Post-larval specimens of 12–20 mm standard length (SL) were collected in the Bransfield Strait and adjacent waters during the 2010–11 summer. Catches were unevenly distributed across the surveyed area, yielding a relative abundance of 0.3–3.6 specimens per 1000 m3 of filtered sea water. Age estimates ranged from 34 to 67 days, with good consistency and no apparent bias between readings. Based on an exponential model fitted to the age-length dataset, the growth rate was 0.17 mm day-1, corresponding to a daily percentage increment in size of 1.07% SL. In agreement with previous studies, larval hatching occurred at a mean size of 9.0 mm and was spread over a relatively short period, lasting from late December to late January. Prey composition consisted exclusively of copepods, mainly larval stages of copepodites. Feeding intensity ranged from 1–14 prey items per stomach, being positively correlated with larval fish size. In summary, T. scotti shares a common early life history strategy with several other notothenioids, consisting of small larvae hatching in summer and overwintering as pelagic early juveniles until the following summer season.
Age, growth and feeding habits of early life stages of Chionodraco rastrospinosus Dewitt & Hureau, the most abundant channichthyid in the larval fish assemblages of the Bransfield Strait, were studied by otolith microincrement counts and stomach content analyses. Individuals measuring 39–69 mm standard length were caught in the uppermost depth strata down to 300 m from Brabant to Joinville islands along the northern Antarctic Peninsula. The sample consisted of post-larvae and juveniles aged 105–211 days, with a mean growth rate of 0.25 mm day-1. Larval size at hatching was estimated to be c. 17.2 mm. Hatching was spread over a relatively long period from August–November. Sagittal otoliths were characterized by a strong check located at 23–52 microincrements of distance from the core, tentatively associated with the onset of first exogenous feeding. The relatively long period during which larvae can rely on yolk reserves and the large size at hatching enable them to utilize a wide size range of prey, as well as cope with occasional food shortages. The stomach contents consisted exclusively of euphausiids (furcilia and adults) and larvae of Pleuragramma antarcticum Boulenger. Based on growth rate, the residence time in pelagic waters of juvenile C. rastrospinosus was estimated to be about a year and a half.
We reviewed photographic images of fishes from depths of 381–2282 m in Marguerite Bay and 405–2007 m in the Amundsen Sea. Marguerite Bay fishes were 33% notothenioids and 67% non-notothenioids. Channichthyids (47%) and nototheniids (44%) were the most abundant notothenioids. The deep-living channichthyid Chionobathyscus dewitti (74%) and the nototheniid genus Trematomus (66%) were the most abundant taxa within these two families. The most abundant non-notothenioids were the macrourid Macrourus whitsoni (72%) and zoarcids (18%). Amundsen Sea fishes were 87% notothenioids and 13% non-notothenioids, the latter exclusively Macrourus whitsoni. Bathydraconids (38%) and artedidraconids (30%) were the most abundant notothenioids. We observed that Macrourus whitsoni was benthopelagic and benthic and infested by large ectoparasitic copepods. Juvenile (42 cm) Dissostichus mawsoni was not neutrally buoyant and resided on the substrate at 1277 m. Lepidonotothen squamifrons was seen near and on nests of eggs in early December. A Pogonophryne sp. from 2127 m was not a member of the deep-living unspotted P. albipinna group. Chionobathyscus dewitti inhabited the water column as well as the substrate. The pelagic zoarcid Melanostigma gelatinosum was documented in the water column a few metres above the substrate. The zoogeographic character of the Marguerite Bay fauna was West Antarctic or low-Antarctic and the Amundsen Sea was East Antarctic or high-Antarctic.
The Tristan klipfish, Bovichtus diacanthus, an endemic species at Tristan da Cunha Island was successfully aged using whole sagittal otoliths. The annulation pattern was clear, resulting in an alternating combination of opaque and translucent zones that form an annulus. Although sampling limitations did not allow direct validation of annual ring deposition, most otoliths showed a translucent edge, perhaps suggesting that the deposition of translucent zones is a synchronous process which takes place in winter coincident with the spawning season. The reliability of ageing methodology was supported by the good agreement between readings. The maximum age was estimated to be five years in females and four years in males. In order to increase the small number of direct readings, the length at age was back-calculated for each fish by fitting growth curves. The resulting growth of B.diacanthus was described by the von Bertalanffy growth model, as summarized by the following parameters: L∞ = 207.1 mm TL, k = 0.49 per year, and t0 = 0.04 years. The size at which 50% of the population spawns for the first time was c. 147 mm, corresponding to 2.5 years of age.
The Antarctic silverfish Pleuragramma antarcticum Boulenger is the dominant fish species in the high Antarctic zone, playing a key role in the Ross Sea midwater shelf ecosystem. Unlike other notothenioids, it is holoplanktonic species, spending its entire life cycle in the water column. Early life stages of P. antarcticum are generally found in the upper 200 m and their spatial distribution is largely affected by water masses and general circulation. To understand better the mechanisms involved in the geographical distribution of the Antarctic silverfish within the western Ross Sea, an analysis of abundance and distribution was carried out in relation to oceanographic conditions. Samples were collected in summer during the 1998, 2000 and 2004 Italian cruises, covering the majority of the western sector of the Ross Sea. Overall 127 stations were sampled using standard plankton nets for biological samples and CTD and XBT to record abiotic parameters. Although all surveys were in December–January, the yearly results differed in terms of relative abundance of larval developmental stages and of oceanographic characteristics. The 1997–98 samples were characterized by very low abundance overall and by the virtual absence of early larvae. In summers 1999–2000 and 2003–04 the abundance of P. antarcticum was one order of magnitude higher than in the earlier season. In 1999–2000 catches were mainly composed of pre-flexion larvae and late postlarvae, while in 2003–04 catches were made up of pre-flexion larvae and juveniles. In January 2000 the Ross Sea summer polynya was fully open as the pack ice was almost completely melted, whereas in January 1998 and 2004 the opening of the polynya was considerably delayed. As a consequence, a delay in phytoplankton blooms and a decrease in primary production were observed in the summer seasons 1998 and 2004 with respect to 2000. The spatial distribution of early life stages, that were confined to the continental shelf and shelf break of the Ross Sea, generally appeared to be positively influenced by transition zones (oceanographic fronts). In addition, most of catches were recorded on or in close proximity to the banks (Pennell, Mawson, Ross and Crary) that characterize the continental shelf of the Ross Sea. On the basis of present findings and literature data, a link between the general circulation in the western Ross Sea and the distribution pattern of the early life stages of P. antarcticum has been developed.
Age and growth of the brown meagre Sciaena umbra (Sciaenidae) collected (n = 532) in the north-western Adriatic Sea was studied by means of transverse otolith sections. The maximum age estimated was 19 and 16 years for males and females, respectively. The length at age estimated for each sex indicated that females attain a larger size and grow slightly faster than males. Both marginal increment analysis and edge analysis confirmed that annuli are formed once a year, with opaque zones laid down in summer (June-July). Age readings were very precise, with a percentage agreement of 98% between readers and low values of the index of average percent error (0.9%) and coefficient of variation (1.3%). Von Bertalanffy growth function was fitted to age-length data, and growth parameters were estimated for males (L∞ = 44.9 cm; k = 0.27,t0 = −2.17 years) and females (L∞ = 47.2 cm; k = 0.28, t0 = -1.82 years). In both sexes of brown meagre, growth rate was high until they attain 2–3 years of age, i.e. at sexual maturity. Most of catches were obtained in late summer-early autumn. On the basis of present data, the sampled population of brown meagre mostly consisted of small fish younger than three years of age (about 90%). In addition, landings of this species in recent years largely decreased. As reported elsewhere in north-western Mediterranean, brown meagre stock in the Adriatic Sea showed clear signs of depletion, thus specific management measures for this species are urgently required.
Trematomus eulepidotus and T. loennbergii are two of the most common epibenthic fish in the waters of the High Antarctic continental shelf. Since the reproductive biology of these species has not been studied in the Ross Sea, we provide a macroscopic and histological analysis of the reproductive effort and gonadal development in both sexes. Most samples were collected during benthic trawl surveys in the south-western Ross Sea in the 1996 and 1997 summer seasons. The aim of the study was to define the reproductive characteristics of these two sympatric species and to examine the hypothesis that different reproductive strategies mitigate interspecific competition. We found that, in common with most Antarctic notothenioids, both species possess a suite of similar reproductive strategies including delayed sexual maturity, prolonged gametogenesis, group-synchronous oocyte maturation, a single spawning event per year and iteroparity. Both species show a comparable reproductive effort in terms of potential fecundity with between 2000 and 20 000 eggs per female per season. Nevertheless, the two species exhibited a considerable difference in the timing of the breeding season, spawning in summer (T. eulepidotus) and in autumn (T. loennbergii). This gives rise to a mismatch in the time of appearance of larvae in the environment and probably leads to reduced competition.
We analysed histological characteristics of gonads and reproductive effort of the small deep-living dragonfishes Akarotaxis nudiceps (Waite) and Bathydraco marri Norman collected in the south-western Ross Sea. From a macroscopic point of view, most specimens of B. marri were juveniles in early stages of gonad maturity, except for a maturing female. Conversely, the sample of A. nudiceps was composed of both immature and adult fish in different stages of maturity. A single A. nudiceps female was mature with a gonadosomatic index of 9.8%. Its absolute and relative fecundity was 260 oocytes and 31.5 oocytes g−1 TW, respectively, with a mean size of ripe oocytes of 1.9 mm. Gametogenesis in both species closely resembled that observed in other notothenioids, with females possessing two well-defined groups of oocytes. One group consisted of previtellogenic oocytes as a reserve stock while the other group was maturing oocytes to be ovulated in the current spawning season. A distinctive feature of oogenesis in recovering and maturing females of A. nudiceps was the presence of both postovulatory follicles in different stages of reabsorption and atretic oocytes. Based on low absolute fecundity, it is possible that A. nudiceps provides parental care and egg guarding.
The Antarctic plunderfishes Artedidraco lönnbergi and A. skottsbergi are small, bottom dwelling species inhabiting the continental shelf of the High Antarctic Zone. During cruise 97–9 of the US RV Nathaniel Palmer during the summer in the south-western Ross Sea, samples of both species were collected by means of bottom trawling. On the basis of macroscopic and histological analysis, we present the first data on the reproductive characteristics of these two plunderfishes, including gametogenesis, spawning period and absolute fecundity. Histologically, we found immature (stage I and II) and mature (stage V) females in both species, whereas developing females (stage III) were found only in A. skottsbergi. All examined male specimens of A. skottsbergi were in the final stage of spermatogenesis (stage III), whereas male A. lönnbergi were immature (stage I), mature (stage IV) and post-reproductive (stage V) individuals. In both species, spawning takes place in summer during December and January. Absolute fecundity was very low, with less than 100 and 200 oocytes in A. lönnbergi and A. skottsbergi, respectively. These data are compared with those reported in literature for other artedidraconids.
The nototheniid Pleuragramma antarcticum (Boulenger, 1902), is the dominant pelagic fish in waters of the continental shelf in High Antarctic regions. Larvae and juveniles of this species comprise the majority of ichthyoplankton at many locations around Antarctica including the Weddell Sea and the western Ross Sea, where it may amount to 98% of the ichthyoplankton. Its life cycle has been the subject of a number of studies but spawning and embryological development are still uncertain. Eggs with embryos and newly hatched larvae of P. antarcticum were collected in November 2002 near the Italian Antarctic station at Terra Nova Bay through holes drilled in the sea ice. Eggs and yolk-sac larvae were floating among the platelet ice below the solid cap of congelation ice. Eggs were 2.2–2.5 mm in diameter and contained embryos at an advanced stage of development. Hatching occurred from mid-November onwards, and newly hatched larvae averaged 9.3 mm SL. This paper provides the detailed description of embryos and newly hatched larvae in terms of pigmentation pattern and morphometric characteristics, thus allowing a significant advance in our understanding of the early life history of P. antarcticum in the Ross Sea, and extending the knowledge of the life cycle of this key Antarctic species.
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