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Krill (Euphausia superba) catch is currently the most relevant fishery industry in Antarctic waters. This resource is a keystone species in the Antarctic food web, sustaining the contribution to the trophic ecology of many invertebrate and vertebrate species. To catch krill, part of the fleet in this fishery uses large mid-water nets that also retain a diversity of other organisms like plankton, meroplankton, and fish species as bycatch. Therefore, it is necessary to understand and evaluate the magnitude of this incidental catch, as well as the potential interactions between krill fishing gear with seabirds and mammals. To estimate the composition and extent of bycatch for this fishery included 784 samples of 25 kg and an equal number of 1 kg sub-samples obtained from Antarctic krill catches in Subarea 48, between years 2012 and 2016. A total of 15 fish species were identified along with the record of five other taxa and other unidentified specimens. The most relevant fish species bycaught by weight were mackerel icefish Champsocephalus gunnari, South Georgia icefish Pseudochaenichthys georgianus, and painted notie Lepidonotothen larseni. Additionally, 20 interactions with seabirds and nine interactions with Antarctic fur seals (Arctocephalus gazella) were registered. In the five years of operations, only three seabirds died, and only two individuals of A. gazelle caught by the net were killed.

Knowledge of the essential and nonessential elements distribution and behavior in Antarctic fish is important for understanding the essentiality and requirements of minerals in the diet of those species, as well as the bioavailability and storage of trace minerals in the tissues of fishes. In this study, the levels of zinc, copper, cadmium, and lead in muscle, intestine, liver, gill, and skin of mackerel icefish Champsocephalus gunnari (Lonnberg, 1905) in South Georgia were determined during winter. The following element concentration ranking was recorded: Zn > Cu > Cd > Pb in the muscle, Zn > Cu > Cd > Pb in the skin, Zn > Cu > Cd > Pb in the intestine, Zn > Cu > Pb > Cd in liver, and Zn > Cu > Cd > Pb in the gill. The concentration of Zn is higher by about two orders of magnitude than that of the other three elements for the tissues. Generally, apart from Cu levels in the liver and Cd levels in the intestine, the correlation of elements in tissues with both size and weight of C. gunnari is not observed. The levels of elements were compared to those reported for C. gunnari in the Kerguelen waters.

International audience; Diet analyses can reveal important changes in seabird foraging ecology and, by inference, resource availability and predator–prey dynamics within the wider marine ecosystem. Here, we analysed stomach contents of 1544 grey-headed albatross Thalassarche chrysostoma (GHA) and black-browed albatross T. melanophris (BBA) chicks from Bird Island, South Georgia. We describe dietary shifts (1996–2017), and link those to annual prey availability indices and breeding success. Annual variability in diet was high, and long-term trends in the main components were broadly similar in both albatrosses. Fish consumption (by mass) generally increased over time. Mackerel icefish Champsocephalus gunnari occurrence increased in GHA diets, but was unrelated to local densities derived from fisheries/research cruises. Cephalopod consumption declined until the early 2000s, then plateaued, and the occurrence of the ommastrephid squid Martialia hyadesi declined over time in both albatrosses. In BBAs, Antarctic krill Euphausia superba consumption decreased over time. Conversely, Antarctic krill consumption by GHAs increased until the early 2000s, decreased until the mid-2010s, and increased again in 2017. Antarctic krill consumption was unrelated to local densities based on acoustic surveys, and did not correlate with breeding success. Remotely sensed chlorophyll-a within core foraging areas showed a positive relationship with Antarctic krill in GHA diets, but a negative relationship with M. hyadesi occurrence in both albatross diets. Dietary shifts had consequences for GHA breeding success, which was negatively related to the importance of the cranchiid Galiteuthis glacialis and positively related to M. hyadesi importance. These results highlight the complex mechanisms linking prey availability, diet and breeding success in albatrosses.

Three species of skate, Bathyraja eatonii , B. irrasa and B. murrayi, are commonly taken as incidental by-catch in Patagonian toothfish ( Dissostichus eleginoides ) longline and trawl fisheries, and the mackerel icefish ( Champsocephalus gunnari ) trawl fishery on the Kerguelen Plateau (KP) in the southern Indian Ocean. Data from fishery observations for 1997–2014 shows that the three skates were widely distributed across the Kerguelen Plateau, showing different spatial distributions, linked mainly with depth. Off Heard Island and McDonald Islands (HIMI), in the southern part of the KP, B. eatonii and B. irrasa were most abundant to the north and northwest of Heard Island, out to the edge of the Australian Exclusive Economic Zone (EEZ), and were caught down to depths of 1790 m and 2059 m respectively. The smallest species, B. murrayi , occurred mainly in the shallower waters down to 550 m, and was most abundant to the north and northeast, close to Heard Island. Around Kerguelen Islands, in the northern part of the KP, skates were most abundant between the 500 m and 1000 m contours circling and extending from the islands. Catch rates were modelled using zero-inflated GAMs and GLMs. The catch rates of skates from the trawl fisheries in the Australian EEZ surrounding Heard Island and McDonald Islands have shown little evidence of depletion on the main trawl fishing grounds, although there is evidence of a decrease in the average total length of B. eatonii . The marine reserves and the conservation measures employed by the Commission for the Conservation of Antarctic Marine Living Resources in the HIMI fisheries, appear to provide effective protection for the skates, at least in the shallower waters where the trawl fisheries operate. B. irrasa taken in the deeper waters where longline fishing occurs have shown a slight decline in catch rate over the years of the HIMI fishery. Although all skates are returned to the water from this fishery, survival rates are unknown and careful monitoring should continue to assess the status of these stocks. There appears to be little change in the abundance of the skate species at Kerguelen in the time period. This study provides the first review of skate by-catch across both the HIMI and Kerguelen fisheries. Ongoing monitoring of species specific by-catch levels and further research to determine the important life history parameters of these species are required, particularly for B. irrasa which is taken in both trawl and longline fisheries.

Myctophids are the most abundant and diverse mesopelagic fishes in the Southern Ocean. They are a conduit of energy between primary consumers and higher marine predators, and between the surface layers and the mesopelagic depths. However, there remain major uncertainties about their ecology, particularly regarding their role in Southern Ocean food webs in waters south of the Antarctic Polar Front, which are often regarded as dominated by Antarctic krill. Limited data on the feeding ecology of myctophids has made it difficult to assess the importance of myctophids as consumers of krill and how they fit in the traditional view of a krill-dominated system (diatom-krill-higher predator). We provide a new assessment of the role of myctophids in Southern Ocean food webs using information from recent trophodynamic studies of myctophids conducted in the Scotia Sea, one of the most productive regions of the Southern Ocean and a region that sustains both major populations of higher predators (sea birds, seals, whales) and important commercial fisheries (krill, toothfish, and mackerel icefish). Collectively, these data show that myctophids have a central role in Southern Ocean food webs as both predators and prey. Large myctophid species are prevalent consumers of krill throughout their distributional range and in different seasons in the Scotia Sea. Moreover, best estimates of both myctophid and higher predator consumption of krill to date indicate that large myctophids are the greatest predators of krill in this region, consuming almost as much krill as do all other vertebrates. Nevertheless, there are several smaller myctophid species that do not eat krill, instead consuming copepods and other small euphausiids. Myctophids therefore link primary producers to higher predators through both krill-dependent and krill-independent trophic pathways, emphasizing their importance in regional food webs. Consequently, trophic pathways involving large myctophids are unlikely to be exempt from the direct consequences of projected redistribution and/or reduction in krill population biomass under scenarios of ocean-warming, whilst trophic pathways involving small myctophids may be more resilient to such effects and become increasingly important to higher trophic levels. Further studies are required to determine the extent to which myctophids can maintain food web stability and sustain higher predator populations during periods of prolonged reductions in krill abundance. Including knowledge of the role of myctophids in Southern Ocean food webs will be important for developing robust projections of the impacts of future change to inform decision making for conservation and management.

Target identification remains a challenge for acoustic surveys of marine fauna. Antarctic krill, Euphausia superba, are typically identified through a combination of expert scrutiny of echograms and analysis of differences in mean volume backscattering strengths (SV; dB re 1 m−1) measured at two or more echosounder frequencies. For commonly used frequencies, however, the differences for krill are similar to those for many co-occurring fish species that do not possess swimbladders. At South Georgia, South Atlantic, one species in particular, mackerel icefish, Champsocephalus gunnari, forms pelagic aggregations, which can be difficult to distinguish acoustically from large krill layers. Mackerel icefish are currently surveyed using bottom-trawls, but the resultant estimates of abundance may be biased because of the species' semi-pelagic distribution. An acoustic estimate of the pelagic component of the population could indicate the magnitude of this bias, but first a reliable target identification method is required. To address this, random forests (RFs) were generated using acoustic and net sample data collected during surveys. The final RF classified as krill, icefish, and mixed aggregations of weak scattering fish species with an overall estimated accuracy of 95%. Minimum SV, mean aggregation depth (m), mean distance from the seabed (m), and geographic positional data were most important to the accuracy of the RF. Time-of-day and the difference between SV at 120 kHz (SV 120) and that at 38 kHz (SV 38) were also important. The RF classification resulted in significantly higher estimates of backscatter apportioned to krill when compared with widely applied identification methods based on fixed and variable ranges of SV 120–SV 38. These results suggest that krill density is underestimated when those SV-differencing methods are used for target identification. RFs are an objective means for target identification and could enhance the utility of incidentally collected acoustic data.

Annual phytoplankton blooms on the northern region of the Kerguelen Plateau fuel a productive food web that supports highly valuable commercial fisheries for Patagonian toothfish (Dissostichus eleginoides) and mackerel icefish (Champsocephalus gunnari). The food web on the plateau is understudied in comparison to other regions of the Southern Ocean. Major linkages and energy pathways have not been explored, and the combined effects of fishing and a changing climate on the ecosystem are largely unknown. Single species studies on the plateau have shown that the combined effects of climate change and fisheries are impacting populations, however, it is unclear how these impacts translate to the ecosystem. We extended an existing Ecopath model to describe food web dynamics on the plateau and investigate food web interactions with the fishery. Results from our model highlight, for the first time, the properties of the food web, major energy pathways and energy transfers between trophic levels. Energy transfer from detritus was most efficient at the lowest trophic level while energy from primary production was more efficient at higher trophic levels. Consumption and respiration were high in our system, most likely due to the inclusion of bacteria and microzooplankton. Killer whales, cephalopods and myctophids were key functional groups for energy transfer in the system. These groups were relatively data poor, suggesting a useful focus area for future updates to the model. Patagonian toothfish and mackerel icefish were heavily consumed in the food web, however, the inclusion of fisheries catches and by-catch had little to no impact on food web dynamics.

Mackerel icefish (Champsocephalus gunnari) is a semi-pelagic finfish species inhabiting shelf areas in the Southern Ocean. The population at South Georgia is currently exploited by pelagic trawlers fishing close to the seabed. Annual catches peaked at 150,000 t in 1983 and have declined since the mid-to-late 1980s. Bottom-trawl surveys have been conducted since 1987, providing a time series of abundance and size distribution for use in assessing the status of the stock and setting quotas. Food web models suggest that estimates of the biomass from survey data are substantially lower than the amount of icefish required by the local ecosystem. The aim of this study was to assess the uncertainty around current estimates of density and variance, using alternative nonparametric stratified bootstrapping methods. The stratified rescaling bootstrap estimator was identified as the most appropriate method of those tested: in comparison with the existing method, confidence intervals and the inter-annual variability of the estimates were reduced. Numbers-at-age were estimated from mixture distribution models fitted to length-disaggregated density data in order to determine whether individual cohorts were consistently detected by the surveys. Estimates of numbers-at-age could not consistently delineate cohorts in successive years indicating that survey-based estimates of density were biased. These biases may have arisen because the trawl gear did not select individuals of all sizes equally, or because sampling was restricted to the demersal component of the stock. Estimates of abundance of the pelagic component of the stock should be derived from acoustic data to improve the assessment.

This paper submits the results of the acoustic cruise made in the austral summer of 2016. The survey delivers the bottom bathymetry, the geographic distribution of krill concentrations, and the presence of fish shoals in 1,500 nautical miles over the shelf and the continental slope (100–500 m), around Elephant Island and South Orkney Islands. Krill concentrations are described, as well as those of mackerel icefish (Champsocephalus gunnari), the latter representing the most abundant fish species in the research, besides other finfish species in the area. The mackerel iceflsh was found between 160 and 449 m depth, associated to steep bottom, showing demersal characteristics during the day and semi-pelagic overnight. During the darkness hours, the mean determined depth in the schools was 207.1 ± 62.6 m around Elephant Island and 219.4 ± 55.8 m in South Orkney Islands.

In order to allocate quotas for sustainable harvests, that account for climate warming, it is important to incorporate species vulnerabilities that will underlie likely changes in population dynamics. Hotspots, regions with rapidly changing climate, are important locations for rapid advances in mechanistic understanding of the factors driving these changes, particularly if they coincide with regions with a high incidence of range limits, such as the sub-Antarctic Island of South Georgia. This archipelago is at the Northern limit of the Southern Ocean and therefore the northern distribution limit for many Southern Ocean shallow water marine species, which are amongst the most sensitive fauna to increasing temperature. At range limits species may either be living close to their physiological limits, or they may have more resistant phenotypes. In case studies, the northern range limit population of the gastropod limpet, Nacella concinna, has greater physiological plasticity at South Georgia than those from further south, allowing them to cope better with the warmer and more variable seasonal temperatures. Bivalve species, however, alter their depth distributions at South Georgia, to avoid the warmer water masses, indicating that they may not be able to cope with the warmer temperatures. Mackerel icefish, Champsocephalus gunnari, has a unique Antarctic trait, the loss of haemoglobin. A combination of temperature driven change in food web structure, and this extreme physiological cold adaptation, may explain why rapid warming at its northern range limit of South Georgia, has prevented stocks fully recovering from over fishing in the 1980s, despite highly conservative management strategies.

Waters around South Georgia are amongst the most productive in the Southern Ocean, and support internationally important fisheries. However, there is significant inter-annual variability in fish stocks, and some species have failed to recover from historical overfishing. Dispersal and retention of the planktonic eggs and larvae of marine fish can play a key role in the maintenance of adult stocks. We use a numerical modelling approach to examine the influence of oceanographic and life-history variability on the dispersal and retention of 2 Antarctic fishes: Champsocephalus gunnari (mackerel icefish) and Notothenia rossii (marbled rockcod). Mean retention of N. rossii larvae was predicted to be 5.3%, considerably lower than that of C. gunnari (31.3%), a difference related to the longer planktonic period of the former. Such apparent loss of larvae from local recruitment grounds may contribute to the failure of the N. rossii population to recover from its collapse in the 1970s. However, retention of both species showed high inter-annual variability. Dispersal and retention of C. gunnari were strongly influenced by location of the spawning site, with the greatest contribution to overall retention from spawning sites on the southwest South Georgia shelf. In addition, a consistent feature in C. gunnari was a lack of larval exchange between the proximate South Georgia and Shag Rocks shelves, regions separated by only 240 km. Our findings provide insights into the demographic dynamics and connectivity of C. gunnari and N. rossii populations at South Georgia in relation to prospects for recovery and ongoing responses to environmental variability and change in the region.

Fatty acid analysis was used to study the trophic ecology of 10 demersal fish species in the South Georgia region. Principal component analysis grouped the species into three general clusters, revealing resource partitioning between species. Two groups were characterised by large proportions of either monounsaturated or polyunsaturated fatty acids, separating species according to their predominant feeding habitat. The third group showed fatty acid signatures overlapping with either or both of the previous two groups, suggesting a more opportunistic feeding behaviour for these species. Intraspecific comparisons furthermore revealed dietary variability with size, year and geographical location in several species. Mackerel icefish (Champsocephalus gunnari) in particular showed inter-annual differences in muscle lipid concentrations closely linked to prey availability with low lipid contents found in years of low krill (Euphausia superba) abundance. Despite the intraspecific differences the majority of species could be easily distinguished from each other, which indicates the utility of this method in the dietary analysis of higher predators.

The mackerel icefish (Champsocephalus gunnari Lonnberg, 1905) is a ray-finned fish living in the Southern Ocean around Antarctica. We sequenced the complete mitochondrial (mt) genome of the mackerel icefish and a segment from cytochrome b to the control region (CR) in 32 individuals. The mt genome of the mackerel icefish was rearranged, containing two nicotinamide adenine dinucleotide (reduced form) dehydrogenase subunit 6 (ND6), two tRNAGlu, and two CRs. However, variations in numbers of ND6 and tRNAGlu were observed amongst individuals. These variations included type 1 (containing two ND6 and two tRNAGlu), type 2 (containing one ND6, one incomplete ND6, and one tRNAGlu), and type 3 (containing one ND6 and one tRNAGlu). The gene orders of types 1 and 2, and variations in numbers of ND6 and tRNAGlu were not previously found in any Antarctic notothenioids, whereas type 3 is the same as that of Racovitzia glacialis. Phylogenetic analyses of CR DNA sequences showed that duplicated CRs of the same species formed a monophyletic group, suggesting that duplication of CRs occurred in each species. The frequent duplication of mt genomes in Antarctic notothenioids is an unusual feature in vertebrates. We propose that interspecific hybridization and impairment of mismatch repair might account for the high frequency of gene duplications and rearrangement of mt genomes in Antarctic notothenioids.

The population dynamics, individual growth rates and productivity of the hyperiid amphipod Themisto gaudichaudii were studied in Cumberland Bay, a fjord on the north-eastern coast of South Georgia. The study involved the weekly sampling of the population and its potential food with a Rectangular Midwater Trawl (RMT1) at weekly intervals between May 2005 and October 2006. Offshore samples were taken the previous austral summer using an RMT8 net. T. gaudichaudii produced one main cohort in the spring (September/October) and a secondary weaker cohort in the late summer (January/February). The main cohort appeared at the time of phytoplankton increase, about 3 weeks before an increase in mesozooplankton. Larger individuals (20–25 mm) resided offshore but not within the Bay during the main recruitment period. T. gaudichaudii grew at an average rate of 0.1 mm d−1 during the first four months after release and then by 0.07 mm d−1 for the subsequent four months. At these rates, it is possible that the main cohort reaches reproductive size in time to spawn the autumn cohort, although the relative weakness of the autumn cohort suggests that few manage this. T. gaudichaudii biomass reached its highest levels in November at a mean value of 5.4 mg DW m−3 while daily production was also highest in November at a mean value of 0.17 mg DW m−3 d−1. The P/B d−1 ratio varied between 0.03 d−1 in the summer to below 0.01 d−1 in the winter. Daily mortality rate was initially 0.12 d−1 during the first two months post-recruitment, falling to 0.03 d−1 for the remainder of the year. Integrating production over the year gave a value of 10.04 mg DW y−1, equivalent to 3.62 mg C m−3 y−1, making T. gaudichaudii a relatively productive component of the pelagic community. During the summer, the species was estimated to consume around 1–2% of available mesozooplankton biomass and a mean of 15% of mesozooplankton daily productivity. T. gaudichaudii may have a controlling influence on mesozooplankton dynamics as well as on the recruitment of the larval stages of important commercially fished species such as Antarctic krill (Euphausia superba) and Mackerel icefish (Champsocephalus gunnari).

Ontogenetic, inter-annual and regional variations in diet were investigated for mackerel icefish, Champsocephalus gunnari, in three successive summer seasons around South Georgia. Stomach contents from 2239 C. gunnari (130–560 mm total length) were examined. A bootstrapping technique was used to calculate confidence intervals for an index of relative importance of prey categories (% IRIDC). Diet varied significantly between years and age classes but there was little regional difference in diet. In general, diet was dominated by krill, Euphausia superba and by the amphipod Themisto gaudichaudii. Smaller (younger) fish tended to prey on a higher proportion of T. gaudichaudii and small euphausiids such as Thysanoessa sp. and took smaller quantities of E. superba. In a season of poor krill availability (summer of 2003–2004) the proportion of krill in the diet, stomach fullness and fish condition (indicated by length–weight relationships) were significantly lower than in the other summer seasons. A large reduction (>80%) in the estimated annual (2005) biomass of the C. gunnari stock directly followed the season of poor krill availability. This decline was largely because of mortality of 2+ and 3+ fish, which were more krill dependent than 1+ fish. Younger fish appear to have survived, leading to an increase in the estimated population biomass in 2006.

The diet of pinnipeds is most commonly inferred from morphologically diagnostic remains of prey in their scats. Although this method can generate quantitative estimates of diet simply, important prey types may not always be detected. DNA-based methods improve detection of prey in scats, but they are not quantitative. While some studies have combined morphological and DNA-based methods, these have only assessed prey that are represented by their hard remains in scats. To overcome this bias, we apply molecular and morphological analyses to the soft and hard portions of faecal samples respectively, to estimate the diet of lactating Antarctic fur seals (Arctocephalus gazella) on Heard Island. The diet of this population is of particular interest because it is expanding rapidly and may rely to some extent on mackerel icefish (Champsocephalus gunnari), which are subject to commercial fisheries. Based on results from morphological analysis and likely important prey types, we tested for DNA remains of C. gunnari, myctophids and squid in faecal samples. The proportion of samples (n = 54) yielding no dietary information was reduced from around 25.9% using either method alone, to 9.3% when combined. Detection of all prey types tested for was notably improved by integrating molecular and morphological data. Data from either method alone would have underestimated the number of animals consuming C. gunnari by around 25.7%. Detection of multiple prey types in samples increased from 9.3% when using morphological analysis only, to 33.3% when using DNA only, to 46.3% when using both methods. Taken in isolation, morphological data inferred that individual seals consume either C. gunnari or myctophids, probably foraging in separate locations characteristic of those prey. Including molecular data demonstrated that while this may be true of some individuals, many other seals consume a mixed diet of at least C. gunnari, myctophids and squid. This new approach of combining DNA-based and morphological analyses of diet samples markedly increased the number of samples yielding dietary information, as well as increasing the amount of information attained from those samples. Our findings illustrate the broad potential of this technique to improve insight into trophic interactions in marine ecosystems.

Chaenocephalus aceratus (Family Channicthyidae) is one of the dominant species of demersal fish living on the South Georgia shelf where it is caught in low numbers as by-catch in the mackerel icefish and Antarctic krill commercial fisheries. Data collected during 14 demersal fish surveys, from 1986 to 2006, are analysed to investigate biomass, distribution, growth and diet. Biomass estimates from a swept area method ranged from 4,462 to 28,740 tonnes on the South Georgia and Shag Rock shelves although few fish were caught at Shag Rocks. Analysis of length frequency data indicated that growth was fast in the first five years with males and females attaining lengths at first spawning of 440 mm TL and 520 mm TL. The diet was comprised of fish and crustaceans, with an ontogenetic shift in diet from Euphausia superba and mysids to benthic fish and decapods observed to begin at 250 mm TL. In larger fish (>500 mm TL) the diet was dominated by fish. C. aceratus diet is sufficiently different from the other species of channichthyids around South Georgia to suggest that these species have undergone resource partitioning.

The mackerel icefish (Champsocephalus gunnari Lonnberg E (1905) The Fishes of the Swedish South Polar Expedition. Wiss. Ergebnisse Schwedische Sudpol- Exped. 1901–1903, vol 5, p 37 is widely distributed south of the Antarctic convergence and over shelf areas surrounding sub-Antarctic Islands. In order to evaluate global population structure in this species, we examined DNA sequence variation in four mitochondrial regions and four nuclear genes in icefish from four locations in the Atlantic Ocean sector and one location in the Indian Ocean. Despite small sample sizes, mitochondrial and nuclear gene data indicated the existence of at least three genetically distinct stocks: Heard Island, South Shetland Islands, and the remaining Atlantic populations (Shag Rocks, South Georgia, and Bouvet Island). The mitochondrial and nuclear SNP markers developed here will be useful for more extensive analyses of population structure in this species.

This study analysed the total length (LT)-frequency distribution of mackerel icefish Champsocephalus gunnari at South Georgia and Shag Rocks from nine bottom trawl surveys at South Georgia and eight at Shag Rocks between 1987 and 2002. The estimated mean LT of age-classes 1+, 2+, 3+ and 4+ years during January were, respectively, 14·7, 23·5, 29·8 and 35·1 cm at South Georgia. Age-classes 1+, 2+ and 3+ years were 18·3, 26·2 and 33·8 cm at Shag Rocks. The derived Bertalanffy growth parameters for South Georgia were: L∞ = 51·7 cm, k = 0·27 and t0 = −0·26. The mean LT of each age-class of C. gunnari at Shag Rocks was significantly larger than at South Georgia, equivalent to c. 5 months growth, although the annual growth in LT was similar. This is further evidence that C. gunnari hatched earlier at Shag Rocks. At South Georgia, the mean LT of age-classes 1+ and 3+ years were correlated, and significantly decreased between 1987 and 2002, and were smaller following warmer summers. This decrease in the size of C. gunnari may be the result of reduced food availability linked to climate warming.

The South Georgia population of mackerel icefish (Champsocephalus gunnari) is exploited by both a fishery and predators, including gentoo penguins (Pygoscelis papua). Because considerable uncertainty surrounds recent estimates of stock size, there is a need to consider novel approaches that may give insight into the population dynamics of this species. We derive two indices of recruitment based on the occurrence of mackerel icefish in gentoo penguin diets, one of which is scaled by a survey estimate of the density of Antarctic krill (Euphausia superba), an alternative prey species for gentoo penguins. The closure of the fishery for much of the 1990s allowed the relationship between environmental conditions and recruitment to be studied without the confounding effects of fishing mortality. The recruitment indices were positively correlated with sea surface temperature with a lag time equal to the age of the fish. These results suggest strong links between mesoscale environmental processes and the smaller scale interaction between gentoo penguins and their mackerel icefish prey.

Mackerel icefish, Champsocephalus gunnari, larvae were sampled by weekly plankton tows in Cumberland Bay South Georgia. The formation of daily growth increments in sagittal otoliths was validated from larvae sampled 34 days apart. A mean increase of 33.7 increments occurred during this period, one per day. A major growth check 25–26 days after the hatch check may indicate the length of the yolk sac stage.

The mackerel icefish Champsocephalus gunnari Lonnberg is an important component of the ecosystem at South Georgia. Its diet is dominated by Antarctic krill Euphausia superba; in turn, it is also an important prey of a number of upper trophic level predators, as well as being the target of commercial fisheries. Data on the frequency and size structure of mackerel icefish in the diet of Antarctic fur seals Arctocephalus gazella and gentoo penguins Pygoscelis papua from 1991 to 2002 were used to examine trophic interactions involving this species and to evaluate the potential impact of predators on its population. Mackerel icefish occurred in 10 to 20% of scats from Antarctic fur seals and comprised 48% of the diet by mass of gentoo penguins. The contribution of mackerel icefish to the diet of predators was in inverse proportion to the contribution of Antarctic krill. The length- frequency distributions of mackerel icefish indicated a dominant mode at 130 to 180 mm total length (the 1+ age class), with apparent strong cohorts entering the population in 1993, 1996, 1999 and 2001. Extrapolation of diet data to produce a consumption estimate for the South Georgia population of Antarctic fur seals and gentoo penguins suggests that scientific trawl surveys may underestimate the standing stock of mackerel icefish by 1 order of magnitude. Changes in the South Georgia ecosystem over the past 2 decades may have increased the level of consumption of mackerel icefish by predators, providing a potential ecosystem-based explanation for the lack of a recovery to the pre- exploitation population size.

The summer diet of two species of icefishes (Channichthyidae) from the South Shetland Islands and Elephant Island, Champsocephalus gunnari and Chaenocephalus aceratus, was investigated from 2001 to 2003. Champsocephalus gunnari fed almost exclusively on krill (Euphausia superba) in all years. The importance of other taxa (Themisto gaudichaudii, mysids, myctophids) in the diet was negligible. The average feeding rate of Champsocephalus gunnari inferred from an exponential gastric evacuation model was between 1.0 and 1.5% body weight per day. Most of the stomachs of Chaenocephalus aceratus were empty. Stomachs with food contained mainly krill, mysids and fish. Among the fish taken, locally abundant species formed the bulk of the diet: Gobionotothen gibberifrons in 2001, Lepidonotothen larseni and Champsocephalus gunnari in 2002 and L. larseni in 2003. An ontogenetic shift in feeding preference of Chaenocephalus aceratus was observed: fish smaller than 30 cm fed on krill and mysids, while larger animals relied primarily on fish.

Mackerel icefish have a widespread distribution in the Atlantic and Indian Ocean sectors of the low-Antarctic region. Biological characteristics differ considerably between populations in the southern Scotia Arc and those living further to the north. Fish living in the north mature 1 year earlier than in the south. They have a much shorter life span and die after they have spawned two to three times. The number of eggs produced per gram of body mass is higher in the north. Stocks have declined in most parts of the distributional range due to the impact of fishing and due to natural causes. Increases in populations of Antarctic fur seals at South Georgia and parts of the Indian Ocean appear to have led to increased predation on stocks of icefish. Shifts in hydrological regimes in the northern part of the distributional range have either started to lead or will lead to deteriorating living conditions for mackerel icefish in the near future. Fish stock assessment needs to take these constraints into consideration when providing advice on total allowable catches for fisheries management.

The lipid content, fatty acid composition and calorific value of seven species of mesopelagic deep-sea fish of the family Myctophidae and the mackerel icefish, Champsocephalus gunnari, important in the diet of Southern Ocean marine predators, are presented. Fish were sampled at the Kerguelen Plateau (KP) and Mac- quarie Ridge (MR) in the Indian and Pacific sectors of the Southern Ocean respectively, to examine geographic variation in lipid compositon. All species of myctophid from KP and Electrona antarctica from MR were high in lipid content (6-18% wet mass), particularly Gymnoscopelus nicholsi (18%) and E. antarctica (15%). The mackerel icefish, and G. fraseri and Protomyctop- hum tenisoni from MR were generally lower in lipid content (3-5%) and varied significantly in fatty acid composition from KP species. KP myctophids were high in calorific content (9.3 kJ g -1 wet mass) when compared with icefish (5.4 kJ g -1 wet mass) and other published values for prey items of marine predators such as squid (1.7-4.5 kJ g -1 ). KP myctophids were distinguished from each other and from C. gunnari and MR specimens by cluster and discriminant function analysis using six fatty acids (16:0, 18:1x9, 20:1x9, 22:1x11, 20:5x3, 22:6x3). Findings presented here highlight trophic links between high-latitude fish and their prey and emphasise the im- portance of myctophids as a significant energy source for marine predators foraging in the Polar Frontal Zone.

Spawning by Antarctic fish is generally considered to be seasonal and restricted to a brief period during the autumn and winter. Arising from this it has been assumed that the gonadal maturation cycle is also closely associated with the time of year. The gonad maturation cycle of the mackerel icefish Champsocephalus gunnari was investigated using data collected during research surveys and from sampling the commercial fishery. Spawning appears to occur at the same time each year, but the timing of gonadal development is subject to a considerable inter-annual variation. The implications of this variation are discussed with respect to feeding conditions.

During the last decade, methods for detecting DNA polymorphisms have proliferated at a bewildering pace. Today the investigator must choose among various types of genetic markers as well as between a variety of methods for discovering and screening polymorphisms. Polymorphisms useful for the analysis of population structure are found in both mitochondrial and nuclear genomes. Marker development may proceed along two routes: 1) discovery of species–specific markers, and 2) application of universal methods. Species-specific markers are based on sequence data from the target species or close relatives, whereas universal markers are based on the use of polymerase chain reaction (PCR) primers targeted to regions highly conserved across diverse taxa. Markers commonly employed include mitochondrial DNA polymorphisms, microsatellites, anonymous nuclear loci and known genes (both coding and noncoding regions). Methods for detecting polymorphisms range from technically simple (RFLP analysis) to more sophisticated mutation scanning methods. We review the application of these approaches to several key Antarctic species (the Patagonian toothfish Dissostichus eleginoides, the mackerel icefish Champsocephalus gunnari, and the squid Martialia hyadesi Rochebrune & Mabille, 1889) and present preliminary data on genetic polymorphisms in toothfish and icefish.

1. Bottom trawl surveys on the South Georgia shelf indicate episodic declines in the abundance of the mackerel icefish Champsocephalus gunnari that, since 1990, are not directly attributable to commercial fishing. 2. The greatest effect has been observed in years when krill are known to have been scarce on the South Georgia shelf. 3. It is thought that C. gunnari survivorship is closely related to, but indirectly influenced by, krill availability. 4. Examination of food chain relationships indicates that predation by fur seals is likely to play a major role in the observed changes in C. gunnari abundance. 5. Periodic shifts in food chain relationships, involving krill, C. gunnari and fur seals, are postulated that could explain the episodic declines in the abundance of C. gunnari in the absence of fishing. 6. The study demonstrates how a key predator can have an over-riding impact on a commercial fish species, highlighting the importance of the ecosystem approach to fisheries management.

The diet composition of king penguins (Aptenodytes patagonicus) at Heard Island (53°05′S; 73°30′E) was determined from stomach contents of 98 adults captured as they returned to the island throughout 1992. During the two growth seasons, the diet was dominated by the myctophid fish Krefftichthys anderssoni (94% by number, 48% by mass). The paralepidid fish Magnisudis prionosa contributed

South Georgia and the South Sandwich Islands (SGSSI) are surrounded by oceans that are species-rich, have high levels of biodiversity, important endemism and which also support large aggregations of charismatic upper trophic level species. Spatial management around these islands is complex, particularly in the context of commercial fisheries that exploit some of these living resources. Furthermore, management is especially complicated as local productivity relies fundamentally upon biological production transported from outside the area. The MPA uses practical management boundaries, allowing access for the current legal fisheries for Patagonian toothfish, mackerel icefish and Antarctic krill. Management measures developed as part of the planning process designated the whole SGSSI Maritime Zone as an IUCN Category VI reserve, within which a number of IUCN Category I reserves were identified. Multiple-use zones and temporal closures were also designated. A key multiple-use principle was to identify whether the ecological impacts of a particular fishery threatened either the pelagic or benthic domain.

The mackerel icefish (Champsocephalus gunnari) is a prominent member of the coastal fish fauna of the Seasonal Pack-Ice Zone and the islands north of it. Separated into a number of stocks, its distribution ranges from the Scotia Arc region, namely South Georgia, in the Atlantic Ocean sector to the Kerguelen-Heard Plateau in the Indian Ocean sector. Mackerel icefish have been heavily exploited since the beginning of the 1970s with reported annual catches exceeding 50–100,000 tonnes in some years. C. gunnari has many characteristics typical of Antarctic fish species with respect to life history characteristics, such as egg size, fecundity and growth. These fall well within the range of other sympatric red-blooded notothenioids; eggs are large and yolky. Egg diameter, egg production per gram body weight and growth performance at South Georgia and the Kerguelen Islands was comparable with similar-sized nototheniids and channichthyids. Stocks of C. gunnari have a number of biological characteristics in common, such as feeding mode and food, early life history and growth in the first years of life. However, stocks in sub-Antarctic waters differ in reproductive characteristics, such as length and age at first spawning, natural mortality and life expectancy from those on more southerly grounds. They may grow to more than 60 cm and may become 13–15 yrs old. However, in the vicinity of South Georgia and the Kerguelen Islands, fish apparently do not exhaust this growth potential fully. Their ability to reproduce at an early age coupled with a comparatively high fecundity and growth performance may mean stocks of C. gunnari in sub-antarctic waters have far more resilience and a greater capacity to rebuild than species with the slower reproductive rates common in other notothenioids.

When fish are patchily distributed, unstratified trawl surveys give highly variable abundance estimates. If the patches are small and mobile, it is impractical to use pre-stratified or multi-stage adaptive designs to reduce variability. Based on recent trawl surveys for mackerel icefish Champsocephalus gunnari, we have modified the survey design and analysis to reduce estimation variance at minimal cost, using concomitant qualitative acoustic data. We have also developed new confidence interval procedures that should be accurate even for small samples. In computer simulations, the new method substantially outperformed a standard trawl survey.

During the early ontogeny of fishes, the timing and duration of key events such as larval hatching and the switch from endogenous to exogenous feeding largely determine the offspring viability and survival. The aim of the present study was to investigate the life history traits of the early larvae of the mackerel icefish, Champsocephalus gunnari, collected in summer south of the South Shetland Islands in the Bransfield Strait and north of Elephant Island. Through the analysis of sagittal otolith microstructure, we assessed the timing and duration of egg incubation, larval hatching, first exogenous feeding, rate of yolk resorption and body growth rate. Compared to populations living further north (i.e. around South Georgia and Kerguelen Islands), mackerel icefish in the southern Scotia Arc exhibits longer egg incubation (lasting 90-120 days from winter to summer) and delayed hatching time spread over a relatively short period lasting 26 days between January and February. The first exogenous feeding takes place between 13 and 24 days after hatching still in the presence of the yolk-sac, indicating a prolonged mixed feeding afterward. The specific growth rate or daily percentage change in size (G) was 1.9 % SL day(-1), corresponding to a daily growth rate at mean size of 0.31 mm day(-1). While showing significant differences in early life history traits across their geographical distribution, C. gunnari populations share a common strategy, spawning a small number of large eggs that hatch in relatively large-sized larvae, at a time which may be independent of the timing of pack-ice retreat and onset of the production cycle.

The diet of mainly adult and sub-adult male Antarctic fur seals (Arctocephalus gazella) was investigated during the austral winters of 1992 and 1993 using faecal (scat) analysis. Of samples containing identifiable prey remains (n = 376), c. 28% contained krill alone, c. 37% contained fish alone and c. 35% contained both krill and fish. The mean size of krill taken was smaller than in summer due to the absence of large adult krill of 55–60 mm total length. Of the 25 fish taxa taken, krill-feeding species predominated, especially in scats that also contained krill. Non-krill feeding species were more abundant in scats containing fish alone. The mackerel icefish (Champsocephalus gunnari Lönnberg 1905), which feeds mainly on krill, was the most important fish species in terms of absolute frequency (53%), frequency of occurrence (69%) and estimated biomass (47%); fur seals may have a significant impact on local stocks of this species. Cephalopods and pelagic fish (myctophids) were of minor importance in the diet (

Wide-ranging, surface-feeding pelagic seabirds are the most numerous functional group of birds in the Southern Ocean. The mesoscale habitat use of these birds is increasingly being quantified by relating their movements to remotely sensed, near surface properties of the ocean. However, prey availability at the sea surface may also be determined by habitat characteristics not measurable from space. For instance, benthic-pelagic coupling, which occurs when seabed processes affect productivity in the epipelagic zone, can link benthic habitat type to availability of surface prey. We combined acoustically derived maps of the substrate of the South Georgia shelf with GPS tracking to quantify the sub-mesoscale habitat use of breeding black-browed albatrosses. We show that albatrosses preferentially used waters overlaying glacial moraine banks near the shelf edge and that this was unrelated to the presence of trawlers targeting mackerel icefish, which are also associated with these features. Stomach temperature profiles suggest that albatrosses primarily caught krill and fish over the banks. We hypothesize that black-browed albatrosses target waters overlaying moraine banks due to upward benthic-pelagic coupling, mediated by an increase in abundance of zooplankton such as Antarctic krill. Our findings suggest that the potential effects of such processes on pelagic seabird distribution warrant wider investigation.

The diet composition and feeding intensity of mackerel icefish Champsocephalus gunnari around Shag Rocks and the mainland of South Georgia was analyzed from ca 8700 stomachs collected in January/February 1985, January/February 1991 and January 1992. Main prey items were krill Euphausia superba, the amphipod hyperiid Themisto gaudichaudii, mysids (primarily Antarctomysis maxima), and in 1985 also Thysanoessa species. The proportion of krill and T gaudichaudii in the diet varied considerably among the 3 years, whereas the proportion of mysids in the diet remained fairly constant, Krill appears to be the preferred food. In years of krill shortage, such as in 1991, krill was replaced by T gaudichaudii. The occurrence of krill in the diet in 1991 was among the lowest within a 28 yr period of investigation. Variation in food composition among sampling sites was high. This high variation appears to be primarily associated with differences in prey availability, but much less with prey size selectivity. Feeding intensity varied considerably among seasons. It was highest in 1992. The proportion of empty stomachs was uncommonly high and stomach content weight was uncommonly low in January 1991, a period when energy-rich food was needed for the final maturation of gonads. At the same time, an unusually high proportion of sexually mature fish showed no signs of the gonad development necessary for spawning in that season. It was hypothesized that as in some other non-Antarctic fish species the shortage of suitable food may have forced the fish to sacrifice gonad maturation in order to maintain body size.

Management of Antarctic krill,Euphausia superba, and the mackerel icefish,Champsocephalus gunnariunder the Commission for the Conservation of Antarctic Marine Living Resources (CCAMLR) is discussed in relation to changes in their distribution and abundance arising from variation in circulation of the circumpolar current. It is concluded that on a Southern Ocean scale it is currently not possible to detect change but on a local scale, such as at South Georgia, major changes are detectable. These changes affect the krill fishery directly in terms of total catch and the way the fleets are deployed. Major local reductions in krill are thought to have a significant effect on natural mortality of the icefish.

Yields of exploited fish stocks from South Georgia Island (Antarctic) have been recorded to be declining since the early 1970s. However, the probable effects of exploitation on the demersal fish community structure have not been investigated. We used a non-parametric method (FAUNSIM) to test for statistically significant changes in that community based on 3 years of survey data. A significant shift in the community was detected between 1986–1987 and 1987–1988. This change was due mostly to reductions in the abundance of mackerel icefish, Champsocephalus gunnari , and gray rockcod, Notothenia squamifrons , and the increased presence of South Georgia icefish, Pseudochaenichthys georgianus . The observed change was from a community dominated by a few large-bodied species to fewer, mostly small-bodied species of more even abundance. Possible causes for this change in the community were explored. Fishing mortality was believed to be the most likely mechanism affecting the observed change.

Gonad maturation in Antarctic notothenioid fish is a biennial process although spawning is likely to take place annually. However, part of the populations of Champsocephalus gunnari in the Atlantic Ocean sector do not spawn each year. Gonadosomatic index (GSI) of females is 15–40% at spawning. Apart from a few nototheniid species of the GSI of males is much less and typically only 15–20% of that of females. Length at first spawning may be from 55% of Lmax onwards, but in many species it is not attained until 70–80% of the maximum length. The only exception is Champsocephalus gunnari at South Georgia which may begin spawning at about 40% of Lmax. Most species of the Seasonal Pack-ice Zone are autumn/winter spawners, whereas in the High-Antarctic Zone more species spawn in summer and autumn. Spawning time is remarkably constant among populations of some species, in others a latitudinal shift in spawning time is apparent. Fecundity is commonly positively correlated with fish length and weight. It exceeds 100 000 eggs only in a few nototheniid species and is commonly in the order of 1000 to 15–20 000 eggs. Ova diameter varies from 0.8 to 5.0 mm. Egg size distribution among fishes of the Seasonal Pack-ice Zone is bimodal. There is a general trend in nototheniids of increasing egg size and decreasing relative fecundity towards higher latitudes. Incubation time may be up to five months. Eggs of most species are probably left unattended for the long incubation period. Nest guarding has been observed in three species but may be more common in particular among the artedidraconids. A number of reproductive strategies associated with nest guarding, egg size and the duration of the pelagic phase have been identified.

The Commission for the Conservation of Antarctic Marine Living Resources (CCAMLR) is bound by its Article II, 3 to follow an ecosystem approach to management. This approach has been extended to the application of a precautionary approach in the late 1980s. In our review, we deal primarily with the science-related aspects of CCAMLR and its development towards an ecosystem approach to the management of the living resources of the Southern Ocean. To assist the Commission in meeting its objectives, as set out in Article II, 3, the Scientific Committee established the CCAMLR Ecosystem Monitoring Programme to detect possible effects of krill fishing on the performance of top-level predators, such as albatrosses, penguins, petrels and fur seals. Fisheries in the Southern Ocean followed the fate of other fisheries worldwide in which target species were depleted to low level one after the other. Currently, two types of fisheries are open: the longline fisheries on Patagonian toothfish ( Dissostichus eleginoides ) and Antarctic toothfish ( Dissostichus mawsoni ) and the trawl fisheries on mackerel icefish ( Champsocephalus gunnari ). Both fisheries are managed in a single-species context, however, with conservation measures in place to protect by-catch species, such as rattails (Macrouridae) and skates and rays (Rajidae). Two major problems still exist in fisheries in the Southern Ocean: the by-catch of birds in longline fisheries primarily in the Indian Ocean and the high level of IUU fishing again in the Indian Ocean. Both, the by-catch of birds and high IUU catches undermine the credibility of CCAMLR to safeguard the marine living resources in the Southern Ocean.

International audience; We studied the dietary preferences of Antarctic fur seals Arctocephalus gazella from Cap Noir, Îles Kerguelen, foraging in the Antarctic Polar Frontal Zone (PFZ) in February of 1998, 1999 and 2000. Scats were collected and analyzed for remaining prey hard parts in each of the 3 years, and in 1999 and 2000, the fatty acid (FA) composition of fur seal milk samples was also examined for longerterm dietary preferences. Scat analyses revealed that seals foraged primarily on fish and some squid in all 3 years with 25 species of fish being taken during the study. Myctophid fish accounted for an average of 94% by number of all fish consumed with 3 species, Gymnoscopelus nicholsi, G. piabilis and Electrona subaspera, forming the core diet. Inter-annual differences in dietary species composition were apparent, however, with the presence of the mackerel icefish Champsocephalus gunnari in 1998 and the myctophid Protomyctophum tenisoni in 1999 accounting primarily for the differences observed between years. While reconstituted prey biomass per scat was similar between years, scats from 1998 represented less energy per gram than those from 1999 and 2000. This study highlights the usefulness of using FA signature analysis to confirm longer-term shifts in dietary intake of fur seals using milk samples. Polyunsaturated fatty acids (PUFAs) were significantly more prevalent in the 1999 milk samples, which were also lower in overall lipid content (43% vs 53%). G. nicholsi, a particularly oily fish, occurred in higher proportions in the diet in 2000, perhaps explaining the higher incidence of monounsaturated fatty acids (MUFAs) in this year and the generally higher lipid levels present in milk samples. The inter-annual variation in the diet of Antarctic fur seals confirmed by these 2 techniques lends support to the hypothesis that previously identified variations in oceanographic conditions surrounding Îles Kerguelen in 1998, 1999 and 2000 affect the availability of fur seal prey resources.

Mackerel icefish Champsocephalus gunnari are widespread on the South Georgia (54deg; S, 36deg; W) shelf. Analysis of condition indicated a strong interannual variation. High condition indices, indicative of good feeding conditions, were present when krill were abundant in the region. Years when krill were scarce and condition index was consequently low, were consistent with years when indices from land-based krill predators also indicated that krill were scarce.