Feeding ecology of three species of midwater fishes associated with the continental slope of eastern Tasmania,Australia

The feeding ecology of Maurolicus muelleri, Lampanyctodes hectoris and Diaphus danae was examined from samples collected from continental slope waters of eastern Tasmania between April 1984 and April 1985. A total of 2 232 stomachs was analysed. M. muelleri, L. hectoris and D. danae fed primarily on euphausiids and secondarily on copepods, although larger D. danae (> 60 mm standard length) fed on other lanternfish (chiefly L. hectoris). The diets of M. muelleri and L. hectoris overlapped substantially. Diet overlaps between D. danae and the former species was low, however, due to the large biomass of fish present in D. danae. The prey taxa consumed changed with time of year and predator size. Stomach fullness (feeding intensity) varied seasonally in all three species, but only M. muelleri showed significant diel differences in fullness. The synchronization of the size structure of the predator populations and their feeding intensity, with seasonal variations in preferred prey, is proposed as a mechanism whereby each species maximizes its share of the available food resources.     

Distribution and community structure of midwater fishes in relation to the subtropical convergence off eastern Tasmania,Australia

Midwater fishes were sampled at night to a depth of 400 m in oceanic waters east of Tasmania, Australia, in May/June of 1992, 1993 and 1994. We examined whether there were differences in the distribution and density of these fishes in relation to the subtropical convergence (STC) separating East Australia Current (EAC) water from subantarctic water (SAW) to the south. A total of 23999 fishes from 107 taxa and 43 families were identified. Four new records were identified from the study area. Myctophids and stomiatoids were the main fish taxa captured. We found no evidence of increased density of the combined fish catch in any particular area. However, differences between areas in the density of individual species were noted, although no species was confined to any one area. Ceratoscopelus warmingi, Lobianchia dofleini and Vinciguerria spp. were most abundant in the EAC, whereas Lampichthys procerus was most abundant in the SAW. Only Diaphus danae was found in significantly higher numbers in the STC. Multivariate analysis revealed that community structure in the EAC was significantly different from that of the SAW but not from the STC. Further, when separated by depth, the shallow EAC group was significantly different in all group comparisons bar that with the shallow STC. Our data indicated that the thin EAC layer above 200 m could be distinguished by its fauna from the SAW. However, we could not detect a separate community in the STC. We conclude, therefore, that the STC is not an area of increased micronekton abundance, nor does it contain a distinct community, during the Australian autumn/winter.     

Diets of fishes of the upper continental slope of eastern Tasmania: content,calorific values,dietary overlap and trophic relationships

Diets of 15 species of demersal and pelagic fishes on the upper continental slope (420 to 550 m) were determined, based on samples taken every two months over 13 mo (April 1984 to April 1985) off eastern Tasmania. The calorific contribution of each prey item to the diets was determined. The fish could be divided into four trophic categories: pelagic piscivores, epibenthic piscivores, epibenthic invertebrate feeders and benthopelagic omnivores. Dietary overlap between the groups was low. The pelagic piscivores Apogonops anomalus, Trachurus declivis, Brama brama, Lepidopus caudatus and Macruronus novaezelandiae primarily consume the shelf-break myctophid Lampanyctodes hectoris; their diet is narrow, with a large overlap between species. The epibenthic piscivores Deania calcea and Genypterus blacodes both take a greater variety of prey, but have little dietary overlap. The fish feeding on epibenthic invertebrates, Coelorinchus sp. 2 and Centriscops humerosus, obtain most of their energy from benthic Crustacea and Ophiuroidea, supplemented with Lampanyctodes hectoris; the diet is broad, with little overlap. Among the benthopelagic omnivores (Cyttus traversi, Coelorinchus sp. 4, Lepidorhynchus denticulatus, Neocyttus rhomboidalis, Helicolenus percoides, Epigonus denticulatus and E. lenimen), most diets are broad and show slight overlap. All but E. denticulatus consume significant quantities of Lampanyctodes hectoris as well as Crustacea, particularly Polychaeta, Euphausiacea and Pyrosoma atlanticum. Seasonal changes in diet occurred in G. blacodes, T. declivis, Lepidopus caudatus, Coelorinchus sp. 4, Lepidorhynchus denticulatus, H. percoides, E. denticulatus and E. lenimen; these were related to changes in abundance of particular prey species, not to alterations in feeding habits. Only three species, Lepidopus caudatus, Coelorinchus sp. 2 and H. percoides, showed significant diel feeding periodicity. Ontogenetic dietary changes were evident in Cyttus traversi, Coelorinchus sp. 2, Lepidorhynchus denticulatus and H. percoides. Cyttus traversi and H. percoides progressively changed from crustaceans to fish as their size increased. The diets of size classes within species showed little overlap, except for Lepidorhynchus denticulatus, which eats chiefly euphausiids and Lampanyctodes hectoris at all sizes. In addition to describing the diets and trophic relationships of 90% of the fish biomass, the results emphasize the importance to the entire fish community of mesopelagic food resources, particularly Lampanyctodes hectoris. Many benthopelagic species undertake extensive vertical migrations in search of prey, thus playing a major role in the transport of energy from midwater regions to the benthos of the continental slope.     

Benthic and pelagic fish biomass of the upper continental slope off eastern Tasmania

Benthic and pelagic fishes were sampled east of Maria Island, Tasmania, at two-monthly intervals from April 1984 to June 1985, from the surface to the bottom (500 m depth), using commercial-sized trawls. Biomass was calculated by the area swept/volume filtered method and divided by estimated catchability coefficients so that catches from the two sampling gears could be combined. Of the 54 families caught, three (Myctophidae, Squalidae, Sternoptychidae) contributed 25% of the 115 species. Most benthic and dispersed species were caught regularly, whereas most pelagic species occurred only occasionally and in low numbers, although a core group was always present. Total fish biomass was high (range=77 to 532 g m^-2; x= 390 g m^-2), due almost entirely to the myctophid Lampanyctodes hectoris (over 90% of the biomass). Benthic biomass was relatively low and stable, but derived from many species. Pelagic biomass was high, fluctuated widely and was composed of a few species. Biomass was highest in summer: Maurolicus muelleri increased by a factor of 200, Diaphus danae by 50, and L. hectoris, Macruronus novaezelandiae and Lepidorhynchus denticulatus by almost 10. Peaks in biomass may correlate with the interactions of the subtropical convergence and the East Australian Current and the resultant marked seasonal cycle in water temperature, nutrients and primary productivity.With an appendix by T.J. Kenchington, CSIRO Division of Fisheries, Hobart, Tasmania 7001, Australia     

Oxygen consumption of midwater fishes and crustaceans from the eastern Gulf of Mexico

Oxygen consumption was measured as a function of temperature, oxygen partial-pressure (P_O2)and species depth of occurrence for twenty-three species of midwater fishes and crustaceans collected from the eastern Gulf of Mexico from June 1981 to July 1985. Q₁₀s (7° to 20°C) of 3.90 and 3.24 were recorded for myctophid and non-myctophid fish groups, respectively, while values of 2.22, 2.19, 2.19 and 2.54 were calculated for sergestid, penaeid, carid and euphausiid crustacean groups, respectively. Q₁₀s were consistent for species within each group. All of the species tested regulated their oxygen consumption to P_O2levels normally encountered within the eastern Gulf. Values of critical partial pressure (P_c) ranged from 20 to 40mm Hg and increased slightly with increasing temperature and respiration rate. Declining respiration with increasing minimum depth of occurrence was primarily a function of temperature alone. Changes in size, dry weight and water content contributed only a small fraction to the observed decrease. This finding contrasts with studies from the eastern Pacific Ocean, where temperature is a minor contributor to changes in respiration rate with depth.     

Feeding ecology of five fishes from the mid-slope micronekton community off southern Tasmania, Australia

We examine the feeding ecology of characteristic micronekton species inhabiting the mid-slope region (~700 to 1,500 m depths) off southern Tasmania. Five fishes, Diaphus danae, Hygophum hanseni, Lampanyctus australis (Myctophidae), Phosichthys argenteus (Phosichthyidae) and Chauliodus sloani (Chauliodontidae), were sampled by depth-stratified midwater trawling on a diel and seasonal basis. Overall, 74 prey taxa were identified from 2,132 stomachs. Euphausiids (mostly Euphausia spinifera and E. similis) and calanoid copepods (mostly Pleuromamma species) were the main prey of the three myctophids; P. argenteus ate fishes and decapods in addition to the euphausiids, while C. sloani ate only fishes. Copepods were less important in the diets of larger D. danae, L. australis and P. argenteus and were replaced by euphausiids in the myctophids and by fishes in P. argenteus. In autumn, when euphausiid biomass increased six-fold, all three myctophids and P. argenteus fed most intensively and consumed a high proportion of euphausiids. The three myctophids appeared to feed nocturnally. Differences in the timing and duration of feeding corresponded to differences in their spatio-temporal overlap with key prey. Daily rations of H. hanseni (1.93% dry-weight biomass) and L. australis (1.43%), estimated from data on stomach fullness, were typical for temperate myctophids and higher than that of the non-migratory P. argenteus (0.43%). The vertical flux of near-surface plankton production to the mesopelagic food web is based primarily on diel feeding in the upper water column (<500 m) rather than consumption of species that migrate seasonally into the deeper mesopelagic zone. Because species such as P. argenteus and C. sloani feed above the third trophic level, their predators, including the commercially important orange roughy, are feeding between levels four and five.     

Age and growth of the lanternfish Lampanyctodes hectoris (Myctophidae) from eastern Tasmania,Australia

The age, growth and mortality of the lanternfish Lampanyctodes hectoris (Gunther) were investigated by examining the otoliths, length-frequency data, and seasonal abundance of fish collected from continental slope waters of eastern Tasmania between April 1984 and June 1985. Although L. hectoris can live to 3 yr old, it has a high annual mortality (79%) and few live past their first year. Growth, described here by a variation of the Von Bertalanffy model, takes place mainly in the first six months of life and only during late spring and summer. The hypothesis of daily deposition of primary growth increments in myctophids was supported by comparing the back-calculated birth dates of daily-aged individuals of L. hectoris with the spawning season of this species.     

Food availability as a selective factor on the chemical compositions of midwater fishes in the eastern North Pacific

In September and October 1980 we examined the relationships between food availability, depth, and chemical composition among 12 midwater fish species, from three adjacent areas of the eastern North Pacific: the eastern gyre, the California Current, and the transitional region between them. By comparing trends in chemical composition across a geographical productivity gradient, the influence of food availability could be examined both with and without depth as a dependent variable. In general, caloric density, lipids, and water content showed consistent trends along both vertical and horizontal gradients of food availability. Lipids and caloric contents were lowest among bathypelagic species and among fishes from the gyre. Water content was highest in the gyre and among deeper-living species. While protein content as a percentage of the total wet weight declined with depth, there was no clear trend among mesopelagic species between stations. Protein as percent of the total organic content showed no decrease with depth, but was significantly higher in the gyre among mesopelagic species. These findings suggest that compositional trends in water, lipids, and caloric content are correlated with food availability, regardless of any depth-related factors. In contrast, protein did not vary consistently with food availability and thus may be linked to other factors.     

Reproductive biology in two species of deep-sea squids

Deep-sea squids, Moroteuthis ingens and Gonatus antarcticus, were collected in the slope waters off the Falkland Islands and their reproductive systems preserved and investigated onshore. Changes in oocyte length-frequencies at maturation and spawning, and their fecundity were studied. These squids, as well as many other species, are characterised by a synchronous oocyte growth and ovulation. Oviducts are not used for ripe egg accumulation and consequently the universal scale of Lipinski (1979) cannot be applied to assign female maturity. M. ingens spawns near the bottom; its fecundity is 168–297 thousand eggs. Maximum egg size is 1.8–2.7 mm. G. antarcticus spawns midwater; its fecundity is 10–25 thousand eggs. Egg size is 3.2–3.3 mm. In M. ingens spawning takes place in the austral autumn and winter, in G. antarcticus—in austral winter. Our data and the literature data show that the so-called “synchronous ovulation” probably occurs in all deepwater squids. This pattern is very rare among fish, but is quite common among benthic octopods that brood their egg masses.     

Feeding strategies of some demersal fishes of the continental slope and rise off the Mid-Atlantic Coast of the USA

Stomach contents of 729 fishes comprising 16 species were examined from the continental slope and rise off the Middle Atlantic States of the USA. Two main feeding modes among demersal deep-sea fishes were evident: those feeding primarily on pelagic food items, and those feeding on benthic invertebrates. Both pelagic and benthic predators were euryphagous. Most pelagic predators also fed on the epibenthos. These findings support Dayton and Hessler's (1972) suggestion that benthic predators should have a generalized diet which may be responsible for the high diversity found in the deep-sea infauna. The mesopelagic fauna is an important food source for some demersal fishes on the continental slope. Pelagic prey, which are also important to ecologically dominant demersal fishes on the lower slope and continental rise, may be nutritionally supported by suspended particulate organic matter in a nepheloid layer close to the bottom, and they may exist in much higher concentrations than in the bathypelagic zone above.Virginia Institute of Marine Science Contribution No. 835.     

Food habits and intestinal parasites of deep demersal fishes from the upper continental slope east of Newfoundland,northwest Atlantic Ocean

Stomach contents and intestinal parasite faunas of 471 individuals of demersal fishes in 14 species were examined from the Carson Canyon region (Lat. 45°30N; Long. 48°40W) of the upper continental slope of the Grand Banks off Newfoundland, Canada. Individual species tended to feed either on benthic or on pelagic/benthopelagic organisms, but pelagic prey assumed the greatest importance overall. Data from stomach contents were supported by the parasite information. Prevalence of parasites was higher in benthic feeders (53.1%) than in pelagic feeders (28.9%), and relative abundance by major group was: Digenetic Trematoda 5.8% benthic vs 27.8% pelagic, Nematoda 53.1% vs 72.2%, and Acanthocephala 40.9% vs 0%. Of the dominant fishes, there were more species of benthic feeders (5) than pelagic feeders (3), but pelagic feeders were numerically more abundant (pelagic 70.9%, benthic 20.5%). Benthic feeders were on average larger (=270.6g) than pelagic ones (=130.6g), but pelagic feeders represented a larger proportion of the biomass (pelagic 43.3%, benthic 25.9%). The results of this study combined with those from other areas suggest that feeding from the pelagial by demersal fishes at upper continental slope depths is probably the general rule.     

Lipid compositions,water contents,swimbladder morphologies and buoyancies of nineteen species of midwater fishes (18 myctophids and 1 neoscopelid)

Lipid compositions, water contents, swimbladder morphologies and specific gravities were studied for 19 species of oceanic midwater fishes, chiefly myctophids (lanternfishes) collected offshore from Oregon and California from 1975–1979. Three groups of species were recognizable. The first group had bodies low in both lipid and water content; they were denser than seawater, regardless of swimbladder morphology, which varied from absent through non-inflated to inflated. The second group had bodies low in lipid but high in water content; they were neutrally buoyant, evidently because of their high content of water, although their swimbladders were never inflated. The majority of the members of the aforementioned groups contained higher proportions of triglycerides than wax esters. The third group had bodies with high lipid but low water content; those with high triglyceride content had swimbladders ranging from non-inflated to (less commonly) inflated, were denser than seawater, and the lipid percentage of their body weights varied both with size and season, indicating that triglycerides function mainly as an energy store. The adults of species in the third group with a higher content of wax esters than triglycerides lacked inflated swimbladders, were neutrally buoyant, and the lipid percentage of their body weights was relatively constant with size and season, indicating that wax esters permit these fishes to attain neutral buoyancy in seawater.     

Reproductive cycles of three dominant Calanus species in Disko Bay, West Greenland

This study describes the annual reproductive cycles of the three dominant Calanus species, C. finmarchicus, C. glacialis and C. hyperboreus, in Disko Bay (West Greenland) in relation to seasonal phytoplankton development. Relative abundance of females, copepodite stage V (CV) and males, and the developmental stage of the female gonad were examined from plankton samples collected at weekly to monthly intervals from May 1996 to June 1997 with a WP2 net or a pump. During spring and summer, egg production rates were determined. Females of all three species were present year round. Maximum relative abundance was reached by C. hyperboreus females at the beginning of February, by C. glacialis in mid-February, and by C. finmarchicus in April. All three species reproduced successfully in Disko Bay. Their reproductive cycles were considerably different with respect to the timing of final gonad maturation and spawning, and hence in their relation to seasonal phytoplankton development. In all three species, early gonad development took place during winter, before living food became plentiful, suggesting that these processes were largely food independent. Final gonad maturation and spawning in C. finmarchicus was related to the phytoplankton concentration, reflecting that final gonad maturation processes are food dependent in this species. C. glacialis females matured and spawned prior to the spring bloom. Our results indicate that first internal lipid stores and later ice alga grazing supplied final gonad maturation and egg production. Maximum egg production rates of C. glacialis were found in spring and summer, when the chlorophyll a concentration was high. Mature female C. hyperboreus were found from February until mid-April, when the chlorophyll a concentration was still low. In this species, reproductive activity was decoupled from phytoplankton development, and final maturation processes and spawning were solely fuelled by internal energy stores.     

Reproductive biology of commercially exploited deep-water royal red prawns (Haliporoides sibogae,Solenoceridae) in south-east Australia

Deep-water royal red prawns (Haliporoides sibogae de Man, 1907; Solenoceridae: Penaeidea) were sampled from commercial catches off south-east Australia between November 1988 and May 1990. Four stages of sexual development were distinguished macroscopically for each sex. For the females, histology and analysis of oocyte size were used to validate the macroscopic staging of ovaries into undeveloped, developing, early ripe and ripe. Females were larger than males, reaching a maximum size of 46 mm carapace length (CL) compared to 32 mm CL for males. Average size at sexual maturity was 30.8 mm CL for females and 25.8 mm CL for males. Females apparently breed several times in their life, whereas males probably breed only once. Individual fecundity (ranging from about 58 000 to 140 000 oocytes) was relatively low compared with other penaeids, but oocytes were larger (mean diameter=0.41 mm). Two short breeding seasons occurred per year (February to April and July to August). There was a clear size-related latitudinal distribution of the prawns, with largr prawns (spawners) being more abundant in the northern region of the fishing area, and juveniles (CL20 mm) being more abundant in the south. Most of the spawning stock probably lies outside the main fishing ground, while the fishery targets mainly prawns below the size at maturity.     

Association of Syscenus infelix (Crustacea: Isopoda: Aegidae) with benthopelagic rattail fishes, Nezumia spp. (Macrouridae), along the western North Atlantic continental slope

During submersible surveys along the continental slope (summers of 1991 and 1992, 184–847 m) between False Cape, Virginia, and Cape Hatteras, North Carolina, USA, we observed the aegid isopod, Syscenus infelix Harger, attached to the macrourid Nezumia bairdii (Goode and Bean). This is the first report of S. infelix attached to fishes in the western North Atlantic. The association of this blind isopod with its host appears species specific. The large, conspicuous isopod always attached to a fish in the same location, the dorsal midline, immediately behind the first dorsal fin. Attachment appears to be long term, with the isopod forming a characteristic scar consisting of a distinct discolored oval depression with seven small, dark impressions that coalesce as the fish grows. Only one S. infelix was found on each host fish. The isopod occurred on 23.7% of N. bairdii observed from submersible on the middle continental slope off Virginia and North Carolina, compared with 16.6% of 1236 museum specimens of the same species (based on inspection for scars) collected at latitudes 26°–64°N. Prevalence of the fish–isopod association was not correlated with depth or latitude. We also found identical scars on preserved specimens of N. aequalis (2.6% of 660 specimens), N. sclerorhynchus (1.2% of 86 specimens), and N. suilla (14.3% of 7 specimens), mostly from areas outside the range of N. bairdii. No scars were found on museum specimens of N. atlantica (n=27), N. cyrano (n=57), or N. longebarbata (n=7). The low incidence of isopod attachment on these species suggests that N. bairdii is the preferred host. Infestation by the isopod appears to result in erosion of host fish scales and tissue. We propose that S. infelix is an obligate associate of its host fish and should be considered parasitic. Received: 9 June 2000 / Accepted: 21 October 2000     

Triacylglycerols and wax esters in the lipids of deep midwater teleost fishes of the Southern California Bight

Total lipid contents and the proportions of triacylglycerols and wax esters were examined in 23 species of deep-sea fishes collected between 1977 and 1980 in the Santa Catalina, San Clemente and San Pedro Basins off Southern California, USA. Mean total lipid content ranged from 1 to 22% of wet weight. Triacylglycerols accounted for 1 to 91% of total lipids present and were more abundant than wax esters in the lipids of all but a single species. Wax ester contents ranged from 0 to 24% of total lipids but in 20 species were 10% of the lipids. These results do not substantiate the previously suggested idea that wax esters are important constituents of the lipids of deep-sea fishes. Moreover, the hypothesis that wax ester content increases with increasing depth of occurrence is not supported. To assess the importance of the lipids present in bouyancy regulation, the water content and swimbladder inflation of these species were also examined. Neither total lipids nor wax esters appear to play an important role in buoyancy regulation in the majority of these fishes. Eight species have gas-filled swimbladders. Of the species lacking inflated swimbladders, 7 have watery bodies (87 to 95% H₂O) and 6 with low lipid (8.3%) and water (85%) contents may need to swim continuously to maintain their position in the water column. Lipids, largely in the form of triacylglycerols, may be important in the buoyancy of three species with lipid contents>10% of wet weight. Two of these species also possess inflated swimbladders. The reasons for the production of large quantitites of was esters by certain deep-sea teleosts, especially gonostomatids and myctophids, remain unknown.     

Reproductive biology of a deposit-feeding amphipod, Casco bigelowi, with extended parental care

The reproductive traits of a deposit-feeding amphipod that engages in extended parental care were examined. At the study site in Lowes Cove, Maine, USA, Casco bigelowi (Blake, 1929) occurred in highest densities in soft sediments just below mean low water (MLW). During most months, the sex ratio was ≃1. Many females hosted males in their burrows throughout the summer, but after fertilization of females in September, all adult males disappeared from the study area. In October almost 80% of the females were ovigerous, and in November about the same percentage was parental, i.e. caring for juveniles in their burrows. The females produced only one brood each in late fall which they accommodated in their burrows for 2 mo or longer. The average number of juveniles per female was ∼20 in November, and continuously decreased until January. Juveniles reached sizes >10 mm length in the maternal burrows. In early December the first juveniles were found in their own burrows, but major recruitment took place in late December and January. It is concluded that for C. bigelowi, the delay of recruitment into the winter months with low predation pressure and the large offspring size at this time are major advantages gained by extended parental care. C. bigelowi is host to the peritrich ciliate Cothurnia sp. on its gills, and during the summer months >70% of all amphipods had ciliates on their gills. Juveniles still living in their mother's burrows showed infestation rates similar to that of the parent; those of highly infested mothers were more heavily infested than those of “clean” mothers. Facilitated epibiont transmission during intimate and long-lasting (2␣mo) parent–offspring associations may be a consequence of extended parental care. Received: 25 November 1997 / Accepted: 14 April 1998     

Species composition, biomass and vertical distribution of micronekton over the mid-slope region off southern Tasmania, Australia

Seasonal sampling was carried out based on day/night, vertically stratified tows (100 or 125 m strata) in the upper 900 m of the water column over the mid-slope commercial fishing grounds south of Tasmania. A large midwater trawl (105 m² mouth area) was used with an opening/closing cod-end. Subtropical convergence and subtropical species dominated the fauna, but many less abundant, more widely-distributed species were also present. Fishes, which contributed 89% of micronekton biomass and 135 of 178 species, were dominated by the Myctophidae (48% biomass and 48 species). Twenty micronekton species made up 80% of the total biomass. Overall, the micronekton fish biomass in this region was 2.2 g m⁻² wet weight. A pronounced day/night shift in the distribution of biomass was attributable to diel migratory species. During the day, <0.2% of the total micronekton biomass was found in 0 to 300 m; most biomass was below 400 m, with peaks at 400 to 525 m and 775 to 900 m. At night, 53% of the biomass was found in 0 to 300 m, with progressively less in each deeper stratum. The vertical ranges of individual species typically exceeded 400 to 500 m during the day and night and were non-coincident, although nyctoepipelagic migrators were concentrated in the surface 200 m at night. Distinct epipelagic, lower and upper mesopelagic assemblages were identified, and patterns of epipelagic migration, limited migration and non-migration were categorised for species from each of the lower and upper mesopelagic assemblages. The vertical distribution of these assemblages was coincident with the primary water masses: subantarctic mode water (∼250 to 600 m) and antarctic intermediate water (below ∼700 m). The flux of migrating micronekton, estimated at 0.94 to 3.36 g C m⁻² yr⁻¹ to the lower mesopelagic and 1.14 to 4.06 g C m⁻² yr⁻¹ to the upper mesopelagic, appeared to be considerably outweighed by the consumption needs of aggregated mid-slope benthopelagic predators. We suggest that advection of mesopelagic prey in antarctic intermediate water may sustain aggregated populations of orange roughy (Hoplostethus atlanticus) and other predators on the micronekton in mid-slope depths at this site. Received: 2 April 1997 / Accepted: 21 August 1997