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Scavengers perform an important role in deep-sea ecosystems, recycling carrion. However, scavenged material can be difficult to discern from predation. The synaphobranchid eel Diastobranchus capensis Barnard, 1923, is common bycatch in deep-water fisheries for orange roughy ( Hoplostethus atlanticus Collett, 1889) in New Zealand waters. Despite anecdotal reports of scavenging in D. capensis , their ecology and feeding are little known. Scavenged material was estimated to be approximately 39 per cent (by weight) of the diet of D. capensis from northeastern Chatham Rise, New Zealand, from water depths of 1062–1196 m. Scavenged material was defined as fragmentary food items from animals and plants either not normally available to the eel, or from animals larger than the eel consuming them (e.g., fish heads and tails, spent squid spermatophores, beaks and tentacle crowns, and seaweed). D. capensis were primarily piscivorous, with the diet supplemented by squid, natant decapods and mysids. The weight or number of food items did not increase with the size of the eel, nor was there any major ontogenetic shift in the composition of the diet. D. capensis probably plays an important role in mid-slope communities, recycling carrion.

Deep-sea fishes are the target of directed fisheries and are considered a conservation concern. Yet, we still know little about the factors that affect deep-sea fish distributions and assemblage patterns on relatively small spatial scales. We used results from remotely operated vehicle surveys that observed 105 km (∼346 960 m 2 ) of seafloor over a depth range of 351–2245 m in three canyons off Newfoundland to examine the occurrence, behavior, habitat specificity, and regional assemblage patterns of deep-sea fishes in this region. We found distinct assemblages based on both depth and habitat classifications. The most obvious unique assemblage was that associated with outcrops, which served as habitat for relatively rare species such as Neocyttus helgae , Hoplostethus atlanticus , and Lepidion eques . Several coral habitats hosted distinct assemblages when compared to other habitats with low or medium structural complexity. Our results illustrate that any program targeted at protecting deep-sea ecosystems must protect a wide-range of habitats and depths to conserve a variety of fish species and assemblages.

Microsatellite and otolith chemistry variability were analysed to assess fine scale genetic structure in the deep-sea teleost orange roughy (Hoplostethus atlanticus). The Porcupine Bank located on the continental shelf west of Ireland, comprises a complex system of mounds and flat areas that are broken up by canyons. Orange roughy form spawning aggregations on mounds and flat areas, and were heavily fished until the resource was depleted. By analysing adults in spawning condition and juvenile orange roughy from six mounds and one flat area, shallow but significant genetic population structure was evident (FST=0.0031, Dest across loci=0.0306 and G-test). Most of the structure was accounted for by inclusion of a sample from the flats (six of ten significant pairwise FST estimates and G-tests, and five of the highest Dest estimates included the flat sample). While the flat sample contributed most to the genetic structure, there was still significant (albeit weaker) structure among mound samples. The observed structure was supported by otolith analyses. Fish caught as late juveniles in either the flat or mound areas showed consistent differences in chemistry at the otolith core throughout the initial 10 years of growth, which could indicate site fidelity. We hypothesise that seafloor topographic structures (mounds and flats) may provide discrete spawning areas for orange roughy and that the limited gene flow between these spawning areas is insufficient to counteract genetic drift.

Otoliths of pre-recruit orange roughy (Hoplostethus atlanticus) were sampled from different deep-water habitats (‘flat’ and ‘hill’) and areas (north and south) on the Porcupine Bank. Age-based profiles for stable isotopes of carbon (d13C) and oxygen (d18O) in these otoliths were developed, which provide a fish life-history record of water depth and metabolic activity. These profiles were consistent among all individuals implying endogenous (ontogenic) influences on the pattern. The data indicate that post-larval orange roughy are mesopelagic active foragers, early juvenile fish move into a low energy deep-demersal phase, and older pre-recruit orange roughy assume the habitat depth and metabolic rate typical of adults. Comparison of otolith stable isotope profiles among areas and habitats on the Porcupine Bank suggest that juvenile orange roughy from South ‘hill’ and ‘flat’ habitats experience differing temperature and metabolic status at certain life stages. This may reflect oceanographic and ecological divergence between the two environments and suggests fine-scale population structure that may reduce resilience to exploitation.

The response of the benthopelagic fish, orange roughy ( Hoplostethus atlanticus ), to lowering an underwater camera was monitored acoustically. Acoustic layers of the fish at 660–790 m depth dispersed rapidly at least 30–40 m when the camera was ∼130 m above them. The reaction occurred day and night and prior to activation of the strobe lights, so it was presumably mediated by the low-frequency sound of the system being lowered rather than visually. Orange roughy contain a pronounced lateral line and extensive frontal sensory canal system that may be used to sense low-frequency sound. Our observations indicate that some marine species are highly sensitive even to non-capture sampling gears, so use of non-remote methods of sampling may lead to highly biased estimates of density. The avoidance response is consistent with the relatively high metabolic levels that have been reported for this species, as well as with their very low estimated rates of natural mortality. We speculate that the response has evolved to facilitate escape from large, highly mobile predators.

The mid-slope (800–1200 m) demersal fish community off southeastern Australia was sampled at 376 random, depth-stratified trawl stations. The mean density of demersal species was 4.82 g m −2 . Thirty-seven families and 111 species of demersal fish were represented in the catch. The density of mid-slope fishes off southeastern Australia was comparable to that observed in the Northern Hemisphere. However, landings and acoustic and egg surveys of orange roughy ( Hoplostethus atlanticus ) indicate that densities of that species alone are an order of magnitude higher than the total fish density indicated by trawl surveys. Water-column productivity over the mid-slope region appears insufficient to support the higher range of density estimates, implying a significant flux of energy into the region either from offshore or downslope. The dominant mid-slope demersal fishes appear to comprise an identifiable community within a biogeographic province that extends at least from the Great Australian Bight to the Chatham Rise (New Zealand), a distance of ∼5000 km. Distinct assemblages of demersal fish were found at upper (500 m) and mid-slope (800–1200 m) depths off southeast Australia. The mid-slope community could be sub-divided into assemblages by depth (shallow, intermediate and deep) and area (east and west Tasmania), which were statistically robust although with considerable overlap of species composition. There was no overlap in species composition of the southeast Australian mid-slope demersal fish community with fish communities at similar latitudes and depths in the North Pacific, but there were affinities with those in the North Atlantic. These biogeographic patterns, which appear consistent with oceanic circulation at intermediate depths, provide strong evidence that negates the recent hypothesis that deepwater fish communities cannot be defined over broad areas and are only random assemblages ( Haedrich and Merrett , 1990, Progress in Oceanography , 24 , 239–250).