Darren W. Stevens

Age, growth, and sexual maturity of two New Zealand endemic skates, Dipturus nasutus and D. innominatus

Abstract Rough and smooth skates (Dipturus nasutus (Banks 1841) and/), innominatus (Garrick & Paul 1974)) were aged by counting growth bands on X‐rays of thick sections of vertebral centra. Band counts were imprecise, but there was no between‐reader bias. Age estimates were not validated. The oldest rough skate was 9 years old, but few were more than 6 years old. Females may live longer than males. The combined sexes von Bertalanffy growth curve was Lr = 91.3 (1 ‐ e−0.16[t + 1.20]). Half the males matured by c. 52 cm pelvic length (PL) and 4 years, and females by 59 cm PL and 6 years. The oldest smooth skate in the sample was 24 years, but longevity probably exceeds that. Females appear to live longer than males. The combined sexes von Bertalanffy growth curve was: Lt = 150.5 (1 ‐ e−0.095[t + 1.06]). Half the males matured by c. 93 cm PL and 8 years, and females by 112 cm PL and 13 years. Smooth skate are late maturing and long‐lived relative to other skates, whereas rough skate are early maturing with a moderate life span.

Diet of Two Large Sympatric Teleosts, the Ling (Genypterus blacodes) and Hake (Merluccius australis)

Ling and hake are tertiary consumers, and as a result both may have an important structuring role in marine communities. The diets of 2064 ling and 913 hake from Chatham Rise, New Zealand, were determined from examination of stomach contents. Ling was a benthic generalist, and hake a demersal piscivore. The diet of ling was characterised by benthic crustaceans, mainly Munida gracilis and Metanephrops challengeri, and demersal fishes, mainly Macrourids and scavenged offal from fishing vessels. The diet of hake was characterised by teleost fishes, mainly macrourids and merlucciids. Multivariate analyses using distance-based linear models found the most important predictors of diet variability were depth, fish length, and vessel type (whether the sample was collected from a commercial or research vessel) for ling, and fish length and vessel type for hake. There was no interspecific predation between ling and hake, and resource competition was largely restricted to macrourid prey, although the dominant macrourid species predated by ling and hake were different. Cluster analysis of average diet of intraspecific groups of ling and hake confirmed the persistent diet separation. Although size is a central factor in determining ecological processes, similar sized ling and hake had distinctly different foraging ecology, and therefore could influence the ecosystem in different ways, and be unequally affected by ecosystem fluctuations.

The Antarctic toothfish (Dissostichus mawsoni): biology, ecology, and life history in the Ross Sea region

The Antarctic toothfish (Dissostichus mawsoni, Norman) is a large notothenioid fish that supports valuable fisheries around the Antarctic continent. The Ross Sea fishery, which started in 1997, is managed by the Commission for the Conservation of Antarctic Marine Living Resources (CCAMLR). Whilst a large amount of research into the biology of this species has been carried out over the past decade, much of this work has been presented in CCAMLR working group papers and has not been published in the primary literature. In this paper, we bring together and summarise the extensive published and unpublished literature on the biology and ecology of Antarctic toothfish in the Ross Sea region including distribution, stock structure, reproduction, age and growth, and trophic ecology in a single document for the first time. We also review and further develop testable hypotheses surrounding its life cycle and identify gaps in our knowledge including spawning behaviour and early life history which need to be addressed.

Diet of Antarctic toothfish ( Dissostichus mawsoni ) from the continental slope and oceanic features of the Ross Sea region, Antarctica

Abstract The diet of Dissostichus mawsoni captured by bottom longline in the Ross Sea region was examined during 2003, 2005 and 2010. The diet of sub-adult toothfish was similar to adult toothfish, comprising mainly benthic fishes and cephalopods. Sub-adult toothfish ate a greater variety of smaller prey than adults, including smaller fish and prawns. Grenadiers (Macrourus spp.) were the most important fish and overall prey species. On the continental slope, icefish (Channichthyidae) and eel cods (Muraenolepididae) were also important fish prey, while Psychroteuthis glacialis was the most important cephalopod prey. On oceanic features, toothfish fed mainly on Macrourus spp. but also fed on Antimora rostrata, cephalopods and the occasional mesopelagic to epipelagic fish. Diet varied significantly with toothfish size and location on northern parts of the Mawson and Iselin banks of the Ross Sea continental slope. There was no significant temporal change in diet composition.

Trophic Interactions and Distribution of Some Squaliforme Sharks, Including New Diet Descriptions for Deania calcea and Squalus acanthias

Squaliforme sharks are a common but relatively vulnerable bycatch in many deep water fisheries. Eleven species of squaliforme shark are commonly caught at depths of 200–1200 m on Chatham Rise, New Zealand, and their diversity suggests they might occupy different niches. The diets of 133 Deania calcea and 295 Squalus acanthias were determined from examination of stomach contents. The diet of D. calcea was characterised by mesopelagic fishes, and S. acanthias by benthic to pelagic fishes, but was more adaptive and included likely scavenging. Multivariate analyses found the most important predictors of diet variability in S. acanthias were year, bottom temperature, longitude, and fish weight. The diet of the nine other commonly caught squaliforme sharks was reviewed, and the spatial and depth distribution of all species on Chatham Rise described from research bottom trawl survey catches. The eleven species had a variety of different diets, and depth and location preferences, consistent with niche separation to reduce interspecific competition. Four trophic groups were identified, characterised by: mesopelagic fishes and invertebrates (Centroselachus crepidater, D. calcea, and Etmopterus lucifer); mesopelagic and benthopelagic fishes and invertebrates (Centroscymnus owstoni, Etmopterus baxteri); demersal and benthic fishes (Centrophorus squamosus, Dalatias licha, Proscymnodon plunketi); and a generalist diet of fishes and invertebrates (S. acanthias). The trophic levels of the species in each of the four groups were estimated as 4.18–4.24, 4.20–4.23, 4.24–4.48, and 3.84 respectively. The diet of Oxynotus bruniensis and Squalus griffini are unknown. The different niches occupied by different species are likely to influence their vulnerability to bottom trawl fisheries. Some species may benefit from fisheries through an increased availability of scavenged prey.

Diet and trophic niche of Macrourus spp. (Gadiformes, Macrouridae) in the Ross Sea region of the Southern Ocean

Both Macrourus caml and M. whitsoni are probably abundant and widespread in the Ross Sea region at depths between approximately 500 and 2100 m. We present the analysis of stomach contents (including regurgitated stomach contents) from an undifferentiated mixture of 33 specimens of these species (henceforth Macrourus spp.) in the Ross Sea region of the Southern Ocean. Samples of stomach contents were obtained from the Ross Sea slope (between about 71° and 73° S) and on the Scott Island and seamount chain. Overall, amphipods were the dominant prey found in Macrourus spp. stomachs with an index of relative importance (IRI) of 43%. About half the amphipods were Eurythenes gryllus and about half could not be identified. Based on stable isotope results, unidentified amphipods were probably herbivorous rather than carnivorous. Other important prey were copepods (especially for fish with total length less than 30 cm), krill (mainly Euphausia superba for larger fish) and fishes (Pleuragramma antarcticum and Gymnoscopelus opisthopterus). Minor prey found in stomachs included isopods, mysids, salps and polychaetes, with rocks and fragments of coral, echinoderm and shell also found. Stomach contents data provide evidence of both pelagic and benthic feeding. Carbon and nitrogen stable-isotope analysis of 161 Macrourus spp. muscle tissue samples from the Ross Sea slope, Admiralty and Scott seamounts and two parts of the Pacific-Antarctic fracture zone (near 65° S) are presented. Nitrogen isotope results (δ15N = 10.6 ± 1.0‰, mean ± SD) were generally consistent with stomach contents. Carbon isotope results (δ13C = −24.7 ± 0.8‰) were consistent with Southern Ocean residence but higher than phytoplankton values (−29.9 ± 1.2‰). We found that Macrourus spp. in the Ross Sea region have a trophic level of 3.8 ± 0.3, placing them well below the top of the food chain. Trophic level of Macrourus spp. was significantly affected by location and by size of fish (especially for fish less than 30 cm), whereas sex, fatness and bottom depth were not significant.

Some observations on the biology of two rarely seen deep‐sea chimaerids, Chimaera carophila and Hydrolagus homonycteris

Chimaera carophila (n = 45) and Hydrolagus homonycteris (n = 11), two deep-sea chimaerids rarely caught in the waters off New Zealand, were collected from research trawl catches and commercial fishery catches around New Zealand at depths between 400 and 1300 m, between 2014 and 2016. Additional preserved specimens of both species (n = 58) from museum collections were analysed for size, sex and maturity. External assessment of male claspers and a combination of internal assessments of female gonad mass and oviducal gland width, were used to determine maturity. For both species, length at first maturity was 0·70-0·82 of their maximum observed chimaera length (LC ), with females maturing at a larger size. Length at maturity for C. carophila (LC range: 28·7-103·9 cm) was estimated at 72·5 cm LC for males (n = 163) and 82·5 LC for females (n = 58). In H. homonycteris, length at maturity (length range: 78·6-99·8 cm LC ) was estimated at 79·1 cm LC for males (n = 51) and 80·1 cm LC for females (n = 17). Ovarian fecundity was up to 31 for C. carophila and sperm storage was confirmed in the oviducal gland of this species. Both species preyed on benthic invertebrates. Some C. carophila and H. homonycteris inhabit depths beyond most current fisheries, but both species appear to be relatively rare and have reproductive parameters characteristic of low productivity, which may make these species vulnerable to population decline if mortality was to increase in the future.

Diets of deepwater oreos (Oreosomatidae) and orange roughy Hoplostethus atlanticus.

The diets of black oreo Allocyttus niger, smooth oreo Pseudocyttus maculatus, spiky oreo Neocyttus rhomboidalis and orange roughy Hoplostethus atlanticus were determined from examination of contents of 240, 311, 76 and 415 non-empty stomachs, from fishes sampled by bottom trawl on Chatham Rise to the east of South Island, New Zealand. Hoplostethus atlanticus had an opportunistic predatory strategy with a broad diet dominated by prawns and mesopelagic teleosts, but with substantial components of mysids and cephalopods. Pseudocyttus maculatus was strongly specialized on gelatinous zooplankton (jellyfish and salps). Allocyttus niger consumed mainly salps and hyperiid amphipods, and to a lesser extent fishes, prawns, mysids and copepods. Neocyttus rhomboidalis primarily consumed salps, along with mysids, euphausiids and fishes. Only P. maculatus did not exhibit significant ontogenetic variation in diet. The diets were also influenced by year and bottom depth. Differences in the distributions and diets of the four species probably reduce conflicts in resource use.

Notes on the diet of seven grenadier fishes (Macrouridae) from the lower continental slope of Chatham Rise, New Zealand

Diets of 8 rarely sampled species of grenadiers were examined from the lower continental slope (1178 to 1837 m) on Chatham Rise, New Zealand and includes the first known records of the diet of Bathygadus cottoides, Coryphaenoides mcmillani, C. murrayi, and Idiolophorhynchus andriashevi. Most species fed benthopelagically on small crustaceans, e.g., calanoid copepods, amphipods and mysids and others.

Surface egg structure and early embryonic development of the Antarctic toothfish, Dissostichus mawsoni Norman 1937

The Antarctic toothfish (Dissostichus mawsoni Norman 1937) is the largest notothenioid inhabiting high-latitude Antarctic waters, where it is an important fishery resource and plays a key ecological role at a high trophic level. Despite the considerable amount of data on D. mawsoni biology and distribution developed since the fishery began in 1997, crucial aspects of the life cycle, including spawning and early life history, remain undescribed. During the first winter longline survey to the northern Ross Sea region in 2016, ripe male and female D. mawsoni were collected for the first time, and in vitro fertilisation of eggs was performed. Here, we report on the first characterisation of D. mawsoni egg structure and initial embryonic development. The duration of the egg cleavage period was similar to that of other nototheniid species releasing pelagic eggs. The structural features of fertilised eggs, including chorion thickness and structure, support the hypothesis that eggs of D. mawsoni are pelagic. The data presented here contribute to the description of the potential habitat of the eggs of this species, and provide the first diagnostic information to recognise the eggs of D. mawsoni.