Peter L. Horn

Age and growth of Patagonian toothfish (Dissostichus eleginoides) and Antarctic toothfish (D. mawsoni) in waters from the New Zealand subantarctic to the Ross Sea, Antarctica

Abstract The margins of otoliths of Patagonian toothfish ( Dissostichus eleginoides ) from several samples collected throughout the year were classified as either opaque or translucent. The margins were generally opaque in summer and translucent in winter. Thus, this species appears to deposit one translucent zone in its otoliths each year, and counts of these zones are probably a valid method to determine fish age. Comparisons of readings of D. eleginoides otoliths by workers from various institutions indicated a reasonable between-reader consistency, but still suggested that the otoliths were difficult to read. Von Bertalanffy growth parameters were calculated from the author’s readings only, separately by sex, for D. eleginoides caught from waters south of New Zealand to the Ross Sea, Antarctica, by longline and trawl fisheries. D. eleginoides appear to be moderately fast growing, at least to about age 10, and reasonably long-lived, reaching at least 50 years. Females grow at a faster rate and reach a larger size than males, but both sexes exhibit comparable maximum ages. Von Bertalanffy growth parameters were also calculated, separately by sex, for Antarctic toothfish ( Dissostichus mawsoni ) caught by the longline fishery in the northern Ross Sea. Otoliths of this species were interpreted similarly to those of D. eleginoides , but this method of ageing D. mawsoni is invalidated. D. mawsoni appears to be moderately fast growing, at least to about age 10, and can live for at least 35 years. This species probably grows at a slightly faster rate, and reaches a larger size than D. eleginoides .

Multiple techniques for determining stock relationships between orange roughy, Hoplostethus atlanticus, fisheries in the eastern Tasman Sea

Abstract Five techniques were used to determine stock relationships between four spatially isolated but geographically close orange roughy fisheries in the eastern Tasman Sea: the Lord Howe Rise (HOWE), Northwest Challenger (NWCH), Southwest Challenger (SWCH), and Westpac Bank (WPAC). The techniques included life history traits (age and length at maturity), population length frequency analysis, otolith shape analysis, genetic makers, and a comparison of spawning times. The estimated ages and lengths at onset of maturity did not identify clear stock differences between the four areas. Otolith shape revealed two groups: HOWE/NWCH and SWCH/WPAC. There were significant size differences between HOWE and NWCH (and between HOWE–SWCH and HOWE–WPAC) with larger fish on HOWE. One out of six nuclear DNA markers revealed significant heterogeneity among sites. Mitochondrial DNA polymorphisms of the control region revealed no heterogeneity among areas, but restriction digests of the whole mitochondrial genome revealed differences between HOWE and NWCH/SWCH. There was considerable between-year variation in the time of the onset of spawning at SWCH (3 weeks) and WPAC (4 weeks). In both areas, the time of spawning was later in the early 1990s than in the late 1990s. No major oceanographic features that might isolate stocks were identified in this region of the Tasman Sea. The biological differences between orange roughy taken from HOWE, NWCH, and SWCH indicate that these fisheries could be managed as independent stocks. There were no biological differences between SWCH and WPAC and these fisheries probably exploit one straddling stock.

Background and review of ageing orange roughy (Hoplostethus atlanticus, Trachichthyidae) from New Zealand and elsewhere

Abstract Studies on New Zealand orange roughy (Hoplostethus atlanticus) otoliths, and of orange roughy ageing conducted in New Zealand and elsewhere are described. Ageing studies have concentrated on three aspects: the interpretation of daily growth increments, the interpretation of annual growth increments, and radiometric analyses. All the methods suffer from problems relating to validation. Daily growth zones have not been validated, annual zones have been validated for juvenile fish only, and assumptions necessary for the application of radio‐metric techniques may be flawed. However, the weight of current evidence indicates that orange roughy are a slow‐growing, long‐lived species. A review of otolith morphology and microstructure studies, and a summary of the productivity parameters used in stock assessments of orange roughy, are also presented. Standard protocols used to prepare and interpret otoliths in current investigations are described. This review highlights the complexities of ageing long‐liv...

Validated aging methodology using otoliths, and growth parameters for hoki (Macruronus novaezelandiae) in New Zealand waters

Abstract The age of hoki was determined by counting bands in otolith cross‐sections. The technique was validated using the progression of length modes in length‐frequency distributions (for age classes 0+ to 4+) and the progression of strong and weak year classes in age‐frequency distributions from commercial catches sampled off the west coast of the South Island, New Zealand, from 1988 to 1994. Von Bertalanffy growth parameters for the west coast and Cook Strait spawning populations are significantly different, providing further support for a two‐stock hypothesis. In both areas female hoki have a growth rate significantly faster than males, and also a greater life expectancy.

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.

Feeding habits of alfonsino Beryx splendens.

The diet of the alfonsino Beryx splendens was determined from examination of stomach contents of 287 specimens of 17 to 48 cm fork length (L(F)) sampled by bottom trawl on the Chatham Rise to the east of New Zealand. Prey items were predominantly crustaceans and mesopelagic fishes. The most important prey species by mass was Sergestes spp. prawns, followed by the myctophid Lampanyctodes hectoris, and then Pasiphaea spp. prawns. Multivariate analyses indicated that small crustaceans (euphausiids and amphipods) were most important in the diet of smaller B. splendens (100-424 g, 17-26.5 cm), with larger prawn species and mesopelagic fishes most important for larger fish (425-2070 g, 27-46 cm). Moon phase and bottom temperature also explained some of the variability in diet, but the moon phase effect was difficult to explain, and the bottom temperature effect may have been confounded, to some extent, with L(F). The results indicated that B. splendens were moderately selective feeders that foraged primarily in the mesopelagic layers. The diet of New Zealand B. splendens is generally similar to those reported from other areas, i.e. dominated by mesopelagic crustaceans and fishes, and with a transition from small crustaceans to fishes with increasing predator size.

Diet and trophic niche of Antarctic silverfish Pleuragramma antarcticum in the Ross Sea, Antarctica

The diet of Antarctic silverfish Pleuragramma antarcticum was evaluated by examining stomach contents of specimens collected in the Ross Sea (71°-77° S; 165°-180° E) in January to March 2008. Pleuragramma antarcticum (50-236 mm standard length, L(S)) and prey items were analysed for stable-isotopic composition of carbon and nitrogen. According to index of relative importance (I(RI) ), which incorporates frequency of occurrence, mass and number of prey items, the most important prey items were copepods (81%I(RI) over all specimens), predominantly Metridia gerlachei and Paraeuchaeta sp., with krill and fishes having low I(RI) (2·2 and 5·6%I(RI) overall). According to mass of prey (M) in stomachs, however, fishes (P. antarcticum and myctophids) and krill dominated overall diet (48 and 22%M, respectively), with copepods being a relatively minor constituent of overall diet by mass (9·9%M). Piscivory by P. antarcticum occurred mainly in the extreme south-west of the region and near the continental slope. Krill identified to species level in P. antarcticum stomachs were predominantly Euphausia superba (14·1%M) with some Euphausia crystallophorias (4·8%M). Both DistLM modelling (PRIMER-permanova+) on stomach contents (by I(RI)) and stepwise generalized linear modelling on stable isotopes showed that L(S) and location were significant predictors of P. antarcticum diet. Postlarval P. antarcticum (50-89 mm L(S)) consumed exclusively copepods. Juvenile P. antarcticum (90-151 mm L(S)) consumed predominantly krill and copepods by mass (46 and 30%M, respectively). Small adult P. antarcticum (152-178 mm L(S)) consumed krill, fishes and copepods (37, 36 and 15%M, respectively). Large adult P. antarcticum (179-236 mm L(S)) consumed predominantly fishes and krill (55 and 17%M, respectively), especially in the north (near the Ross Sea slope) and in the SW Ross Sea. Amphipods were occasionally important prey items for P. antarcticum (western Ross Sea, 39%M). General concordance between stomach contents and trophic level of P. antarcticum and prey based on δ(15) N was demonstrated. Pleuragramma antarcticum trophic level was estimated as 3·7 (postlarval fish) and 4·1 (fish aged 3+ years).

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.

Age validation and growth of bluenose Hyperoglyphe antarctica using the bomb chronometer method of radiocarbon ageing

Age validation of bluenose Hyperoglyphe antarctica was sought using the independent bomb chronometer procedure. Radiocarbon ((14) C) levels were measured in core micro-samples from 12 otoliths that had been aged using a zone count method. The core (14) C measurement for each fish was compared with the value on a surface water reference curve for the calculated birth year of the fish. There was good agreement, indicating that the line-count ageing method described here is not substantially biased. A second micro-sample was also taken near the edge of nine of the otolith cross-sections to help define a bomb-carbon curve for waters deeper than 200-300 m. There appears to be a 10 to 15 year lag in the time it takes the (14) C to reach the waters where adult H. antarctica are concentrated. The maximum estimated age of this species was 76 years, and females grow significantly larger than males. Von Bertalanffy growth curves were estimated, and although they fit the available data reasonably well, the lack of aged juvenile fish results in the K and t(0) parameters being biologically meaningless. Consequently, curves that are likely to better represent population growth were estimated by forcing t(0) to be -0·5.

Dietary partitioning by two sympatric fish species, red cod (Pseudophycis bachus) and sea perch (Helicolenus percoides), on Chatham Rise, New Zealand

Abstract The diets of red cod Pseudophycis bachus and sea perch Helicolenus percoides were determined from examination of stomach contents of 246 and 494 specimens, respectively, sampled by bottom trawl on Chatham Rise to the east of South Island, New Zealand. The distributions of both species on Chatham Rise overlapped geographically and by depth; the entire P. bachus distribution was encompassed by the H. percoides distribution. The diets of both species were dominated by crustaceans, but fish was also an important dietary component for P. bachus, while pelagic tunicates were important for H. percoides. Despite the overlap in spatial distribution, P. bachus and H. percoides had distinct diets with non-significant overlap, which will markedly reduce resource competition between these two species. The most important crustacean prey were Galatheidae (Munida sp.) and the pandalid shrimp Notopandalus magnoculus in P. bachus, and two-spined crab (Pycnoplax victoriensis) and isopods in H. percoides. Within-species variation in diet was influenced by predator location and ontogeny. The main ontogenetic shifts in diet were from small shrimps to larger fish prey for P. bachus, and from small crustaceans (mysids and galatheids) to larger crustaceans (scampi and crabs) for H. percoides.