Matthew R. Dunn

Lead-radium dating of orange roughy (Hoplostethus atlanticus): validation of a centenarian life span

Life-span estimates for orange roughy (Hoplostethus atlanticus) range from ~20 years to well over 100 years. In this study, an improved lead-radium dating technique provided independent age estimates from sagittal otoliths. This tech- nique used the known properties of radioactivity for lead-210 and radium-226 to determine the validity of fish age esti- mates. An improvement to lead-radium dating using mass spectrometry allowed the use of smaller samples than previously possible; therefore, an application was made to otolith cores, the first few years of otolith growth. This ap- proach circumvented the use of whole otoliths and alleviated many of the assumptions that were necessary in previous lead-radium dating applications. Hence, it was possible to critically evaluate lead-radium dating as a tool in fish age vali- dation. The measurement of lead-radium ratios for a series of age groups that consisted of otolith cores, grouped based on growth-zone counts from thin sections, showed a high degree of correlation to the expected lead-radium ingrowth curve. This finding provided support for age estimation procedures using thin otolith sectioning. As independent estimates of age, the results indicated that fish in the oldest age group were at least 93 years old, providing robust support for a centenarian life span.

Feeding habits of the ommastrephid squid Nototodarus sloanii on the Chatham Rise, New Zealand

Abstract The diet of the squid Nototodarus sloanii was determined from examination of stomach contents of 388 specimens of 14.3–41.0 cm dorsal mantle length sampled at depths of 211–760 m on the Chatham Rise, New Zealand. Prey items were predominantly mesopelagic fishes, with some crustaceans and cephalopods. The most important prey species identified was Maurolicus australis, followed by Lampanyctodes hectoris, and unidentified squids (Teuthoidea). Multivariate analyses using distance‐based linear models, non‐parametric multi‐dimensional scaling, analysis of similarities, and similarity percentages, indicated crustaceans were more important in the diet of smaller squid (83–480 g), the fish component of the diet was dominated by L. hectoris on the east Chatham Rise and M. australis on the west Chatham Rise, and there may be a difference in diet with sex, with crustaceans and cephalopods more important for females. The results indicated that N. sloanii, similar to other ommastrephid squids, foraged primarily in the mesopelagic layers.

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.

Spatial management of deep-sea seamount fisheries: balancing sustainable exploitation and habitat conservation

Seamounts throughout the worlds oceans can support diverse and abundant fish communities. Many have been subject to commercial deep-sea bottom trawl fisheries and have exhibited ‘boom and bust’ characteristics. There is growing concern about the effect of fishing on fragile and vulnerable benthic invertebrate species. This review examines why deep-sea fisheries have generally failed, and recommends measures that are necessary to improve their sustainability. Much is based on lessons learned in the south-west Pacific that may be more generally applicable to global deep-sea fisheries. Sustainable fisheries require highly precautionary feature-based catch limits, and credible and timely stock assessment advice. Management also needs to consider fishing impacts on the benthic habitat, and while reducing and spreading fishing effort on seamounts is beneficial for fish stocks, it can have a negative effect on the benthos. To balance exploitation and conservation, elements of spatial management are required, whereby some seamounts are protected before any fishing has occurred. Protected areas should include entire seamounts, and multiple seamounts in a network. A management regime should incorporate closed seamounts, open seamounts for fishing, and management of adjacent slope areas where these are important for the productivity of fish and invertebrate populations.

Diet and scavenging habits of the smooth skate Dipturus innominatus.

The diet of smooth skate Dipturus innominatus was determined from examination of stomach contents of 321 specimens of 29·3-152·0 cm pelvic length, sampled from research and commercial trawlers at depths of 231-789 m on Chatham Rise, New Zealand. The diet was dominated by the benthic decapods Metanephrops challengeri and Munida gracilis, the natant decapod Campylonotus rathbunae and fishes from 17 families, of which hoki Macruronus novaezelandiae, sea perch Helicolenus barathri, various Macrouridae and a variety of discarded fishes were the most important. Multivariate analyses indicated the best predictors of diet variability were D. innominatus length and a spatial model. The diet of small D. innominatus was predominantly small crustaceans, with larger crustaceans, fishes and then scavenged discarded fishes increasing in importance as D. innominatus got larger. Scavenged discards were obvious as fish heads or tails only, or skeletal remains after filleting, often from pelagic species. Demersal fish prey were most frequent on the south and west Chatham Rise, in areas where commercial fishing was most active. Dipturus innominatus are highly vulnerable to overfishing, but discarding practices by commercial fishing vessels may provide a positive feedback to populations through improved scavenging opportunities.

Hypotheses of Spatial Stock Structure in Orange Roughy Hoplostethus atlanticus Inferred from Diet, Feeding, Condition, and Reproductive Activity

We evaluate hypotheses for meso-scale spatial structure in an orange roughy (Hoplostethus atlanticus) stock using samples collected during research trawl surveys off the east coast of New Zealand. Distance-based linear models and generalised additive models were used to identify the most significant biological, environmental, and temporal predictors of variability in diet, proportion of stomachs containing prey, standardised weight of prey, fish somatic weight, fish total weight, and reproductive activity. The diet was similar to that observed elsewhere, and varied with ontogeny, depth, and surface water temperature. Smaller sized and female orange roughy in warmer bottom water were most likely to contain food. Fish condition and reproductive activity were highest at distances more than 20 km from the summit of the hills. Trawl survey catches indicated greater orange roughy densities in hill strata, suggesting hill habitat was favoured. However, analyses of feeding, condition, and reproductive activity indicated hill fish were not superior, despite fish densities on hills being reduced by fishing which, in principle, should have reduced intra-specific competition for food and other resources. Hypotheses for this result include: (1) fish in relatively poor condition visit hills to feed and regain condition and then leave, or (2) commercial fishing has disturbed feeding aggregations and/or caused habitat damage, making fished hills less productive. Mature orange roughy were observed on both flat and hill habitat during periods outside of spawning, and if this spatial structure was persistent then a proportion of the total spawning stock biomass would remain unavailable to fisheries targeting hills. Orange roughy stock assessments informed only by data from hills may well be misleading.

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.

A comparison of the diets of silver (Seriolella punctata) and white (Seriolella caerulea) warehou

Abstract The diets of the silver warehou (Seriolella punctata) and the white warehou (S. caerulea), were determined from examination of the stomach contents of 478 and 291 specimens, respectively, sampled by bottom trawl on Chatham Rise to the east of South Island, New Zealand. The diets of both species were dominated by pelagic tunicates, with the remainder comprising mostly small crustaceans. The importance of small crustaceans (amphipods, copepods, and euphausiids) to white warehou was much greater than for silver warehou. An unknown but small component of the crustacean prey was ingested unintentionally owing to a common commensal relationship between some crustaceans (primarily amphipods) and tunicates. The distributions of silver and white warehous on Chatham Rise overlapped geographically and by depth; the entire white warehou distribution overlapped that for silver warehou. While some between-species differences in dietary composition were apparent, these are unlikely to markedly reduce any competition between species for the tunicate resource. It is possible that the niches of silver and white warehou overlap considerably. There may be some behavioural or biological mechanisms not identified in this study that reduce the resource competition between these two species, but understanding the mechanisms that promote the coexistence of silver and white warehou requires further study.

Moon phase influences the diet of southern Ray's bream Brama australis

Diet composition of the southern Rays bream Brama australis was examined from stomach contents of 399 specimens sampled by bottom trawl on Chatham Rise to the east of South Island, New Zealand, over 3 years. Prey items were predominantly mesopelagic fishes and crustaceans. Multivariate analysis indicated that moon phase explained more of the diet variability than any other predictor examined. It appears likely that diet composition is influenced by a combination of changes in both tidal flows and illumination. Different combinations of prey were consumed by B. australis at different times of the lunar cycle. An influence of moon phase on feeding by fishes has rarely been reported, but it is likely that moon phase influences the diets of other species that specialize in mesopelagic prey. The most important prey group by mass for B. australis was Myctophidae (primarily Lampanyctodes hectoris), followed by Stomiiformes (primarily Maurolicus australis) and shrimps (Sergestes spp). An ontogenetic shift in diet was observed, from numerical dominance by small crustaceans including amphipods and euphausiids (with some fishes) in smaller (mass <1045 g) B. australis to pelagic teleost prey (with a few larger crustaceans) in larger (>1440 g) B. australis.