Stuart Hanchet

Pilot trophic model for subantarctic water over the Southern Plateau, New Zealand: a low biomass, high transfer efficiency system

Abstract The Southern Plateau subantarctic region, southeast of New Zealand, is an important feeding area for birds, seals and fish, and a fishing ground for commercially significant species. The Southern Plateau is a major morphometric feature, covering approximately 433,620 km2 with average depth of 615 m. The region is noted for its relatively low levels of phytoplankton biomass and primary production that is iron-limited. In order to evaluate the implications of these attributes for the functioning of this ecosystem a steady-state, 19-compartment model was constructed using Ecopath with Ecosim software of Christensen et al. [ www.ecopath.org ]. The system is driven by primary production that is primarily governed by the supply of iron and light. The total system biomass of 6.28 g C m−2 is very low compared with systems so far modelled with a total system throughput of 1136 g C m−2 year−1. In the model, the Southern Plateau retains 69% of the biomass in the pelagic system and 99% of total production. Although fish are caught demersally, most of their food is part of production in the pelagic system. Top predators represent about 0.3% of total biomass and account for about 0.24 g C m−2 year−1 of food consumed made up of birds 0.058 g C m−2 year−1, seals 0.041 g C m−2 year−1, and toothed 0.094 g C m−2 year−1 and baleen whales 0.051 g C m−2 year−1. This amounts to 105,803 tonnes carbon over the whole of the Southern Plateau and is about 17% of the total amount of food eaten by non-mesopelagic fish. Mean transfer efficiencies between trophic levels II and IV of 23% are at the high end of the range reported in the literature. In the model, adult fish production is almost completely accounted for by the fisheries take (32%), consumption by seals (7%), toothed whales (21%), other adult fish (13%), and squid (20%). Fish and squid catches are at the trophic levels of 4.8 and 5.0, respectively. The gross efficiency of the fishery is 0.018% (catch/primary production). Although not all data come from direct knowledge of this system, the model reflects its general characteristics, namely a low primary production system dominated by the microbial loop, low sedimentation to the seafloor, high transfer efficiencies, a long food web and supporting high-level predators.

ALBATROSS PREDATION OF JUVENILE SOUTHERN BLUE WHITING (MICROMESISTIUS AUSTRALIS) ON THE CAMPBELL PLATEAU

Abstract Dietary samples collected at Campbell Island in summer 1997 indicate that southern blue whiting (Micromesistius australis) formed the bulk of the food of black‐browed albatrosses (Diomedea melanophrys impavida) during the chick‐rearing period. Birds preyed upon a single size class of fish with a mode at 80–90 mm standard length; fish were 4–5 months old and belonged to the 0+ age group. Satellite tracking showed that, when performing trips of short duration, adult albatrosses foraged within the 1000 m depth contour in the subantarctic zone north of Campbell Island. The feeding ecology of albatrosses thus suggests that juvenile (0+) southern blue whiting are pelagic and occur in dense schools in the top 5 m of the water column over the Campbell Plateau during the summer months. The high reliance of birds on juvenile southern blue whiting during the chick‐rearing period has implications for the management of the southern blue whiting fishery and the conservation of black‐browed albatrosses and othe...

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.

Stock structure of southern blue whiting (Micromesistius australis) in New Zealand waters

Abstract Data on the reproduction, growth, and morphometric characteristics of southern blue whiting (Micromesistius australis Norman) were examined to determine its stock structure in New Zealand waters. Consistent differences in the size distribution of fish, and in the timing of spawning were found between spawning grounds on the Bounty Platform, Pukaki Rise, Auckland Islands Shelf, and Campbell Island Rise. Significant differences in morphometric characteristics were found between fish from the Bounty Platform, Pukaki Rise, and Campbell Island Rise and significant differences in growth rates were found between the Bounty Platform and Campbell Island Rise. The results provide strong evidence that fish return to spawn on the grounds to which they first recruit, and suggest that the four areas should be assessed and managed as separate stocks.

Have Antarctic toothfish returned to McMurdo Sound

Abstract A dramatic reduction in catch rates of Antarctic toothfish in McMurdo Sound, Antarctica, has led to conclusions that the commercial bottom longline fishery for toothfish in the Ross Sea has drastically altered the toothfish population with cascading effects on the McMurdo Sound ecosystem. However, results from a new monitoring programme for Antarctic toothfish and other top predators carried out in McMurdo Sound in 2014 have shown toothfish catch rate, fish size and fish age similar to those observed prior to 2002. These results suggest that either large and old fish have returned to McMurdo Sound following a temporary environmentally driven absence or that they remained locally present but were not detected in the areas sampled. These findings highlight the importance of continued standardized monitoring for detecting the potential effects of fishing on the Ross Sea ecosystem.

Diversity, relative abundance, new locality records, and updated fish fauna of the Ross Sea region

Abstract Two surveys were carried out in the Ross Sea region during February and March 2004 and 2008 from the New Zealand RV Tangaroa. Fishes were sampled on the continental shelf and slope of the Ross Sea, and on adjacent seamounts to the north, mainly using a large demersal fish trawl and a large mesopelagic fish trawl. Parts of the shelf and slope were stratified by depth and at least three random demersal trawls were completed in each stratum, enabling biomass estimates of demersal fish to be calculated. Fish distribution data from these two surveys were supplemented by collections made by observers from the toothfish fishery. A diverse collection of over 2500 fish specimens was obtained from the two surveys representing 110 species in 21 families. When combined with previous documented material this gave a total species list of 175, of which 135 were from the Ross Sea shelf and slope (to the 2000 m isobath). Demersal species-richness, diversity and evenness indices all decreased going from the shelf to the slope and the seamounts. In contrast, indices for pelagic species were similar for the slope and seamounts/abyss but were much lower for the shelf.

Latitudinal variation of demersal fish assemblages in the western Ross Sea

Abstract Demersal fishes were sampled using a large fish trawl during two surveys carried out in February and March 2004 and 2008 in the Ross Sea, and around seamounts and islands just to the north at 66°S. The distribution and abundance of 65 species collected in these surveys were examined to determine if demersal fish communities varied throughout the area, and what environmental factors might influence this. Species accumulation with sample frequency did not reach an asymptote, but the rate of new species was low suggesting data were adequate for describing the main components of the communities. Three broad assemblages were identified, in the southern Ross Sea (south of 74°S), central–northern Ross Sea (between latitudes 71°–74°S), and the seamounts further north (65°–68°S) where some species more typical of sub-Antarctic latitudes were observed. Multivariate analyses indicated that environmental factors of seafloor rugosity (roughness), temperature, depth, and current speed were the main variables determining patterns in demersal fish communities.

Age validation and growth of southern blue whiting, Micromesistius australis Norman, in New Zealand

Abstract Over 3000 otoliths and 100 000 length and sex records were examined of southern blue whiting caught from commercial fishing grounds on the Campbell Island Rise, south of New Zealand, during August and September between 1981 and 1989. Ages of juveniles were validated by following modes in length‐frequency data over a 13‐month period between September 1981 and 1982. Ages of adults were validated up to at least 10 years by following strong year classes both from otolith‐based age frequency distributions and from length‐frequency data from 1981 through to 1989. Independent analysis of the length‐frequency data using MULTIFAN further supported the adult otolith ages. Above age 10 there was less confidence in otolith ages, less agreement between readers and a greater degree of bias between readers. When using the data in catch‐at‐age models it is recommended that ages greater than 10 be grouped as a single plus group.

CCAMLR’s precautionary approach to management focusing on Ross Sea toothfish fishery

Abstract Several recent papers have criticized the scientific robustness of the fisheries management system used by the Commission for the Conservation of Antarctic Marine Living Resources (CCAMLR), including that for Ross Sea toothfish. Here we present a response from the wider CCAMLR community to address concerns and to correct some apparent misconceptions about how CCAMLR acts to promote conservation whilst allowing safe exploitation in all of its fisheries. A key aspect of CCAMLR’s approach is its adaptive feedback nature; regular monitoring and analysis allows for adjustments to be made, as necessary, to provide a robust management system despite the statistical uncertainties inherent in any single assessment. Within the Ross Sea, application of CCAMLR’s precautionary approach has allowed the toothfish fishery to develop in a steady fashion with an associated accumulation of data and greater scientific understanding. Regular stock assessments of the fishery have been carried out since 2005, and the 2013 stock assessment estimated current spawning stock biomass to be at 75% of the pre-exploitation level. There will always be additional uncertainties which need to be addressed, but where information is lacking the CCAMLR approach to management ensures exploitation rates are at a level commensurate with a precautionary approach.

Mercury concentrations of two toothfish and three of its prey species from the Pacific sector of the Antarctic

Abstract Muscle tissue samples were collected from Antarctic toothfish (Dissostichus mawsoni Norman) and Patagonian toothfish (D. eleginoides Smitt) in 1998 and from D. mawsoni and three of its prey species - Whitsons grenadier (Macrourus whitsoni (Regan)), ice fish (Chionobathyscus dewitti Andriashev & Neyelov), and blue antimora (Antimora rostrata (Günther)) - in 2006 to determine their mercury. Mercury levels were highly variable both within and between the five species studied but were positively correlated with fish length in four of the species. Once the factors length and year had been accounted for, the mercury levels in D. eleginoides were more than four times greater than in D. mawsoni. The low levels of mercury in D. mawsoni relative to its prey species and the four-fold difference in mercury concentrations between it and D. eleginoides were unexpected. Reasons for these different levels of bioaccumulation were explored including differences in diet, growth and longevity, and location. Differences in bioaccumulation between the two toothfish species could be explained partly through differences in their geographic distribution and differences in trophic position. However, the low levels of mercury in D. mawsoni relative to its prey species can only be explained by a lower rate of mercury assimilation and/or a higher rate of mercury elimination by D. mawsoni.