Alistair Dunn

Orbitally induced oscillations in the East Antarctic ice sheet at the Oligocene/Miocene boundary

Between 34 and 15 million years (Myr) ago, when planetary temperatures were 3–4 °C warmer than at present and atmospheric CO2 concentrations were twice as high as today, the Antarctic ice sheets may have been unstable. Oxygen isotope records from deep-sea sediment cores suggest that during this time fluctuations in global temperatures and high-latitude continental ice volumes were influenced by orbital cycles. But it has hitherto not been possible to calibrate the inferred changes in ice volume with direct evidence for oscillations of the Antarctic ice sheets. Here we present sediment data from shallow marine cores in the western Ross Sea that exhibit well dated cyclic variations, and which link the extent of the East Antarctic ice sheet directly to orbital cycles during the Oligocene/Miocene transition (24.1–23.7 Myr ago). Three rapidly deposited glacimarine sequences are constrained to a period of less than 450 kyr by our age model, suggesting that orbital influences at the frequencies of obliquity (40 kyr) and eccentricity (125 kyr) controlled the oscillations of the ice margin at that time. An erosional hiatus covering 250 kyr provides direct evidence for a major episode of global cooling and ice-sheet expansion about 23.7 Myr ago, which had previously been inferred from oxygen isotope data (Mi1 event).

Promising signs of regeneration of blue cod and oyster habitat changed by dredging in Foveaux Strait, southern New Zealand

Abstract Epifaunal reefs in Foveaux Strait are oyster (Ostrea chilensis Philippi, 1845) habitat. One hundred and thirty years of oyster dredging has diminished the complexity and distribution of these reefs. Commercial densities of blue cod (Parapercis colias (Forster in Bloch and Schneider, 1801)) were discovered on epifaunal reef habitat in 1989 and became the focus of a major blue cod fishery. We document habitat changes that followed the closing of the oyster fishery in 1993 and interactions between the blue cod and oyster fisheries after the oyster fishery was reopened in 1996. Evidence from blue cod fishers and oyster surveys suggests that the benthic habitat of some oyster beds regenerated in the absence of dredging and that the relative density of blue cod, and then oysters, rebuilt to commercial levels. Benthic habitat was modified once more when oyster dredging restarted and the relative density of blue cod on oyster beds fell again. The observations suggest that rotational fishing of oysters could mitigate the effects of dredging on habitat and that marine protected areas could expedite habitat recovery. Increasing habitat complexity and blue cod density on a reef of oyster shells formed by an oyster fisher suggests that habitat enhancement might remedy effects of dredging. The questions raised by the observations could be answered by management experiments on the scale of the fisheries.

Mean circulation and hydrography in the Ross Sea sector, Southern Ocean: representation in numerical models

Abstract Three models were used to look at the Southern Ocean Ross Sea sector circulation and hydrography. Two were climate models of low (1°) to intermediate resolution (1/3°), and one was an operational high resolution (1/10°) ocean model. Despite model differences (including physics and forcing), mean and monthly variability aspects of off-shelf circulation are consistently represented, and could imply bathymetric constraints. Western and eastern cyclonic gyral systems separated by shallow bathymetry around 180°E redistributing water between the wider Southern Ocean and the Ross Sea are found. Some model seasonal gyral transports increase as the Antarctic Circumpolar Current transport decreases. Model flows at 900 m at the gyral eastern end compare favourably with float data. On-shelf model depth-averaged west–east flow is relatively consistent with that reconstructed from longline fishing records. These flows have components associated with isopycnal gradients in both light and dense waters. The climate models reproduce characteristic isopycnal layer inflections (‘V’s) associated with the observed Antarctic Slope Front and on-shelf deep water formation, and these models transport some 4 Sv of this bottom water northwards across the outer 1000 m shelf isobath. Overall flow complexity suggests care is needed to force regional Ross Sea models.

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.

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.

Revised estimates of natural mortality for the Foveaux Strait oyster (Ostrea chilensis)

Abstract We present revised estimates of the instantaneous natural mortality rate of recruited (legal sized) Foveaux Strait dredge oysters (Ostrea chilensis, Philippi, 1845) for the years 1974–86. A mark‐return experiment was analysed, assuming constant rate of fishing (and hence sampling) over 6 months of the year (i.e., the period of the fishing season). The natural mortality between 1974 and 1986 of recruited oysters increased from 0.02 to 0.19 year1 for a sample of oysters released in 1974, and from 0.01 to 0.20 year‐1 for those released in 1973. We explored possible causes for the observed increases including the impact of indirect fishing mortality, rising disease mortality ahead of the Bonamia sp. epidemic of 1986, and systematic mis‐reporting in returns of living and dead tagged oysters. We find no direct evidence for any of these, and hence speculate that the observed increase in natural mortality of the tagged individuals is associated with senescence.

Response to the opinion paper by Ainley et al.

We respond to comments made in the opinion paper by Ainley et al. (Hydrobiologia, 10.1007/s10750-015-2607-4, 2016) regarding our recent publication on the biology, ecology and life history of Antarctic toothfish (Dissostichus mawsoni) in the Ross Sea region. We focus in particular on the spatial and temporal extent of data collected from the fishery; results of research to date on changes in abundance and size of Antarctic toothfish at McMurdo Sound; and the subject of neutral buoyancy and inferences about vertical distribution. We conclude by re-iterating the need for well-designed, standardised research programmes to address remaining uncertainties in its life history.