The diet of Antarctic toothfish (Dissostichus mawsoni) in the Ross Sea region was examined based on stomach samples obtained from observers on New Zealand fishing vessels in 2003 and 2010. Overall, the diet of sub-adult toothfish was similar to that of adult toothfish, with a dominance of piscivory on benthic fishes and cephalopods, augmented by benthic invertebrates. Sub-adult toothfish ate a greater variety of smaller prey than adults, including smaller fishes (such as Trematomus sp., dragonfish, mainly Bathydraco spp.), and large decapod prawns (Nematocarcinus). By weight and index of relative importance (IRI), grenadiers (Macrourus spp.) were the most important fish and overall prey species. The study did not distinguish between M. whitsoni and M. caml, but this should occur in future. Over the Ross Sea continental shelf, icefish (probably mainly Chionobathyscus dewitti), and eel cods (probably mainly Muraenolepis evseenkoi) were important prey items. Glacial squid, P. glacialis, were the most important cephalopod prey species, especially over the northwest Iselin Bank. On oceanic seamounts, toothfish fed substantially on Macrourus spp. but also deep sea cod (Antimora rostrata), a variety of cephalopods and the occasional mesopelagic to epipelagic fish.
A key aim of this study was to see whether stomach contents of toothfish could be used to detect ecosystem change in the Ross Sea region, with a focus on the area of highest fishing intensity – the northern parts of the Mawson and Iselin Banks on the Ross Sea continental slope (70°–73°S, 175°E–175°W). Variation of stomach contents with toothfish length, weight, sex, month, depth and location was analysed to investigate whether a residual year effect was discernible in stomach contents in this region between 2003 and 2010. Although these factors explained some variation in stomach contents in the Mawson and Iselin Bank block, the proportion of variance explained was relatively small, and we found no statistically significant change in toothfish diet between 2003 and 2010. Holling type I and type II feeding relationships were determined for the Mawson and Iselin Bank on the continental slope using catch per unit effort (CPUE, fish per 1000 hooks) for grenadiers, ice fish and eel cods. Compared to CPUE, icefish and eel cods were over-represented in toothfish stomachs relative to grenadiers. The fitted feeding selectivity for toothfish relative to the selectivity of baited autoline fishing gear was found to be nearly 11 times higher for icefish than for grenadiers; the relative selectivity was more than 4 times higher for eel cods than that for grenadiers. It is not known whether this is due to lower catchability by longlines, greater availability to toothfish predation for a given abundance, or higher toothfish feeding preference for icefish and eel cods relative to grenadiers.