Results of the first winter longline survey to the northern Ross Sea region to investigate toothfish reproductive life history

The first winter longline survey of Antarctic toothfish in the northern Ross Sea region was successfully completed during June and July 2016. A total of 55 longlines sets were completed in the 4 strata, and 55.2 t of Antarctic toothfish and 3.4 t of Patagonian toothfish were caught. Spawning and spent Antarctic toothfish were captured late in the survey on undersea features to the northwest of stratum 1. Gonad staging and gonadosomatic indices suggest that males in spawning condition may aggregate earlier than females and that spawning begins in early July. There were a higher proportion of male fish caught during the survey than during the summer fishery (73% vs. 60–65%), and the sex ratio varied substantially among sets. Nineteen toothfish eggs were captured using a plankton net in the top 200 m of the water column, and eggs from two running ripe females were successfully fertilised and reared for several days in flow through incubators. Egg buoyancy measurements, conducted with fertilised eggs in density gradient cylinders, are being analysed and compared with CTD data to indicate the depth of neutral buoyancy. We recommend that a survey be carried out from mid-July through August to further document the temporal extent of spawning, although much of the likely spawning habitat is under sea ice at that time.