For the 2014 assessment of Patagonian toothfish (Dissostichus eleginoides) around the Heard Island and McDonald Islands (HIMI) in Division 58.5.2, a prior for catchability q of the random stratified trawl survey (RSTS) was estimated by comparing abundance estimates of the survey with abundance estimates calculated from the tag-recaptures data on the main trawl ground. Here, we use simulations to address potential bias in the estimation of q and conclude that the potential bias that may arise from fishing selectivity cannot be corrected for given the available data.  

In the absence of an unbiased estimate of survey catchability, we recommend that a non-informative prior with uniform distribution be used for abundance estimates from trawl surveys in a toothfish stock assessment. The bounds of such as uniform prior should be wide, e.g. 0.1-1.5, since many processes act on catchability at the same time which collectively result in an estimate of catchability that can be well below or above 1.

Toothfish in SSRUs 882.A–B are currently managed as part of the Ross Sea region stock, but the stock structure and fish movement patterns are uncertain and more information is required for the management across all of the region. Recognising this requirement, the Scientific Committee endorsed two years of spatially stratified longline surveys in the northern region of SSRUs 88.2A–B (SC-CAMLR XXXII, paragraph 3.76 (iv-c)). A multi-year, multi-Member research program was therefore established to map and sample the fishable areas of SSRUs 88.2 A–B north of 66° S latitude. Following its successful completion of the first part of the two year survey, this paper presents the details of the vessel that will conduct the second year of research during the 2015/16 season and summarises the research plan, previously agreed at CCAMLR XXXIII.

A multi-year, multi-Member research program was established to map and sample the fishable areas of SSRUs 88.2 A–B north of 66° S latitude. During the 6^th - 10^th February 2015, the UK fishing vessel Argos Froyanes surveyed SSRU 88.2 A research block 4 defined in the multi-Member 88.2 offshore research survey. Bathymetric data was collected and longline fishing conducted to release tagged fish and collect information on the characteristics of the toothfish inhabiting the region. The vessel set 7 longline sets with all vessel operations conforming to the required research criteria. A catch of 49 tonnes of D mawsoni and small catch of 29 kg of two D eleginoides was taken; 150 tagged fish were released at 3.1 fish tagged per tonne and a tag overlap statistic of 85%.

A link between D. eleginoides stocks in Subareas 48.3 and 48.4 has been speculated for some time. Different growth rates and maturity suggest that there is no regular exchange between the two areas, but tag recapture data clearly show a small number of toothfish moving between them, while genetic analysis also indicates that both stocks belong mostly to the same genetic population. Here we examine the information currently available. 

Characterisation of fishery selection is key to reducing uncertainty in the assessment and management advice provided for CCAMLR toothfish stocks. Toothfish migrate towards deeper waters as they age and increase in size and, in combination with the practical difficulties associated with deploying fishing gear at increasing depths, there is the potential for a dome shaped selection with age and size to arise. However this type of selection pattern is associated with the estimation of highly uncertain quantities of cryptic biomass. A review of the fishery and tagging data from the Patagonian toothfish (D. eleginoides) in Subarea 48.3 is used to determine whether there is sufficient information to determine the most likely fishery selection profile. A simple metric, the distribution of tag age with depth, is suggested as an indicator that a dome selection pattern is unlikely in the 48.3 fishery.   

Marine depredation (removal of fish from longline hooks by marine predators) is a growing issue in fisheries around the world. Within the CCAMLR management killer whales and to a lesser extent sperm whales cause greatest disruption to longline fishing activities for toothfish (Dissostichus sp.). Several measures have been developed and tested over the years to mitigate depredation by toothed whales, which include devices such as acoustic harassment devices (AHDs) emitting a deterring sound, the physical protection of the catch by nets, hooks or wires or changes in fishing practise such as moving to a different area when orcas are present, changing offal dumping practises or using lines of different length. However, deterrents and in particular acoustic deterrents have the potential to also affect non-target species such as fish, diving birds or pinnipeds. The different mitigation measures employed in the CCAMLR management area and their potential for disruption both in target and non-target species are reviewed below.

Depredation, the removal of fish from longlines by large marine predators, has been observed throughout the CCAMLR management area. Loss of fish through depredation can cause financial loss to the toothfish fishing industry and if not considered appropriately may lead to a bias in stock assessments through the underestimation of total mortality resulting from fishing and depredation combined. In some CCAMLR subareas this occurrence is well studied and included into stock assessments, while in other subareas the problem is known but not currently addressed. The areas in which the establishment of depredation has been studied have trialled a range of mitigation methods and developed approaches to minimize fishery-mammal interactions. This review summarises the current state of knowledge on depredation development and its impact on stock assessment, and concentrates on the four subareas that have either depredation occurring regularly or only sporadically but have the potential for development of a depredation situation, and could thus benefit of the methods and approaches tested in other subareas.