CCAMLR

Population-genetic structure of Antarctic toothfish (Dissostichus mawsoni) was investigated not once during the last decade and before, but in this work we present our data on DNA-analysis of recent samplings of toothfish from CCAMLR Subareas – 88.1, 58.4.1, 58.4.2 and 48.6. Our findings show difference with publishes data for several genes and present new view onto toothfish population genetic structure. Our results, based on analysis of nuclear genetic markers presented in Kuhn and Gaffney, 2008, do not support genetic differentiation of samples from Ross Dependency. Difference in allelic frequency with previously published data was also found for several loci.

Commercial exploitation of fish stocks around Elephant Island and the lower South Shetland Islands ceased after the 1989/90 season. One of the larger and most abundant species in the area, Gobionotothen gibberifons, was only lightly exploited compared to more northerly fishing grounds such as South Georgia. Six surveys were conducted by the US and Germany from 1998 to 2012 to investigate if and to what extent Antarctic fish stocks recover from exploitation and how natural causes may add to man-made perturbations on a fish stock by recruitment failure as one of them.

Data from the 1980’s and 1990’s (1983, 1985, 1986, 1987, 1996 and 1998: Kock, 1986, Kock 1987, Kock, 1998, Jones et al., 1998) demonstrated that recruitment of G. gibberifrons was normal. Recruitment began to decline substantially at the turn to the 2000’s. In 2012, the proportion of immature fish (<30 cm) in the population was less than 10%. Recruitment failure was restricted to Elephant Island and the lower South Shetland Islands. Surveys in the Antarctic Peninsula in 2006 and in the South Orkney Islands in 2009 demonstrated normal recruitment. What caused a number of years of very low recruitment around Elephant Island and the lower South Shetland Islands is still unknown.

The marbled notothenia (Notothenia rossii) was the first species targeted by commercial fishing in the Southern Ocean. The species has been heavily fished around South Georgia and Iles Kerguelen in the first half of the 1970’s and at Elephant Island in 1979/80. The closure of the fishery for this species was one of the first conservation measures the ‘Commission for the Conservation of Antarctic Marine Living Resources (CCAMLR)’ adopted in 1985. Fish biomass within a CCAMLR Subarea or Division or part of it, is commonly estimated by targeting a number of species including N. rossii simultaneously in the same survey.  Surveys are conducted under the assumption that the target fish species are more or less evenly distributed over the area at the time of the survey. This assumption is violated in the case of N. rossii which shows an extremely skewed abundance in that a large proportion of the population tends to aggregate in small areas while most of the area of distribution is only thinly populated. In order to provide more accurate estimates we suggest modification to the way the survey is designed.

By way of simulation, this study investigated how the bias and precision of biomass estimates from an integrated tag-based assessment are influenced by various aspects of a multi-year tagging program, in particular the effects of the size (tag size-overlap) and numbers of tagged fish, the duration of the tagging program, the type of auxiliary data used in the assessment, and the catch history. Biomass estimates generally improved with more and better quality tagging data, however important nuances emerged within this overall trend. The results show that in the early stages of the tagging program, a high tag size-overlap is imperative to maximise the likelihood of a robust assessment. Tagging fish with a low tag size-overlap, even with large numbers, is likely to result in overestimates of biomass and the resulting data should not be used in stock assessments with only few years of data available. In contrast, with a 100% tag size-overlap even low numbers of releases and subsequent recaptures collected in short tagging programs were sufficient for relatively robust assessments. Estimation errors of SSB0 and SSB status in the assessment year stabilized or were relatively small with a tag size-overlap over 60-70%. Biomass estimates were largely unaffected by the type of catch history. Therefore, creating ‘contrast’ in the data by strongly reducing the fish biomass is not needed for tag-based assessments, since data obtained from a tagging program in recent years only can be sufficiently informative as long as estimates of the catch history are available.

The analysis of the data on size composition of Antarctic toothfish (Dissostichus mawsoni) from catches in different statistical areas of the Ross Sea was given. Biological data for two fishery periods were selected for comparison: 2003-2005 and 2008-2012. It allowed estimating changes in the size composition of the fished part of Antarctic toothfish population. It was noted that during both periods of the fishery, in the southern statistical areas (SSRU H, I, K, L), the portion of small-sized immature fish had increased though the average size of fish in catches had not changed significantly. It may evidence to the good state of the whole population.