Genetic Diversity and Population Structure of the Antarctic Toothfish, Dissostichus mawsoni, Using Mitochondrial and Microsatellite DNA Markers

The Antarctic toothfish, Dissostichus mawsoni, serves as a valuable fishery resource around the Antarctic Continent since 1997, managed by the Commission for the Conservation of Antarctic Marine Living Resources (CCAMLR). Although delineating genetic or stock structure of populations is crucial for improving fishery management of this species, its number of genetic populations and genetic diversity levels remain ambiguous. In the present study, we assessed the population genetic and phylogeographic structure of the Antarctic toothfish across 20 geographic localities spanning from Subareas 88 (88.1, 88.2, and 88.3) to Subareas 58 (58.4 and 58.5) by using mitochondrial DNA (mtDNA) cytochrome oxidase I (COI) and 16S rRNA (16S) sequences and seven nuclear microsatellite loci. MtDNA revealed a low level of polymorphism (h = 0.571, π = 0.0006) with 40 haplotypes in 392 individuals, connected only by 1–5 mutational steps, which is indicative of shallow evolutionary history. Microsatellites showed a range of allelic richness (AR) from 6.328 (88.3 RB3) to 7.274 (88.3 RB6) within populations. Overall genetic diversity was generally higher in Subareas 58 than in Subareas 88, suggesting that effective population size (NE) is larger in Subareas 58. The results of population analyses using microsatellites suggest that the sampled populations are likely to comprise a well-admixed single gene pool (i.e., one genetic stock), perhaps due to high contemporary gene flow occurring during the prolonged larval phase of this fish. However, given weak, but significant microsatellite differentiation found in six population-pairs, the possibility of existence of multiple genetic populations could not be completely excluded. The mtDNA AMOVA suggests a genetic break between the Subareas 88 and 58 groups (FCT = 0.011, P = 0.004). Moreover, mtDNA genetic distances (FST) between populations were proportionally greater as geographic distances increase. The patterns of isolation by distance (IBD) shown only in mtDNA, but not in microsatellites might suggest that population differentiation or divergence processes underwent faster in mtDNA than microsatellites, due to its NE being only one-quarter of nuclear DNA. Temporal stability in the genetic structure of D. mawsoni is also indicated by the results of no genetic differentiation between juveniles and adults. The findings of this study will help to design effective stock management strategies for this valuable fishery resource. We suggest that a long-term genetic monitoring is needed to understand the population structure and dynamics of toothfish in response to ongoing climate changes.

The Challenge to Observe Antarctic Toothfish (Dissostichus mawsoni) Under Fast Ice

In situ observation of Antarctic toothfish (Dissostichus mawsoni) is challenging as they typically live at depths greater than 500 m, in dark and ice-covered Antarctic waters. Searching for adequate methodologies to survey Antarctic toothfish in their habitat, we tested a miniaturized Baited Remote Underwater Video camera (BRUV), deployed through holes drilled in the sea ice in the Ross Sea region, over three field seasons. In 2015 three BRUVs were deployed at McMurdo Sound, and paired with a vertical longline sampling. In 2017, three opportunistic deployments were performed at Terra Nova Bay. In 2018 seven deployments at Terra Nova Bay provided preliminary data on the habitat preferences of the species. The design and configuration of the mini-BRUV allowed to collect high-quality video imagery of 60 Antarctic toothfish in 13 deployments from the fast sea ice. The behaviour of fish at the bait, intra-species interactions, and potential biases in individual counting were investigated, setting baselines for future studies on the abundance and distribution of Antarctic toothfish in sea-ice covered areas. This work represents the first step towards the development of protocols for non-extractive monitoring of the Antarctic toothfish in the high-Antarctica coastal shelf areas, of great value in the Ross Sea region where the largest MPA of the world has recently been established.

Morphological dietary composition of Antarctic toothfish (Dissostichus mawsoni) along the East Antarctic continental slope

To predict how the fishing of Antarctic toothfish, Dissostichus mawsoni , would affect the ecosystem, it is necessary to understand the species’ ecological niche. Morphological analysis of the stomach contents of 960 D . mawsoni specimens collected at depths of 946–1,600 m along the East Antarctic continental slope from December 2016 to March 2017 was used to assess dietary composition according to depth, sex, site, and size. Fishes were the most common prey item for D . mawsoni , comprising 97.8% based on the index of relative importance. Among the nine fish families consumed by D . mawsoni , Macrouridae was the dominant taxon. The size of D . mawsoni increased with depth. The dietary composition of D . mawsoni did not show significant differences by depth or sex, but did differ with site and size. D . mawsoni was the top predator in the ecosystem along the East Antarctic continental slope and can be considered an opportunistic feeder, feeding on abundant food in the environment. Therefore, additional studies of the diet of Antarctic toothfish are necessary to maintain the ecosystem structure and function in a changing environment, and the results of this study can be used as a monitoring baseline.