A multidisciplinary approach to understanding the population structure of an exploited Southern Ocean top predator, the Antarctic toothfish, to improve sustainability and marine spatial planning

The Antarctic toothfish (Dissostichus mawsoni), commonly known as Chilean Sea Bass, has a critical role in Southern ecosystems as a top fish predator. Simultaneously, it represents the most lucrative Antarctic fishery.

Its fishery is managed by the Commission for the Conservation of Antarctic Marine Living Resources (CCAMLR), which introduced the world’s largest Marine Protected Area (MPA) in the Ross Sea region of the Southern Ocean in 2016.

Since 2013, scientists at the Alfred Wegener Institute in Germany have been proposing the creation of an even more expansive MPA in the Weddell Sea region of the Southern Ocean, in order to protect unique ecosystems in this region, which has largely escaped the exploitation seen in the Ross Sea, due to its historic inaccessibility. However, CCAMLR, whose 25-member country composition functions by consensus, has failed to arrive at unanimous support for the various forms of a Weddell Sea MPA (WSMPA) proposed over recent years.

A remaining impediment to the design and acceptance of a WSMPA, is a near total lack of knowledge of the life history and population structure of Antarctica toothfish in the Weddell Sea. Much of the data regarding connectivity and ontogenic movement of Antarctic toothfish derive from the Ross Sea, given the presence of an active fishery there since 1997. Based on the hypotheses that have arisen from the Ross Sea (which remain contentious), a possible life cycle of Antarctic toothfish comprises juvenile development on nutrient rich continental shelf areas, followed by passive transport via gyre systems to offshore sea mounts, where spawning occurs, prior to completion of the cycle as fish are passively transported back towards the coast.

The combination of population genetics and otolith chemistry, methodologies which define population structure via metrics of relatedness and provenance respectively, offers the possibility to fill many of the existing knowledge gaps with regards to Antarctic toothfish life history connectivity in the Weddell Sea region of the Southern Ocean. The integration of hydrographic data on water mass movement which informs both the passive transport of Antarctic toothfish at various life stages, as well as the location of important prey sources, is an integral third point of consideration, completing the development of life history connectivity hypotheses testable via the aforementioned metrics.

Tissue samples from the present study derive from otoliths (fish ear bones), which are a standard tissue extract by CCAMLR observers on Antarctic fishing vessels, historically collected for age determination. Otoliths provide both a source of DNA for genetics work, via tissue traces dried on the otolith exterior, as well as a source for chemistry analysis, via trace element analysis of otolith ring layers from the nucleus (earliest) to edge (latest) elemental depositions.

The aim of the present study is to utilize this readily available tissue source (otoliths) in order to apply both aforementioned methodologies, with the ultimate aim to test between hypotheses of single or multiple populations within the Weddell Sea, while also contextualizing those Weddell Sea population(s) within the greater Southern Ocean distribution of Antarctic toothfish.