A long road to recovery: dynamics and ecology of the marbled rockcod (Notothenia rossii, family: Nototheniidae) at South Georgia, 50 years after overexploitation

Philip R Hollyman,Mark Belchier,Simeon L Hill,Vladimir V Laptikhovsky,Martin A Collins,Alice Clement,Susan Gregory,Olav Rune Godo

doi:10.1093/icesjms/fsab150

Abstract

Abstract Exploitation is one of the major drivers of change in marine ecosystems. Following discovery in 1775, South Georgia saw sequential overexploitation of living resources, including seals, whales, and fish. Although exploitation is now tightly regulated, the ecosystem is still recovering. Marbled rockcod, Notothenia rossii (Richardson 1844), was the first fish species to be commercially exploited and high catches between 1967 and 1972 resulted in dramatic stock decline. Here, we use 30 years of trawl survey data to provide the first evidence of a sustained increase in the N. rossii population starting two decades after the prohibition of targeted fishing in 1985. The way species respond to change is mediated in part by trophic relationships with other organisms. We present the first multi-year, spatially-resolved comparison of adult N. rossii diet at South Georgia, highlighting a variable diet with less reliance on Antarctic krill than previously thought. Life history factors and possible heavy predation on early life stages might have delayed their recovery while diet plasticity potentially supported recent population growth. Due to the dynamic ecosystem at South Georgia and questions over catch reports from the period of heaviest exploitation, it is unlikely the current ecosystem could support a recovery to estimated pre-exploitation levels.

Highlights

Overexploitation of marine resources is an issue found throughout the world’s oceans, with an estimated 34.2% of fish stocks unsustainably fished as of 2017 (FAO, 2020)
No N. rossii were recorded deeper than 335 m and very few mature/adult individuals were collected in waters < 100 m during trammel net sampling, this suggests the preferred depth range for adults at South Georgia is ∼100–350 m
The individual trawl Catch per unit effort (CPUE) (t/h) of N. rossii around South Georgia and Shag Rocks shows an increase in large hauls (>1 t/h) between the early and later period (Figure 3)

Summary

Introduction

Overexploitation of marine resources is an issue found throughout the world’s oceans, with an estimated 34.2% of fish stocks unsustainably fished as of 2017 (FAO, 2020). Whilst some fish stocks may be resilient to moderate overexploitation and recover quickly, for many stocks this recovery has been slow due to a combination of ongoing fishing mortality and the intensity and duration of the initial exploitation (Neubauer et al, 2013). Recovery may well take several decades (Neubauer et al, 2013). Some parts of the Southern Ocean historically experienced serial overexploitation of marine resources, resulting in the establishment of the Commission for the Conservation of Antarctic Marine Living Resources (CCAMLR, a dual conservation and management body) in 1982. The sub-Antarctic island of South Georgia is found within the CAMLR Convention area, and before

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