Does Connectivity Mitigate the Impact of Climate Change on Antarctic Silverfish in the Southern Ocean?

Ocean circulation has been identified as a major process controlling the distribution of biological material in marine systems.  In the Southern Ocean, a relatively simple system with low levels of mesoscale activity facilitates testing of hypotheses concerning the connectivity of marine populations.  The Ross and Weddell Gyres, Antarctic Circumpolar Current, and the Antarctic Coastal Current are circulation features that structure populations of marine Antarctic organisms.  Antarctic silverfish (Pleuragramma antarcticum), a pelagic, neutrally buoyant notothenioid fish species, are distributed around the shelf systems of Antarctica and are considered a keystone species rivaling krill as prey for many birds, seals, whales, and other fish. However, young stages are ice-dependent, making them vulnerable to climate change.  We asked whether silverfish are distributed in independent, discrete populations along the West Antarctic Peninsula (WAP) and shelf areas bordering the Weddell Gyre, or whether the large-scale circulation has led to connectivity among populations. We hypothesized that if populations are independent, recent reductions in sea-ice may lead to extinction of local populations through drastic mortality of early stages. Alternatively, if linked by the large-scale circulation, populations in areas subsidized by migration may be more robust to declining sea ice coverage, although susceptible to shifts in transport pathways.  We found that silverfish abundances have collapsed in an area off the central WAP where ice cover has changed dramatically over the last several decades.  Along the southern WAP, where changes in sea ice have occurred more recently, length and maturity distributions suggested a population with no evidence of local recruitment since 2001, and connectivity southwards along the Antarctic Coastal Current.  However, north of the WAP where colder water from the Weddell Gyre influences the hydrography, the distributions suggested an independent population with recent recruitment and connectivity northwards along the Weddell Front to the South Orkney Islands.  Since δ¹⁸O reflects glacial runoff and subsequent hydrographic mixing, and δ¹³C reflects local prey availability, we tested these hypotheses by analyzing stable isotope concentrations in the otoliths of silverfish.  Differential structuring by the physical environment can potentially influence the impact of climate change on fish populations and their distributions in polar ecosystems.