Connectivity and population structure in Pleuragramma antarcticum along the west Antarctic Peninsula

We asked whether silverfish (Pleuragramma antarcticum) are distributed in independent, discrete populations along the Western Antarctic Peninsula (WAP), or whether the shelf circulation leads to connectivity, by testing between rival hypotheses of population segregation and linkage. If independent, reduction in sea-ice along the WAP may lead to extinction of local populations through drastic mortality of early stages which are ice-dependent. Alternatively, if linked by the large-scale circulation, abundance in areas subsidized by migration may be more robust to declining sea ice coverage. We provide preliminary data from sampling along the WAP on board the R/V Nathaniel B. Palmer, including initial simulations using a circulation model. Differences in length distributions and maturity suggested two discrete populations at 1) the north tip of the Peninsula, and 2) the southern WAP encompassing Charcot Island and Marguerite Bay, with no mixing between. The presence of a single length mode at Charcot Island and Marguerite Bay corresponding to the year class found during GLOBEC 2001 suggested a dominant cohort and no local recruitment since 2001. The collapse in silverfish abundances previously found off Anvers and Renaud Islands, without similar collapses to the north and south, suggest a third population in the central WAP. Analyses of otolith chemistry are scheduled over the next year.