Variability in age of a Southern Ocean myctophid (Gymnoscopelus nicholsi) derived from scat-recovered otoliths

Myctophids are ecologically important in the Southern Ocean, where they occupy a central trophic position and are a key energy resource for top predators. However, understanding their population dynamics is limited by a paucity of data due to sampling challenges. Antarctic fur seal Arctocephalus gazella scats provide large collections of otoliths and other prey remains that can be used to form time series for important mesopelagic taxa such as Gymnoscopelus nicholsi (Gn). Examination of otoliths from scats allowed for a reconstruction of Gn age and length structure from 8 selected sample years between 2000 and 2015. While mean reconstructed length did not change significantly over the time series, mean age declined. Older age classes were scarce in scat samples, and age-6 animals were not found after 2008. During the same time period, Gn otoliths in fur seal scats declined from approximately 2000 to fewer than 200. The decline in the number of otoliths in Antarctic fur seal scat samples coupled with the negative trend in mean age suggests declines in the availability of Gn on the South Shetland Island slope region between 2000 and 2015. This study demonstrates the utility of central-place foragers in assessing populations of unfished but ecologically important mesopelagic fishes, thus allowing evaluation of hypotheses about their population structure and dispersal.